Redefining lumbosacral transitional vertebrae classification: Morphological variation in a South African population

Background: Lumbosacral transitional vertebrae (LSTV) are common anatomical variants where the last lumbar vertebra demonstrates unilateral or bilateral enlargement of the transverse processes, with articulation or fusion to the sacrum. This radio-anatomical study sought to investigate the morphological characteristics of LSTV in the South African population and describe unique variations that enhance the traditional Castellvi classification. Methods: A retrospective radiographic review was performed on 3096 consecutive thoracoabdominal pelvic radiographs from two South African hospitals. Targeted osteological appraisal was conducted on selected skeletons containing LSTV sourced from the Raymond Dart Collection of Modern Human Skeletons. The presence of LSTV was classified according to Castellvi, identifying Types II, III, and IV, unilateral and bilateral subtypes. Results: The prevalence of LSTV was 10 %. Morphological distribution was as follows: Type II (67.9 %), Type III (27.6 %), and Type IV (4.5 %). Subtype frequencies were IIA (41.9 %), IIB (26.0 %), IIIB (21.8 %), and IV (5.8 %). Laterality showed a predominance of bilateral cases (47.7 %), followed by left- (26.6 %) and right-sided (21.1 %) variants. Type IV morphology could not be classified by side using traditional methods. Transverse processes at the LSTV level were significantly longer than those in matched controls (p < 0.05). Conclusion: This study proposes two refinements to the Castellvi system: First, Type IV morphology should be assigned left or right at the side of the lumbosacral fusion. Second, a novel transversoiliac articulation was associated with Type II morphology, representing a previously undescribed feature in LSTV morphology. These modifications may enhance diagnostic precision, support recognition of Bertolotti syndrome, and formulate appropriate treatment strategies.