From the Guidelines
A transitional S1 vertebral body is an anatomical variation where the first sacral vertebra has characteristics of both a lumbar and a sacral vertebra. This variation can be seen in certain individuals and may require verification through imaging studies such as a full spine planar radiograph, CT, or MRI, especially if specific vertebral labeling is needed for surgical intervention 1.
- Key characteristics of a transitional S1 vertebral body may include an extra pair of lumbar-type transverse processes or a larger size compared to the other sacral vertebrae.
- The existence of lumbosacral transition vertebrae, including a transitional S1, is considered a normal anatomic variant and should be taken into account when interpreting spinal imaging studies 1.
- Accurate identification of a transitional S1 vertebral body is crucial in clinical practice for correct diagnosis and treatment of spinal conditions, as it can affect the numbering and identification of lumbar and sacral vertebrae.
From the Research
Definition of Transitional S1 Vertebral Body
- A transitional S1 vertebral body refers to a condition where the first sacral vertebra (S1) exhibits characteristics of a lumbar vertebra, resulting in an anomalous lumbosacral junction 2, 3.
- This condition is also known as lumbosacral transitional vertebrae (LSTV) or S1 lumbarization, where the S1 vertebra is partially or completely separated from the rest of the sacrum 3, 4.
Characteristics of Transitional S1 Vertebral Body
- The transitional S1 vertebra may exhibit a gap between the S1 and S2 bodies, incomplete separation between the S1 and S2 neural arch elements, and well-developed "baseline" (S2-S5) segments 3.
- The auricular surfaces in transitional S1 vertebrae may be situated lower than normal, with negligible contribution from the S1 segments 3.
- The transitional S1 vertebra may also exhibit distinct anatomical alterations, such as smaller overall dimensions, diminished facet depth, and sacral height 3.
Prevalence of Transitional S1 Vertebral Body
- The prevalence of LSTV, including transitional S1 vertebrae, has been reported to be around 12.9% to 35.6% in various studies 2, 5.
- The most common anatomical variant of LSTV is the Castellvi Type IA, which accounts for around 14.7% of cases 5.
Clinical Significance of Transitional S1 Vertebral Body
- The presence of a transitional S1 vertebra can lead to incorrect identification of a vertebral segment, resulting in wrong-level spine surgery and poor correlation with clinical symptoms 2, 4.
- The altered lumbosacral load bearing at the anomalous junction may also result in changes in the number, density, and trajectory of the trabecular bone in transitional lumbosacral vertebral bodies 6.