Curvature of the lower lumbar facet joints: variations at different levels and relationship with orientation.

The curvature of the lower lumbar facet joints in the transverse plane was measured using an innovative computed tomographic approach, and its relationship with the orientation of the joints was evaluated. Two hundred three vertebral levels were examined in 116 patients with low back pain. Reference circles were used to quantify joint curvature. The mean curvature (+/- SD) at the levels L3-L4, L4-L5, and L5-S1 conformed to circles of 27.3 mm +/- 11.5; 35.1 mm +/- 14.6; and 51.0 mm +/- 34.5 in diameter respectively. The ratio between the curvature of the facets and their orientation was 62.6% +/- 19.0; 57.8% +/- 21.6; and 64.9% +/- 48.9. From L3-L4 down to L5-S1 the curvature of the facet joints decreases. The differences between L3-L4 and L4-L5 is smaller than between L4-L5 and L5-S1. The curvatures of the joints conform to circles that are in mean approximately 40% in diameter smaller than those to which their orientations conform.