A geometric model of the lumbar spine in the sagittal plane.

This article describes a geometric model using the skin profile and five anthropometric measurements to estimate the position of S1, T12, and all lumbar vertebrae for all postures assumed in the sagittal plane. This method involves a normalization process by which different skin profiles can be compared between postures and individuals. The skin profile is transformed by taking the differences with the lumbar spine into consideration. The model was developed and validated with 20 and 7 subjects, respectively. An error analysis shows an adequate level of accuracy for the absolute (1.68-1.82 cm) and relative (0.32-0.54 cm) linear positions of vertebrae as well as their absolute (2.6-6.7 degrees) and relative (1.4-3.6 degrees) angular positions except for T12; however, the validity of the model was limited to specific angular motions in flexion for the pelvis (12 +/- 3%), the entire lumbar spine (14 +/- 13%) and the intervertebral motion of L4/L5 (13 +/- 10%). The data obtained are very useful, especially in models designed to evaluate loadings on the lumbar spine.