A Pseudo-Rigid-Body Model of the Human Spine to Predict Implant-Induced Changes on Motion

Injury, instrumentation, or surgery may change the functional biomechanics of the spine. Adverse changes at one level may affect the adjacent levels. Modeling these changes can increase the understanding of adjacent-level effects and may help in the creation of devices that minimize adverse outcomes. The current modeling techniques (e.g., animal models, in vitro testing, and finite element analysis) used to analyze these effects are costly and are not readily accessible to the clinician. It is proposed that the pseudo-rigid-body model(PRBM) may be used to accurately predict adjacent level effects in a quick and cost effective manner that may lend itself to a clinically relevant tool for identifying the adjacent-level effects of various treatment options for patients with complex surgical indications. A PRBM of the lumbar spine (lower back) was developed using a compliant mechanism analysis approach. The global moment-rotation response, relative motion, and local moment-rotation response of a cadaveric specimen were determined through experimental testing under three conditions: intact, fused, and implanted with a prototype total disc replacement. The spine was modeled using the PRBM and compared with the values obtained through in-vitro testing for the three cases. The PRBM accurately predicted the moment-rotation response of the entire specimen. Additionally, the PRBM predicted changes in relative motion patterns of the specimen. The resulting models show particular promise in evaluating various procedures and implants in a clinical setting and in the early stage design process.