The macroscopic and histomorphological properties of periosteal rib lesions and its relation with disease duration: evidence from the Luis Lopes Skeletal Collection (Lisbon, Portugal)

Abstract

Periosteal new bone formation (PNBF) is a common finding in a large spectrum of diseases. In clinical practice, the morphology and location of periosteal lesions are frequently used to assist in the differential diagnosis of distinct bone conditions. Less commonly reported is the presence of PNBF on the ribs. This contrasts with the data retrieved from the study of skeletonized human remains that shows a high frequency of cases and a strong, albeit not specific, association between periosteal rib lesions and pulmonary conditions (e.g. tuberculosis). Despite that, an overall disagreement regarding the specificity and non‐specificity of periosteal reactions exists in the study of dry bone remains. The insufficient number of clinical models exploring the morphology and the pathophysiology of PNBF's and the lack of systematic studies of pathological samples with a known diagnosis are claimed as major reasons for the disagreements. This study aimed to describe and compare the macroscopic and the histomorphologic appearance of periosteal rib lesions and to discuss their usefulness as diagnostic indicators. To pursue this goal, an assemblage of 13 rib samples (males = 11, females = 2, mean age‐at‐death = 36.6 years old) was collected from the Luis Lopes Skeletal Collection (Museu Nacional de História Natural e da Ciência, Universidade de Lisboa, Portugal). The assemblage belongs to individuals who died from pulmonary‐TB (group 1), non‐TB pulmonary infections (group 2) and other conditions (group 3). Prior to sample preparation, the ribs were visually inspected and the PNBF described according to its thickness, the degree of cortical integration and the type of new bone formed (e.g. woven, lamellar or both). After sampling, each bone sample was prepared for histological analysis under plane and polarized light microscopy. Macroscopically, the results showed no differences in the new bone composition between cause‐of‐death groups. Only slight differences in the degree of cortical integration, which was most frequently classified as mild to high in the pulmonary‐TB group, were observed. Histologically, no distinguishing features were identified by pathological group. However, new bone microarchitectures were observed compatible with (1) acute, fast‐growing processes (e.g. spiculated reactions), (2) long‐standing processes with a rapid bone formation (e.g. appositional layering of bone) and/or (3) chronic, slow‐growing processes (e.g. layers of compact lamellae). To some extent, these distinct rates of disease progression resonate with the cause‐of‐death listed for some individuals. Despite the small sample size, the results of this investigation are in agreement with previous studies, according to which the macroscopic and histological appearance of periosteal formations are not specific for a particular pathological conditions. Nevertheless, the results support the conclusion that the morphology of periosteal lesions is a good biological indicator for inferring the rate of progression and duration of pathological processes. This study provides important reference data regarding the histomorphology of periosteal lesions that can be used for comparative purposes, as well as to narrow down the differential diagnosis in unidentified skeletal remains.

Introduction

Periosteal reactions (PR) or periosteal new bone formations (PNBF) are a response of the periosteum, a specialized double‐layered type of connective tissue that covers the external surface of bones except in sesamoid bones and intra‐articular surfaces (Dwek, 2010). The inner ‘cambium’ layer of the periosteum possesses osteogenic potential (Ragsdale et al. 1981; Allen et al. 2004; Malizos & Papatheodorou, 2005; Dwek, 2010; Moore et al. 2014) in the face of many possible insults affecting the underlying bone or surrounding tissues, or generalized pathological processes (Rana et al. 2009; Bisseret et al. 2015).

Periosteal reactions may be caused by any disturbing agent that separates the periosteum from the underlying bone, such as blood, pus, granulation tissue, haematoma, abscess or tumour cells (Kenan et al. 1993; Weston, 2012). Other factors can also contribute to it, namely, the mechanical adaptation or compensation for weakness secondary to osteolysis attempts at tumour containment, disruption of blood circulation (passive hyperaemia) and bone‐stimulating products derived from tumours (Ragsdale et al. 1981). The response invariably consists in the deposition of new bone onto the cortical bone surface (Bisseret et al. 2015). Depending on the modalities and pace of bone production, the newly built bone may assume different configurations (Ragsdale et al. 1981; Bisseret et al. 2015). These are perceived as biological indicators of the intensity, aggressiveness and duration of the underlying insult or stimuli (Ragsdale et al. 1981). For example, slow‐growing disease processes (low intensity) produce lesions that are often solid, thick, organized and uninterrupted. When the periosteum does not have adequate time to respond, interrupted PR may develop (Richardson, 2001). On the other hand, processes that induce a rapid synthesis of woven bone over a short period of time (i.e. acute disease processes) produce a more aggressive PR (Rana et al. 2009). Woven bone is a type of primary bone formed by a matrix with an anarchic texture in which the mineralized collagen fibrils are randomly organized (Steiniche & Hauge, 2003; Junqueira & Carneiro, 2005; Young et al. 2006; Chappard et al. 2011). This lack of organization is the result of a rapid and haphazard production of osteoid by osteoblasts, as observed during foetal bone development (Young et al. 2006), in the presence of tumour and some metabolic (e.g. Paget's disease) and infectious conditions, or during fracture healing (Steiniche & Hauge, 2003; Young et al. 2006). Normally, woven bone is remodelled into lamellar or compact bone when the pathological stimulus is removed or in later stages of a disease process that became chronic (Ortner, 2008). In contrast, conditions with a slow and less intense progression (i.e. chronic or long‐standing processes) tend to develop PR with a nonaggressive appearance, mostly composed of compact lamella (Rana et al. 2009). Lamellar bone is a mature, highly organized and densely packed tissue (Su et al. 2003; Junqueira & Carneiro, 2005) characterized by ‘regular parallel bands (or lamellae) of collagen arranged in sheets’ (Young et al. 2006: 191).

PNBF is a common clinical finding in a variety of pathological conditions (e.g. trauma, infection, tumours, venous lesions, developmental disorders, endocrine disturbances), as well as in normal growth processes (Edeiken et al. 1966; Shopfner, 1966; Kenan et al. 1993; Wenaden et al. 2005; Haun et al. 2006; Tong et al. 2006; Burgener et al. 2006; Rana et al. 2009). PR are also observed in skeletonized human remains from archaeological contexts, most often in long bones, and particularly in the tibiae (Ortner, 1991, 2003; Aufderheide & Rodríguez‐Martín, 1998; Weston, 2008, 2012). In the axial skeleton, new bone formation on the visceral surface of ribs has been identified in human remains from distinct temporal periods and geographical provenances (e.g. Roberts et al. 1998; Pálfi et al. 1999; Roberts & Buikstra, 2003; Nicklisch et al. 2012; Baker et al. 2015; Pósa et al. 2015), as isolated findings (e.g. Pfeiffer, 1984, 1991; Molto, 1990; Wakely et al. 1991; Lambert, 2002; Walker & Henderson, 2010; Wescott et al. 2010) or associated with other bony changes, such as massive vertebral destruction (e.g. Haas et al. 2000; Suzuki & Inoue, 2007; Suzuki et al. 2008; Arrieta et al. 2011). In clinical settings, periosteal rib lesions are an occasional finding, often reported in patients with chronic conditions of the pleura and lungs, such as tuberculosis (Eyler et al. 1996; Guttentag & Salwen, 1999). Similar compelling evidence comes from studies of morgue cadavers (e.g. Kelley & Micozzi, 1984; Roberts et al. 1994) and identified skeletal collections from the pre‐ and post‐antibiotic era (e.g. Santos & Roberts, 2001, 2006; Matos & Santos, 2006; Steyn et al. 2013; Mariotti et al. 2015; Steyn & Buskes, 2016), in which a possible causal relationship between periosteal rib lesions and pulmonary disorders, particularly tuberculosis, has been found. However, periosteal rib lesions have also been documented in other pulmonary and non‐pulmonary conditions such as pneumonia, bronchopneumonia, chronic bronchitis, neoplastic conditions and trauma (e.g. Roberts & Buikstra, 2003; Matos & Santos, 2006; Santos & Roberts, 2006). This fact challenges the differential diagnosis of pulmonary tuberculosis in skeletonized human remains, and ultimately our understanding of host–pathogen interactions (Donoghue, 2009).

