Considering the inferior surface area of lower lumbar vertebrae: determining weight transmission pattern at the lumbosacral junction.

The biomechanical function of the lumbosacral junction (LSJ) is obscure, but its medical significance is not, as it is the most common site of low back pain. In this study, we analyzed the difference between the mean values of the surface areas of the inferior body and total inferior facet areas of the fourth and fifth lumbar vertebrae. We aimed to define the function of the LSJ during weight transmission and clarify its mechanical significance. Vertebral columns of 45 adult male human cadavers from five anatomy departments in Nigeria were cut at the L3-L4 intervertebral disc and macerated. Using the graph paper method, the mean values of the surface area of the inferior body and total facet area of the fourth and fifth lumbar vertebrae were 1356 ± 26 and 329 ± 6 and 1277 ± 27 and 418 ± 8 mm2 , respectively. The relationships between the fourth and fifth lumbar vertebrae paired variables were highly significant (P < 0.001). A sudden reduction in the surface area of the inferior body of the fifth lumbar vertebra was compensated for by a corresponding increase in its total inferior facet area, which indicated that corresponding weight was diverted from the anterior column to the posterior column at the LSJ. This pattern of weight transmission may be a beneficial functional adaptation in man to protect the relatively large intervertebral disc of the LSJ in bipedal posture, or it may predispose the LSJ synovial zygapophyseal joints to mechanical stress.