Lumbosacral transitional vertebra in military aviation candidates: A cross-sectional study

INTRODUCTION

Lumbosacral transitional vertebra (LSTV) is a common congenital spinal anomaly or rather a spectrum of anomalies involving the fifth lumbar and/or the first sacral vertebra. Castellvi et al. have classified LSTV into four subtypes.^[1] These four subtypes incorporate all the types of lumbarization or sacralization that may occur at the lumbosacral junction. Various studies have reported a prevalence rate of 4–35.9% for LSTV.^[2] Mario Bertolotti first described the morphologic characteristics of LSTV and its association with low back pain (LBP) in 1917 and this association has, therefore, been termed as Bertolotti Syndrome.^[3] Certain types of LSTV are associated with alterations in the biomechanics of the spine and the paraspinal structures leading to LBP. The presence of these types of LSTV in a candidate makes him/ her unfit for aviation duties. At present, no study has been published on the prevalence of LSTV among candidates being evaluated for fitness for aviation duties. The primary objective of this study was to determine the prevalence of LSTV among flying aspirants and secondarily, to ascertain the utility of Ferguson’s view as a supplementary radiography in the evaluation of the lumbosacral junction to identify the condition.

MATERIAL AND METHODS

RESULTS

The study included retrospective and prospective analysis of AP and lateral radiographs of the whole spine of candidates done for a period of 2 years. Radiographs of a total of 1071 candidates were examined, out of which 1031 (96.3%) were male and 40 (3.7%) were female. The youngest candidate was 16 years old while the oldest was 33 years old. The analysis revealed a total 148 cases of LSTV with a prevalence of 13.9%.

Ferguson’s view was done for a total of 30 candidates for better evaluation of LSTV, whenever there was ambiguity on AP radiographs. Out of these 30 cases, the presence and type of LSTV was confirmed by Ferguson’s view in 27 cases while in the remaining 3 cases, LSTV was excluded.

Further analysis of the data revealed that Type IIa was the most common pattern identified followed by Type IIIb. The number of candidates in various subtypes of LSTV is given at Table 2. As per the existing guidelines, 36.2% of cases (Type 1a, 1b, and IIIb) were considered fit, whereas rest (63.8%) were considered unfit for flying duties.

DISCUSSION

LSTV is a spectrum of common congenital anomalies with a prevalence rate of 4.0–35.9%.^[2] This wide variation in the prevalence of this entity is due to the varying diagnostic criteria as well as the different imaging modalities and techniques used to evaluate it, as also the clinical setting in which the anomaly is picked up. It is one of those clinical conditions which may have been underreported, as it is most often reported as an incidental finding. When it is the fifth lumbar vertebra which fuses or assimilates with the sacrum, the condition is called sacralization, whereas, the separation of the S1 segment from the rest of the sacrum is called lumbarization. It is postulated that the development of transitional vertebra depends on the weight transmitting ability of the sacrum and the surface area of its articular surface with the ilium. Sacralization or addition of a vertebra results when this area is inadequate, while if the surface area is more, release of a sacral vertebra or lumbarization occurs.^[4] Both the processes occur at a very rudimentary stage in the development of the sacrum. Increase incidence of LSTV in some families suggests a genetic component. HOX10/HOX11 is genes influencing the vertebral segmentation and mutations in these genes may play a role in the formation of LSTV.^[5]

The asymptomatic radiological conditions detected during entry medicals of the flying aspirants are given in the Indian Air Force Publication, Manual of Medical Examinations and Medical Boards, 5^th edition. As per the existing guideline,^[6] cases of LSTV with complete bilateral sacralization or lumbarization are only considered fit for flying duties. Thus, identification of the condition and appropriate classification becomes important. The classification of LSTV is generally followed as given by Castellvi et al. in 1984. It is based on the varying degree of articulation between the sacral ala and the transverse process [Table 1]. The classification, though widely followed, does not provide information regarding the accurate numbering of the involved segment. There are other morphological changes that are observed in a case of transitional vertebra. These include squaring of the upper sacral segment when sacralized and wedging of the sacral segment when lumbarized.^[7] This “squaring” or “wedging,” however, is not enough to diagnose a transitional vertebra as there is considerable overlap with the normal variants. Reduction in the disc height is noted when a lumbar vertebra is sacralized, and similarly, there may be an increase in the disc space between S1 and S2 when the former is lumbarized.^[8] Changes in the facet joints are observed at the lumbosacral junction in cases of LSTV. It may be hypoplasia or aplasia of the facet joints which may be unilateral or bilateral, asymmetrical, or symmetrical.^[9] While identifying the condition in our study, all the above variables were taken into consideration. However, the analysis was done as per Castellvi classification.

