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

Study Design. The efficacy of solid hydroxylapatite (HA) blocks for ob taining cervical interbody fusion was studied in a canine model. Each of 21 adult colony-reared hounds received a hydroxylapatite implant an d an autogenous bone graft in the C3-C4 and C5-C6 disc interspaces. Se ven dogs each were killed at 6, 12, and 26 weeks postoperation. Object ive. This study determined the rate, mechanical strength, and histolog ic characteristics of cervical interbody fusions achieved using a hydr oxylapatite (HA) block and compared the results to those obtained with autogenous iliac crest bone graft. Summary of Background Data. The us e of corticocancellous autograft has been successful in a high percent age 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 wi th bone because of their chemical similarity to bone mineral. Methods. Radiographic evaluation including plain radiographs, computed tomogra phy (CT), and magnetic resonance imaging (MRI) studies were used to de termine fusion quality. Specimens were also subjected to mechanical te sting to determine bending and torsional stiffness as well as ultimate load to failure. Histologic evaluation included integrity and incorpo ration of the graft materials and interface characterization. Results. The CT images were well correlated with plain radiographs and demonst rated progressive incorporation of the graft materials with time. Magn etic resonance images were of little value in evaluating quality of fu sion; the autograft sites demonstrated progressive disc height loss wi th time. Minimal disc height loss was observed with HA blocks. There w as no statistical difference in torsional stiffness for the HA blocks and autogenoUs bone grafts at any time period. In bending, the HA bloc k sites were significantly stiffer at 6 weeks (P < 0.005). There was n o statistical difference in the ultimate failure load for the HA and a utogenous bone grafts. Histologically, the HA blocks demonstrated area s of direct bone apposition with increased bone appositions and implan t incorporation with time. At 6 and 12 weeks postoperation, the autogr aft sites demonstrated areas of graft resorption and some new bone for mations. By 26 weeks, new bone was continuous with the vertebral endpl ates. Conclusions. The results indicate that HA blocks may provide an alternative to autogenous graft materials for anterior cervical interb ody 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.