Use of percutaneous transpedicular external fixation pins to measure intervertebral motion

Study Design. Direct measurement of intervertebral motion was compared to m otion determined by measuring the position of the exposed ends of the exter nal fixation pins. Objectives. To verify the accuracy of this technique, so that this protocol can be used to study intervertebral motion in the clinical setting. Summary of Background Data. The transpedicular external fixation test has b een shown to be a test that can predict the outcome of spinal fusion. In pa tients who are candidates for this test, intervertebral motion can be calcu lated from motion at the external ends of these pins. Methods. Six fresh cadaveric spinal segments from L2 to L5 were instrumente d with titanium Schanz screws. Reflective markers were placed on the tips o f the pins, and intervertebral motion was measured using a noncontacting ca mera system. Computed tomography data were used to determine the position o f the vertebra relative to the reflective markers. Intervertebral distances were calculated and compared with direct measurements obtained using a thr ee-dimensional digitizing arm. Results. There was an excellent correlation (r(2) = 0.931) between the dire ctly measured intervertebral motions and those that were indirectly calcula ted from measurements of motion at the end of the Schanz screws. Conclusions. Intervertebral motion can be measured by monitoring motion of the ends of transpedicular external fixation pins. Motion of anatomic landm arks on the vertebrae can be calculated from the pin end's motion if comput ed tomography data are used to determine the geometric relation between the vertebrae and the external fixation pins. This validation study supports t he use of this method in clinical investigations of intervertebral motion i n patients with low back pain and external fixation.