Transitional Changes at the Lumbosacral Junction on X-ray
Transitional changes at the lumbosacral junction are normal anatomic variants that require recognition but typically do not necessitate treatment unless symptomatic; the primary clinical significance lies in accurate vertebral labeling to prevent wrong-level surgery.
What Transitional Changes Mean
Lumbosacral transitional vertebrae (LSTVs) represent congenital vertebral anomalies at the L5-S1 junction where the fifth lumbar vertebra may be partially or completely fused to the sacrum (sacralization) or the first sacral segment may appear more lumbar-like (lumbarization) 1. These variants occur in approximately 35.6% of the general population, making them relatively common findings 2.
Classification and Recognition
The most common anatomical variant is Castellvi Type IA, occurring in 14.7% of cases 2. When evaluating these changes on imaging:
- Correct vertebral counting is essential - the iliac crests typically align with the L4-L5 intervertebral space, and counting should proceed from bottom to top 1
- Full spine imaging may be required - plain radiograph, CT, or MRI may be needed for definitive vertebral identification when surgical intervention is planned 1
- Document the variant clearly - anatomical variants such as 4 or 6 lumbar vertebrae should be explicitly mentioned in reports 1
Clinical Management Approach
When No Treatment is Needed
Most transitional vertebrae are asymptomatic and require no intervention 1. The existence of lumbosacral transition vertebrae should be considered a normal anatomic variant 1.
When Imaging is Indicated
If lumbosacral plexopathy or neurologic symptoms develop in the context of transitional anatomy:
- MRI lumbosacral plexus without IV contrast (rating 8/9) is the preferred initial imaging modality 1
- MRI lumbosacral plexus with and without IV contrast (rating 9/9) provides optimal evaluation for acute or chronic nontraumatic plexopathy 1
- MRI lumbar spine without IV contrast (rating 8/9) may be complementary to assess for neural foraminal stenosis or epidural compression 1
Surgical Considerations
When surgical intervention becomes necessary due to symptomatic pathology at the transitional level:
Accurate preoperative imaging is mandatory - magnetic resonance imaging and computed tomography should be obtained to determine the degree of stenosis and plan surgical trajectory with >90% accuracy 3.
Specific anatomical considerations at transitional levels:
- Facet joints at L5-S1 with transitional anatomy demonstrate altered morphology, including smaller dimensions and more coronal orientation, particularly with accessory articulations 4
- Disc herniations show specific patterns: Type II transitional vertebrae present herniated discs at the transition level and increased incidence just above the transition 5
- Sacra with fused L5 vertebrae possess significantly smaller heights and narrower interauricular distances compared to normal anatomy 6
Surgical approach selection:
- For degenerative pathology at the lumbosacral junction, both posterolateral fusion (PLF) and interbody fusion techniques (PLIF, TLIF, or ALIF) achieve >90% fusion rates 3
- Posterior approach with pedicle screw fixation provides greater biomechanical stability 3
- Intraoperative electrophysiological monitoring during pedicle screw placement has 100% sensitivity for detecting screw malposition 3
Critical Pitfalls to Avoid
Wrong-level surgery is the most serious complication - transitional anatomy contributes to incorrect vertebral segment identification 2. Always verify vertebral levels with full spine imaging when planning surgical intervention at or near the lumbosacral junction 1.
Do not assume symmetry - facet tropism (asymmetry) occurs predominantly with accessory L5-S1 articulations 4. Evaluate both sides independently when assessing for pathology.
Recognize altered biomechanics - the transition from mobile lumbar spine to stiff sacroiliac segment creates high biomechanical stresses that can lead to accelerated disc degeneration, ligamentum flavum hypertrophy, and neural foraminal stenosis 7. Adjacent segment degeneration may occur more rapidly cranially into the thoracolumbar spine 7.