Periprosthetic bone densitometry of the hip: influence of prosthetic design and hydroxyapatite coating on regional bone remodelling.

OBJECTIVE To determine the influence of femoral hip prosthesis design and composition on regional periprosthetic bone remodelling as a function of time in asymptomatic patients. MATERIALS AND METHODS Bone mineral density (BMD) changes surrounding two differently designed porous-coated femoral prostheses (S-ROM, n=111; and Multilocki, n=62) were determined serially over a 24 month interval. There was a subset of patients with Multilock implants that were coated with a 50 micron film of hydroxy-apatite (n=23) over the porous surface. Seven Gruen zones defined the periprosthetic bone regions (LUNAR ORTHO software). Inclusion criteria were primary implants, Harris hip scores >or=95 and no radiographic evidence of loosening. RESULTS For the S-ROM femoral component, at 6 months all zones showed a significant decrease in BMD relative to the immediate postoperative baseline measurements; mean loss varied from 4.3% to 17.4%. At 24 months mineral change varied from -10.8% to +1% for Gruen zones 1 to 6. Gruen zone 7, the calcar, differed in that it was the site of greatest mineral loss, attaining a mean of 17% at 24 months, and it was also significantly greater than the losses registered in the other zones at that time point. For the Multilock, maximal mineral losses were registered at zones 1 and 7. There were significant differences in mineral losses between the S-ROM and Multilock implants in zones 1, 2 and 3 at 24 months; S-ROM losses of 7.3%, 10.8% and 3.5% respectively, compared to Multilock losses of 23.8%, 15.9% and 6.9% respectively. Comparison of hydroxyapatite coated and uncoated Multilock implants demonstrated significantly less loss in the coated implants in zones 1 (greater trochanter) and 7 at 24 months. Coated losses were -16.9% and 11.3% for zones 1 and 7, respectively, whereas uncoated losses were 23.8% and 20.5% respectively.