Anterior cervical discectomy, fusion, and plating. A comparative animal study.

Thirty-five goats were used as animal models for three-level anterior cervical discectomy and fusion. The goats were divided equally into five experimental groups: Group I, three-level anterior cervical discectomy without fusion; Group IIa, three-level discectomy with autogenous bone (Smith-Robinson technique); Group IIb, autogenous bone grafting plus anterior plate application; Group III, three-level discectomy and fusions with fresh frozen allograft bone; Group IIIa, graft only; and Group IIIb, anterior plating. The goats were killed, and spines were removed and analyzed for evidence of fusion. Radiographic union was judged to have occurred in 0% of Group I disc spaces, 48% of Group IIa, 52% of Group IIb, and 38% of Groups IIIa and IIIb. Histologic fusion was judged to have occurred in 0% of Group I specimens, 38% of Group IIa, 45% of Group IIb, 0% of Group IIIa, and 19% of Group IIIb. The histologic fusion rate was significantly higher in Groups IIa and IIb than in Groups I and IIIa. There was no statistically significant increase in the histologic fusion rate between goats with anterior cervical plating and goats without plating. Biomechanically, the spines in Groups IIb and IIIb, ie, those with anterior plates applied, were stiffer in axial load, torsion, and flexion/extension. Peri-end-plate vascularity was significantly diminished in those groups that had anterior plates applied. Qualitative analysis of fluorochrome labels showed that autografts revascularized more rapidly than did allografts in both the nonplated and plated groups. Histomorphometric analysis failed to reveal any significant device-related osteopenia in those vertebrae spanned by the anterior plate. We found that although autograft bone led to a significantly higher rate of union than did allograft bone, the addition of anterior plate fixation did not significantly increase union rate. Biomechanical rigidity was significantly increased in all modes of testing by the use of an anterior plate. The decreased vascular response seen in spines that underwent plating may be responsible for the lack of increased union. The increased rigidity found with anterior plating supports its use in traumatic conditions. The failure, however, to increase significantly the union rate in this model fails to lend support to the use of anterior cervical plating for degenerative discectomy and fusion procedures.