Differentiation and classification of thoracolumbar transitional vertebrae

Abstract

The literature states that transitional vertebrae at any junction are characterized by features retained from two adjacent regions in the vertebral column. Currently, there is no published literature available that describes the prevalence or morphology of thoracolumbar transitional vertebrae (TLTV). The aim of this study was to identify the qualitative characteristics of transitional vertebrae at the thoracolumbar junction and establish a technique to differentiate the various subtypes that may be found. A selection of vertebral columns from skeletal remains (n = 35) were evaluated in this study. Vertebrae were taken based on features that are atypical for vertebrae in each relative region. The transitional vertebrae were qualitatively identified based on overlapping thoracic and lumbar features of vertebrae at the thoracolumbar junction. The following general overlapping characteristics were observed: aplasia or hypoplasia of the transverse process, irregular orientation on the superior articular process and atypical mammillary bodies. The results show that the most frequent location of the transitional vertebrae was in the thoracic region (f = 23). The second most frequent location was in the lumbar region (f = 10). In two specimens of the selection (f = 2), an additional 13th thoracic vertebra was present which functioned as a transitional vertebra. This study concluded that one can accurately identify the characteristics of transitional vertebrae at the thoracolumbar junction. In addition, the various subtypes can be differentiated according to the region in the vertebral column the vertebra is located in and the relative number of vertebral segments in the adjacent regions of the vertebral column. This provides a qualitative tool for researchers to differentiate the transitional vertebrae from distinctly different typical thoracic or lumbar vertebrae at the thoracolumbar junction.

Introduction

The vertebral column collectively refers to 33 vertebrae that are subdivided into five regions (Drake et al. 2009; Hansen, 2010; Moore et al. 2010; Standring, 2015). Typically, there are seven cervical (C₁–C₇), 12 thoracic (T₁–T₁₂), five lumbar (L₁–L₅), five sacral (S₁–S₅) and four coccygeal vertebrae (Rawls & Fisher, 2010). The vertebrae are numbered according to their sequence in a specific vertebral region. Vertebrae vary in size and morphology from one region of the vertebral column to the other. The region is demarcated by specific anatomical features present on the vertebrae that is unique to each region (Moore et al. 2014; Standring, 2015).

The vertebral column originates from the pre‐somatic mesoderm under regulation of the notochord (Greene & Copp, 2009). The number of vertebrae in a region may vary congenitally. Variability in the vertebral column may arise from cranial–caudal border shifts. Border shifts take place when there is a somatic shift from the typical distribution of vertebral segments in a region. This may cause an anomalous total number of vertebrae in the vertebral column (Thawait et al. 2012).

Transitional vertebrae are abnormal vertebrae that resulted from the overlap of developing somites during a cranial–caudal border shift. The affected vertebra has combined anatomical morphology of the two adjacent vertebral regions. Thoracolumbar transitional vertebrae (TLTV) are vertebrae that result from the overlap or shift of thoracic and lumbar somites at the thoracolumbar junction (Savage, 2005; Chang et al. 2007; Konin & Walz, 2010; Khairnar & Rajale, 2013; Nakajima et al. 2014; Sekharappa et al. 2014). This may alter the number of vertebrae present in the thoracic region (T₁–T₁₁ or T₁–T₁₃) or lumbar region (L₁–L₄ or L₁–L₆). In addition, cranial–caudal border shifts alter the morphology of either the thoracic or lumbar vertebrae at the thoracolumbar junction.

The thoracolumbar junction is a weak biomechanical structure (Kim et al. 2015; Maulida et al. 2015; Shin et al. 2016). This often leads to spinal instability that result in injuries, such as spine fractures and thoracolumbar junction syndrome. Thoracolumbar junction syndrome is characterized by referred pain originating at the thoracolumbar junction due to functional abnormalities. Clinical manifestations include irritable bowel symptoms, back pain, pseudo‐visceral pain and pseudo‐pain on the posterior iliac crest (Kim et al. 2013). In addition, the majority (90%) of spine fractures are related to the thoracolumbar region. Severe instability, structural damage and post‐traumatic deformity are the most frequent indications for surgical intervention (Beisse, 2006; Kim et al. 2015; Maulida et al. 2015; Shin et al. 2016).

