Orthopaedic Prostheses Fixation

Introduction

1. The fixation and maintenance of a stable interface between the prostheses and tissues is a most difficult problem of orthopaedic joint prostheses implantation. Frequent prostheses fixation problems are related to infection, wear and wear particles, loosening of prostheses and failure of implants. The failure of implants can be (1) mismatch of properties of tissue and biomaterial (2) wrong design of implants (3) improper surgery and fixation (4) post-surgical improper care (5) loosening of implant. Prostheses fixation can be mechanical or bone cement fixation.

2. Mechanical fixation : Bolts and nuts ( to fix femoral components to the femur bone in total hip prostheses) and metal pegs (to fix acetabular correspondent with metal to metal bearing surfaces) are the mechanical methods of fixation of prostheses. This method of fixation and bearing surfaces have many drawbacks like (1) massive tissue reaction (2) harmful release of wear particles during metal to metal friction and (3) stress concentration around the holes resulting into loosening of fixation. The passive mechanical fixation is a better method of fixation in which press fit is used to fix the femoral ceramic stems of a hip joint instead of bolts and nuts. The large size of the stem also helps in distributing the stresses on a large area. The passive fixation also includes the formation of a membrane at the interface of the bone and implant which prevents any relative movement between them or loosening of joint.

3. Bone cement fixation : Bone cement is made of PMMA (polymethyl methacrylate) powder and MMA (methyl methacrylate) monomer liquid. When the powder and liquid are mixed, the monomer liquid wets the polymer powder particle surfaces and links them by polymerisation. The mixture has a dough state when it is injected into the prepared intramedurally cavity. The prostheses is then placed over the cement as shown in the figure. The setting time of the bone cement takes 5 to 15 minutes. The properties of cured bone cement are comparable to those of acrylic resins (compressive strength at least 70 Mpa).