Failure of the locking mechanism between the acetabular metal shell and the polyethylene liner after primary total hip arthroplasty--a case report.

The short and intermediate-term clinical and radiographic results associated with Harris-Galante II porous-coated acetabular modular cup without cementing have been excellent in most patients. Nevertheless, its modular design introduced the potential for failure of the locking mechanism. There are four pairs of locking ties on the rim of metal shell to fix the polyethylene liner within it. Failure of any locking tine would result in less rigid fixation between the metal shell and the polyethylene liner and potentially produce instability or even disassembling of the liner. Only nine cases have been reported previously about the fractures of the locking tine, but it has never been reported in literature in Taiwan. We report a case with fracture of one tine following minor trauma at five years after primary total hip arthroplasty. During reoperation, scratching injury on the outer surface of the polyethylene liner in contact with the inferior tines of the metal shell was noted. Although no disassembling of the liner was noted, the remaining locking mechanism was found to be less rigid when the liner was taken out of the shell. The locking mechanism of HG II cup may be mechanically weak and may fail without major trauma; therefore, its design ought to be improved in an attempt to prevent postoperative dissociation of the polyethylene liner.