Posterior Cervical Fusions Using Cerclage Wires, Methylmethacrylate Cement and Autogenous Bone Graft

An Experimental Study of a Canine Model

Abstract

Forty-eight adult mongrel dogs underwent posterior exposure of C4–C5, fixation of the two posterior spinous processes together with a no. 20-gauge cerclage wire, posterior element decortication, wound irrigation and the following: bone fusions (application of a standard volume of iliac crest autograft), polymethylmethacrylate (PMMA) fusions (application of a standard volume of methylmethacrylate cement), Combination 1 fusions (application of onehalf the volume of graft used in the bone fusions, over the facet joints. Methylmethacrylate cement was pressed into position centrally to surround the posterior spinous processes and cerclage wire), Combination 2 fusions (application of the same volume of graft used in the bone fusions, over the facet joints. Methylmethacrylate cement was applied as in the Combination 1 fusions). For each preparation, six animals survived 2 weeks or 3 months. All had monthly lateral cervical radiographs. At the appropriate times, they were killed and their C4–C5 segments excised and studied mechanically and histologically. At 2 weeks all of the above preparations were mechanically inferior to normal C4–C5 segments in respect to at least one of the parameters studied. At 3 months, the bone fusions and both combination fusions had developed sufficient mechanical stability so that they were equivalent to normal segments. At this time, the PMMA fusions remained inferior to the “normals.” The mechanical data for the PMMA and both combination fusions was corroborated by the histology which demonstrated a fibrosynovial layer between the cement masses and underlying posterior element bone. In the 3-month combination fusions, the lateral aspects of the posterior elements had been spanned by a fusion mass. Clinical relevance: Previously, the authors defined some of the problems associated with constructs modeled by their PMMA fusions. This work confirms the previous research. It also demonstrates that ultimate spinal stability is produced by combination constructs. Because of the 2-week mechanical data, it is recommended that when combination constructs are used clinically, the patient's neck be protected by an external orthosis in the early postoperative period.