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Effects of alendronate-immobilized calcium phosphate coating on bone growth into porous tantalum : a gap model animal study Hu, Youxin

Abstract

Porous tantalum has been shown to be a promising orthopaedic implant material because of its similarity to bone in both mechanical properties and its three-dimensional porous structure. However, in some circumstances, bone quality or quantity is insufficient to allow adequate bone ingrowth. Alendronate, one of the bisphosphonate families, affects the activities of bone cells and enhances bone formation. In this thesis, we hypothesized that the addition of alendronate could increase the osteoconductivity and bone-ingrowth of porous tantalum and overcome the challenges of bone-implant gaps. To facilitate local delivery of alendronate, a micro-porous calcium phosphate coating was deposited onto the tantalum surface by an electrolytic deposition technique, which was followed by alendronate adsorption. Coating structures and morphologies were confirmed by scanning electron microscopy, X-ray diffraction, and Fourier transform infrared spectroscopy. The presence of alendronate was confirmed by high performance liquid chromatography. To study the effects of alendronate-immobilized calcium phosphate coating on bone reaction to porous implants, an animal gap model, with a fixed gap of 0.6 mm between implants and bone, was developed. Three types of surfaces, which were non-coating (Ta), calcium phosphate coating (Ta-CaP), and alendronate-immobilized calcium phosphate coating (Ta-CaP-AIN), were compared. Two fluorochromes were adopted to track the front of bone formation. After four weeks of healing and following standard histology techniques, the implants were analyzed with backscattered electron microscopy and fluorescent optical microscopy for bone-implant interactions. The relative volume increase of gap filling, bone ingrowth and total bone formation were 124 % (2.24-fold), 232% (3.32-fold) and 170% (2.7-fold) respectively in Ta-CaP-ALN compared with Ta controls. The contact length of newly formed bone on porous tantalum was increased by 700% (8-fold) in Ta-CaP-ALN compared with Ta plugs, suggesting enhanced osteoconductivity of Ta-CaP-ALN implants. The bone formation mechanism analysis found that bone growth initiated on both surfaces of the Ta-CaP-ALN implants and host bone, while little bone initiation on the Ta implant surface was detected. These significant enhancements of Ta-CaP^ALN may have direct applications in orthopaedics. For revision arthroplasty with insufficient bone stock, the local delivery of alendronate would enhance biological fixation of the implant and promote the healing of bone defects.

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