It should be stressed that the diagnosis of bone changes in skeletal remains, which falls under the scope of palaeopathology and allied sciences, is not a straightforward exercise. Most bone changes observed are circumscribed to disease‐related processes of circulatory, metabolic and mechanical stress origin (Ragsdale & Lehmer, 2012; Ragsdale et al. 2018). Bone tissue also has a limited number of ways to respond to internal or external assaults (Ragsdale & Lehmer, 2012), producing abnormalities of size, shape, density, bone formation and/or bone destruction (Ortner, 2003, 2008). Consequently, a single pathology may present a broader spectrum of morphological changes or, in contrast, different conditions may produce the same pattern of bone lesions, which impacts our ability to establish an accurate diagnosis (Wood et al. 1992; Grauer, 2008; Ragsdale & Lehmer, 2012; Ragsdale et al. 2018).

If diagnosing a pathological bone based on the macroscopic features is a challenging task, the complementary histological analysis is not free of debate. This is particularly notorious in the case of PR. No population‐based investigations and few systematic studies have been conducted, specifically in the palaeohistopathological characterization of bone lesions with known diagnoses, which imposes considerable comparative limitations (Weston, 2009). Moreover, few clinical models exploring the morphology of periosteal lesions and the processes that mediate its formation are available (Weston, 2012). As a result, some disagreement exists, mainly concerning the extent to which specific or pathognomonic histomorphological features are retained in dry bone (De Boer & Van der Merwe, 2016). Recent studies focusing on the macroscopic (e.g. Weston, 2008) and the histological proprieties (e.g. von Hunnius et al. 2006; Weston, 2009; Van der Merwe et al. 2010) of PR in long bones found an absence of specific diagnostic traits in a large array of pathologies. These findings contrast with previous studies that described ‘special telltale signs at the micro‐level’, for example, in cases of chronic treponemal diseases or leprosy (e.g. Schultz, 2001: 126; Schultz & Roberts, 2002, for a review of the diagnostic value of palaeohistopathology see De Boer et al. 2013). With regard to the histomorphology of periosteal rib lesions, single to multiple layering of new bone and increased vascularity have been observed in skeletal remains from archaeological contexts, with evidence of respiratory infections (e.g. Wakely et al. 1991; Nicklisch et al. 2012). Other authors (e.g. Schultz & Schmidt‐Schultz, 2015) state that no significant histological differences in rib lesions affected by specific (tuberculous) or non‐specific infections exist. Using a dry bone sample retrieved from individuals with known cause‐of‐death (pulmonary tuberculosis (TB), non‐tuberculosis infectious diseases and other conditions of non‐infectious origin), this study aims to describe and compare the macroscopic and the histomorphological appearance of periosteal rib lesions and to discuss its usefulness as a diagnostic indicator. Furthermore, it aims to provide information on the duration and possible aggressiveness of the underlying disease process that could be used to narrow down the differential diagnosis in unidentified skeletal remains.

Materials and methods

Thirteen ribs from individuals of known sex (males, n = 11; females, n = 2) and age at death (mean age = 36.6 years old) were selected for analysis from the Luis Lopes Skeletal Collection (LLSC) housed at the Museu Nacional de História Natural e da Ciência from the Universidade de Lisboa (Lisbon, Portugal) (Table 1). The LLSC is composed of individuals that were buried in the modern cemeteries of the city of Lisbon and whose remains were not claimed by their relatives after the stipulated period of body interment. It is assumed that these individuals most likely derived from a low to a middle socio‐economic segment of the population (Cardoso, 2006). For each individual, biographical data are available, including the individual's name, age at death, cause‐of‐death, place and year of death and occupation (Cardoso, 2006). The sample belongs to individuals that were born between 1862 and 1969 and died between 1926 and 1972. Of the individuals selected, about seven died prior to the introduction of antibiotics in Portugal, specifically penicillin and streptomycin, which were instituted as antibacterial therapy in 1944 and 1947, respectively (Pereira & Pita, 2005; Bell, 2014). The criterion that guided rib selection was the presence of PNBF. Ribs were collected from individuals who died from pulmonary‐TB (group 1, n = 9), non‐TB pulmonary infections (group 2, n = 3) and other conditions of non‐infectious origin (group 3, n = 1). Regarding bone sectioning, some limitations were imposed, a fact that has affected the number of rib samples collected. For example, most of the ribs authorized for sampling exhibited some form of postmortem damage, such as bone breakage and minor cortical flaking (at the macroscopic level). As a result, seven rib samples were retrieved from the sternal end, five from the vertebral end, and one from the rib shaft.

Table 1.

List of the rib samples collected at the Luis Lopes Skeletal Collection (Lisbon, Portugal) by cause‐of‐death group

Samples	Sk. no.	Sex/age	Year of birth	Year of death	Cause‐of‐death	Rib no.	Portion sampled
Group 1	102	M/48	1909	1958	Pulmonary TB	3rd right	Shaft
	154	M/35	1890	1926		6th right	Vertebral end
	332	M/53	1904	1958		5th left	Sternal end
	470	M/68	1864	1933		Un. right	Vertebral end
	1227	M/21	1925	1947		7th left	Sternal end
	1235	M/50	1881	1932		5th right	Sternal end
	1299	M/26	1930	1957		4th right	Sternal end
	1383	F/22	1924	1947		5th left	Sternal end
	1583	F/9	1933	1943		11th right	Vertebral end
Group 2	270	M/50	1890	1941	Bronchopneumonia	7th right	Sternal end
	1429	M/26	1907	1934	Pulmonary congestion	Un. left	Vertebral end
	1534A	M/2	1969	1972	Pneumonia	Un. right	Vertebral end
Group 3	457	M/66	1862	1929	Rectal carcinoma	5th left	Sternal end

Age at death in years. F, female; M, male; Un., unidentified rib sample.

Each rib was visually inspected with the aid of a magnifying lamp and the lesions described according to their thickness (thin deposits – flat, plaque‐like PNBF with a thickness < 1 mm, aproximately; and thick deposits – elevated PNBF with a thickness ≥ 1 mm, aproximately, and with expanded or enlarged surface); degree of cortical integration (detached appearance – a slight gap between the PNBF and the bone surface is visible; mild cortical integration – the margins of the PNBF are round and smooth, but not fully merged with the underlying cortex; and high cortical integration – the margins of the PNBF are indistinct from the cortex); and type of new bone formed (woven, lamellar, or a combination of woven and lamellar bone), following some of the recommendations presented in Buikstra & Ubelaker (1994). The presence of erosive foci was also recorded. Afterwards, each rib sample was prepared for histological analysis by plane and polarized microscopy, following the protocol of FitzGerald & Saunders (2007), as already outlined in Assis & Keenleyside (2016). It should be noted that for each rib sample, two contiguous slices of bone were cut and prepared for analysis. Several qualitative features of the PNBF were recorded and compared, such as the general microarchitecture, the extent of the bone changes and the type of bone activity observed (e.g. evidence of bone tissue remodelling). As badly preserved bone remains may negatively impact histological analysis, an assessment of the bone tissue quality was conducted using the Oxford Histological Index (OHI; Hedges et al. 1995), and by an analysis of the bone birefringence (BI; Jans, 2005). The OHI measures the extent of loss of the bone microstructure caused by microbial attack (Hedges et al. 1995) and BI categorizes the intensity of birefringence which can be affected during burial due to the loss of the organic component of the bone, with resultant loss of orientation of the hydroxyapatite crystals (Schoeninger et al. 1989: 286).

Results

Periosteal rib lesions were observed on the visceral surface of the ribs of those individuals who died from pulmonary‐TB (group 1) and non‐pulmonary TB infections (group 2). Only the rib sample of the Sk. 457 individual, whose recorded cause‐of‐death was ‘rectal neoplasm’ (group 3), showed PNBF in all rib cross‐sections. The lesions were equally observed in the rib samples of individuals who lived before and after the introduction of antibiotics in Portugal. In this section, a detailed description of the macroscopic and microscopic features of PNBF, as well as of the bone preservation and general bone microstructure, will be presented by cause‐of‐death group. A descriptive summary of the findings is also provided in Table 2.