Deviation from typical vertebral anatomy can result in confusion by clinicians, specifically orthopedic surgeons (Thawait et al. 2012). It is clinically relevant to study anatomical variations at the thoracolumbar junction, such as TLTV. Any deviation from typical morphology can result in significant clinical errors during medical procedures.

According to Thawait et al. (2012), TLTV in the thoracic region were defined by Wigh (1980) as the presence of hypoplastic ribs that are < 38 mm in length on the lowest rib‐bearing segment. This technique is insufficient as it is based on the morphology of ribs and not on the overlapping features of vertebrae. The prevalence of TLTV is unknown (Carrino et al. 2011; Thawait et al. 2012). Currently, there is no published literature available that describes the morphology of TLTV or identifies the various TLTV subtypes that may exist.

The classification according to Wigh (1980) is not sufficient to identify or differentiate TLTV. The technique described by Wigh (1980) is based on rib morphology and not on the overlapping features of the vertebra. This contradicts literature stating that intermediate vertebrae must possess intermediary features of adjacent vertebral regions. In addition, the study only considers TLTV in the thoracic region. This method was, therefore, not applied in this study, as the presence of TLTV in the lumbar region would be disregarded.

The aim of this study was to identify the qualitative characteristics of TLTV and establish a technique to classify the various subtypes of TLTV that may be found.

Methods

Ethical considerations

This study was ethically cleared by the Health Research and Ethics Committee of Stellenbosch University, which conforms to the principles within the Declaration of Helsinki (1964). The allocated ethics number is S13/05/100.

Materials

Materials for this study were provided from the Kirsten Collection at Stellenbosch University, Tygerberg Medical Campus. A selection of vertebral columns with TLTV (n = 35) were taken for this study. The selection was made from a subset of skeletal remains with random congenital defects in the vertebral column. The selection represents random individuals from a subset of the South African Western Cape population. Age, ethnicity and sex were not taken into consideration for the selection of vertebral columns.

The selection was cross‐evaluated with a control group consisting of typical thoracic and lumbar vertebrae. The anatomy of typical vertebrae is clearly defined in the published literature (Drake et al. 2009; Moore et al. 2010, 2014; Standring, 2015).

The instrument used to record the qualitative data in this study was an electro‐optical system digital© Canon camera. Qualitative data sources include observations using interpretive techniques.

Methods

This was a qualitative descriptive research study (Kothari, 2004). Data was gathered by evaluating the morphology of vertebrae.

A control was used to recognize distinctive features of typical thoracic and typical lumbar vertebrae at the thoracolumbar junction, as already established in the reviewed published literature.

Vertebrae included in the selection of TLTV were chosen based on features that are atypical for vertebrae in each relative region. Subsequent qualitative observations were made, highlighting the differences and similarities of TLTV relative to typical thoracic or lumbar vertebrae in the adjacent regions.

Statistical analysis

Statistical analysis was performed using the following program: ©Microsoft excel xlstat extension pack. This program is a free ©Microsoft excel hold‐on that can be used to analyze both qualitative and quantitative datasets. The frequencies of overlapping features identified on TLTV were listed in qualitative frequency charts. Stacked bar graphs were constructed to illustrate the frequencies of the various features (Fig. 1). This method effectively demonstrates the frequencies of defects in specimens and the overlap observed between adjacent thoracolumbar regions.

Figure 1.

Stacked frequency chart of TLTV characteristics.

Results

Identification of TLTV was based on any overlapping thoracic and lumbar features in vertebrae at the thoracolumbar junction (T₁₂–L₁).

The following observations were made about the general characteristics of TLTV in this study. The superior articular facets are directed posteriorly in thoracic vertebrae and medially in lumbar vertebrae. The orientation of superior articular facets in TLTV was observed to be similar to that in both thoracic and lumbar regions. Lumbar vertebrae have mammillary bodies on the superior articular processes not present in thoracic vertebrae. This study observed that TLTV have irregular mammillary bodies present in both the thoracic and lumbar regions. Lastly, hypoplasia or aplasia of the transverse processes was observed in transitional vertebrae at the thoracolumbar junction.

