Dissociation of modular hip arthroplasty components after dislocation. A report of three cases at differing dissociation levels.

Modular hip arthroplasty systems, currently widely employed, offer the advantage of increased intraoperative flexibility in component selection with reduced inventory, as well as the disadvantage of modular component dissociation. Dissociation during closed reduction for dislocation is reported in three patients at three different interface levels: (1) fixed acetabular shell-polyethylene linear interface, (2) bipolar acetabular component-femoral head interface, and (3) femoral head-neck interface. Subsequent open reduction was required in each case. Although this potential disadvantage of modular hip systems does not outweigh the benefits, it does warrant that certain precautions be taken when implanting modular components. The acetabular linear should lie flush within the metallic shell after impaction. The femoral head should be firmly impacted onto the neck. Both should resist reasonable manual force of disassembly. Should a modular hip arthroplasty component dislocate, gentle reduction under general anesthesia and fluoroscopic control is warranted. Careful inspection of pre- and postreduction roentgenograms for signs of modular component dissociation is mandatory.