Table 2.

Macroscopic characterization of the periosteal new bone formation (PNBF) by cause‐of‐death group

	Sk. no.	Rib no. (portion)	Bone preservation	PNBF
				Location	Macroscopic description	Histological description	Other observations
Group 1	102	3rd right (S)	Macro: well‐preserved Histo: OHI = 5 (> 95%), BI = 1	Visceral	Thick PNBF (enlarged appearance). Mixture of woven and lamellar bone. Multiple small vascular pores (centre of the lesion). Mild cortical integration	Appositional layering of new bone (mostly of woven type), intersected by primary vascular channels. Layers of distinct thickness. Multiple osteocyte lacunae. Large resorption defects between the PNBF and the cortical bone	Cortical bone: secondary osteons and interstitial bone. Haversian remodelling. Endosteal resorption
	154	6th right (VE)	Macro: well‐preserved Histo: OHI = 5 (> 95%), BI = 1	Visceral	Thick PNBF (enlarged appearance). Mostly composed of woven bone. Lamellar bone at the edges of the lesion. Widespread small vascular pores. Mild cortical integration	Appositional layering of new bone intersected by primary vascular canals. Distinct tissue organization: lamellar bone (inner layers), woven bone (outer layers). Multiple osteocyte lacunae. Resorption defects between the PNBF and the cortical bone	Cortical bone: remnants of primary lamellar bone, drifting and secondary osteons, and interstitial bone. Haversian remodelling. Slight endosteal resorption
	332	5th left (SE)	Macro: cortical flaking (>dorsal surface) Histo: OHI = 5 (> 95%), BI = 1	Visceral	Thick PNBF. Lamellar bone. High cortical integration. Small vascular pores and erosive focus (lower edge)	Layer of thick compact lamella overlying the cortical bone. Resorption defects on the periosteal surface and crossing the compact lamellae	Cortical bone: secondary osteons and interstitial bone. Haversian remodelling. Cortical and endosteal resorption
	470	Und. right (VE)	Macro: cortical flaking (> upper edge)Histo: OHI = 5 (> 95%), BI = 1	Visceral	Thin plaque‐like PNBF. Woven bone. High cortical integration. Widespread small vascular pores. Erosive foci (upper edge)	Appositional layering of new bone intersected by primary vascular canals. Distinct tissue organization: lamellar bone (inner layers), woven bone (outer layer). Multiple osteocyte lacunae (> outer layers). Resorption defects between the PNBF and the cortical bone, and affecting the inner lamellae	Cortical bone: secondary osteons, interstitial bone. Haversian remodelling. Endosteal resorption
	1227	7th left (SE)	Macro: periosteal and cortical flaking Histo: OHI = 4 (> 85%), BI = 1	Visceral	Thin plaque‐like PNBF. Woven bone. Multiple small vascular pores (centre of the lesion). Mild cortical integration	Appositional layering of new bone combining dense, compact lamella and immature bone. Resorption defects between the layers of new bone and at the interface between the PNBF and the cortex	Cortical bone: secondary osteons, interstitial bone. Haversian remodelling. Endosteal resorption
	1235	5th right (SE)	Macro: well‐preserved Histo: OHI = 3 (> 50%), BI = 0.5	Visceral	Thin plaque‐like PNBF. Lamellar bone. High cortical integration	Single layer of bone with a dense appearance. At some points, compact lamellae and vascular channels are visible. Resorption defects between the PNBF and the cortical bone	Cortical bone: secondary osteons (some poorly‐defined), interstitial bone. Extensive endosteal and cortical resorption
	1299	4th right (SE)	Macro: periosteal and cortical flaking Histo: OHI = 5 (> 95%), BI = 1	Visceral	Thin plaque‐like PNBF. Woven bone. Widespread small vascular pores. Detachable appearance	Thick bone formation composed of two almost indistinct layers: compact lamellae (inner layer); woven bone with an undulating appearance and showing primary vascular canals (outer layer)	Cortical bone: secondary osteons, interstitial bone. Endosteal resorption. Haversian remodelling
	1383	5th left (SE)	Macro: periosteal and cortical flaking Histo: OHI = 5 (> 95%), BI = 1	Visceral	Thin plaque‐like PNBF. Woven bone. Widespread small vascular pores. Margins with irregular contours. Detachable appearance	‘Arcade‐like’ bone formation (woven type). New bone formed by a ruffled rim attached to a substrate layer by small bone pillars. Presence of primary vascular canals and multiple osteocyte lacunae. Between the PNBF and the cortex, a small line of demarcation is visible; no resorption defects are present	Cortical bone: drifting and secondary osteons, interstitial bone. Haversian remodelling
	1583	11th right (VE)	Macro: well‐preserved Histo: OHI = 5 (> 95%), BI = 1	Visceral	Thin plaque‐like PNBF. Woven bone associated with an erosive focus. Mild cortical integration	Single layer of bone (Woven type). Multiple osteocyte lacunae. Resorption defects on the visceral surface	Cortical bone: primary lamellar and woven bone. Primary vascular canals
Group 2	270	7th right (SE)	Macro: periosteal flaking Histo: OHI = 5 (> 95%), BI = 1	Visceral	Thin plaque‐like PNBF. Mixture of woven and lamellar bone. Widespread multiple vascular holes. Mostly detachable from the supporting cortex. Areas with mild cortical integration	‘Arcade‐like’ bone formation attached to the underlying cortex by bone pillars. At the edges of the lesion, a substrate layer is visible. Lesion mostly composed of woven bone. Compact lamellae and osteocyte lacunae are also present	Cortical bone: circumferential lamellar bone (periosteal surface), secondary osteons, interstitial bone. Endosteal resorption. Haversian remodelling
	1429	Un. left (VE)	Macro: cortical flaking Histo: OHI = 5 (> 95%), BI = 1	Visceral	Thick PNBF (swollen appearance). Lamellar bone. High cortical integration	Appositional layering of bone intersected by primary vascular canals. Distinct tissue organization: lamellar bone (inner layers), woven bone (outer layers). Multiple osteocyte lacunae. Resorption defects between the PNBF and the cortical bone	Cortical bone: remnants of primary lamellar bone, secondary osteons. Intracortical bone. Haversian remodelling. Extensive cortical and endosteal resorption
	1534A	Un. right (VE)	Macro: periosteal and cortical flaking Histo: OHI = 5 (> 95%), BI = 1	Visceral	Thin plaque‐like PNBF. Woven bone. Widespread small vascular pores. Detachable appearance	‘Arcade‐like’ bone formation (woven type). New bone with an undulating appearance and attached to a substrate layer by bone pillars. Multiple primary vascular canals and osteocyte lacunae	Cortical bone: woven and primary lamellar bone. Primary vascular canals
Group 3	457	5th left (SE)	Macro: periosteal flaking Histo: OHI = 5 (> 95%), BI = 1	Visceral/dorsal	Thick PNBF. Thin bony spicules with a perpendicular development. Woven bone. Mild cortical integration	Spiculated bone formation. Woven bone with a spiculated morphology. Multiple osteocyte lacunae. Resorption defects at cortical level (collapsing microanatomy)	Cortical bone: secondary osteons and interstitial bone. Haversian remodelling. Extensive cortical resorption

Histo, histological observation; Macro, macroscopic observation; S, shaft; SE, sternal end; VE, vertebral end.

Group 1: Pulmonary tuberculosis

Bone preservation

Of the nine ribs analysed, five showed postmortem damage, namely, bone breakage, and periosteal and cortical bone flaking. The remaining ribs were well‐preserved. Histologically, the majority of the samples exhibited a well‐preserved bone tissue architecture (> 95% of the microstructure intact, OHI = 5) and a good bone birefringence (BI = 1). Only two samples (Sk. 1227 and Sk. 1235) showed postmortem damage, such as artifactual cracks, probably resulting from the embedding and grinding process, minor bone tissue disaggregation and crystal deposits.

