Isoelastic total hip replacement: clinical evaluation of prosthetic isoelasticity

By using the lower modulus material, polyacetal, ie, prosthesis made by R. Mathys', isoelastic hip prosthesis is claimed to reduce stress shielding effects and secure long-term, stable fixation of prostheses. In Tri-Service General Hospital from October 1985 to October 1989, 23 patients (25 hips) were treated using this prosthesis. Twenty patients (22 hips) were followed for clinical and radiographic evaluation for an average of 44.8 months (29-58 months). In clinical evaluation, the overall results were comparable to other cemented or cementless hip prostheses. The Merle d'Aubigne and Postel score for pain, motion and walking were improved from 1.6, 3.9, and 2.4 to 4.6, 4.3, and 4.9, respectively. The acetabular cup showed more favorable radiographic evaluation with a total mean score by combined Charnley and Engh's system of 7.9. It was not optimal on the femoral side and it showed that 20 of the femoral stems were stable fibrous fixations with a mean score of 15.8. Early complications included two dislocations, two significant femoral shaft fractures, and five medial neck cracks at surgery. The late complications included one loosening, one screw breakage, and one screw back out. The isoelastic hip always gained an inferior score in clinical evaluation from the "self comparison" in three patients with one side isoelastic hip and the other side different cemented or cementless prosthesis. There is no obvious benefit in radiographic evaluation, both in elimination of stress shielding and enhancement of fixation stability. As a result, the theoretical advantages of the isoelasticity concept are difficult to justify in our clinical practice.