Anterior Cervical Plate Fixation: A Biomechanical Study to Evaluate the Effects of Plate Design, Endplate Preparation, and Bone Mineral Density

Abstract

A biomechanical study using multidirectional flexibility testing in a human cadaveric cervical spine model of a flexion-distraction injury. To compare the immediate postoperative stabilizing effect of dynamic and rigid anterior cervical plates and to assess the confounding effects of bone mineral density (BMD) and endplate preparation technique. Dynamic plate designs presumably increase load sharing between the plate and graft, but their effect on spinal stabilization has not been assessed in a traumatic flexion-distraction model. Twenty-four fresh frozen human cervical functional spinal units were dual-energy x-ray absorptiometry scanned for bone mineral density and allocated into 4 groups by the type of plate, dynamic (ABC, Aesculap, Germany) versus rigid (Cervical Spine Locking Plate, Synthes USA, Paoli, PA), and the technique of endplate preparation, intact versus removed. Each functional spinal unit had all posterior ligaments transected and both inferior facets excised, after which anterior discectomy, grafting, and plating was performed. Nondestructive testing applied a 1.5 Nm pure moment, whereas ranges of motion and neutral zones were measured in flexion/extension, lateral bending, and rotation. Ratios of the range of motion and neutral zone of the plated to the intact site were analyzed. The load sharing between the plate and the functional spinal unit was measured via strain gauges mounted on the plate. There were no significant differences in the range of motion or neutral zone ratios between the 2 plate designs, except for the range of motion ratio in extension, where the dynamic plate exhibited better stabilization than the rigid plate (P = 0.02). There was a consistent interaction whereby endplate removal resulted in better stabilization for the dynamic plate, but less stabilization for the rigid plate. Significantly less motion was observed with increasing bone mineral density in all loading directions. In flexion and extension, the dynamic plate measured one-third less strain than the rigid plate. The dynamic plate appeared to provide better stabilization in extension, and the technique of endplate preparation has some effect on immediate stabilization, dependent on the type of plate employed. Bone mineral density of the specimen was a strong determinant of the degree of stabilization achieved, regardless of the type of plate used.