Bone structure

In regard to the general bone microstructure, a number of qualitative features were shared. Six rib samples presented a mature bone microstructure composed of secondary osteons of different sizes and shapes, and interstitial bone. Enlarged Haversian canals were also observed. A combination of drifting osteons, secondary Haversian systems and interstitial bone was observed in two rib samples: Sk. 154 (male, 35 years old) and Sk. 1383 (female, 22 years old). The former also exhibited remnants of primary lamellae. One rib sample (Sk. 1583: female, 9 years old) presented an immature bone structure mostly composed of primary lamellar bone, woven bone and primary osteons. Apart from the samples Sk. 1383 and Sk. 1583, all remaining cases showed signs of endosteal resorption. The Sk. 332 sample (male, 53 years old) also presented massive Howship's lacunae affecting the cortical bone and periosteal surface.

Macroscopic PNBF observations

The macroscopic inspection of the nine rib samples revealed PNBF with variable thicknesses and degrees of cortical integration. In general, thin deposits of new bone (n = 6, 66.7%) were more frequent than thick periosteal formations (n = 3, 33.3%).

Of the six rib samples with thin plaque‐like periosteal formations, five showed lesions composed of woven bone and one presented a lamellar deposit. Differences in the degree of cortical integration were noticed. Most lesions composed of woven bone appeared slightly detached from the underlying cortex (n = 2, Sk. 1299, Fig. 1 and Sk. 1383, Fig. 2) or exhibited mild cortical integration (n = 2, Sk. 1227 and Sk. 1583). Only two samples, one presenting a thin layer of woven bone (Sk. 470, Fig. 3) and the other a lamellar deposit (Sk. 1235, Fig. 4) showed high cortical integration. Thick deposits of new bone were observed in three rib samples. The new bone formed ranged from lamellar deposits almost indistinct from the cortex (Sk. 332, Fig. 5) to a mixture of woven and lamellar bone with an enlarged appearance and mild cortical integration (Sk. 102 and Sk. 154, Fig. 6). In the two latter cases, the woven bone was deposited on the centre of the lesion, whereas the lamellar formation was seen at the edges. In addition to the new bone formation, erosive foci were also observed in three rib samples (Sk. 332, Sk. 470 and Sk. 1583).

Figure 1.

The 4th right rib of the Sk. 1299 individual (male, 26 years old). (A) Thin plaque‐like PNBF of the woven type (white arrows) on the visceral surface. Detachable appearance. Detail of the sample collected for histological analysis, before (A1) and after slide preparation (A2). (B) Micrograph showing two layers of bone with scattered osteocyte lacunae (white arrows): inner layer composed of lamellar bone (LB); outer layer composed of woven bone (WB) with a wavy‐like appearance and primary vascular canals (VC). A line of demarcation (‘cement‐line’) is seen between layers of new bone (black arrowed). Haversian systems (HS) with enlarged vascular canals (black asterisk), interstitial bone (IB) separated by cement‐lines (white arrowheads), and postmortem changes (white asterisk) are seen in the cortical bone. PS, periosteal surface. Plane light. Magnification ×100.

Figure 2.

The 5th left rib of the Sk. 1383 individual (female, 22 years old). (A) Thin plaque‐like PNBF of the woven type (white arrows) on the visceral surface. Detachable appearance. Detail of the sample collected for histological analysis, before (A1) and after slide preparation (A2). (B) Micrograph showing an irregular rim of woven bone (WB) attached to a substrate layer by small bone pillars (black arrows). Between the PNBF and the cortical bone (CB), a small line of demarcation is visible (white arrows). (C) Detail of the PNBF (black arrows) revealing some vascular canals (VC) and osteocyte lacunae (white arrows). Cortical bone composed of Haversian systems (HS) and interstitial bone (IB) separated by cement‐lines (white arrowheads). PS, periosteal surface; ES, endosteal surface. Plane light. Magnification ×40; ×100.

Figure 3.

Fragment of a right rib of Sk. 470 individual (male, 68 years old) who died from pulmonary‐TB. (A) PNBF of the woven type associated with erosive foci (white arrows). High cortical integration. Detail of the sample collected for histological analysis, before (A1) and after slide preparation (A2). (B) Micrograph displaying the cortical bone (CB) and the superimposed layers of new bone (black arrows) with distinct arrangements: innermost layers – lamellar bone (LB); outmost layers – woven bone (WB). (C) Image showing the cortical bone formed by Haversian systems (HS) and the distinct layers of new bone intercepted by longitudinal vascular canals (VC). Between the cortex and the new bone formation, a line of demarcation is visible (white arrows). (D) Another image detailing the new bone organization and the shape of the osteocyte lacunae: elongated and with a linear orientation in the lamellar bone (LB); round, more numerous, and less organized in the woven bone (WB). (E) Micrograph highlighting the exuberant PNFD (black arrows), which is almost three times thicker than the underlying cortical bone (CB). Note the thick layers of lamellar bone (LB) occupying an innermost position, whereas the woven bone (WB) is located close to the periosteal surface. Bays of osteoclastic activity were seen at the interface between the new bone and the cortex (black asterisks). (F) Detail showing a thick layer of compact lamellae (LB). Note the linear alignment of the collagen fibres, the osteocyte lacunae (white arrows) and the presence of resorption spaces (black asterisks). ES, endosteal surface; PS, periosteal surface. Polarized light. Magnification ×40, ×100.

Figure 4.

The 5th right rib of the Sk. 1235 individual (male, 50 years old) who died from pulmonary‐TB. (A) Thin plaque‐like PNBF with a compact appearance (white arrows) on the visceral surface. High cortical integration. Detail of the sample collected for histological analysis, before (A1) and after slide preparation (A2). (B,C) New bone formation with a dense appearance (black arrows), preserving Haversian and Volkmann's canals (white arrowheads), and compact lamellae (white arrows). Presence of large osteoclastic defects displaying abundant Howship's lacunae (black asterisks) between the new bone and the cortex (CB). Possible crystal deposits (white asterisks, postmortem changes). ES, endosteal surface; PS, periosteal surface. Polarized light. Magnification ×40.

Figure 5.

The 5th left rib of the Sk. 332 individual (male, 53 years old) who died from pulmonary‐TB. (A) Visceral surface slightly enlarged due to new bone deposition (lamellar type, white arrows), and showing erosive foci. High cortical integration. Image of the sample collected for histological analysis, before (A1) and after slide preparation (A2). (B,C) Micrographs showing thick compact lamellae (LB) at the periosteal surface (black arrows). Presence of multiple bays of bone resorption in the endosteal and periosteal surfaces, and in the cortical tissue (black asterisks). (D) Thick layer of lamellar bone (LB) bordering the pleural surface. Note the presence of resorption spaces displaying multiple Howship's lacunae (black asterisks). Cortical bone composed of preserved Haversian systems (HS) and interstitial bone (IL). (E) Another detail of the periosteal lamellar bone exhibiting several Howship's lacunae (black asterisks). Note the presence of a partially preserved secondary osteon (HS). ES, endosteal surface; PS, periosteal surface. Polarized light. Magnification ×40, ×100.

Figure 6.

The 6th right rib of Sk. 154 individual (male, 35 years old) who died from pulmonary‐TB. (A) Thick plaque‐like PNBF with a porous appearance (woven bone) overlying a more compact formation (lamellar bone) on the visceral surface (white arrows). Mild cortical integration. Image of the sample collected for histological analysis, before (A1) and after slide preparation (A2). (B) Micrograph showing the cortical bone (CB) and the multiple layers of bone, most of the woven type (WB). A line of demarcation between the cortex and the PNBF is seen (white arrows). (C) Detail of the superimposed layers of new bone separated by multiple vascular canals (VC). The outer layers are mostly formed by immature bone (WB), whereas the innermost layers of lamellar bone (LB). Numerous osteocyte lacunae are also present (white arrows). (D) Another micrograph showing bays of bone resorption (black asterisks) at the interface between the cortical bone (CB) and the PNBF. (E) Detail of a resorption space displaying Howship's lacunae (black asterisks) and some lamella (black arrow). Note the linear alignment of the collagen fibres in the inner layers of new bone (LB) and the presence of vascular canals (VC). (F) Image displaying a secondary osteon not completely formed located between the cortical bone (CB) and the lamella bone formed (LB). ES, endosteal surface; PS, periosteal surface. Polarized light. Magnification ×40, ×100.

