Dynamic Properties and Screw Loosening Prevention Effect of a Posterior Dynamic Stabilization System With Polyethylene Articulating Surfaces for Motion Preservation: An In Vitro Biomechanical Study

Study Design: Biomechanical in vitro study. Objective: To investigate the dynamic characteristics of a posterior spinal stabilization system incorporating polyethylene as a bearing surface and evaluate its potential in preventing pedicle screw loosening. Summary of Background Data: Although various motion preservation systems have been clinically implemented, there are no clinical reports documenting posterior dynamic stabilization systems utilizing polyethylene. This unique material was selected to overcome the limitations of conventional fusion techniques and offer a new modality for dynamic stabilization. Methods: The system consists of a pedicle screw, a polyethylene ball, a rod, and a set screw. The rod passes through the polyethylene ball, which is clamped by the set screw, allowing 15° of mobility on each side. Axial and torsional gripping capacity and flexion-extension moment tests were conducted using three dynamic and three rigid screws. In addition, fatigue tests were performed by inserting screws into polyurethane foam blocks simulating healthy (20 PCF) and osteoporotic (7.5 PCF) bone, applying cyclic loading of 0.88 mm amplitude at 1 Hz for one million cycles. Maximum pullout strength was measured postloading. Fatigue tests were conducted using four dynamic and four rigid screws per bone model, and controls without fatigue loading were also assessed. Results: Compared with rigid screws, dynamic screws exhibited significantly lower axial (23±6 N vs. 1660±68 N) and torsional (0.08±0.01 N·m vs. 4.79±0.51 N·m) gripping capacities, and significantly lower flexion-extension moment (0.08±0.02 N·m vs. 8.74±0.46 N·m). After fatigue testing, pullout strength decreased by 12% for dynamic and 38% for rigid screws in healthy bone, and by 32% versus 39%, respectively, in osteoporotic bone. Conclusions: This polyethylene-based system permits controlled motion in flexion-extension, axial rotation, and cranio-caudal translation. By distributing mechanical loads, it may effectively reduce the risk of pedicle screw loosening.