MECHANICAL VIBRATION AS A MEANS OF OPTIMISING CEMENT MICRO INTERLOCK IN CEMENTED HIP ARTHROPLASTY

Abstract

Introduction: Total hip replacement is a commonly performed surgical procedure with at least 50,000 operations taking place each year in Britain The Swedish Hip Registry quotes a success rate of 94.6% at 10years, decreasing with time. The major reason for failure of the total hip replacement is aseptic loosening of the femoral stem, which accounts for 75.7% of revision. Polymethylmethacrylate cement is used in THR, unlike glues, which form a chemically adhesive bond, cement forms a mechanical bond with bone. In order to maximize the strength of this mechanical bond, an optimum interlock between the cement and the irregular, lattice-like, structure of the cancellous bone must be achieved. Current surgical methods to insert the stem into the bone cavity are purely manual; it is proposed to increase the amount of cement penetration by using a vibration technique.

Materials and Methods: A model was developed to approximate the conditions under which a cemented THR would be performed. The model consisted of a Zimmer Collarless Polished Tapered size 1 stem and a reusable mould, designed to simulate the femoral cavity into which a cemented stem would be inserted. This was then used to compare the cement micro-interlock induced under normal and mechanically vibrated conditions. To impart mechanical vibration to the stem of the prosthesis during insertion into the cement filled mould a custom built device was employed. PMMA cement was mixed and inserted into the mould according to the manufacturer’s instructions. The stem was then lowered into the cement mantle. The cement was allowed 10minutes to cure, the mould was then separated and the cement mantle retrieved and removed from the prosthesis stem. The resulting cast had dimples across the surface that corresponded to the holes in the mould. These dimples were then counted and classified into one of four categories, formed, semi-formed, unformed and voids. Two variables were studied, frequency and amplitude. All other known variables were controlled or monitored.

Results: The frequency and amplitude of vibrations at which optimum interlock was achieved in this test was at 19Hz and 4mm.The maximum force needed to insert a stem without vibration was found to be approximately equal to 185N, the same test was performed when the stem was vibrated during insertion, and the force was found approximately equal to 125N.

Conclusion: The results found indicate that vibration of the femoral stem during insertion into PMMA cement has a beneficial effect on the cement-bone interface. An increase in the frequency of formed and semi-formed dots with the use of vibration has been noted. Another effect of vibrating the prosthesis as it is inserted into the cement is to significantly lower the force needed for insertion by about 50N at an insertion depth of 100mm. This is a considerable force, roughly the weight of a gallon of water. Evidence presented here and from previous studies suggests mechanical vibration of an implant during cemented joint replacement has a beneficial effect on the cement bone interface.