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Studies using roentgen stereophotogrammetry and bone scintigraphy support a narrow definition of prosthetic fixation (viz., nonmigration), and consequently a broad definition of loosening. Roentgen stereophotogrammetric research indicates that if loosening occurs, it is initiated at an early stage; insufficient initial fixation or loss of fixation by resorption of a layer of heat-injured bone may cause prosthetic instability and progressive bone resorption. Migration of one or both prosthetic components can be revealed by roentgen stereophotogrammetry in many asymptomatic hips during the first postoperative year--some of these components will probably fail in the future. The femoral component is exposed to greater shear stresses than the acetabular component, and it is uncertain whether fixation of the femoral component can be achieved more than occasionally without the use of bone cement.

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Twenty patients were examined by standard radiography and roentgen stereophotogrammetric analysis (RSA) during a two-year period after total hip arthroplasty. Eleven of the acetabular components migrated cranially and three femoral components migrated distally. This migration was most rapid during the first four months after operation. Our findings support the possibility that mechanical loosening is initiated by thermal injury during polymerisation of the cement; the less frequent migration of the metallic femoral component compared with the polyethylene acetabular component may be because the metal acts as a heat sink. Standard radiographs were inadequate for assessment of early mechanical loosening, whereas RSA could reveal migration within four months of the arthroplasty.