The efficacy of interconnected porous hydroxyapatite in achieving posterolateral lumbar fusion in sheep

Study Design. An animal study was performed to evaluate lumbar spinal fusio n radiologically and mechanically. Objectives, To assess the efficacy of interconnected porous hydroxyapatite in achieving posterolateral lumbar arthrodesis in sheep. Summary of Background Data. Posterolateral spinal arthrodesis with autologo us bone graft is the gold standard procedure for lumbar fusion, The procedu re for harvesting bone from the iliac crest increases morbidity. Interconne cted porous hydroxyapatite has been used effectively as an alternative to c ancellous bone graft material in metaphyseal bone defects. Little is known about the efficacy of interconnected porous hydroxyapatite in achieving lum bar spinal fusion. Methods. Four groups of seven sheep underwent segmental posterolateral lumb ar fusion with instrumentation using different intertransverse graft materi al. In group 1, no graft material was used. in group 2, autologous bone was used. Group 3 had interconnected porous hydroxyapatite. Group 4 had an equ ip of interconnected porous hydroxyapatite and autologous bone. The animals were killed at 20 weeks after surgery. Radiographs and computed tomography images were obtained. The fusion masses were graded for bone resorption an d trabecular connectivity on the computed tomography images, Mechanical tes ting of the specimens was performed, and the three-dimensional segmental mo tion was measured in flexion/extension, axial rotation, and lateral bending . Results. The radiographic images were difficult to interpret because of the radiodense interconnected porous hydroxyapatite granules. According to mec hanical stability criteria, the fusion rate for the different groups was as follows: 100% (14/14) for the autologous bone group, 72% (10/14) for the b one/interconnected porous hydroxyapatite group, 50% (7/14) for the pure int erconnected porous hydroxyapatite group, and 15% (2/14) for the sham group. Conclusions. Spinal arthrodesis using interconnected porous hydroxyapatite alone or mixed with bone as graft material reduced segmental motion. It was not, however, as effective as autologous bone graft material in achieving spinal arthrodesis. The sheep model using autologous bone achieved a 100% f usion rate. Because the non-union rate for a single level in humans may be as high as 40%, the fusion rate with bone/interconnected porous hydroxyapat ite in humans may be lower than the 72% found in the sheep model. The littl e resorption of the radiodense interconnected porous hydroxyapatite granule s made the radiologic evaluation of the fusion masses difficult.