Osteogenic Effect of Ashed Bone in Fixation of Hip Implants
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OSTEOGENIC EFFECT OF ASHED BONE IN FIXATION OF HIP IMPLANTS ALEXIS S.
NASH,
Marquette University
ABSTRACT The purpose of this work is to discuss the role of ashed bone reinforced Ultra-High Molecular Weight Polyethylene, UHMWPE, biocomposite in bringing about early biological fixation of hip implant by combining the high damping capacity of this composite with the osteogenic characteristic of ashed bone. This paper will present the damping results obtained for this composite containing 65 weight% ashed bone over a range of frequencies, 0.1-100 rad/s, using a forced vibration technique. Some of the data obtained for lower ash content compositions are compared. The structural characteristics of bone and ashed bone are illustrated using x-ray diffractometry, XRD, and scanning electron microscopy, SEM. The optimal ashing temperature and volume fraction of ashed bone are also significant parameters which will be discussed. The 65 weight % ashed bone composite was elected to simulate the bone mineral content and use as a bioactive agent for the enhancement of bone ingrowth in the early days post-implantation. INTRODUCTION An understanding of the problems associated with the two techniques of fixation of hip implants would provide the underlying reasons for this work. The popularity of biological fixation of total hip arthroplasty, THA, over acrylic bone cement in recent years may be attributed to the fact that 40 % incidence of stem failures have been related to the acrylic sheath of the prostheses. Amongst the various problems are short term toxic and long term mechanical drawbacks, as discussed previously [1]. The loosening of hip implants may be due to the mechanical failure of the cement itself as microcracks develop or it may occur as a result of loss of mechanical interlocking at the bone-cement or cement-implant interface. This argument is supported by the fact that a polymerization shrinkage of 4.5 um for a 2mm thick cement layer occurs on cooling from 67 C to body temperature. The exothermic reaction in a 3 mm thick and in a 10 mm thick cement is known to produce temperatures around 60 C and 107 C respectively. Necrosis of tissue around the implant develops deep sepsis in the region. Around 1% of THA cases develop septic conditions which necessitates retrieval surgery and long term antimicrobial treatment before replacement surgery. Because of the problems associated with bone cement, a gap filler, biological fixation offers a more attractive alternative, although this technique may also fail due to the formation of weak interfacial bonding. The presence of micromotion at bone-implant interface stimulates the formation of fibrous tissues in place of bone ingrowth and a weak bonding results in stem failure, [1]. It is therefore clear that early stability of total hip implant is an essential criterion for the success of the implant that depends on the formation of a strong interfacial bone ingrowth which is possible in the absence of heel strike transients and micromotion at the interface. In this work the issu
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