The various subtypes of TLTV were identified based on the location in the vertebral column. The frequency chart of TLTV shows that 65.71% (f = 23) of TLTV were located at T₁₂. This location was the most frequent position of TLTV in the selection. The second most frequent location of TLTV in the vertebral column was L₁. It was noted that 28.57% (f = 10) of the TLTV were located at L₁. In two specimens of the selection, 5.71% (f = 2), T₁₃ were transitional at the thoracolumbar junction.

Discussion

Thoracic vertebrae (Fig. 2) form the midline of the posterior wall of the thoracic cavity. There are 12 thoracic vertebrae; corresponding to 12 pairs of ribs (Oostra et al. 2005; Hansen, 2010; Moore et al. 2010; Rawls & Fisher, 2010; Standring, 2015). A distinct feature of thoracic vertebrae are facets for costal articulation on the vertebral bodies and transverse processes (Rawls & Fisher, 2010). In the thoracic region the superior articular facets are directed posteriorly, at a slight lateral angle. The inferior articular facets are directed anteriorly, and slightly medially (Drake et al. 2009; Moore et al. 2010, 2014; Rawls & Fisher, 2010).

Figure 2.

Morphology of typical thoracic vertebrae.

There lumbar vertebrae (Fig. 3) form the skeletal support of the posterior abdominal wall. The five lumbar vertebrae are distinguished from vertebrae in other regions by their large size (Oostra et al. 2005; Drake et al. 2009; Moore et al. 2010; Rawls & Fisher, 2010; Standring, 2015). Vertebrae in the lumbar region are characterized by large vertebral bodies. The robust structure of the vertebral bodies in lumbar vertebrae provides strong weight‐bearing structures. There are no costal facets on lumbar vertebrae for articulation with ribs (Drake et al. 2009; Moore et al. 2010, 2014; Rawls & Fisher, 2010; Standring, 2015).The superior articular facets of lumbar vertebrae are directed medially, at a mild posterior angle. The inferior articular facets are directed laterally, at a mild anterior angle. Lumbar vertebrae have unique mammillary processes on the lateral surface of the superior articular processes. The transverse processes are generally thin and long, with the exception of the L₅ (Drake et al. 2009; Hansen, 2010; Moore et al. 2010, 2014; Standring, 2015).

Figure 3.

Morphology of typical lumbar vertebrae.

This study observed that TLTV are atypical vertebrae with many variations. The vertebrae were selected based on overlapping morphology of adjacent thoracic and lumbar regions. The following observations regarding the general features of TLTV were made in this study.

In normal thoracic vertebrae, the superior articular facets are directed posteriorly (Fig. 2). Conversely, the superior articular facets in normal lumbar vertebrae are directed medially (Fig. 3). This study observed that the superior articular facets of TLTV are directed in directions that resemble both regions. This can be bilateral where the angles lie posterior‐medially or unilaterally where the facets are asymmetrical (Figs 4 and 5).

Figure 4.

Morphology of TLTV in the thoracic region.

Figure 5.

Morphology of TLTV in the lumbar region.

Normal lumbar vertebrae have mammillary bodies on the superior articular processes(Fig. 3). This feature is not present in normal thoracic vertebrae (Fig. 2). In TLTV, there were intermediary or unilateral mammillary bodies located between the transverse and superior articular processes (Figs 4 and 5).

Vertebrae in the lumbar region are characterized by long slender transverse processes(Fig. 3). Vertebrae in the lower thoracic region have prominent short, stout transverse processes (Fig. 2). In general, hypoplasia or aplasia of the transverse processes was present in transitional vertebrae at the thoracolumbar junction (Figs 4 and 5).

This study found that TLTV may be located in either the thoracic or lumbar regions of the vertebral column. According to Bron et al. (2007), vertebrae with costal facets for rib articulation are thoracic vertebrae. Contrarily, lumbar vertebrae do not have costal facets on the vertebral bodies. Transitional vertebrae can, therefore, be classified according to the region in the vertebral column that it is located in.

