Design, Evaluation, and Application of a System for Mechanical Evaluation of the Lumbar Spine

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Abstract

From a mechanical perspective, the spinal column is a complex system in both anatomy and function. The difficulty of accurately or at least representatively manipulating the spinal column in an experimental setting, has made physiologically relevant analysis challenging.

An approach to evaluating the mechanical behavior of the spine including a 3 D.O.F. system for ex vivo testing of cadaveric spine specimens, and an implantable transducer for direct measurement of intervertebral motion in vivo, during normal activities has been designed. Transducer systems were implanted in goats to evaluate motion patterns in normal, injured, and surgically stabilized groups for 12 weeks after surgery. The relative contribution of active muscle and the passive osteo-ligamentous structure, to segmental mobility, and the ability of muscle to stabilize a motion segment after surgical injury were evaluated. The mechanical environment during healing of posterolateral intertransverse process spinal fusion, with bone graft alone and with constrained, and semi-constrained internal fixation was studied. The effect of muscle action on the mechanical behavior of spinal fusion with internal fixation implants, and the mechanics of healing of posterolateral spinal fusions were investigated.

Muscle action was found to reduce motion during normal activity to one half of the motion allowed by passive osteo-ligamentous structures in normal spines. Deformation of 10% in the fusion mass of uninstrumented posterolateral lumbar fusions resulted in psuedarthroses. Augmentation with constrained pedicle screw fixation implants reduced the rate of deformation to 3% resulting in a 100% rate of fusion. Semi-constrained fixators resulted in an 83% rate of fusion. The fusion mass with semi-constrained implants, however, was stiffer under all modes of load tested.

Muscle action is important to the loading and deflection of internally fixed spinal segments. Spinal arthrodesis in posterolateral lumbar fusion requires a "chain of fusions" in the healing bone graft. Variability seen in clinical and research outcomes are likely to result from varying healing patterns in the chain of fusions. Improving local control of mechanical behavior may improve the outcome of these procedures.

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