Microscopic PNBF observations

When the microanatomy of the PNBF was evaluated and compared, three main types were identified: ‘arcade‐like’ new bone formation, single layers and appositional layering of bone.

In the thin sections of the rib sample Sk. 1383 (Fig. 2), an irregular rim of bone with an ‘arcade‐like’ structure attached to a substrate layer by pillars of bone tissue was observed. This ‘arcade‐like’ formation presented an immature arrangement of the collagen fibres and multiple osteocyte lacunae. The cortical bone was continuous; that is, no resorption spaces were seen between the new bone formed and the adjacent cortex.

In four rib samples, the periosteal formation appeared as single layers of new bone of variable thickness and distinct compositions. For example, a thin layer of bone with a haphazard arrangement and multiple osteocyte lacunae, and almost indistinct from the other immature tissues, was observed in the Sk. 1583 sample. Signs of osteoclastic activity that substantially compromised the rib structure on the visceral surface (costovertebral extremity) were also present. In contrast, two layers of new bone with distinct organization and separated, at some points, by ‘cement‐lines’, were observed in the Sk. 1299 thin section (Fig. 1). The innermost layer was thick and showed a lamellar orientation, whereas the outmost layer was thin, less organized and presented an irregular, wave‐like contour. In some areas, the inner layer almost merged with the cortical bone. All the above‐mentioned cases displayed an intact cortical surface and no resorption defects between the cortical and the new bone formation. In contrast, large bays of osteoclastic activity were seen beneath the PR of the Sk. 1235 rib sample (Fig. 4). In spite of the presence of considerable postmortem changes, intact Haversian and Volkmann's canals and preserved lamellar bone were observed in the new bone formation. In the Sk. 332 thin section (Fig. 5), a thick compact lamella, extending from the periosteal surface and into the cortical bone, was observed. The degree of osteoclastic activity was elevated not only in the new bone formed but also at the cortical level. Cortical involvement was manifested by endosteal resorption and cortical destruction, which also permeated the Haversian canals.

The appositional layering of bone was observed in the remaining four samples. This feature was particularly notorious in those ribs (Sk. 102 and Sk. 154) that exhibited, macroscopically, a markedly enlarged outer surface. In these cases, the new bone appeared to be organized in superimposed layers with entrapped primary vascular canals. For example, in the thin section of the Sk. 154 individual (Fig. 6), a massive PNBF was observed whose thickness was almost three times the thickness of the underlying cortical bone. This was due to the apposition of multiple layers of bone (~ 10 distinct layers), which also revealed distinct structural organizations: the innermost layers presented a linear alignment of the collagen fibres, whereas the newest layers just under the periosteum exhibited a more haphazard organization. In the Sk. 102 rib thin section, thick layers of immature bone densely populated by osteocyte lacunae were observed. Only at the interface with the cortex, did the new bone show a more lamellar organization.

As described earlier, two adjacent slices of bone were prepared and analysed by rib sample. In contrast to the previous samples that showed contiguous slices with a similar microarchitecture, different new bone arrangements were observed for the thin sections of the Sk. 470 individual. In one thin section (Fig. 3), the deposit of woven bone revealed a microstructure formed by superimposed layers of bone intercepted by primary vascular canals and with distinct levels of organization. Whereas the oldest layers (located farthest from the periosteum) contained compact lamellae arranged parallel to the bone surface, those located close to the periosteal surface were composed of woven type. The latter also showed numerous osteocyte lacunae with a round morphology, in contrast to those from lamellar bone, which displayed a more elongated form. At or near the junction between the cortical and periosteal new bone, small bone spaces were seen histologically. Similar defects were observed in the lamellar new bone formation. These spaces were recognized as the product of osteoclastic activity. In the adjacent thin section, the new bone formation appeared as a thick, bulky layer with an immature composition.

In Sk. 1227 thin sections, a less exuberant bone microarchitecture formed by the apposition of layers of compact lamellae and immature bone, particularly on the outer layers, was observed. Between the new bone formed and the underlying cortex, large spaces of osteoclastic resorption were seen.

Group 2: Pulmonary non‐tuberculosis conditions

Bone preservation

All three rib samples showed periosteal and cortical bone flaking. Black‐stained spots, probably resulting from contact with fungus, were also seen at the vertebral end of the Sk. 1429 rib sample. Histologically, all samples showed an almost intact tissue architecture (> 95% preserved) and a good bone birefringence (BI = 1). Artifactual cracks, possibly produced during sample preparation, were observed in the Sk. 270 rib sample.

Bone structure

The Sk. 1534 A rib sample (male, 2 years old) displayed an age‐related microstructure mainly composed of woven bone, primary compact lamellae and primary vascular canals. A mature bone microstructure formed by secondary osteons and interstitial bone was observed in the remaining samples. Signs of Haversian remodelling and endosteal resorption were also visible. Remnants of primary lamellar bone occupying an intracortical position were observed in the Sk. 1429 rib sample (male, 26 years old).

Macroscopic PNBF observations

Of the three rib samples analysed, two displayed thin plaque‐like periosteal formations composed of woven bone (Sk. 1534A) and a mixture of woven and lamellar bone (Sk. 270, Fig. 7). A thick deposit of new bone of lamellar type was observed in the Sk. 1429 rib sample (Fig. 8). Whereas the lesions composed of woven bone or a mixture of woven and lamellar bone appeared slightly detached from the underlying cortex; those formed by lamellar bone were characterized by high cortical integration.

Figure 7.

The 7th right rib of the Sk. 270 individual (male, 50 years old) who died from bronchopneumonia. (A) Thin plaque‐like PNBF with a mixture of woven and lamellar bone (white arrows) on the visceral surface. Slight detachable appearance (visceral surface). Detail of the sample collected for histological analysis, before (A1) and after slide preparation (A2). (B) Micrograph displaying PNBF with an ‘arcade‐like’ architecture, connected to a thick substrate layer by bone pillars (black arrows). Note the woven bone (WB) composition and the presence of numerous vascular canals (VC). (C) Micrograph of another thin section (adjacent) showing a similar new bone architecture (black arrows) composed of woven (WB) and lamellar bone (LB), and pinpointed by vascular canals (VC). No clear substrate layer is visible. Some artifactual cracks are present. In both thin sections, remnants of Haversian systems (HS) lined by Howship's lacunae (black asterisks) were observed in the cortical bone. ES, endosteal surface; PS, periosteal surface. Polarized light. Magnification ×40.

Figure 8.

Fragment of a left rib of the Sk. 1429 individual (male, 26 years old) who died from pulmonary congestion. (A) Rib with an irregular and enlarged visceral surface due to new bone deposition (lamellar type, white arrows). High cortical integration. Image of the sample collected for histological analysis, before (A1) and after slide preparation (A2). (B) Micrograph displaying a bulky PNBF (black arrows) mostly composed of woven bone (WB). Presence of primary intracortical lamellae (IL) and resorption spaces (black asterisks) between the cortex (CB) and the PNBF. (C) Detail of the new bone formed. Note its haphazard arrangement (WB) and the presence of osteocyte lacunae (white arrows). (D) Detail of the cortical bone displaying some osteons and primary intracortical lamellae (IL). (E) Micrograph of another thin section (adjacent) showing the cortical bone (CB), severely affected by bays of bone resorption (black asterisks), and the appositional new bone formation (black arrow). At some points, a line of demarcation between tissues is visible (white arrows). The inner layers of new bone present a lamellar organization (LB), the outer layers a more disorganized pattern (WB). Note the alignment of the osteocyte lacunae in the innermost layers and the presence of vascular canals (VC) between layers. ES, endosteal surface; PS, periosteal surface. Polarized light. Magnification ×40, ×100.

