Spontaneous vertebral fractures are a large and growing health care problem. Biomechanical factors, specifically, abnormal posture or gait-related spinal motion may interact with age-weakened bone to induce altered spinal biomechanics that in turn increase the likelihood of vertebral body fracture. This research takes steps towards the goal of reducing the number of vertebral fractures in two phases: (1) Validation of a noninvasive spinal motion measurement system in cadaver torsos and 2) Application of the measurement system in human subjects.

The cadaver study compared vertebral motion at 4 levels (T7,T12,L3,L5) as measured by adhesive skin markers versus motion measured by bone pins implanted into the vertebrae. Cadaver torsos were tested in lateral-bending, flexion and axialrotation. Mean differences in vertebral body angular motion between skin markers and bone pin markers were <0.5° around the anterior-posterior and medial-lateral axes and <0.9° around the superior-inferior axis. This measurement method was able to accurately quantify vertebral body motion in cadaver torsos thus allowing for application to human subject testing.

X-rays and 3D motion capture were employed to quantify spinal posture and motion parameters during gait in 12 older and 12 younger normal, females. Vertebral motion around 3 axes was measured at 4 levels (T7,T10,T12,L2) using noninvasive retroreflective markers during treadmill gait at 3 speeds (0.5,0.7,0.9m/s). The average angular motion of all gait cycles at each speed was determined for each level. The triplanar ranges of motion and variability of motion were compared as a function of age. Older subjects had 31.7% larger frontal Cobb angles and up to 30.9% and 33.5% smaller ranges of spinal motion in the frontal and sagittal planes. Variability of motion in the sagittal plane was up to 42.9% less in older subjects.

Decreased ranges of motion and variability of spinal motion observed in older subjects may imply that vertebral loading in these subjects may not be as uniformly distributed across the vertebrae as in younger subjects. Greater stresses may result from the abnormal motion, thus increasing fracture risk. Confirmation of this hypothesis requires a longitudinal study, but if verified, may lead to the development of inexpensive countermeasures to prevent fractures.

KEYWORDS: spontaneous fractures, spinal kinematics, spinal loading, skin markers, gait-related spinal motion