Normal Lumbosacral Angle Size on X-ray, CT & MRI
The lumbosacral angle (also called the sacral base angle or Ferguson’s angle) is formed between a horizontal reference line and a line drawn along the superior endplate of S1 on a lateral projection. It quantifies the inclination of the sacrum relative to the horizontal plane and serves as a key indicator of lumbosacral alignment. Accurate measurement is clinically important because abnormal values are associated with altered lumbar lordosis, increased shear stress at the L5–S1 segment, and a range of degenerative and developmental spinal conditions.
Normal Reference Values
| Orientation | Measurement |
|---|---|
| Lateral | 26-57¡ |
Clinical Significance
A normal lumbosacral angle of 26–57° reflects balanced sagittal alignment at the lumbosacral junction. Values below 26° (hypolordotic or flat sacrum) reduce the mechanical advantage of the posterior spinal musculature and are associated with reduced lumbar lordosis and chronic low back pain. Conversely, angles exceeding 57° (hyperlordotic or steep sacrum) increase anterior shear forces at L5–S1, predisposing patients to spondylolysis, spondylolisthesis, and accelerated facet joint degeneration.
Pitfalls include patient positioning: rotation or non-true lateral positioning can artificially alter the measured angle. Pelvic incidence and pelvic tilt should be assessed concurrently to interpret the lumbosacral angle within the broader context of global sagittal balance.
- Spondylolisthesis (L5–S1) — commonly associated with an elevated lumbosacral angle
- Degenerative disc disease (L5–S1) — disc collapse may reduce the angle over time
- Spondylolysis — increased angle raises shear stress at the pars interarticularis
- Flat-back deformity — associated with reduced lumbosacral angle
- Transitional lumbosacral anatomy — sacralization or lumbarization may affect angle measurement
Reference: Yochum TR, Rowe LJ. Essentials of Skeletal Radiology. (2005).
Imaging Notes
On lateral radiography, the lumbosacral angle is measured by drawing a line along the superior endplate of S1 and comparing it to a true horizontal line; a standing weight-bearing lateral view is preferred to reflect physiologic loading. On CT, sagittal reformats provide equivalent anatomical landmarks with higher endplate definition, which is useful when plain films are degraded by overlying bowel gas or body habitus. On MRI, T1-weighted sagittal sequences clearly delineate the S1 endplate and are increasingly used in preoperative planning; however, MRI is typically performed supine, which may underestimate the angle compared to weight-bearing radiography. Ensure the entire S1 superior endplate is included in the field of view on all modalities to avoid measurement error.