In vitro flexibility of an experimental pedicle screw and plate instrumentation system on the porcine lumbar spine

The positive correlation between spinal construct stiffness and fusion rate has led to the use of increasingly rigid surgical spinal instrumentation s ystems. Unfortunately, however, these rigid systems have also been correlat ed with sub-optimal fusion quality measures. To date, in vivo studies to ex plore these relationships have involved the use of different implants and s urgical procedures to influence the biomechanical environment at the fusion site. In order to avoid these confounding variables, a novel experimental instrumentation system has been developed which is capable of independently controlling spinal construct flexibility (inverse of stiffness). In the pr esent study, this experimental pedicle screw and plate system was subjected to rigorous pure moment flexibility testing in flexion-extension and later al bending using an in vitro porcine lumbar spine model. Analysis of the da ta showed that the system provided a reproducible, stepwise-modulated spina l construct flexibility as measured by neutral zone flexibility, laxity ang le and range of motion parameters. Differences in flexibility were most evi dent using the neutral zone parameters (neutral zone flexibility and laxity angle). This is of particular interest given that the clinical instability of a spinal segment may be related to its behaviour within the neutral zon e. This information will ultimately guide the design of improved human spin al instrumentation systems. (C) 2001 IPEM. Published by Elsevier Science Lt d. All rights reserved.