LOAD-TRANSFER THROUGH A HYDROXYAPATITE-COATED CANINE HIP IMPLANT

Strain transfer near hydroxyapatite (HA) coated canine hip implants wa s examined using simulated anatomical loading based on in vivo strain measurements. Strain changes near implants relative to intact control values were in excess of 100% for transverse and principal strains for zero time period (immediate postimplant) specimens. They were general ly smaller (100% or less) for axial, transverse, and principal minimum s in the same locations for 4 months postimplantation specimens. Corti cal bone loss occurred in all implanted femora, The most extensive los s, up to 47%, occurred adjacent to the proximal section of the implant . Extensive trabecular bone formation, over 300% in some regions of ea ch femur, was noted in all implanted femora. Backscattered electron im aging along the HA-coated sections of the implants showed extensive bo ne bonded to the coating. Normal light and UV light micrographs showed direct bone apposition to the implant surfaces and extensive bone for mation in all test animals. Microscopy revealed no evidence of any sof t tissue layer between the implant and bone. Bone was typically found in direct contact with the implant surface. Histomorphometry indicated that bone formation rates in the implanted femora were elevated, up t o 850%, relative to controls. Fewer formation sites were noted on the posterior and lateral (in two cases zero sites). Mineral apposition ra tes (MAR) from two of the dogs were slightly elevated (from 110-113%) in the implanted femora relative to controls and depressed (to about 8 3% of controls) in a third. (C) 1994 John Wiley & Sons, Inc.