Predicting the vertebral inclination of the lumbar spine

The objectives of this study were to examine the accuracy of the external stick marker method in the assessment of sagittal plane vertebral inclination (L1 to S1) during trunk flexion and to develop regression equations for predicting vertebral inclinations of the lumbar spine. Lateral radiographs of 16 subjects were taken from the upright position to a trunk flexion of 90 degrees, in 30 degrees increments. Each subject was radiographed in only three of the four torso positions to minimize the risks of radiation. The inclinations of the vertebrae in the radiographic view were then obtained. The results show that the stick marker technique is a poor protocol for measuring vertebral inclination of the lumbar spine. During trunk flexion, the upper vertebrae incline linearly and the lower vertebrae incline exponentially. This is verified by the finding that the best-fit equations selected by regression techniques were linear at the upper vertebrae (L1, L2 and L3) and non-linear at the lower ones (L4, L5 and S1), with a mean R2 value of 0.964. The inherent difference in motion pattern between the vertebrae of the lumbar spine during trunk flexion is discussed for clinical and ergonomic purposes.