FRACTURE OF THE ACETABULUM DURING INSERTION OF AN OVERSIZED HEMISPHERICAL COMPONENT

A hemispherical metal-backed acetabular component that was oversized by two or four millimeters was introduced without cement into thirty fresh or embalmed cadaveric acetabula that were fifty, fifty-two, fifty-four, fifty-six, or fifty-eight millimeters in diameter. Manual tapping was used to insert the component into ten fresh and ten embalmed specimens, and a materials testing machine was used for the remaining five fresh and five embalmed specimens. The magnitude of force and the number of pulses necessary to seat the component were determined for the insertions performed with the materials testing machine; 2000 and 3000 newtons of force were required to insert the cups that were oversized by two and four millimeters, respectively. Visual and radiographic assessments were used to identify any resulting acetabular fractures. Over-all, eighteen (60 per cent) of the thirty acetabular specimens fractured. Of the eighteen fractures, only fifteen were detectable radiographically: ten on en face or lateral radiographs of the cup and five only on oblique radiographs of the cup, The remaining three fractures were apparent only after the removal of the acetabular component. No specific relationship was observed between the tendency of a specimen to fracture and the status of the specimen with regard to whether it was fresh or embalmed, the technique of insertion of the component, or the size of the specimen: a fracture occurred in nine of the fifteen embalmed specimens and in nine of the fifteen fresh specimens, in twelve of the twenty in six of the ten specimens for which the materials testing machine had been used, and in at least one-half of the specimens of each size. The components that were oversized by four millimeters were associated with significantly more fractures (p = 0.01) than were those oversized by two millimeters. CLINICAL RELEVANCE: The present data suggest that the use of a grossly oversized component - that is, one that is four millimeters larger than the acetabulum - is associated with a high risk of acetabular fracture. Even a component that is oversized by only two millimeters requires substantial force - 2000 newtons - to be seated within the acetabulum. Multiple radiographic views may be needed to detect a fracture in a patient who has pain in the groin after the insertion of an oversized acetabular component.