Load-sharing characteristics of stabilized lumbar spine segments

Study Design. Load sharing in stabilized spinal segments was evaluated usin g sequential injury and stabilization with a posterior instrumentation syst em under an in vitro flexibility protocol. Objective. To analyze the partitioning of applied loads between anatomic an d implanted structures of lumbar functional spinal units stabilized with a posterior instrumentation system. To identify surgical indications for whic h the risk of fixator breakage in vivo is high. Summary of Background Data. Relatively few groups have experimentally measu red the in vitro and in vivo forces and/or moments supported by posterior i nstrumentation systems, and no analysis, of the load sharing in these syste ms has been performed. This information will provide novel insight into imp lant fatigue life, and the degree to which the spinal anatomy is shielded f rom the applied load and will allow the verification of mathematical models for new injury scenarios. Methods. Specimen kinematics were determined using an optoelectronic tracki ng system. Intradiscal pressure and forces and moments supported by the imp lants were measured using, respectively, a needle-mounted pressure sensor a nd strain gauges mounted on the spinal implants. Results. A large majority of the applied moments were supported by an equal and opposite force pair between the intervertebral disc and fixator rods i n flexion and extension and an equal and opposite force pair between the le ft and right fixator rods in lateral bending. Torsional moments were shared approximately equally between the posterior elements, intervertebral disc an equal and opposite shear force fair in the transverse plane between the right and left fixators and internal fixator moments. Conclusions. When posterior instrumentation devices are used to stabilize s evere anterior column injuries, they are at risk of fracture secondary to r eversed bending moments.