Total joint replacement has become one of the most common procedures in the area of orthopedics and is often the solution in patients with, diseased or injured hip joints. Component loosening is a significant problem in these cases. When implant components become insecure, they may cause the patient pain, debilitation, and typically lead to shortened implant life which necessitates revision surgery. Implant loosening is primarily caused by bone resorption at the bone-cement interface in cemented implants.

Many factors leading to this bone resorption have been suggested including mechanical environment and cement wear debris. It is our hypothesis that localized shear stresses are responsible for the resorption. Given this, it was previously shown analytically that local stresses at the interface could be reduced by using a cement of lower modulus. A new reduced modulus cement, polybutyl methylmethacrylate (PBMMA), was developed in order to test the hypothesis. PBMMA was formulated to exist as polybutyl methacrylate filler in a polymethyl methacrylate matrix. Thesuccess of PBMMA cement is based largely on the fact that the polybutyl component of the cement will be in the rubbery state at body temperature. The toughness of the polybutyl methacrylate is responsible for the lower modulus of the new cement. In vitro characterization of the cement was undertaken previously and demonstrated a modulus of approximately one-eighth that of conventional bone cement, polymethyl methacrylate (PMMA) and favorable fracture toughness.

The purpose of this experiment was to perform an in vivo comparison of the two cements. A sheep model was selected. Total hip arthroplasty was performed on 50 ewes using either PBMMA or PMMA. The sheep were maintained in stalls for one year. Radiographs were taken at 6 month intervals. Prior to sacrifice, the contralateral femur of the sheep were implanted so that each animal served as its own control. The tightness of the femoral component within the femur was assessed during internal and external hip rotation using mechanical methods.

The torsion tests demonstrated a significant difference in loosening between the cements when the femurs were tested in external rotation (p < 0.007). Evaluation of the mechanical data also suggests that the PBMMA sheep had a greater amount of loosening for each subject, 59 % versus 4 % for standard PMMA. A radiographic analysis demonstrated more signs of loosening in the PMMA series of subjects. A second study of the radiographs was used to rule out the possibility of subsidence of the implant leading to a relative tightening effect. A brief histological examination showed similar bony reaction to both cements, however, study of the interface membrane was not accomplished. Reasons for the rejection of the hypothesis based on the results of the mechanical testing are discussed.