Role of hydroxyapatite coating in resisting wear particle migration and osteolysis around acetabular components

The main problem facing the longevity of total joint replacements is wear particle-induced osteolysis, particularly around the acetabular component. Ovine Total Hip Replacement surgery was performed with roughened femoral heads in order to enhance wear debris generation in vivo. The resistance to aseptic loosening of acetabular components with different surface coatings was investigated. Implants remained in vivo for 1 year. Sheep were randomly assigned to one of six experimental groups where the acetabular cup was fixed utilising an: (a) cemented polyethylene acetabular cup, (b) metal backed grit blasted surface, (c) metal backed plasma sprayed titanium porous coating, (d) metal backed sintered beaded coating, (e) Hydroxyapatite (HA)-coated grit blasted surface (f) and HA-coated porous components. Ground Reaction Force (GRF) was used to asses the functional performance of the implants and data was collected pre-operatively and at 12, 24, 36 and 52 weeks post op. Wear debris generated was analysed and radiographs taken prior to preparation of thin sections. Fibrous tissue (FT) thickness and bone contact at 1 mm intervals along the acetabular bone-implant interface was calculated. GRF data demonstrated significant differences between experimental groups. In all groups there was an increase in the function of the hip after surgery and up to 24 weeks but thereafter the function of the group with the grit blasted surface reduced whereas the function of the other groups did not significantly change. Average wear particles generated were <1 microm in size. The cemented group demonstrated a significantly thicker average FT layer (2.69 mm) when compared with all other groups (p<0.05 in all cases) except the grit blasted group (1.56 mm). HA porous coated cups demonstrated significantly least fibrous tissue adjacent to its interface when compared with all other groups (cemented p<0.05, grit blasted p=0.029, porous p<0.05, sintered beads p<0.05 and HA grit blasted p<0.05). Significantly increased bone contact to HA-coated porous cups (73.33%) when compared with all groups was identified except HA-coated grit blasted cups where no significant difference was demonstrated. Radiographic signs of loosening were visible in all groups except the HA-coated porous group. Results demonstrated that HA porous coated acetabular components significantly enhanced bone ingrowth in the presence of wear particles, preventing their migration and reducing osteolysis. Non-HA-coated porous and sintered beaded components provided a more effective seal against the ingress of wear debris when compared with cemented cups.