W e investigated the implant-bone interface around one design of femoral stem, proximally coated with either a plasma-sprayed porous coating (plain porous) or a hydroxyapatite porous coating (porous HA), or which had been grit-blasted (Interlok). Of 165 patients implanted with a Bimetric hip hemiarthroplasty (Biomet, Bridgend, UK) specimens were retrieved from 58 at post-mortem.We estimated ingrowth and attachment of bone to the surface of the implant in 21 of these, eight plain porous, seven porous HA and six Interlok, using image analysis and light morphometric techniques. The amount of HA coating was also quantified.There was significantly more ingrowth (p = 0.012) and attachment of bone (p < 0.05) to the porous HA surface (mean bone ingrowth 29.093 ± 2.019%; mean bone attachment 37.287 ± 2.489%) than to the plain porous surface (mean bone ingrowth 21.762 ± 2.068%; mean bone attachment 18.9411 ± 1.971%). There was no significant difference in attachment between the plain porous and Interlok surfaces. Bone grew more evenly over the surface of the HA coating whereas on the porous surface, bone ingrowth and attachment occurred more on the distal and medial parts of the coated surface. No significant differences in the volume of HA were found with the passage of time.This study shows that HA coating increases the amount of ingrowth and attachment of bone and leads to a more even distribution of bone over the surface of the implant. This may have implications in reducing stress shielding and limiting osteolysis induced by wear particles. The use of hydroxyapatite (HA) coating has been advocated in order to increase the attachment of bone to metal implants. Many animal as well as clinical studies have demonstrated the osseoconductive properties of HA and the results at six to eight years are excellent.1,2 Bone apposition appears to be well advanced as early as three weeks, 3 and some studies have shown it to be greater than 90% at 96 weeks. 4 Although there is concern that HA resorbs with time and that the release of HA debris may have adverse effects, 5 the clinical results reported so far for HA-coated components suggest that at present this is not a significant problem. Several reports of the outcome after the insertion of porous-coated uncemented implants have shown good results.6-8 Most studies found some degree of bone ingrowth into the pores 9-12 with the mean extent reported to be in the range of 5% 9,12 to 39.2% 13 of the available pore volume. An HA coating has been advocated in order to reduce the effects of debonding and to encourage bone ingrowth and attachment to a porous surface. Several studies have examined the use of an HA surface coating on porous-coated implants. Some have reported that there is no clinical advantage 14 while others have demonstrated a significant increase in bone ingrowth. 15,16 We have investigated bone ingrowth and attachment to an HA-coated porous titanium surface, a plain porous titanium surface and a roughened titanium (Interlok) surface finish in one femoral design from s...