Spinopelvic Parameters and Sagittal Alignment of Symptomatic Degenerative Adult Spinal Disorder Patients With 6 Lumbar Vertebrae

This was a cross-sectional, observational study.

The main objectives of this study were to observe the prevalence of a true L6 among patients with symptomatic adult spinal degeneration, and to evaluate similarities of their radiographic spinopelvic parameters to L5 patients.

Spinopelvic parameter values used for diagnosis and surgical planning are different between individuals with 5 or 6 lumbar vertebrae. The difference has not been studied in patients with symptomatic spinal degenerative conditions.

A total of 775 consecutive symptomatic patients with degenerative spinal disorders were classified as having 5 or 6 lumbar vertebrae in full spine radiographs. Pelvic incidence minus lumbar lordosis (PI-LL), sagittal vertical axis (SVA), pelvic tilt, and T1 pelvic angle were measured in 3 groups: sacral (L5 and L6s) and L6 upper endplate (L6e). Oswestry Disability Index (ODI) was obtained.

In total, 715 (92.3%) patients had L5 and 60 (7.7%) had L6. LL values were comparable between the L5 and L6s (P=0.355) and SVA between all groups (P=0.869). Only SVA had excellent concordance correlation (ρc=0.91) between the L6s and L6e groups. PI-LL had significantly different values (P<0.001 all groups) and distributions between L5 and L6s (P=0.038), and L6s and L6e (P<0.001) groups. In patients with severe disability (ODI>40%) the parameters that deteriorate with increasing degeneration (pelvic tilt, T1 pelvic angle, SVA, LL, PI-LL) were not significantly different between L5 and L6s groups unlike patients with ODI<40% while PI remained similarly different (P<0.001). PI and LL had a multivariate relationship in L6 patients computable as regression model equation: PI (sacrum)=-0.92×LL (L6e)+0.91×LL (sacrum) +1.11×PI (L6e)+10.81 (R=0.88).

L6 variant is fairly common. The radiographic L6 parameters were different from L5 except for SVA and values of patients with severe disability measured from sacrum. PI and LL have a mathematic relationship in L6 patients. The cutoff values for radiographic modifiers need further studies combining radiology and clinical outcome.

Level III.