Transitional Vertebrae: A Review by Dr Sherief Elsayed

Abstract

Transitional vertebrae are vertebral anomalies that exhibit features of two different vertebral types due to developmental errors during somitogenesis. This article reviews the anatomy, clinical presentation, classification, imaging findings, management, and potential for wrong level surgery in patients with transitional vertebrae. The accurate identification and characterization of transitional vertebrae is paramount to preventing neurological deficits from incorrect spinal level localisation prior to instrumentation.

Introduction

Transitional vertebrae arise during foetal development when errors occur in the complex segmentation process of somites that form the axial skeleton (1). As a result, the vertebral body and/or posterior elements exhibit features of two different vertebral types, most commonly between the lumbar and sacral regions (2). Transitional lumbosacral vertebrae have a reported prevalence between 7-30% (3). Clinically, these anomalies can complicate spinal surgery by obscuring or altering anatomy, leading to inaccurate level counting and potential neurological injury (4). This review covers the anatomy, clinical presentation, imaging findings, classification, management, and wrong level surgery risks associated with transitional vertebrae.

Anatomy

In a normal spine, the fifth lumbar (L5) vertebra rests superior to the sacrum, separated by the lumbosacral intervertebral disc. The L5 vertebra typically has the following features: a squared vertebral body, mammillary processes, and transverse processes that lack lumbar ribs (5). The first sacral (S1) segment has a trapezoidal body shape, exhibits fused mammillary and accessory processes, and possesses lateral sacral wings with sacral foramina (5).

In transitional lumbosacral vertebrae, the L5, S1 or an intermediate segment exhibit a mixture of lumbar and sacral characteristics due to errors in somitogenesis (6). The most common finding is an enlarged transverse process (occasionally with a rudimentary lumbar rib), suggesting partial sacralization of L5 (7). Complete lumbarization of S1 is less common but demonstrates a well-formed, square body and patent disc space(8). Anomalies can also involve the posterior elements, such as partial or bilateral lumbarization/sacralization of the L5/S1 facets.

Clinical Presentation

Transitional vertebrae are often asymptomatic and discovered incidentally on imaging obtained for other reasons. However, several studies have explored potential links between transitional lumbosacral anomalies and back pain or radicular symptoms:

• Low back pain - Multiple studies have found either no association or only a weak correlation between transitional vertebrae and chronic low back pain (11,12). However, other analyses noted a higher prevalence of transitional lumbosacral anomalies in patients with mechanical low back pain versus controls (27,28). The hypothesized mechanism involves altered biomechanics and accelerated degenerative changes.
• Radiculopathy - Enlarged transverse processes and abnormal articulations in transitional vertebrae may cause foraminal stenosis and nerve root compression, resulting in sciatica or radicular leg pain (10,29). The L5 nerve root is most commonly affected in sacralised L5 vertebrae, while S1 impingement occurs with lumbarised S1 segments (30). Symptoms typically arise in middle age as degeneration progresses.
• Neurogenic claudication - Narrowing of the spinal canal from transitional segmentation abnormalities can also lead to symptoms of neurogenic claudication. Patients report bilateral lower extremity pain, weakness, and paraesthesia exacerbated by walking and relieved with sitting or bending forward (31).
• Segmental instability - In rare cases, transitional vertebrae may exhibit pathological mobility leading to mechano-neuropathic symptoms. Pseudoarticulations between transverse processes permit painful microinstability (13). Traumatic spondylolisthesis is also associated with sacralisation anomalies (14).

Careful clinical evaluation and targeted diagnostic imaging are key to distinguishing symptomatic versus incidental transitional vertebrae findings. Treatment is only necessitated when a clear link exists between the anomaly and neural compression or instability causing a patient's pain.

