What is the diagnosis and treatment approach for a transitional vertebral complex?

Transitional Vertebral Complex: Diagnosis and Management

Definition and Clinical Significance

A transitional vertebral complex refers to congenital vertebral anomalies at the lumbosacral junction where the fifth lumbar vertebra may be partially or completely fused to the sacrum (sacralization) or the first sacral segment appears more lumbar-like (lumbarization), occurring in approximately 35.6% of the general population. 1

• These anatomical variants are also termed lumbosacral transitional vertebrae (LSTVs) and represent normal anatomic variations rather than pathologic conditions 1
• The prevalence in the American general population is approximately 35.6%, with most individuals being asymptomatic 2
• The most common anatomical variant is Castellvi Type IA (14.7%) 2

Diagnostic Approach

Initial Imaging Strategy

For suspected transitional vertebrae, full spine imaging using plain radiograph, CT, or MRI is required for definitive vertebral identification, particularly when surgical intervention is planned. 1

• Correct vertebral counting is essential, with the iliac crests typically aligning with the L4-L5 intervertebral space, and counting should proceed from bottom to top 1
• Lumbar MRI alone has a 14.1% diagnostic error rate for vertebral segmentation without whole-spine imaging 3
• Three-dimensional CT images are most suitable for detecting transitional vertebrae and revealing their morphological features 4

Advanced Imaging for Symptomatic Cases

For symptomatic transitional vertebrae with neurologic symptoms, MRI lumbosacral plexus without IV contrast is the preferred initial imaging modality (rating 8/9). 1

• MRI lumbosacral plexus with and without IV contrast provides optimal evaluation for acute or chronic nontraumatic plexopathy (rating 9/9) 1
• Spinal and paraspinal anatomic markers (renal artery position, aortic bifurcation, conus medullaris) are not completely reliable for diagnosing LSTVs or identifying vertebral levels 3

Critical Diagnostic Pitfalls

Wrong-level surgery occurs in 15% of LSTV cases when whole spine X-ray is not obtained preoperatively, with S1 lumbarization being miscounted more frequently than L5 sacralization (28 vs. 10 cases). 5

• Vertebral miscounting at initial outpatient evaluation occurs in 54% of LSTV cases 5
• Surgical spinal levels were altered from original plans in 15% of cases after recognizing true spinal counts 5
• Documentation of the variant must be explicit in radiology reports, with anatomical variants such as 4 or 6 lumbar vertebrae clearly mentioned 1

Classification and Morphological Features

Castellvi Classification System

• Castellvi LSTVs (located above the promontory on the arcuate line of the ilium) are most common, with Type III being the most frequent subtype (37.8%) 4
• On sagittal CT images, Castellvi LSTVs resemble normal L5-S1 anatomy 4
• On axial CT images, most Castellvi LSTVs exhibit S2-like appearances in the S1 upper segments 4

Alternative Classification

• "S6 LSTV" (with 6 sacral vertebrae and 5 foramina below the promontory) accounts for 33.3% of cases 4
• S6 LSTV resembles normal S1-S2 anatomy on sagittal CT and exhibits L5-like appearances on axial CT 4

Associated Thoracolumbar Findings

Thoracolumbar transitional vertebrae (TLTV) coexist with LSTV in 78.4% of cases, most commonly at T13 (67.8%), and overlooking TLTV causes additional vertebral numbering errors. 4

Clinical Management

Asymptomatic Cases

Most transitional vertebrae are asymptomatic and require no intervention, with their existence considered a normal anatomic variant. 1

• No specific treatment or follow-up is necessary for incidentally discovered asymptomatic LSTVs 1
• The significance to pain, degenerative changes, stenosis, and disc disease remains controversial 2

Symptomatic Cases

For symptomatic transitional vertebrae causing neurologic symptoms, treatment follows standard protocols for the specific pathology (stenosis, radiculopathy, instability) rather than treating the transitional vertebra itself. 1

• Transitional vertebrae may cause pain due to biomechanical changes in addition to progressive neurological symptoms 6
• Tethered cord syndrome patients have a 62.9% prevalence of transitional vertebrae, though no significant difference exists in most studied anomalies between TV and non-TV groups 6

Surgical Considerations

Preoperative Planning Requirements

Accurate preoperative imaging with whole-spine visualization is mandatory, using magnetic resonance imaging and computed tomography to determine vertebral levels with >90% accuracy before any spinal surgery. 1

• Intraoperative electrophysiological monitoring during pedicle screw placement has 100% sensitivity for detecting screw malposition 1
• The vertebra below Castellvi type-IIIb LSTV should be recognized as S1 anatomically, but clinically it is better to recognize it as S2 to avoid surgical errors 4

Fusion Techniques

Posterolateral fusion (PLF) and interbody fusion techniques (PLIF, TLIF, or ALIF) achieve >90% fusion rates for degenerative pathology at the lumbosacral junction. 1

• Posterior approach with pedicle screw fixation provides greater biomechanical stability 1
• Surgical planning must account for altered biomechanics at the transitional level 1

Prevention of Wrong-Level Surgery

To prevent wrong-level surgery, obtain whole-spine radiographs on admission for all patients undergoing lumbar spinal surgery, as 12.9% have LSTV and 54% of these are initially miscounted. 5

• Count vertebrae from bottom to top using the sacral promontory as the reference point 1
• Verify vertebral levels intraoperatively using fluoroscopy with anatomic landmarks 5
• Document the transitional anatomy explicitly in operative reports 1