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Thirty-two stainless steel Charnley total hip replacements were examined; 23 of these had been removed from patients after a period of implantation, twelve as a result of fracture. All the components were forgings produced by the same manufacturer. The majority of the components (Type 1) were about eight years old and were manufactured from air-melted EN58J stainless steel with a maximum permitted carbon content of 0.06 percent (this steel is similar to Type 316 stainless steel). The remainder (Type 2) were about four years old and were manufactured from vacuum-melted material with a maximum permitted carbon content of 0.03 percent (similar to 316L stainless steel). The following results were noted:
1. The material in the Type 1 implants was often sensitized and contained many inclusions. The material in the Type 2 implants was never sensitized and contained fewer inclusions but had a larger grain size than the Type 1 material.
2. In both types of implant, fracture initiation was not obviously associated with corrosion, and the influence of the patient' weight and activity seemed to be more important than that of metallurgical defects.
3. In the Type 1 components, intergranular corrosion occurred in sensitized regions. and the severity of the corrosion increased with the period of implantation. The incidence of corrosion of all kinds was 35 percent for an average implantation time of 46 months. In the Type 2 material, pitting corrosion occurred, but the sample size was too small to allow a meaningful estimate to be made of the incidence of this kind of corrosion.
4. Other surface markings were observed, including fretting due to abrasion by acrylic cement.
5. It is the authors' opinion that the main consequence of corrosion may be an eventual undermining of the integrity of the clinical procedure rather than the destruction of the component itself.
implant materials, fracture (materials), corrosion fatigue (materials), stainless steel, total hip replacement
Research fellow, Institute of Orthopaedics (University of London), Royal National Orthopaedic Hospital, Middlesex,
Professor, Institute of Orthopaedics (University of London), Royal National Orthopaedic Hospital, Middlesex,