Effect of stem stiffness and bone stiffness on bone remodeling in cementedtotal hip replacement

The hypothesis in this study is that the stem stiffness-to-bone stiffness r atio influences the incidence and type of bone remodeling and fixation with cemented total hip arthroplasty. Ninety-one patients with 99 hips had ceme nted stems using 3 different anatomic porous replacement designs. The APR I and APR II titanium stems with proximal porous coating on the proximal one fourth of the stem were cemented into 49 and 35 patients. The APR TT-C ste m, which is a cobalt-chrome stem only for cemented fixation, was cemented i nto 15 patients. These 3 different stem designs were used to study differen t metals as well, as different stem shapes. The average follow-up was 4.3 y ears (range, 2-10 years) with all hips having 2 years' follow-up and 42 hip s at least 5 years' follow-up. Bone remodeling was measured as stress shiel ding, calcar resorption, and distal hypertrophy on anteroposterior and late ral radiographs of the hip. Stress shielding was measured by the 4 grades d escribed by Engh. A stem stiffness-to-femoral bone stiffness ratio was calc ulated from the plain radiographs with the stem stiffness known from the ma nufacturer and the bone stiffness calculated using measurements of the oute r and inner diameters of the femur. There was no statistical difference for bone remodeling and fixation between the 3 stem shapes or 2 metal types us ed in these hips. No stem was loose, and only 10 had radiolucent lines. Str ess shielding was statistically related to stem stiffness but was more stro ngly related to the axial stiffness ratio, mediolateral bending stiffness r atio, anteroposterior stiffness ratio, and torsional stiffness ratio. Stres s shielding grade 3 and 4 was present in 20% of hips with a torsional stiff ness ratio < 0.33, in 38% of hips with a torsional stiffness ratio of 0.34 to 0.5, and in 70% of hips with a torsional stiffness ratio > 0.5. Five-yea r results showed no statistical change in stress shielding, calcar resorpti on, and distal hypertrophy from the 2-year observations. The stem stiffness -to-bone stiffness ratio influenced bone remodeling but not fixation of the se cemented stems.