Superior sealing effect of hydroxyapatite in porous-coated implants: experimental studies on the migration of polyethylene particles around stable and unstable implants in dogs.

BACKGROUND: Migration of wear debris to the periprosthetic bone is a major cause of osteolysis and implant failure. Both closed-pore porous coatings and hydroxyapatite (HA) coatings have been claimed to prevent the migration of wear debris. We investigated whether HA could augment the sealing effect of a porous coating under both stable and unstable conditions.
METHODS: We inserted porous-surfaced knee implants, with and without HA coating, in 16 dogs, according to a paired, randomized study design. 8 dogs had 2 implants inserted into each knee using a stable implant device and 8 dogs received 1 implant in each knee using a micromotion (500 microm) implant device. Implants had a peri-implant gap of 0.75 mm. We then injected polyethylene (PE) particles or a control solution into the knee joints on a weekly basis.
RESULTS: After 16 weeks, the rating of particles around stable implants was reduced by the HA coating froma median value of 2 (1-4) to 1 (0-1) (p = 0.01) and during micromotion from 3 (2-4) to 1 (0-3) (p = 0.002). HA-coated implants had superior bone ongrowth during stable and unstable conditions. We found no difference in bone ongrowth between PE-exposed and vehicle-exposed implants.
INTERPRETATION: Compared to a pure plasma-sprayed porous coating, a layer of HA coating provides bettter bone ongrowth and protects the bone-implant interface against the migration of wear debris under both stable and unstable conditions.