Within the selection, 65.71% (f = 23) of the transitional vertebrae were located at T₁₂ and are referred to as T₁₂TLTV (Fig. 5). In these cases, the transitional vertebrae assume the position of the 12^th thoracic vertebrae in the vertebral column. Transitional vertebrae at T₁₂ result from segmental shifts that affect the 12th thoracic vertebrae with no additional segments present. Despite the corresponding intermediary features of T₁₂TLTV to vertebrae in the lumbar region, the vertebrae have costal facets for rib articulation and are, therefore, classified as thoracic. When normal thoracic vertebrae are compared with TLTV located at T₁₂, the variations in morphology can be seen. The superior articular facets of T₁₂TLTV are not directed posteriorly as in normal T₁₂. Unlike T₁₂, there are structures resembling mammillary bodies located between the superior articular and transverse processes. The most accurate manner in which to describe the structures are ‘intermediary’ mammillary bodes. Lastly, T₁₂TLTV do not have prominent transverse processes, instead aplasia or hypoplasia of the transverse processes was seen. At least one additional vertebral column defect was seen in specimens with T₁₂TLTV.

The less frequent location of TLTV was at L₁, present in 28.57% (f = 10) of the selection. The transitional vertebrae are referred to as L₁TLTV (Fig. 5). In cases such as these, the transitional vertebrae assume the position of the first lumbar vertebra. The remaining specimens in the selection (f = 15) showed normal L₁ morphology. The L₁TLTV resembles vertebrae in the thoracic region but does not have costal facets for rib articulation. When the features of lumbar vertebrae are compared with those of L₁TLTV, the variation in morphology can be seen. This study found that L₁TLTV do not have long slender transverse processes characteristic of the lumbar region. Instead, aplasia or hypoplasia of the transverse processes was present. The superior facets of L₁TLTV are not orientated in a symmetrical medial direction but are asymmetrical and more posterior. Lastly, L₁TLTV do not have typical mammillary bodies present but rather remnant features of mammillary bodies that are located between the superior articular and transverse processes. In every specimen with L₁TLTV, one additional congenital defect was noted in the vertebral column.

In this study, two types of L₁TLTV were identified. The features of both L₁TLTV types remain the same. If TLTV located at L₁ result from segmental shifts that alter the morphology of the first lumbar vertebrae, without any additional lumbar segments present, this was referred to as L_1aTLTV. Majority of L₁TLTV in the study (f = 13) were L_1aTLTV.

In rare cases it was observed that an individual developed an additional somite segment at the thoracolumbar junction. If the somite develops in the lumbar region, an additional lumbar vertebra will be present (f = 2). The additional somite (L_1bTLTV) forces a caudal shift of other somites and resumes the position of L₁. The vertebral column will, therefore, be characterized by six lumbar vertebrae. The number of vertebrae in the other regions remains unchanged. There will be seven cervical, 12 thoracic and five sacral vertebrae in the vertebral column.

Alternatively, if the additional somite is located in the thoracic region, it will develop into an additional thoracic vertebra. The individual will, therefore, have 13 pairs of ribs correlating to 13 thoracic vertebrae, and five lumbar vertebrae in the adjacent region. The additional vertebrae forces a caudal shift of all lumbar vertebrae, elongating the vertebral column. This study identified two specimens (f = 2) with 13 thoracic vertebrae in the vertebral column. The transitional vertebrae were T₁₃TLTV. The number of vertebrae in the other regions remains unchanged.

Conclusion

In conclusion, transitional vertebrae at the thoracolumbar junction are congenital anomalies in the vertebral column of which little has been published in prior literature. This study successfully identified and evaluated the morphological characteristics of TLTV. This provides a qualitative tool for researchers and clinicians to differentiate TLTV from typical thoracic or lumbar vertebrae at the thoracolumbar junction. Identification of TLTV is based on the overlapping morphology of the adjacent thoracic and lumbar regions. This study concludes that various TLTV subtypes can be differentiated by the region of the vertebrae and the relative number of vertebral segments in the adjacent regions of the vertebral column. This is clinically relevant for surgeons that work at the thoracolumbar junction, as they should be aware of congenital anomalies that may complicate surgical procedures.