Effect of neck resection on torsional stability of cementless total hip replacement.

Loosening of the femoral component in total hip arthroplasty commonly results from inadequate resistance to torsional loads. We evaluated 20 adult human cadaver femora to determine the effect of different neck-resection levels on torsional resistance of the femoral component. All specimens were prepared for fixation with the Impact modular total hip replacement. Each femoral diaphysis was overreamed 2 mm to achieve only proximal fixation. The specimens were then divided into groups of five and implants were inserted with the precision press-fit technique. Each specimen was loaded in an Instron stress-testing device. A linearly variable differential transducer was then attached to the specimen to measure micromotion at the medial interface between the implant and bone. Each specimen was loaded until failure occurred. When all of the neck was preserved, torsional load to failure was significantly better than in the 50%, 15%, and 0% neck-preservation specimens. At a 20 N-m torsional load, the 100% and 50% neck preservation specimens had similar micromotion, but the 15% and 0% specimens had gross motion and a large standard deviation at this load level. Without distal fixation, the femoral component is highly dependent on proximal geometry for resistance to torsional loading. Preserving the femoral neck provides an effective means of resistance. Maintaining the entire femoral neck most effectively reduces miromotion at low loads, but maintaining the midshaft area of the femoral neck appears to most effectively control micromotion at higher torsional loads. Resection below the midshaft of the neck markedly decreases the torsional load-bearing capacity of the proximal femur.