IN-VIVO EVALUATION OF ANTERIOR CERVICAL FUSIONS WITH HYDROXYLAPATITE GRAFT MATERIAL

Study Design. The efficacy of solid hydroxylapatite (HA) blocks for obtaining cervical interbody fusion was studied in a canine model. Each of 21 adult colony-reared hounds received a hydroxylapatite implant and an autogenous bone graft in the C3-C4 and C5-C6 disc interspaces. Seven dogs each were killed at 6, 12, and 26 weeks postoperation. Objective. This study determined the rate, mechanical strength, and histologic characteristics of cervical interbody fusions achieved using a hydroxylapatite (HA) block and compared the results to those obtained with autogenous iliac crest bone graft. Summary of Background Data. The use of corticocancellous autograft has been successful in a high percentage of anterior cervical interbody fusions. Calcium phosphate ceramics may provide an alternative to autogenous and allogenous tissue. These materials are biocompatible and capable of direct intimate bonding with bone because of their chemical similarity to bone mineral. Methods. Radiographic evaluation including plain radiographs, computed tomography (CT), and magnetic resonance imaging (MRI) studies were used to determine fusion quality. Specimens were also subjected to mechanical testing to determine bending and torsional stiffness as well as ultimate load to failure. Histologic evaluation included integrity and incorporation of the graft materials and interface characterization. Results. The CT images were well correlated with plain radiographs and demonstrated progressive incorporation of the graft materials with time. Magnetic resonance images were of little value in evaluating quality of fusion; the autograft sites demonstrated progressive disc height loss with time. Minimal disc height loss was observed with HA blocks. There was no statistical difference in torsional stiffness for the HA blocks and autogenoUs bone grafts at any time period. In bending, the HA block sites were significantly stiffer at 6 weeks (P < 0.005). There was no statistical difference in the ultimate failure load for the HA and autogenous bone grafts. Histologically, the HA blocks demonstrated areas of direct bone apposition with increased bone appositions and implant incorporation with time. At 6 and 12 weeks postoperation, the autograft sites demonstrated areas of graft resorption and some new bone formations. By 26 weeks, new bone was continuous with the vertebral endplates. Conclusions. The results indicate that HA blocks may provide an alternative to autogenous graft materials for anterior cervical interbody fusion. Block slippage and/or fracture may occur in a small number of patients but is primarily related to insertion technique and does not appear to significantly alter the final result.