Study Design. A three-dimensional, noninvasive motion analysis method
was developed by monitoring the orientation of the principal axes of e
ach vertebra. Objectives. To develop a method of performing three-dime
nsional, noninvasive motion analysis of the spine using computed tomog
raphy data. Summary of Background Data. The concept of using principal
axes of the moment of inertia tensor to measure the orientation and p
osition of a rigid body has been applied to the wrist and subtalar joi
nts, but has not yet been applied to the spine. Methods. Scans were ta
ken of two isolated vertebrae in various known positions. Centroids, a
rea, moments, and product of inertia of each scan were determined usin
g a commercial program. Custom software combined data using the parall
el axis theorem to give three-dimensional data for each vertebra. Chan
ges in the centroid and principal axes were used to calculate translat
ion and rotation? respectively. Results. The system accuracy was withi
n 1.0 degrees in rotation and 1.0 mm in translation. Some errors occur
red in minor motions when a smaller number of scans were used System r
esolution was 0.43 mm. Conclusions. A system has been developed capabl
e of calculating three-dimensional spinal motion based on measurements
of a series of computed tomography images. The system has an accuracy
similar to that of current motion analysis methods, but future studie
s will be necessary to apply this system in vivo.