What does it mean when an MRI of the lumbar spine shows transitional anatomy at the lumbosacral junction with a transitional segment referred to as L5 (Lumbar 5), a fully formed intervertebral disc space at L4-5 (Lumbar 4-5), and a rudimentary disc between L5 (Lumbar 5) and S1 (Sacrum 1)?

Understanding Transitional Anatomy at the Lumbosacral Junction

Transitional anatomy at the lumbosacral junction, as described in your MRI report, indicates a congenital vertebral anomaly where the L5 vertebra shows partial sacralization with a rudimentary disc between L5 and S1.

What This Means

Lumbosacral transitional vertebrae (LSTV) are common anatomical variants that occur at the junction between the lumbar spine and sacrum. This condition affects approximately 15-35% of the general population 1, 2.

Key Features of Your MRI Finding:

• Transitional segment labeled as L5: The radiologist has identified a vertebra with transitional features at the lumbosacral junction
• First fully formed disc at L4-5: Normal discs are present above the transitional segment
• Rudimentary disc between L5-S1: The disc between the transitional vertebra and sacrum is not fully developed

Clinical Significance

1. Altered Biomechanics

   • The transitional vertebra changes normal load distribution in the spine
   • May accelerate degeneration of the disc immediately above the transitional segment 3
   • Can lead to altered movement patterns and stress on surrounding structures

2. Nerve Root Considerations

   • Potential for nerve root entrapment, particularly the S1 nerve root 3
   • Coronal MRI views are particularly valuable for identifying nerve compression in these cases

3. Surgical Planning Implications

   • Critical for correct level identification during spine surgery
   • Prevents wrong-level surgical interventions
   • Affects how vertebrae are counted and labeled

Diagnostic Considerations

The iliolumbar ligament is typically used as an anatomical marker to identify the L5 vertebra, as it consistently originates from L5 in patients with normal lumbosacral segmentation 4. However, in patients with transitional anatomy, this ligament is not a reliable marker, as it originates from L5 in only about 29.3% of cases with transitional lumbosacral junctions 2.

MRI is the preferred imaging modality for evaluating the lumbosacral plexus and identifying transitional anatomy 5. For patients with radicular symptoms that cannot be explained by conventional sagittal and axial MRI sequences, adding coronal T1-weighted imaging can be valuable, especially for identifying S1 nerve root entrapment at the level of LSTV pseudoarthrosis 3.

Clinical Management Implications

For patients with transitional anatomy experiencing low back pain:

1. Conservative Management First

   • Physical therapy focusing on core strengthening
   • Pain management with non-narcotic analgesics
   • Targeted injections if radicular symptoms are present 6

2. Surgical Considerations

   • Surgical intervention should only be considered after 6 months of failed conservative management
   • Correct identification of vertebral levels is crucial for surgical planning
   • Minimally invasive decompression may be considered before fusion if neural compression is identified 6

Key Takeaways

• Transitional lumbosacral anatomy is a common variant (15-35% of population)
• It may predispose to accelerated disc degeneration above the transitional segment
• It requires careful identification for accurate diagnosis and treatment planning
• MRI with coronal views is particularly valuable for evaluating potential nerve entrapment
• This finding explains why your spine has a rudimentary disc at L5-S1 rather than a fully formed one

This anatomical variation is not necessarily pathological by itself but may contribute to symptoms and requires consideration in any spine-related treatment planning.