Fusion Rate and Biomechanical Stiffness of Hydroxylapatite Versus Autogenous Bone Grafts for Anterior Discectomy

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Abstract
The fusion rate and biomechanical stiffness were evaluated for 56 goat spinal units from 14 animals that had anterior discectomies and grafting procedures completed using hydroxylapatite and autogenous bone and survived for 6, 12, and 24 week healing times. Harvested spinal units underwent radiographic imaging to assess fusion, biomechanical testing in axial compression, flexion, extension, lateral bending, and axial rotation to assess strength, and histological analysis. The above results were compared for the two procedures and the different healing times. Because of some of the complications associated with the use of autogenous iliac crest bone graft in spine fusions, there has been considerable interest in the use of calcium phosphate ceramics as a possible substitute for a grafting material. One of the attractive features of calcium phosphate ceramics is the resulting strong bond that is formed with the host bone unlike other inert compounds. Surgeries were done at four sites on each animal with two in the cervical spine and two in the lumbar spine. Radiography was done during the survival time and postsacrifice. Biomechanical testing was done on the day of sacrifice under physiological loads. Both hard tissue sections and decalcified sections were histologically evaluated. 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 were 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 biomechanical stiffness between HA versus autogenous bone units for any mode of loading. Our results indicate that these dense, nonresorbable hydroxylapatite blocks perform as well as autogenous bone for anterior spinal fusions in this animal model. The use of this hydroxylapatite material in anterior spine fusions may have some clinical validity.

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