57 J Musculoskel Neuron Interact 2000; 1:57-60 Original Article Hylonome Periprosthetic bone densitometry of the hip: Influence of prosthetic design and hydroxyapatite coating on regional bone remodelling L. Rosenthall, D.J. Bobyns, M. Tanzer McGill University Health Center, Montreal, Canada Abstract 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-ROMì, n=111; and Multilockì, n=62) were determined serially over a 24 month interval. There was a subset of patients with Multilockì 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 ORTHOì software). Inclusion criteria were primary implants, Harris hip scores ≥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. Keywords: Bone Densitometry, Bone Remodelling, Total Hip Arthroplasty Objective To determine the effect of design and hydroxyapatite coating on bone remodelling adjacent to the porous-coated cementless femoral component of hip prostheses in asymptomatic patients as a function of time. Materials and Methods Bone mineral density (BMD, gr/cm 2 ) for the seven Gruen zones (Fig. 1) surrounding the femoral component were determined by dual energy x-ray absorptiometry (DXA) using the LUNAR DPX scanner and LUNAR ORTHO TM software (LUNAR CORPORATION, Madison, WI, USA). Two prosthetic designs of titanium alloy composition were studied: S-ROM TM (Johnson & Johnson, Raynham, MA, USA) and Multilock TM (Zimmer, Warsaw, IN, USA). The S-ROM consists of two separate femoral components, a porous-coated sleeve proximally and a stem which is fluted and slotted distally (Fig. 2), whereas the Multilock is a single component femoral design which is porous-coated proximally and fluted distally (Fig. 3). There were 111 S- ROM and 62 Multilock hip prostheses. Twenty-two of the Multilocks had a 50Ìm thick coating of hydroxyapatite sprayed over the porous surface and 39 were uncoated. Inclusion criteria required that the patient be asymptomatic with Harris hip scores ≥95, primary implants, no osteotomy of the greater trochanter, no bone graft applications, no fractures during surgery, no radiographic evidence of subsidence and no other complications during follow-up. BMD determinations were made within a week of surgery to Corresponding author: L. Rosenthall, M.D., McGill University Health Center, 1650 Cedar Ave., Montreal, Canada H3G 1A4. E-mail: rosenthl@rad.mgh.mcgill.ca Accepted 15 March 2000.