Intersegmental spinal flexibility with lumbosacral instrumentation. An in vitro biomechanical investigation

Flexibility of the porcine lumbosacral spine was measured after application of six different types of surgical instrumentation, and in a control state. Fifteen adult pig spines were tested with flexion, extension, lateral bending, and axial rotation torques applied to the upper end with the pelvis fixed. Instrumentation was applied across two lumbar segments and the lumbosacral level (L5-6, L6-7, and L7-S1). Stereophotogrammetry was used to track markers applied to each vertebra. Intersegmental motion was measured as three angles and as the relative linear translation of adjacent transverse processes and spinous processes. Results showed that all instrumentation systems reduced intersegmental motion compared with the control state, except for minimal reduction at L5-6 by Harrington instrumentation in all loading directions, especially axial rotation. The pedicle screw systems were always the most rigid. After applying instrumentation, there were differences in the motion occurring at different anatomic levels, most commonly with the least motion occurring in the middle of the instrumented segment (L6-7). When intervertebral motion was expressed as the linear motion between adjacent spinous and transverse processes, the usual site of posterolateral fusion, it was 0.6 to 1.8 mm per degree of angular motion at the transverse processes and 1.3 to 2.1 mm per degree at spinous processes.