Abstract
In arthritic joints, prostheses are used to replace degenerated articular surfaces thereby relieving the pain and gaining the near-normal function of the joint. The long-term performance of joint replacement systems depends, amongst others, on the short-term fixation stability of the prostheses. Excessive motions at the bone/porous-surfaced metal interface are generally recognized as a crucial factor to limit bone proliferation into surface pores of implants, resulting in fibrous ingrowth. Recent studies have identified relative micromotions of ∼20–40μm to be compatible with the bone ingrowth while larger motions are associated with less stable bone ingrowth and the formation of the fibrous tissue around the implant [1–3]. In recent years, periprosthetic osteolysis due to the wear debris produced from the implant bearing surfaces has been identified as the leading problem in total joint arthroplasty. To reduce the wear debris, substantial research studies have been and are being performed on the development of surface treatments and new materials for more resistant bearing surfaces. Despite the current general recognition of particulate debris as the principal cause of implant failures, the synergy between the implant motion and wear debris mechanism in the formation of periprosthetic fibrous membranes has only recently been stressed [4]. To provide initial and long-term stability of tibial components, the existing implant designs utilize a wide range of configurations with different number of screws and posts. We have compared the static fixation response of a metal plate resting on a polyurethane block with or without screws and posts under single symmetric/eccentric axial compression loads of up to 1000N [5], In an artificial knee joint, the transmitted articular forces are often inclined generating shear and torsional load components as well. Moreover, the fatigue response was not considered. To overcome these shortcomings of our earlier studies, the current work was set to: -measure the fixation response of a metal plate on top of a polyurethane block for designs with two bone screws or two smooth-surfaced posts under an inclined eccentric load resulting in 1000N axial compression and 250N shear force (the single plate with no fixation cannot be tested due to the destabilizing effect of the shear force which exceeds the frictional resistance); -measure the response of the plate fixated with a novel smooth-surfaced post with adjustable inclinations of nil, 1.5°, and 3°; and -investigate the fatigue response of various fixation systems under repetitive eccentric inclined for up to 4000 cycles at 1 Hz frequency.
