Lumbosacral Junction¶
Definition¶
The lumbosacral junction is the transitional zone between the mobile lumbar spine and the fixed sacrum, centered at the L5–S1 articulation. It bears the greatest biomechanical load of the entire spine, as it transmits the full weight of the upper body to the pelvis. This region is subject to unique pathologies including spondylolysis, spondylolisthesis, and transitional vertebrae.
Anatomy¶
Osseous Structures¶
- L5 vertebra — the largest lumbar vertebra; has a characteristic wedge shape (taller anteriorly) that contributes to lumbar lordosis; the transverse processes are the largest and arise from the pedicle-body junction rather than the pedicle-lamina junction as in other lumbar vertebrae
- S1 vertebra — the superior-most sacral segment; its superior endplate is the sacral promontory, which forms a marked angle with the L5 inferior endplate (the lumbosacral angle, normally 130–160°)
- L5–S1 disc — the most wedge-shaped disc in the spine; bears more compressive and shear load than any other disc
L5–S1 Facet Joints¶
- The L5–S1 facet joints are more coronally oriented than the upper lumbar facets
- This coronal orientation provides some rotational capability but also makes this level susceptible to degenerative spondylolisthesis in some individuals
- The iliolumbar ligaments connect the L5 transverse processes to the iliac crests, providing additional stabilization
Lumbosacral Transitional Vertebrae (LSTV)¶
Transitional vertebrae at the lumbosacral junction are common variants (prevalence ~15–35%):
- Sacralization — the L5 transverse process is enlarged and may partially or completely fuse to the sacrum or ilium
- Lumbarization — the S1 segment is partially separated from the sacrum, appearing as an extra lumbar vertebra
- Castellvi classification: Type I (dysplastic transverse process >19 mm), Type II (pseudoarticulation with sacral ala), Type III (complete fusion), Type IV (mixed)
Clinical Pearl
Lumbosacral transitional vertebrae (LSTV) can cause significant confusion in vertebral numbering, leading to wrong-level surgery. When an LSTV is present, surgeons and radiologists must agree on the numbering convention. MRI alone may not reliably identify the transitional level — correlation with a whole-spine localizer or AP radiograph counting from C2 or T12 (by identifying the lowest rib-bearing vertebra) is essential. Bertolotti syndrome refers to symptomatic pseudoarticulation at a transitional lumbosacral vertebra, which can cause low back and buttock pain.
Imaging Findings¶
Radiography¶
- Lateral view: evaluate the lumbosacral angle, L5–S1 disc height, and spondylolisthesis (anterior slip of L5 on S1)
- AP view: identify transitional vertebrae (enlarged L5 transverse processes, sacralization)
- Oblique views: evaluate the pars interarticularis for spondylolysis (the "broken neck" of the Scottie dog)
CT¶
- Best modality for evaluating bony abnormalities at L5–S1
- Spondylolysis: defect in the pars interarticularis, best seen on sagittal reformats; occurs at L5 in ~85% of cases
- LSTV: axial and coronal reformats demonstrate the degree of transverse process enlargement and articulation/fusion with the sacrum
- Facet arthropathy: joint space narrowing, sclerosis, osteophytes at L5–S1
MRI¶
| Finding | Appearance |
|---|---|
| L5–S1 disc degeneration | Loss of T2 signal (dark disc), decreased disc height, Modic endplate changes |
| Spondylolisthesis | Anterior or posterior translation of L5 on S1 on sagittal images; evaluate for cord/root compression |
| Pars defect (spondylolysis) | Low-signal discontinuity on sagittal T1 through the pars; marrow edema on STIR if active |
| LSTV | Enlarged transverse process; sagittal images help identify the transition |
| Far lateral disc herniation | Disc fragment in or beyond the neural foramen, compressing the exiting L5 root |
Key MRI Finding
L5–S1 is the most common level for disc herniation. However, far lateral (foraminal and extraforaminal) herniations are proportionally more common at L5–S1 than at other levels. These compress the exiting L5 nerve root (not the traversing S1 root), producing an L5 radiculopathy. Sagittal T1 through the foramen and axial images lateral to the canal are key for diagnosis — foraminal herniations can be easily missed on standard axial images through the disc.
Key Points¶
- The lumbosacral junction bears the greatest biomechanical load of the entire spine
- Spondylolysis at L5 is the most common cause of spondylolisthesis in young patients
- Lumbosacral transitional vertebrae are present in 15–35% of people and cause numbering confusion
- LSTVs require correlation with whole-spine imaging to establish accurate vertebral numbering
- L5–S1 is the most common level for disc herniation, with relatively more far lateral herniations
- CT is best for pars defects and LSTV characterization; MRI for disc and neural pathology
References¶
- Konin GP, Walz DM. Lumbosacral transitional vertebrae: classification, imaging findings, and clinical relevance. AJNR Am J Neuroradiol. 2010;31(10):1778-1786.
- Jancuska JM, Spivak JM, Bendo JA. A review of symptomatic lumbosacral transitional vertebrae: Bertolotti's syndrome. Int J Spine Surg. 2015;9:42.
- Leone A, Cianfoni A, Cerase A, Magarelli N, Bonomo L. Lumbar spondylolysis: a review. Skeletal Radiol. 2011;40(6):683-700.
- Le Huec JC, Aunoble S, Philippe L, Nicolas P. Pelvic parameters: origin and significance. Eur Spine J. 2011;20(Suppl 5):564-571.
- Celestre PC, Dimar JR 2nd, Glassman SD. Spinopelvic parameters: lumbar lordosis, pelvic incidence, pelvic tilt, and sacral slope: what does a spine surgeon need to know to plan a lumbar deformity correction? Neurosurg Clin N Am. 2018;29(3):323-329.
- Castellvi classification of lumbosacral transitional vertebrae. Radiopaedia.org. https://radiopaedia.org/articles/castellvi-classification-of-lumbosacral-transitional-vertebrae
- Spondylolysis. Radiopaedia.org. https://radiopaedia.org/articles/spondylolysis