Our study involved evaluation of 1071 candidates in the age group of 16–33 years by radiography of the whole spine in the AP and lateral projection. The Ferguson’s view was added in the prospective part of the study, being done as a supplementary view, to confirm the presence and characterize the type of LSTV. The condition has been classically described on Ferguson’s projection which is AP radiographs with 30° cranial angulation. This view is an established view to delineate the lumbosacral junction.^[10] Other modalities such as CT and MRI are also useful for delineation of LSTV but are not included in the evaluation of candidates. In our study group, we did the Ferguson’s view for 30 candidates. There was an element of doubt in these cases with respect to the presence and type of LSTV. In all these candidates, the Ferguson’s view resolved the diagnostic dilemma by confirming the type of LSTV in 27 cases while excluding it in 3 cases. Figures 1 and 2 illustrate the utility of this view in the evaluation of LSTV. The prevalence of LSTV in our subset of population was 13.9% which is in accordance with the reported prevalence in the literature.^[2]

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Figure 1:

Anteroposterior (AP) and Ferguson’s view done in a candidate. It is difficult to ascertain the type of lumbosacral transitional vertebra (LSTV) based on the AP projection alone. The Ferguson’s view clearly delineates the bilateral pseudoarthrosis making it LSTV IIb. The arrows point to the articulation between the transverse process of L5 and the sacral ala.

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Figure 2:

Anteroposterior and Ferguson’s view in a candidate. The presence and type of lumbosacral transitional vertebra (Castellvi Type IV: Complete fusion on the left and pseudoarthrosis on the right) is clearly delineated on the Ferguson’s view. The arrows point to the articulation between the transverse process and the sacral ala.

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The identification of this condition during radiological examination of the candidates for flying duties is important. This is because of the following reasons; (a) few types of LSTV are not considered fit for flying duties, (b) ability of the spine to withstand aviation stress is suspect in few variants, and (c) there may be a possibility of development of backache in later part of the flying carrier.

As mentioned earlier,^[6] candidates with incomplete unilateral or bilateral sacralization or lumbarization and compete unilateral sacralization or lumbarization are being declared unfit while those with bilateral complete sacralization or lumbarization are being declared fit. As per the Castellvi classification, this translates to Type Ia, Ib, and IIIb being fit while types IIa, IIb, IIIa, and IV are unfit. The basic premise is that symmetric, bilateral, complete assimilation or separation is fit for aviation duties, whereas asymmetry of articulation between the two sides and presence of incomplete articulation or pseudoarticulation between the transverse process and the sacral ala are grounds for making a candidate unfit. Based on the above criteria, 36.2% of candidates were considered fit and 63.8% unfit in our study.

Spine is the most important part of human body which is vulnerable to aviation stress. With LSTV, when the articulations are incomplete or unilateral, spinal geometry becomes unstable. Further, this results in alteration of the transmission characteristics of the forces in case of ejection from a fighter aircraft when there is dynamic overloading of the spine. This may result in increased probability of a spinal injury in case of an ejection.

The association of LSTV with low backache was described almost over 100 years back by Bertolotti. It is postulated that the back pain in a case of LSTV may be due to various causes. It may originate from the incomplete articulation between the transverse process and the sacral ala, from the contralateral facet joint when the transitional vertebral vertebra is unilateral. It may also originate due to the early degeneration of the disc, cephalad to the transitional segment or may arise from the neural compromise caused by the hypertrophic transverse process.^[11] Hence, the possibility of development of backache in future cannot be ruled out specifically on exposure to vibration and awkward flying posture of the helicopter as well as cramped sitting and repetitive G loading of fighter flying.

Our study had some limitations. The candidates coming to us would not volunteer any information with respect to back pain as it would have made them outright unfit for aviation duties. Another limitation was the lack of Ferguson’s view in cases done before April 2019. Further, we could not access data on other types of radiological abnormalities that constituted rejection of flying aspirants; hence, the overall percentage of rejection due to LSTV could not be analyzed. In addition, the data from the other two boarding centers were not accessible to us so we had to limit the study to our establishment only.

CONCLUSION

The prevalence of LSTV on analysis of radiographs of flying aspirants over a period of 2 years was found to be 13.8%. Out of this, 63.8% were rejected as per the existing guidelines. The Ferguson’s view was found to be a useful view for cases of suspected LSTV to identify and characterize the transitional vertebrae and hence, it is strongly recommended that this view be resorted to in the evaluation of LSTV as a supplementary view.