Microscopic PNBF observations

Two distinct new bone microstructures were observed: ‘arcade‐like’ new bone formation (n = 2) and appositional layering of bone (n = 1).

In the rib sample of the young individual Sk. 1534A who died of pneumonia, the new bone revealed a wavy‐like, immature structure with multiple primary vascular canals and osteocyte lacunae. The histological analysis of the rib sample of the individual Sk. 270 who died of bronchopneumonia also revealed an ‘arcade‐like’ new bone formation (Fig. 7). This was mostly composed of woven bone with scattered osteocyte lacunae, although some areas had a lamellar appearance, which indicates limited remodelling activity. Connecting the new bone and a subtract layer and/or the cortex, a set of bony pillars were identified. At some points, the new bone formation also exhibited some detachment from the cortex.

The two contiguous thin sections of the Sk. 1429 individual who died of pulmonary congestion revealed two distinct new bone arrangements (Fig. 8): one formed by a thick, bulky layer with immature organization and scattered osteocyte lacunae, and the other by several layers of bone with variable organizations. In the latter, spaces of bone resorption were seen separating the new bone from the underlying cortex. Dense intracortical lamellae and signs of osteoclastic activity (at the cortical level) were also observed. The presence of primary vascular canals between layers of new bone was a common observation.

Group 3: Other non‐pulmonary conditions

Bone preservation and structure

The rib sample of the individual Sk. 457 (male, 66 years old) who died of rectal neoplasm revealed a well‐preserved bone structure and a good bone birefringence, despite the presence of slight postmortem damage, namely, periosteal bone flaking. Histologically, the rib sample was characterized by a mature bone microstructure composed of secondary osteons, interstitial bone and enlarged Haversian canals. Evidences of osteoclastic activity were also visible at the cortical level.

Macroscopic PNBF observations

A thick PNBF was observed on the visceral and dorsal surfaces of the Sk. 457 rib sample (Fig. 9). As a consequence, the rib body appeared abnormally enlarged. The periosteal bone formation was composed of thin, immature bony spicules with a perpendicular development, and mild cortical integration.

Figure 9.

The 5th left rib of Sk. 457 individual (male, 66 years old) who died from rectal neoplasm. (A) Exuberant new bone formation with a spiculated appearance, affecting the entire rib cross‐section. Image of the sample collected for histological analysis, before (A1) and after slide preparation (A2). (B,C) Micrographs displaying the parallel spicule or streamers of immature bone (white arrows) and the foci of bone resorption (black asterisks) at the interface with the cortical bone. (D) Image of the cortical bone showing Haversian systems (HS) with enlarged vascular canals, interstitial bone (IB) and spaces of osteoclastic resorption displaying Howship's lacunae (black asterisks). Polarized light. Magnification ×40, ×100.

Microscopic PNBF observations

Histologically, a spiculated PNBF was identified in the Sk. 457 sample (Fig. 9). Structurally, the new bone was composed of more or less parallel spiculae of immature bone of different sizes connected at the base by smaller struts of bone. The bone spicules were perpendicularly orientated in relation to the cortex, being thicker at the base and thinner at the extremity. No clear lamellar structure was seen. Large bays of bone resorption (Howship's lacunae) were seen in the transition zone between the periosteal new bone and at the cortical level.

Discussion

Notwithstanding the frequency of periosteal lesions in clinical and palaeopathological contexts, few studies have evaluated the macroscopic, and specifically the histological, appearance of new bone formations on the ribs in order to explore their diagnostic value. In this study, we have characterized and compared the gross appearance and the histomorphology of proliferative rib lesions using a dry bone sample of individuals with known cause‐of‐death.

Periosteal rib lesions and cause‐of‐death

Despite the small sample size, the results showed similar new bone formations between the cause‐of‐death groups. For example, periosteal formations composed of woven bone were present in all groups. Slight differences in the degree of cortical integration were observed, particularly in group 1, which showed more lesions with mild to high cortical integration compared with the other groups. With regard to the histological examination, no distinguishing features were identified by pathological group. The appositional layering of bone and the ‘arcade‐like’ new bone formation was found equally in individuals from group 1 and group 2. It is worth mentioning, however, that the most exuberant periosteal formations, i.e. appositional layering of bone and spiculated new bone formation, were mostly seen in group 1 and group 3 causes of death, respectively. The results of this investigation are in line with previous studies (e.g. Weston, 2008, 2009, 2012), according to which the macroscopic and histological appearance of periosteal formations are not specific for individual pathological conditions.

When considered the year of death of the individuals, i.e. prior to or after the introduction of antibiotics in Portugal, no major differences in the extent and appearance of lesions were found. Steyn et al. (2013), investigating whether bone lesions associated with TB were more or less common in the post‐antibiotic period, found that the skeletal involvement increased considerably with the introduction of antibiotics (from 21 to 38%). Similar results were obtained by Steyn & Buskes (2016) studying a larger South African sample with known cause‐of‐death. According to authors, the high frequency of lesions supports the idea that antibiotics would lead to an extended course of disease, allowing more time for skeletal lesions to develop (Steyn et al. 2013). In the rib sample studied, the presence of similar lesions in individuals who lived and died before and after the introduction of antibiotics suggests that their immune system was strong enough to ensure relative survival, at least until the development of bone lesions.

In the interpretation of the bone changes with regard to the cause‐of‐death, one should also consider the inherent limitations of the skeletal collections. For most documented skeletal collections, the cause‐of‐death is known; however, this information does not indicate whether other illnesses have affected the individuals, and ultimately the skeleton. This is the case for the skeletal collection used in this research. In fact, the lack of comprehensive clinical historiographies constitutes a major impediment to the difficult task of corroborating the relationship between the types of bone lesions and the cause‐of‐death. Compounding this problem is the absence of records on the progression, duration and severity of the conditions listed as the causes of death. Finally, there is always the possibility of an inaccurate record concerning the cause‐of‐death, or misdiagnosis (Brickley & Ives, 2008).

Periosteal rib lesions and aggressiveness of disease processes

Although there is a lack of specific diagnostic traits, the appearance of the periosteal rib lesions unveiled characteristics that are probably related to the duration of the inciting disease process (or processes). Accordingly, the samples studied identified PNBF compatible with long‐standing processes with rapid bone formation (appositional layering of bone), chronic, slow‐growing processes (layers of compact lamella), and acute, fast‐growing processes (single layers or ‘arcade‐like’ new bone formation of woven type and spiculated formations).

Long‐standing processes with rapid bone formation

In four individual who died of pulmonary‐TB (group 1) and in one individual who died of pulmonary congestion (group 2), several superimposed layers of bone were seen enclosing longitudinal and/or round primary vascular channels, mimicking and amplifying the modelling process of the periosteal and endosteal membranes (PEM), as described by Maggiano (2012) in cases of rapid bone growth. In some individuals (e.g. group 1: Sk. 154 and Sk. 470), the formation of multiple layers of bone appeared to have occurred almost uninterruptedly during a period of rapid and intense growth. On the other hand, the observation of layers of bone with distinct compositions (outermost layers – woven or immature bone; innermost layers – lamellar bone) indicates some degree of bone remodelling consistent with the existence of an underlying long‐standing disease. Another common feature was the presence of osteoclastic bone resorption between the inner lamella of new bone and the cortical substance. This observation implies that remodelling was underway at the interface between the appositional new bone and the original cortical surface, suggesting that a long‐standing (e.g. healed) lamellar new bone formation was being integrated into the existing cortical structure. Studying a thin section of an infant femoral mid‐diaphysis, Maggiano (2012) observed a row of irregular resorption spaces. According to the author, these resorption bays were designed either for expansion during endocortical resorption or to be infilled via BMU‐based remodelling (Maggiano, 2012: 57). Figure 6 (E,F) illustrates this process, showing an active resorption space with Howship's lacunae and discrete lamella, and a vascular canal with few circumferential lamellae (Haversian system) that was probably being formed by BMU. The BMU consist of coupled osteoclasts and osteoblasts that act in response to external signals or stimuli, eroding and refilling the bone surface (Ott, 2002; Robling et al. 2006; Robling & Stout, 2008) in order to maintain bone tissue homeostasis (Stout & Crowder, 2012). The bone cell activity described is probably a response to the stage of the disease process. It suggests that the new bone formation was of sufficient long‐standing for the normal remodelling process to be altering it, indicating a lack of acute infectious response. The presence of immature bone in the outer layers, as well as of numerous round osteocyte lacunae, seems to indicate that the underlying condition was still active when the individuals died. According to Franz‐Odendaal et al. (2006), the size and shape of the newly formed osteocytes vary according to the activity and size of the committed osteoblasts, as well as with the type of bone formed; for instance, in woven bone the osteocytes are isodiametric, whereas in lamellar bone they are flattened and oblate, with their long axis parallel to the thickness of the lamellae.

