Femoral revision knee Arthroplasty with Metaphyseal sleeves: the use of a stem is not mandatory of a structural point of view

Purpose Although metaphyseal sleeves are usually used with stems, little is known about the exact contribution/need of the stem for the initial sleeve-bone interface stability, particularly in the femur, if the intramedullary canal is deformed or bowed. The aim of the present study is (1) to determine the contribution of the diaphyseal-stem on sleeve-femur interface stability and (2) to determine experimentally the strain shielding effect on the metaphyseal femur with and without diaphyseal-stem. It is hypothesised that diaphyseal-stem addition increases the sleeve-femur interface stability and the strain-shielding effect on the metaphyseal femur relatively to the stemless condition. Material and methods The study was developed through a combined experimental and finite-element analysis approach. Five synthetic femurs were used to measure cortex strain (triaxial-rosette-gages) behaviour and implant cortex micromotions (Digital Image Correlation) for three techniques: only femoral-component, stemless-sleeve and stemmed-sleeve. Paired t-tests were performed to evaluate the statistical significance of the difference of cortex strains and micromotions. Finite-element models were developed to assess the cancellous bone strain behaviour and sleeve-bone interface micromotions; these models were validated against the measurements. Results Cortex strains are significantly reduced (p < 0.05) on the stemmed-sleeve with a 150 mu strain mean reduction at the medial and lateral distal sides which compares with a 60 mu strain mean reduction (p > 0.05) on the stemless condition. Both techniques presented a mean cancellous bone strain reduction of 700 mu strain (50%) at the distal region and a mean increase of 2500 mu strain (4x) at the sleeve proximal region relative to the model only with the femoral component. Both techniques presented sleeve-bone micromotions amplitude below 50-150 mu m, suitable for bone ingrowth. Conclusions The use of a supplemental diaphyseal-stem potentiates the risk of cortex bone resorption as compared to the stemless-sleeve condition; however, the stem is not essential for the enhancement of the initial sleeve-bone stability and has minor effect on the cancellous bone strain behaviour. Based on a purely structural point view, it appears that the use of a diaphyseal-femoral-stem with the metaphyseal sleeve is not mandatory in the revision TKA, which is particularly relevant in cases where the use of stems is impracticable.