Abstract
Recent introduction of short femoral implants has produced inconsistent outcomes. There have been reports of early aseptic failure as high as 30% within 2 years of implantation. This is in spite of the fact that these short components are shortened versions of existing successful non-cemented designs. The mode of initial fixation in non-cemented implants has been investigated. It has been demonstrated that long term survivability is dependent upon osseous integration; and that osseous integration requires secure initial implant fixation. Traditional non-cemented implants achieve initial fixation analogous to that of a nail in a piece of wood: friction and displacement (with resultant hoop stress). Initial fixation, of a traditional non-cemented femoral component, is directly proportional to surface area contact between the implant and endosteal bone and/or three point fixation. By reducing stem length, contact area may be significantly reduced, thereby increasing stresses over a smaller area of contact. The result of this is to potentially compromise fixation/implant stability against micromotion occurring in the early post-operative period. These stresses are most poorly resisted in flexion/extension and rotational planes about the long axis of the femur. In addition, force applied in an attempt to achieve initial fixation with a short stem may lead to an increased risk of periprosthetic fracture at the time of implantation.
We propose that there is an alternative mode of initial fixation, a “rest fit”, that may avoid both the risk of femoral fracture as well as provide better initial implant stability. To assure a maximal initial fixation and resistance to post-operative stresses which may compromise initial implant stability and osseous integration, a short implant should have three distinct geometric features: a medial and lateral flare, a flat posterior surface and a proximal trapezoidal cross section. The first will provide stability against subsidence and varus migration, by resting upon the proximal femur. A flat posterior surface will maximize load transmission to the femur in flexon/extension activities; and an asymmetrical proximal cross-section will provide resistance against rotational stresses about the long axis of the femur during activities such as stairclimbing. Together these features have been throproughly evaluated by FEA and in vitro testing. We are reporting on the shoprt term follow up (2.5 years avg.) first 300 short stems which have employed a “rest fit”. There have been no aseptic failures or revisions for mechanical failure of these implants.
