EVALUATION OF HYDROXYAPATITE/BIS-GMA BONE CEMENT FOR FIXATION OF CEMENTED HIP STEMS

Participants: E.P. Frankenburg, C.E. Hoffler, T. Shibuya, M. Saito, M. Lavagnino, B. Nolan, J.A. Baker, K.A. Sweet, D.C. Kayner, S.A. Goldstein

Keywords: hip, interface, hydroxylapetite

Introduction

Bone cement has been used as a standard method of fixing implants to bone. The procedure was first developed by John Charnley, M.D. in the 1950's, with the same material being used since that time. The primary purpose for conducting this study is to evaluate the efficacy of a hydroxyapatite (HA) based bone cement to provide fixation for an implant, using a canine animal model. Bilateral hip implants will be used, one implant being fixed with the experimental HA, the other using standard polymethylmethacrylate (PMMA) bone cement.

The HA bone cement used in this study has already been demonstrated to be biocompatible in a static, non-weight bearing study (Saito, et al., Bioceramics V, 1993). In addition, a number of mechanical tests have been performed on this experimental material to evaluate its mechanical properties. In general, the HA bone cement has been shown to set with a lower exothermic reaction than PMMA, and have an endurance limit of approximately five times that of PMMA.

The primary measure of success will be bone apposition at the interface, secure fixation, general healing of the site, and the return of normal function of the animal. The cement/bone interface will be studied through histological/histomorphometric as well as mechanical evaluations. Animals will be sacrificed at intermediate times during the experiment so that samples can be harvested to study bone reaction to the HA bone cement following the course of healing.

Materials and Methods

Twenty-five large adult male mongrel dogs will be randomly assigned to one of five groups, which include sacrifices at 30 days, three months, six months, one year, and two years. Each dog will undergo bilateral implantation of a total hip prosthesis. The implants will be randomly selected to be fixed with either PMMA or the HA cement. The contralateral limb of each dog will receive the other cement, so that each animal will have a prosthesis fixed with PMMA acting as a control. Both components are cemented by following manufacturer's recommendations for the respective materials. After both the acetabulum and femoral components are inserted and the cement hardened, the joint will be relocated, the capsule repaired, and the tissue planes closed in layers.

The femoral component/femur composites will be evaluated for mechanical stability in compression and torsion. The constructs will be positioned anatomically in a specialized fixture, and pre-failure loads applied to the femoral head by a servo-hydraulic testing machine. Relative displacement of the implant stem to the femur will be monitored and plotted as a function of load.

After mechanical testing, standard hard tissue thick sections (approx. 2 mm thick) of appropriate regions of the hip stem and cup will be prepared, surface stained, and observed under light microscopy. Then, they will be coated and examined under the scanning electron microscope. Finally, an image analysis will be performed on the SEM images to quantitatively determine the amount of bone apposition to the cement regions.

Results

Clinically, all 25 dogs reacted well to their bilateral hip arthroplasty. Mechanical tests revealed little statistical differences with respect to time or cement type. However, at one year post-op, the implants fixed with the HA cement had statistically more clip gage displacement when loaded in compression compared to the one and three month groups. Although there was no significant difference with respect to time or cement when considering micromotion at any one LVDT location, local displacements were much greater in the proximal stem than seen distally. Torsional tests revealed a trend that after one month, the HA cemented implants had a greater torsional stiffness than that of the implants fixed with PMMA.

Qualitative histologic data reveals good bony apposition to the HA cement at all time points, whereas a fibrous tissue layer is often seen in PMMA specimens both under light microscopy and SEM (Figure 1). After examining one section from each femur, the investigation revealed the highest bone area fractions at the cortical bone interfaces, while trabecular tissues produced the lowest. Examining the differences between cements, trabecular bone area fractions at the HA cement interface were greater than at the PMMA interface in the six month time point. No other statistically significant differences were noted.

The canine bilateral total hip arthroplasty model was valuable in the evaluation of cements used for implant fixation. Mechanical test results indicated that a hydroxyapatite composite cement could provide comparable stability to that of PMMA over a two year follow-up. Histologically, fibrous tissue is often seen along the bone/cement interface of the specimens fixed with PMMA, whereas good bony apposition is often seen along the HA cement surfaces. Ultimately, the greater apposition along the hydroxyapatite composite may provide a more clinically stable implant.

a b

Figure1: SEM images of HA cement (a) and PMMA (b) at two years post-operative

Progress

All surgeries have been performed, and all specimens have been mechanically tested. Histological analysis with light microscopy as well as SEM imaging have also been completed. Image analysis from the scanning electron microscopy have been developed and completed for the femoral specimens. A manuscript is currently being written.