Classification

Several classification systems exist for transitional lumbosacral vertebrae:

• Castellvi et al. developed a radiographic scale based on dimensions of the transverse processes, lumbar facets, and intervertebral disc (15): Type Ia: Unilateral enlarged transverse process with diarthrodial joint between L5 transverse process and sacral ala Type Ib: Bilateral enlarged transverse processes Type II: Unilateral (IIa) or bilateral (IIb) enlarged transverse processes with incomplete lumbarisation/sacralisation of the neural arch Type III: Complete lumbarisation of S1 with enlarged transverse processes Type IV: Complete sacralisation of L5 with assimilation into the sacrum
• Tini et al. proposed a simpler system based on CT findings of lumbarised S1, sacralised L5, or combined L5-S1 transitional vertebrae (16).
• Bertolotti's syndrome refers to enlargement and articulation of the transverse process of a sacralised L5 vertebra with the sacrum, which may cause symptomatic nerve root compression (22).
• Dysplastic spondylolisthesis describes spondylolisthesis occurring in association with congenital anomalies like sacralization of L5 (14).

Accurate categorization of transitional vertebrae morphology is essential in guiding surgical decision-making and technique.

Imaging

Radiography can detect enlargement or morphologic anomalies of the L5 transverse processes and infer alterations to the neural arch. However, CT best defines osseous anatomy and articulations to distinguish types of transitional lumbosacral vertebrae (16). Important features on CT include:

• Transverse process morphology and articulations
• Presence of lumbar facets
• Rudimentary ribs on enlarged transverse processes
• Pseudoarticulations between L5 and S1
• Disc height and morphology

MRI directly visualizes the neural elements and intervertebral discs to assess effects on neural foramina, nerve roots, and postoperative instrumentation (17). MRI findings associated with transitional vertebrae include:

• Foraminal stenosis from enlarged transverse processes or pseudoarticulations
• Nerve root compression or displacement
• Altered dimensions of the spinal canal, lateral recesses, and neural foramina
• Advanced disc degeneration
• Endplate irregularities and Modic changes

Advanced imaging should ascertain numeric vertebral levels to prevent wrong site surgery.

Management

Most transitional vertebrae require no intervention besides accurate enumeration to avoid surgical errors (18). Symptomatic pseudarthroses may be managed conservatively with rest, bracing, anti-inflammatories, or injections (19).

Surgical treatment is considered for refractory symptoms or instability. Options include:

• Decompression - Foraminal decompression by laminectomy or foraminotomy addresses nerve root impingement (33).
• Discectomy - If transitional anatomy accelerates disc degeneration and herniation, a discectomy may be indicated for intractable radicular symptoms (24).
• Fusion - Lumbar or lumbosacral fusion stabilizes pathological mobility from pseudoarticulations or dysplastic spondylolisthesis related to transitional segments (20,34).
• Osteotomy - Asymmetry from unilateral transitional anomalies can be addressed by spinal osteotomies to correct deformity (35).

Meticulous preoperative planning is mandatory when operating near transitional vertebrae. Surgeons must accurately interpret imaging to guide optimal surgical strategy and prevent incorrect level surgery.

Wrong Level Surgery

Transitional vertebrae increase the risk of wrong level surgery during lumbar procedures, which can cause catastrophic neurological deficits (21-23). Errors result from misnumbering segments or failure to recognize anomalous transitional anatomy. Several techniques help prevent wrong level surgery:

• Thorough preoperative imaging review of bony anatomy and variations from standard vertebral morphology (25,36).
• Intraoperative fluoroscopy with radio-opaque markers to confirm instrumentation placement (26).
• Use of intraoperative neurophysiological monitoring for real-time feedback on nerve root function (37).
• Meticulous exploration prior to decompression or pedicle screw insertion to identify anatomical landmarks and match imaging (38).
• Intraoperative CT or 3D navigation techniques to map patient anatomy and localize spinal levels (39).

Despite precautions, transitional vertebrae remain prone to incorrect level localization and surgical errors. Frank discussion of risks is warranted when obtaining consent for lumbar procedures in this population.

Conclusion

Transitional lumbosacral vertebrae are common vertebral anomalies that can mimic or obscure symptomatic pathology if unrecognized. Accurately diagnosing transitional anatomy on preoperative imaging is crucial to avoiding wrong level surgery. Further study of optimal classification systems and surgical strategies for symptomatic transitional vertebrae is warranted.

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