Loosening of the cemented hip prosthesis. The importance of heat injury.

The cemented hip arthroplasty is one of the most successful orthopedic operations. However, the results do deteriorate with time because of mechanical loosening. Some hips with obviously loose prosthetic components are not painful. This discrepancy between symptoms and radiographic evidence of loosening has caused uncertainty about the diagnostic criteria that should be applied. This uncertainty has in turn led to confusion about the basic cause of loosening. The aims of this presentation were to find a favorable definition of loosening and to investigate the etiology of mechanical loosening of the cemented hip prosthesis. The diagnostic criteria of contrast and radionuclide arthrography, bone scintigraphy, and roentgen stereophotogrammetric analysis (RSA) were evaluated in 14 symptomatic hip arthroplasties examined by these methods. Loosening was found to be best defined as migration; regardless of diagnostic criterion, all loose prosthetic components were migrating, but no nonmigrating component was loose. The development of the radiolucent zone was analyzed in a simple model consisting of 6 cases of giant-cell tumor of bone successfully treated by curettage and cementing. In locations where the cement filling adjacent to cancellous bone was concave, a considerably wider zone developed. There was a positive correlation between the width of the zone and the volume of cement. The radiolucent zone in this model could be explained by heat injury during the polymerization of bone cement. The pattern of migration of the components in 20 cemented hip prostheses was studied with RSA and with conventional radiography over a period of 2 years postoperatively. Eleven acetabular and three femoral components migrated. After an initial 4-month period of rapid migration, most components migrated slowly. The initial rapid migration could be accounted for by the resorption of a layer of necrotic bone following heat injury during the polymerization. The less frequent migration of the metallic femoral component as compared with the polyethylene acetabular component could be explained by the metal acting as a heat sink. Radiographic examination was found to be unsatisfactory as a means of detecting mechanical loosening during the first 2 postoperative years. RSA, on the other hand, was capable of revealing migration as early as 4 months postoperatively. Migration of the components in 16 hip prostheses was studied with RSA for 1 year postoperatively following randomized use of low- and high-viscosity cement. As regards prosthetic migration, no difference was detected between the two types of cement; low-viscosity cement did not improve prosthetic fixation.(ABSTRACT TRUNCATED AT 400 WORDS)