FUSION RATE AND BIOMECHANICAL STIFFNESS OF HYDROXYLAPATITE VERSUS AUTOGENOUS BONE-GRAFTS FOR ANTERIOR DISKECTOMY - AN IN-VIVO ANIMAL STUDY

Study Design. The fusion rate and biomechanical stiffness were evaluat ed for 56 goat spinal units from 14 animals that had anterior discecto mies and grafting procedures completed using hydroxylapatite and autog enous bone and survived for 6, 12, and 24 week healing times. Objectiv es. Harvested spinal units underwent radiographic imaging to assess fu sion, biomechanical testing in axial compression, flexion, extension, lateral bending, and axial rotation to assess strength, and histologic al analysis. The above results were compared for the two procedures an d the different healing times. Summary of Background Data. Because of some of the complications associated with the use of autogenous iliac crest bone graft in spine fusions, there has been considerable interes t in the use of calcium phosphate ceramics as a possible substitute fo r a grafting material. One of the attractive features of calcium phosp hate ceramics is the resulting strong bond that is formed with the hos t bone unlike other inert compounds. Methods. Surgeries were done at f our sites on each animal with two in the cervical spine and two in the lumbar spine. Radiography was done during the survival time and posts acrifice. Biomechanical testing was done on the day of sacrifice under physiological loads. Both hard tissue sections and decalcified sectio ns were histologically evaluated. Results. A 55% fusion rate for bone preparations and a 50% fusion rate for the hydroxylapatite (HA) units was found for the 12 and 24 week preparations. The HA preparations wer e better at maintaining disc space height. The biomechanical analysis revealed significantly higher stiffness values for fused preparations than for nonfused samples under extension, lateral bending, and axial rotation. Fused units demonstrated no statistical difference in biomec hanical stiffness between HA versus autogenous bone units for any mode of loading. Conclusions. Our results indicate that these dense, nonre sorbable hydroxylapatite blocks perform as well as autogenous bone for anterior spinal fusions in this animal model. The use of this hydroxy lapatite material in anterior spine fusions may have some clinical val idity.