Author reply

Dear Editor,

The author read with great enthusiasm the letter written to the editor entitled: ‘Thoracolumbar transitional vertebrae: stages of osteological transition, mechanism of changes in zygapophyseal loading and orientation, and assimilation of mammillary processes’. The author agrees with several of the aspects addressed by the commenting author. The recognition of the significance of this topic warrants full support, in view of its clinical implications.

The commenting author, Mahota, stated in his letter that the authors of the article in question ‘suggest that their findings on transitional morphology are new in literature’. Neither transitional morphology nor transitional vertebrae is a new research topic (Carrino et al. 2011). The article states that transitional vertebrae are characterized in the literature by intermediary overlapping features between two adjacent regions (Savage, 2005; Chang et al. 2007; Konin & Walz, 2010; Khairnar & Rajale, 2013; Nakajima et al. 2014; Sekharappa et al. 2014). The article in question merely focused on bridging the information regarding the morphology of thoracolumbar transitional vertebrae (TLTV) specifically. The intermediate characteristics of TLTV do not refer to the gradual transition of morphology between regions, as transitional vertebrae are abnormal, with atypical morphology compared with vertebrae found at that specific level in the vertebral column.

Transitional vertebrae may be present at any of the regional junctions in the vertebral column and may be located at the cervicothoracic, thoracolumbar, lumbosacral or sacrococcygeal junctions. The most well documented are lumbosacral transitional vertebrae (LSTV), resulting from sacralization or lumbarization (Savage, 2005; Konin & Walz, 2010; Mahato, 2010; Carrino et al. 2011; Murlimanju & Prabhu, 2011; Thawait et al. 2012; Khairnar & Rajale, 2013; Paraskevas et al. 2013; Uçar et al. 2013; Nakajima et al. 2014; Sekharappa et al. 2014; Kershenovich et al. 2015).

The author recognized that TLTV have been identified and classified in prior literature as early as 1980 by Wigh. Bearing in mind that the findings were reported 38 years ago, Wigh (1980) classified TLTV by ‘the presence of hypoplastic ribs that are < 38 mm in length on the lowest rib‐bearing segment’ (Carrino et al. 2011; Thawait et al. 2012). This classification of TLTV is, however, insufficient, as it is based on rib morphology and not on the morphology of the vertebra itself.

As stated by Mahato, other authors have recognized the features of gradual transition between the thoracic and lumbar regions between the years of 1955 and 1999. These authors have according to Mahato ‘attempted to classify these common thoracolumbar transitions’ based on biomechanical load transitions in the thoracolumbar region. However, they did not isolate and differentiate TLTV as a transitional vertebra. They merely described the change in morphology between several vertebrae from transitional regions in the vertebral column. The transition of the thoracolumbar region and TLTV are not mutually inclusive, although approach the same concept.

The author recognizes the bridge that Mahato has provided to link the research conducted by other authors and the published article. The author read with great interest the series of cited papers on load‐bearing in the spine. The explained mechanisms between changes in anatomical features and axial loading of the vertebral components highlight the significance of transitional morphology in the vertebral column. Note that the thoracolumbar junction region is the most biomechanically unstable part of the vertebral column and is most often afflicted with injuries that require surgical intervention (Kim et al. 2013, 2015; Maulida et al. 2015). This explains the repercussions that anatomical variation may have on weight transfer of the spine. All individuals have a gradual transition of vertebral morphology between vertebral regions, with some degree of symmetry. This gradual change in morphology is necessary to accommodate mechanical load transfer, considering the spinal curvatures at the various regional junctions. Individuals with TLTV, however, have an abrupt change in morphology and features typically found in vertebrae from the adjacent region. The anatomical variation will alter the biomechanical transition in individuals with TLTV.

The article by the author reported on some findings of a larger ongoing project. Preliminary findings of this ongoing study have shown that reproducible quantitative methods can be used to differentiate typical thoracic and typical lumbar vertebrae from TLTV using statistical inference, demonstrating the natural transition between the thoracic and lumbar regions. In addition, the study observed that all of the individuals with TLTV had other congenital malformations at various locations in the vertebral column. The resulting congenital anomalies included: LSTV, neural tube defects (NTDs), spondylolysis, meristic/homeotic numeric variations and sacrococcygeal fusion. The malformations affected both the thoracolumbar junctions and other locations in the vertebral column (Fig. 1). In the study, LSTV were considered and identified by their subtypes based on published literature (Table 1).

Figure 1.

Congenital malformations and disruptions correlated to the presence of TLTV in the Western Cape population of South Africa.

Table 1.

Lumbosacral transitional vertebra frequency table

Category	Frequency	Frequency percentage
LSTV categories
Type Ia sacralization	2	5.71
Type IIa sacralization	3	8.57
Type IIIb sacralization	5	14.29
Type IIIa sacralization	1	2.86
Type IV sacralization	1	2.86
Type IIIa lumbarization	2	5.71
Total lumbarization	2	5.71
Total sacralization	12	34.29
Total LSTV	14	40.00

Mahato commented that ‘the study does not state if the samples had extra numerical vertebrae’. The article in question states in the discussion that ‘In rare cases it was observed that an individual developed an additional somite segment at the thoracolumbar junction’, referring to meristic numerical variations or extra numerical vertebrae (Asher et al. 2011). The article further states that ‘This study identified two specimens (f = 2) with 13 thoracic vertebrae in the vertebral column’.

Mahato stated that ‘the article does not discuss any mechanisms explaining these changes’, referring to the changes in TLTV morphology. Unfortunately, the submitted publication was purposefully limited to present a simple report on the qualitative morphology of TLTV. The aim of this article was not to report on the mechanism of TLTV formation or to discuss its clinical correlations of its presence; these aspects have been considered and are being evaluated currently. Based on preliminary findings, the data suggest that TLTV has congenital origins resulting from disrupted development in the vertebral column evident in correlation with other malformations observed in all individuals with TLTV. Only once the parameters are extended beyond the South African population to include a larger selection of individuals, can accurate inferences be made. This will include a conceptual research discussion regarding the congenital origins of TLTV and the clinical correlations that TLTV may have.

In conclusion, the link that Mahato reveals between the research conducted in the past by other researchers and by the author is noted as relevant although not mutually inclusive, the research studies complement each other in several aspects and reveal the significance of this topic.