Influence Of Metaphyseal Sleeve On Stress-strain State Of A Bone-tumor Implant System In The Distal Femur

Objective: Aseptic loosening of distal femoral tumor implants significantly correlates with the resection length.We designed a new “sleeve” that is specially engaged in the metaphysis at least 5 cm proximal to the knee joint line to preserve as much bone stock as possible.This study investigates the influence of a metaphyseal sleeve on the stress-strain state of a bone tumor implant system in the distal femur.Methods: Based on the studies about measurement of morphological parameters of giant cell tumor of bone in the knee and morphological measurement of supracondylar femur based on digital technology in Chinese Han population,a new metaphyseal sleeve would be designed and produced.Cortex strains in intact and implanted femurs were predicted with finite element(FE)models.Moreover strains were experimentally measured in a cadaveric femur with and without a sleeve and stem under an axial compressive load of 1000 N.The FE models,which were validated by linear regression,were used to investigate the maximal von Mises stress and the implanted-to-intact(ITI)ratios of strain in the femur with single-legged stance loading under immediate postoperative and osseointegration conditions.The FE models of femurs without and with imlant with sleeve and stem,which were50 mm,75mm,and 100 in length,would be desigened with the FE models validated previously.Applied loading consisted of six components of force for two different flexion angles(0° and 48°)during the stance phase of gait for a normal walking cycle.The maximal principal strains and maximal von Mises stress would be investigated to the application of metaphyseal sleeve with cementless-diaphyseal engaging stems to determine what influence stemmed implants have on the levels of stress in the distal femur around the implant.Results: The metaphyseal sleeve can be divided into three types: large,medium and small.The height of sleeves are unchanged and can be matched with different forms of metaphyseal of distal femurs.Good agreement was noted between the experimental measurements and numerical predictions of the femoral strains(coefficient of determination(R2)≥ 0.95;root-mean-square error(RMSE%)≈ 10%).The ITI ratios for the metaphysis were between 13 and 28% and between 10 and 21%under the immediate postoperative and osseointegration conditions,respectively,while the ITI ratios for the posterior and lateral cortices around the tip of the stem were 110% and 119% under the immediate-postoperative condition,respectively,and114% and 101% under the osseointegration condition,respectively.The maximal von Mises stress for the implanted femur were 113.8 MPa and 43.41 MPa under the immediate postoperative and osseointegration conditions,which were 284% and 47%higher than those in the intact femur(29.6 MPa),respectively.The ratio at metaphysis of femur implanted with sleeve and three kinds of stem length prostheses was consistent with the range of 2%-20%.In general,the ITI ratios of femur with a 50 mm stem was higher than that of femur with a 100 mm stem,and the absolute difference of the ITI ratios was between 2%-7% at 0° flexion and between 1-6% at48° flexion.The 50 mm and 75 mm stem have a similar ITI ratio.In general,the strain ratio of the femur around the end of the stem is higher than that at the metaphyseal,with a absolute difference of between 3% and 27%.The ITI ratio at flexion 0° was higher than that at flexion 48°,but at the posterior of femoral shaft near metaphyseal,the ITI ratio at flexion 48° was higher,and the ITI ratio of femur with 50 mm,75 mm and 100 mm stems was 156%,162% and 170%,respectively.However,the maximum von Mises of 50 mm,75 mm and 100 mm femur at flexion0° is 40.9,42.4 and 42.2 times higher than that at flexion 48°.However,the maximum von Mises stress of 50 mm,75 mm and 100 mm femur at flexion 48° is40.9,42.4 and 42.2 times higher than that at flexion 0°,respectively.Conclusions: This study reveals that sleeve,which would be matched with different forms of metaphyseal,may cause stress shielding,especially in the distal metaphysis.Stress concentrations might mainly occur in the posterior cortex around the tip of the stem.However,stress concentrations may not be accompanied by periprosthetic fracture under the single-legged stance condition.The stress shielding was more severe in the femur with long length stem,but the stress shielding in the metaphyseal region was not directly related to the length of the stem.The increase of flexion angle has a more serious effect on femoral stress shielding.When flexed at 48°,stress concentration was observed in the posterior femur shaft near metaphysis.