Alternatives to conventional rigid fusion have been proposed for several conditions related to degenerative disc disease when nonoperative treatment has failed. Semirigid fixation, in the form of dynamic stabilization or PEEK rods, is expected to provide compression under loading as well as an intermediate level of stabilization. This study systematically examines both the load-sharing characteristics and kinematics of these two devices compared to the standard of internal rigid fixators. Load-sharing was studied by using digital pressure films inserted between an artificially machined disc and two loading fixtures. Rigid rods, PEEK rods, and the dynamic stabilization system were inserted posteriorly for stabilization. The kinematics were quantified on ten, human, cadaver lumbosacral spines (L3-S1) which were tested under a pure bending moment, in flexion-extension, lateral bending, and axial rotation. The magnitude of load transmission through the anterior column was significantly greater with the dynamic device compared to PEEK rods and rigid rods. The contact pressures were distributed more uniformly, throughout the disc with the dynamic stabilization devices, and had smaller maximum point-loading (pressures) on any particular point within the disc. Kinematically, the motion was reduced by both semirigid devices similarly in all directions, with slight rigidity imparted by a lateral interbody device. 1. Introduction Conventional instrumentation to achieve fusion in the lumbar spine utilizes rigid rods and pedicle screws [1–3]. Rigid rod fixation is criticized to reduce load-sharing and inhibit fusion mass formation because of the stress-shielding effect [4]. Not only does load-sharing influence fusion, but it may also affect a patient’s pain level, adjacent segment kinematics, and the potential for device failure following spine surgery [2]. Semirigid instrumentations, such as polyetheretherketone (PEEK) rods and titanium rods with helical grooves, are designed to increase load-sharing in attempt to induce compression on the bone graft and promote bone remodeling as first credited by Wolff [4]. There is certainly evidence of osteoblastic response to mechanical activation, in various forms, as well as increases in bone formation rate following bouts of loading [5, 6]. Semirigid PEEK instrumentation attempts to allow loading through the anterior column, but most studies show the stiffness of these constructs to be relatively high [7]. One hypothesized benefit of a dynamic device is to restore the loading of the damaged disc to similar thresholds as a
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