EVALUATION OF POROUS BIPHASIC CALCIUM-PHOSPHATE CERAMICS FOR ANTERIORCERVICAL INTERBODY FUSION IN A CAPRINE MODEL

Study Design. This study compared the efficacy of characterized 50/50 hydroxyapatite/beta-tricalcium phosphate ceramics of 30%, 50%, and 70% porosity and autograft to promote interbody spinal fusion at C2-C3 an d C5-C6 in 24 goats: 12 at 3 months and 12 at 6 months. Objectives. Ra diographs, histology, dual energy x-ray absorptiometry analysis, and b iomechanical testing were used to evaluate the ability of the 30%, 50% , and 70% porous 50/50 hydroxyapatite/beta-tricalcium phosphate cerami cs and autograft to promote cervical interbody fusion. Summary of Back ground Data. The conundrum in the use of calcium phosphates for interb ody fusion is what porosity is most effective to promote ingrowth yet strong enough to resist compressive stresses found in the spine? It is known that the ability for bone ingrowth increases and the compressiv e strength decreases as porosity of the ceramic is increased. Dense ce ramics remain intact but may be surrounded by fibrous tissue. Porous c eramics have good ingrowth but may fracture. Methods. Radiographs were evaluated for fusion Methods. Radiographs were evaluated for fusion a cid fracture or collapse of the ceramics or autograft. Dual energy x-r ay absorptiometry was used to evaluate the fusion mass. Treated motion segments underwent biomechanical testing to quantify the flexibility of the segment. Undecalcified and decalcified histologic analysis were performed to evaluate the presence or absence of a bony union. Result s, Thirty percent, 50%, and 70% porous ceramics had better radiographi c fusion scores than the autograft at 3 and 6 months. Incidence of cer amic fracture did not increase with porosity and was equivalent to the collapse of autograft, although ceramics maintained disc height when fracture occurred. No statistically significant differences were found between autograft and the porous ceramics with biomechanical testing and peri-implant bone mineral density values as measured by dual energ y x-ray absorptiometry. At 3 months, histologic analysis showed a unio n rate of 0% for autograft and 30% porous ceramic, 67% for 50% porous ceramic, and 83% for 70% porous ceramic. At 6 months, the union rate w as 67% for the 30%, 50% and 70% porous ceramics and 50% for autograft. Conclusions. Thirty percent, 50%, and 70% porous ceramics performed e qual to or better than autogenous bone after 3 and 6 months. There may be promise for the use of 50/50 hydroxyapatite/beta-tricalcium phosph ate in spine surgery as the need to harvest autograft from the iliac c rest is obviated, and complications and cost associated with the harve st are avoided.