The above‐mentioned new bone characteristics closely parallel the histopathology of lamellated PR, also termed multilamellar or multilayered reactions (e.g. Ragsdale et al. 1981; Wenaden et al. 2005; Rana et al. 2009), as described in the radiology literature. In this form, concentric cylindrical or discoid layers of new bone are deposited on the cortex, producing an ‘onionskin like’ appearance (Wenaden et al. 2005; Rana et al. 2009; Ragsdale et al. 2018). The initial layer of a reactive lamella is composed of immature bone. In indolent or long‐standing conditions, the layers may become progressively thicker by surface apposition of lamellar bone (Ragsdale et al. 1981, 2018). The thickest lamellae occupy an innermost position, close to the cortical bone, whereas the newest layer lies just under the periosteum (Ragsdale et al. 1981). Prominent vascular dilations are normally seen separating the layers of mineralized periosteal bone. During each lamellation, a predominance of osteoblastic activity is found on the periosteal surface, whereas osteoclastic activity prevails on the side facing the cortical tissue (Ragsdale et al. 1981). A lamellated PR is normally seen in rapidly growing processes of intermediate aggressiveness, which may be malignant (more common) or benign (Richardson, 2001; Bisseret et al. 2015). It may form in cases of osteomyelitis, aneurysmal bone cyst, stress fractures, hypertrophic pulmonary osteoarthropathy, Ewing sarcoma, osteosarcoma and chondroblastoma (Ragsdale et al. 1981; Wenaden et al. 2005; Rana et al. 2009). Several hypotheses has been advanced to explain this type of bone formation (for an overview see Ragsdale et al. 1981, 2018; Wenaden et al. 2005; Rana et al. 2009; Bisseret et al. 2015). It has been hypothesized that strong local stimulus such as hyperaemia or infection may activate the osteoblast potential via modulation of sheets of fibroblasts in adjacent soft tissues, leading to the formation of successive layers of new bone (Wenaden et al. 2005; Rana et al. 2009; Bisseret et al. 2015). In the particular case of periosteal rib lesions, it is believed that the formation of new bone occurs as a result of local hyperaemia from an adjacent inflammatory process (Eyler et al. 1996). The exact mechanism is unknown.

Chronic, slow‐growing processes

Signs of long‐standing processes were observed in three samples from group 1. Both the macroscopic and histological analysis of the Sk. 1235 rib sample showed a layer of compact bone (lamella). This type of lesion may be due to a slow process, as chronic conditions may stimulate the formation of compact bone (Ortner, 2008) and/or suggest a condition that was quiescent or overcome at the time of death. In the Sk. 1299 rib thin section, a thick and dense inner layer with a lamellar organization was identified. Overlying it, new bone with a more immature appearance and pinpointed by primary vascular canals was seen. To some extent, the microarchitecture of the inner layer resembles a solid continuous PR compatible with a slow, chronic process. This forms by slow apposition of layers of new bone (compact lamella) to cortex around an indolent lesion within the marrow space, in the cortex or in adjacent soft tissue (Ragsdale et al. 1981, 2018). It is described as a nonaggressive form primarily observed in slow and benign processes (Rana et al. 2009), such as infectious processes (e.g. hypertrophic osteoarthropathy, osteomyelitis, haemorrhage, vascular diseases), benign tumours (e.g. osteoid osteoma, osteoblastoma, eosinophilic granuloma) and healing fractures (De Santos, 1980; Greenfield et al. 1991). Conversely, the histomorphology of the outer layer is consonant with a rapid disease process. It is not clear whether both bone microarchitectures were caused by the same condition. However, it is well known from the literature that a chronic condition may have several episodes of acute new bone formation (Ortner, 2008). The hypothesis that it constitutes an early stage of a lamellated reaction cannot be fully discarded. In the palaeopathological cases reported by Wakely et al. (1991), the new bone formation also ranged from only one layer to up to four superimposed layers. A similar interpretation can be derived from the Sk. 332 rib thin section. The thick and densely packed lamellae observed may be the result of a solid PR. Nevertheless, remnants of appositional new bone deposition, now completely remodelled, may also justify the solid formation. Wenaden et al. (2005: 449) stress that in long‐standing conditions, the matrix between multiple lamellations or between a single layer and the cortex may eventually ossify, originating a continuous, solid layer of periosteal new bone.

The presence of lesions with a lamellar composition and/or with signs of bone remodeling – as also reported for those ribs with rapid bone formation – suggests the existence of a chronic condition, one that the individuals have endured for months or even years, and one that their immune system was able to deal with (Roberts & Buikstra, 2008). This observation may relate to the cause‐of‐death listed for some individuals, e.g. pulmonary‐TB and pulmonary congestion. Tuberculosis is a chronic infectious disease (mainly caused by Mycobacterium tuberculosis), in which the lungs are the prime target, although any organ of the body may be affected (Schwartz, 2012). The course of disease depends on several factors, such as the age and immune competence of the host, and the total burden of organisms (Schwartz, 2012). Nevertheless, only secondary or post‐primary TB, which often culminates in an active, symptomatic disease (Schwartz, 2012), leaves a visible mark on the skeleton (Roberts & Buikstra, 2008). Pulmonary congestion (or oedema) results from the accumulation of fluids in the lung tissues (Shiel et al. 2008). Pulmonary venous engorgement secondary to passive hyperaemia or congestion, i.e. the ineffective exit of blood through venous pathways that causes organ distension, frequently leads to pulmonary oedema (Schwartz, 2012: 268). It may be caused by cardiogenic and non‐cardiogenic causes, some of them with chronic developments (Pappas & Filippatos, 2011). Nevertheless, haemodynamic alterations in the heart are the most common causes (Schwartz, 2012). It is known from the literature that passive hyperaemia may impact bone by creating environmental conditions that favour the action of osteoblasts, and subsequent production of new bone (Ragsdale & Lehmer, 2012).

Acute, fast‐growing processes

Some individuals who died of pulmonary‐TB also showed rib lesions compatible with acute unfolding processes. For example, PNBF with an ‘arcade‐like’ structure and attached to the supporting cortex by pedestals, were observed in one individual who died from pulmonary‐TB (group 1), and in two individuals who died from pneumonia and bronchopneumonia (group 2). In clinical medicine, new bone formations composed of fibro‐osseous trabeculae bound together in the form of arcades are typically found in inflammatory processes, such as acute osteomyelitis (Adler, 2000). This type of new bone formation may resonate with the listed cause‐of‐death, particularly for those individuals from group 2, as pneumonia and bronchopneumonia are inflammatory lung conditions that may show an acute evolution. A different interpretation for this bone formation can be found in palaeopathological literature (e.g. Schultz, 2001, 2003, 2012), where a correlation with haemorrhagic processes, such as subperiosteal haematomas, has been proposed. According to Schultz (2001), the presence of an intact cortical surface is considered a distinguishing feature not seen in cases of inflammation. In the rib samples analysed, the surface of the original cortical surface seems unaffected by the pathological process, which may suggest that the new bone formation was caused by an unknown haemorrhagic process. De Boer et al. (2015) also discussed the presence of a separable periosteal callus, connected to the cortical tissue by bone pillars in a case of a metacarpal bone fracture. Nevertheless, an ‘arcade‐like’ histomorphology was described by Weston (2009) in cases of osteomyelitis and leg ulcer, which are related more to inflammatory processes. Inflammation is a natural reaction of body tissues and microcirculation to internal and external pathological insults. Accordingly, any agent that causes tissue injuries, such as trauma, ischaemia, neoplasm or infectious agents, can trigger an inflammatory response (Murphy, 2012). The hypothesis that similar lesions can be seen at different times in distinct pathologies due to shared physiological responses (e.g. inflammation) may eventually explain the similarities found. Adler (2000) mention that dense, irregular networks of trabeculae with large osteocytes and with an arcade configuration may also develop in inflammatory processes secondary to trauma (Adler, 2000). This corroborates the assumption that a considerable overlap between the appearances of PR may occur between conditions (Rana et al. 2009; Weston, 2012). The overlap occurs because the type and extension of the PR also depend on a combination of factors that comprise the biological activity of the underlying process, the patient age and metabolic state, and the anatomical location (Kenan et al. 1993). Despite the uncertain aetiology of the ‘arcade‐like’ new bone formations, its immature composition and limited remodelling activity suggest a rapid growth process that was probably active at the time of the death of individuals.

