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 systems. Unfortunately, however, these rigid systems have also been correlated with sub-optimal fusion quality measures. To date, in vivo studies to explore these relationships have involved the use of different implants and surgical 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 present study, this experimental pedicle screw and plate system was subjected to rigorous pure moment flexibility testing in flexion–extension and lateral bending using an in vitro porcine lumbar spine model. Analysis of the data showed that the system provided a reproducible, stepwise-modulated spinal construct flexibility as measured by neutral zone flexibility, laxity angle and range of motion parameters. Differences in flexibility were most evident 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 zone. This information will ultimately guide the design of improved human spinal instrumentation systems.