Biomechanics of dynamic rod segments for achieving transitional stiffness with lumbosacral fusion

Transitioning from rigid to flexible hardware at the distal rostral or caudal lumbar or lumbosacral level hypothetically maintains motion at the transition level and protects the transition level and intact adjacent levels from stresses caused by fusion.

To biomechanically compare transitional and rigid constructs with uninstrumented specimens in vitro.

Human cadaveric L2-S1 segments were tested (1) intact, (2) after L5-S1 rigid pedicle screw-rod fixation, (3) after L4-S1 rigid pedicle screw-rod fixation, and (4) after hybrid fixation rigidly spanning L5-S1 and dynamically spanning L4-L5. Pure moments (maximum 7.5 Nm) induced flexion, extension, lateral bending, and axial rotation while motion was recorded optoelectronically. Additionally, specimens were studied in flexion/extension with a 400-N compressive follower load. Strain gauges on laminae were used to extract facet loads.

The range of motion at the transition segment (L4-L5) for the hybrid construct was significantly less than for the intact condition and significantly greater than for the rigid 2-level construct during lateral bending and axial rotation but not during flexion or extension. Sagittal axis of rotation at L4-L5 shifted significantly after rigid 2-level or hybrid fixation (P < .003) but shifted significantly farther posterior and rostral with rigid fixation (P < .02). Instrumentation altered L4-L5 facet load at more than the L3-L4 facet load.

The effect of the dynamic rod segment on the kinematics of the transition level was less pronounced than that of a fully rigid construct in vitro with this particular rod system. This experimental model detected no biomechanical alterations at adjacent intact levels with hybrid or rigid systems.