Clinical experience with a new load-sharing anterior cervical plate

Abstract

Purpose of study: Anterior plate fixation is widely accepted as an adjunct to surgical correction of traumatic or degenerative conditions of the cervical spine. The various systems have evolved from nonconstrained constructs to rigid plate screw constructs. With the popularity of the latter, concerns have increased on both the biomechanical and clinical levels regarding the adverse effects of graft stress shielding which can result in delayed bony union or nonunion. This ultimately can result in hardware or construct failure. A new dynamic plating system (ABC plate) has been developed which seeks to avoid stress shielding of the graft by allowing vertical migration of the fixation screws within the plate. This design can prevent screw back-out and effectively stabilize the spine while allowing full load sharing, thus promoting early fusion and restoring or preserving lordosis. Methods: This study reports on our experience with 486 patients at two different centers over a 3-1/2-year interval utilizing the ABC system. Indications for surgery were herniated disks in 36%, degenerative disease in 45%, trauma in 14%, failed fusion in 2%, and deformity, tumors and OPLL in 1% each. Patients ranged in age from 10 to 80 years (average 47 years). Unicortical screw placement was used in 68%, bicortical in 28%, and a combination of these in 4%, according to the surgeon's preference and judgment. Single-level plates were used in 47%, two-level plates in 39%, and three or more levels were plated in 14%. Corpectomies were used in 19% with the remainder being interbody fusions. Allograft iliac crest bone was used in 61% and autograft iliac crest bone in 39%. Patients were seen in follow-up and radiographs, which include lateral flexion/extension views, were taken at the 1-, 3-, 6-, 12- and 24-month intervals after surgery. Settling was measured and corrected for magnification. Cobb angles were measured to determine changes in angulation from immediately preoperatively to subsequent interval films. Fusion was determined using the rigid criteria of bridging trabecular bone and the absence of motion at the tips of the spinous processes on flexion and extension films. Results: Data analysis was performed on patients with a minimum follow-up of 3 months. Settling: Settling occurred in most patients. It was more pronounced with the use of allograft, but averaged 1 to 2 mm/level overall with a maximum of 7 mm in one patient with a three-level fusion. Ninety-five percent of the settling occurred in the first month, at which time the grafts often were indistinguishable from 3- to 6-month postoperative grafts placed using prior generation rigid plates. No settling was observed after 3 months. Fusion: Virtually all levels showed impressive early graft incorporation. Using the strict criteria defined above, 341 of 508 levels (67%) were fused at 3 months, 310 of 384 levels (81%) at 6 months and 238 of 255 levels (93%) at 12 months, and 69 of 69 levels (100%) were fused at 24 months. No translational instability was observed. Kyphotic angulation: Lordosis was preserved unchanged in almost all patients. In only five patients was loss of lordosis observed ranging from 2 to 8°. Kyphotic correction in 10 patients was maintained within 5° of the postoperative correction. There were rare hardware-related complications. Two multilevel patients, one with poor bone quality, required additional posterior stabilization and one patient with suspected nonunion had that level revised anteriorly at the time of surgery on an adjacent level. Conclusions: This constrained, true load sharing system appears to attain the goals of stabilizing the spine after surgery or trauma while promoting earlier and more substantial bone graft incorporation. Rather than work against the biology of bone healing, it helps optimize the local milieu and maximize the bone healing potential. Since no negative downside has been identified, the use of this type of anterior cervical plating system would seem to confer a significant benefit to the patient and can be recommended as the construct of choice for anterior cervical plating.