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Achieving a stable bone-implant interface is an important factor in the long-term outcome of joint arthroplasty. In this study, we employed an ovine bicortical model to compare the bone-healing response to five different surfaces on titanium alloy implants: grit blasted (GB), grit blasted plus hydroxyapatite (50 microm thick) coating (GBHA), Porocoat(R) (PC), Porocoat(R) with HA (PCHA) and smooth (S). Push-out testing, histology, and backscatter scanning electron microscope (SEM) imaging were employed to assess the healing response at 4, 8, and 12 weeks. Push-out testing revealed PC and PCHA surfaces resulted in significantly greater mechanical fixation over all other implant types at all time points (p <.05). HA coating on the grit-blasted surface significantly improved fixation at 8 and 12 weeks (p <.05). The addition of HA onto the porous coating did not significantly improve fixation in this model. Quantification of ingrowth/ongrowth from SEM images revealed that HA coating of the grit-blasted surfaces resulted in significantly more ongrowth at 4 weeks (p <.05).

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Femoral periprosthetic bone loss following total hip replacement is often associated with stress shielding. Extensive bone resorption may lead to implant or bone failure and complicate revision surgery. In this study, an existing strain-adaptive bone remodelling theory was modified and combined with anatomic three-dimensional finite element models to predict alterations in periprosthetic apparent density. The theory incorporated an equivalent strain stimulus and joint and muscle forces from 45% of the gait cycle. Remodelling was simulated for three femoral components with different design philosophies: cobalt-chrome alloy, two-thirds proximally coated; titanium alloy, one-third proximally coated; and a composite of cobalt-chrome surrounded by polyaryletherketone, fully coated. Theoretical bone density changes correlated significantly with clinical densitometry measurements (DEXA) after 2 years across the Gruen zones (R2 > 0.67, p c 0.02), with average differences of less than 5.4%. The results suggest that a large proportion of adaptive bone remodelling changes seen clinically with these implants may be explained by a consistent theory incorporating a purely mechanical stimulus. This theory could be applied to pre-clinical testing of new implants, investigation of design modifications, and patient-specific implant selection.

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