Adjuvant treatment with osteoconductive agents in a canine model of hip reconstruction

We used a canine model of hip reconstruction to determine whether adjuvant treatment with two osteoconductive agents would enhance bone ingrowth into the porous surface of the prosthesis and the bonding strength of prosthesis to femur. Canine femoral prostheses were coated with 0.9% saline (control), with hydroxyapatite using a new room-temperature electrolytic method, or with chitosan, a linear polysaccharide obtained by partial deacetylation of chitin, and implanted in the right femora of adult mongrel dogs. The dogs were euthanized at either 6 weeks or 6 months and the femur-prosthesis composite was sectioned transversely. Bonding strength was determined by push-out tests on two sections per dog, histology and bone ingrowth (by scanning electron microscopy in the back-scattered electron mode) in adjacent sections, and changes in the host femur by contact radiography of other sections. The main findings in this study were as follows: Compared to controls at 6 weeks, the available pore volume occupied by bone (bone volume) was increased by chitosan treatment (45%, p < 0.0003), but not by the hydroxyapatite treatment. The highest average bone volume (75.6 ± 9.2%) was for the chitosan group, whereas the highest bonding strength (1049 ± 521 N) was for the hydroxyapatite group, at 6 months. Approximately 86% of the bone volume in the latter at 6 months was already attained in the chitosan group at 6 weeks. Histologically, chitosan treatment appears to have produced more volume of immature bone and fibrous tissue quickly, whereas mostly mature bone was produced slowly by hydroxyapatite treatment. Bonding strengths were not significantly different from controls because of variability due to the presence of fibrous tissue (chitosan, 6 weeks), inadequate bone ingrowth (hydroxyapatite, 6 weeks), or small sample size (6 months). We hypothesize that bonding strength may be enhanced further by using chitosan and hydroxyapatite as vehicles for coating prostheses with osteoinductive factors.