The exuberant spiculated new bone formation observed in the Sk. 457 rib sample is also consistent with an aggressive, fast‐growing process. At some extent, it mirrors the pattern seen in some complex PR, such as the divergent spiculated or ‘sunburst’ pattern (Ragsdale et al. 2018). This pattern, as well as the ‘hair‐on‐end’ pattern, forms when the inciting mechanism grows rapidly but steadily. As a result, the periosteum does not have time to produce new bone, at least not as fast as the lesion grows (Richardson, 2001). A spiculated reaction forms along periosteal vascular channels and fibrous bands (Sharpey fibres) that are stretched away from the cortex (Edeiken et al. 1966; Wenaden et al. 2005). The divergent spiculated pattern is a complex PR often seen in aggressive tumours such as osteosarcomas. It is characterized by fine spicules of new bone with variable thickness and a divergent orientation from the cortex (Wenaden et al. 2005; Bisseret et al., 2015; Ragsdale et al. 2018). In the study of dry bone sections of two osteosarcomas, De Boer & Maat (2018) observed a combination of cortical and cancellous bone defects and periosteal depositions of the woven type. The latter exhibited ‘an irregular, trabecular architecture, oriented more or less perpendicularly to the cortical surface’ (De Boer & Maat, 2018: 5). Despite the similarities with the histomorphology of an osteosarcoma, the periosteal rib lesion of the Sk. 457 individual may correlate with the listed cause‐of‐death, i.e. rectal neoplasm. Albeit less frequently, ‘sunburst’ PR also occur in aggressive osteoblastic metastasis and haemangiomas. In a retrospective study of 70 cases of metastatic bone involvement, Bloom et al. (1987) found a higher frequency of sunburst PR in cases of prostatic carcinoma, followed by gastrointestinal and bronchial tumours. Occasionally, the metastases from colon or rectal carcinoma may resemble an osteosarcoma exhibiting PR with a sunburst pattern (Burgener et al. 2006).

Comparing macroscopic and histological observations

Results from this investigation suggest that visual inspection may not be sufficient to characterize the new bone formation, or the degree of remodelling. Moreover, it showed that trying to infer the microarchitecture of lesions based on their macroscopic appearance may be problematic. Although an apparent association between some macroscopic and histological features, such as between thick periosteal formations and appositional layering of bone or mild/high cortical integration and signs of tissue remodelling, contrasting results were also found. For example, of the five rib samples with thick new bone formations, three revealed a histomorphology characterized by appositional layering of bone, one showed an exuberant spiculated new bone formation, and the other showed a single layer of compact lamellae. The appositional new bone formation was equally found in individuals classified macroscopically as having a woven type of bone, and in those with a more compact or lamellar bone. The opposite was also found. For example, in the Sk. 102 sample, the PR showed a combination of woven and lamellar bone; nevertheless, only a more haphazard arrangement of the collagen fibres was observed histologically. New bone microarchitectures formed by single layers or by ‘arcade‐like’ formations were more common in rib samples with thin plaque‐like PRs. An exception was noted for the rib of the individual Sk. 470 (group 1), which showed appositional layering of bone.

The histological analysis was essential to observe bone tissue dynamics related to the regular processes of bone modelling and remodelling. The immature microarchitecture of some rib samples (group 1: Sk. 1583; group 2: Sk. 1534A) agrees with the age at death listed for those individuals. Similarly, the presence of drifting osteons, i.e. primary or secondary osteons presenting an elongated or extended spiral morphology and a non‐centrally located Haversian canal, is often regarded as a remnant of the subadult cortical tissue (Robling & Stout, 1999; Pfeiffer, 2006; Streeter, 2012). The observation of Haversian remodelling and endosteal resorption also resonates with the age‐related changes described in the literature for mature ribs (Pfeiffer, 2006; Streeter, 2012; to name only a few). Nevertheless, one cannot exclude the possible cumulative effect that the disease listed as the cause‐of‐death (or other conditions not recorded) may have had on the progression and/or severity of the changes observed. Some authors (e.g. Hardy & Cooper, 2009) have stressed that systemic inflammation commonly impacts the normal bone resorption/formation, leading to bone loss but rarely to bone gain.

When compared with the visual inspection, histology was useful to evaluate the quality of the bone tissue and to confirm, or discard, the type of bone changes previously identified through visual inspection. Macroscopically, three rib samples from group 1 displayed osteolytic foci (Sk. 332, Sk. 470 and Sk. 1583). However, no corresponding periosteal and cortical defects were recorded in the Sk. 470 sample. This means that the ‘osteolytic foci’ observed macroscopically were not true lytic lesions but probably the result of the coalescence of vascular canals. The osteolytic lesion observed in the Sk1583 sample is similar to those of skeletal tuberculosis. It should be mentioned that this individual also presented severe destructive foci on the thoracic vertebrae (mostly T4–T12), ankylosis and gibbous formation, which is consistent with a case of tuberculous spondylitis, also termed Pott's disease (for a review of the common manifestations of tuberculosis spondylitis, see e.g. Steinbock, 1976; Ortner, 2003; Resnick & Kransdorf, 2005). The application of histological techniques was particularly useful in the Sk. 457 rib sample (group 3). The techniques revealed extensive cortical defects beneath the new bone that was not entirely visible through the naked eye. This histological finding is consistent with previous observations, according to which a combination of osteolysis and new bone formation is frequently recognized in metastasizing lesions, independently of their macroscopic appearance (Adler, 2000).

The application of histological techniques also showed that the appearance of the new bone may change locally, as different microarchitectures may be encountered in adjacent thin sections. This observation highlights the need to analyse different regions in a bone lesion in order to identify possible variations.

Conclusion

In the rib samples studied, no specific PNBF as regards the cause‐of‐death were observed, both at the macroscopic and histological level. However, PR compatible with acute, fast‐growing processes (single layers or ‘arcade‐like’ new bone formation of woven type and spiculated reaction), long‐standing processes with rapid bone formation (appositional layering of bone) and chronic, slow‐growing processes (layers of compact lamella) were observed. For some individuals, the macroscopic and histological proprieties of the periosteal bone resonate with the pathobiology of the disease listed as cause‐of‐death, whereas for others it was more difficult to find any association. The small sample size also hampered the establishment of more conclusive results. Despite that, the periosteal new bone features described here are a useful tool to better understand the range of bone responses and associated inciting processes and thus helping narrow down the differential diagnosis in unidentified cases. This adds further data regarding the histomorphology of periosteal rib lesions for comparative purposes.

Future studies based on well‐documented collections with accurate medical records and/or based on samples retrieved from clinical cases will eventually solve some of the problems and limitations identified in this study. Increasing the sample size will also improve our understanding of the entire spectrum of new bone variation associated with a particular condition and test, for example, the strength of the association between multiple appositional bone growth and TB infection. Further research in the field of pathophysiology may eventually shed light on the exact mechanism (or mechanisms) behind this particular type of bone response.

Conflict of interest

The authors have no conflict of interest to declare.