Mechanical stability of polyethylene liners cemented into acetabular shells

Revision of well-fixed, metal-backed acetabular components for dislocation or poly ethylene failure requires consideration of removing the entire cons truct or replacing the polyethylene liner only. For non-modular or first-ge neration modular components with poor locking mechanisms, one option is to cement undersized liners into well-fixed shells. The purpose of this study was to measure the stability of undersized liners cemented into metal aceta bular shells and compare the results with those of modular components. Hood ed polyethylene liners measuring 28 x 50 mm and 28 x 56 mm were cemented in to 66-mm acetabular shells (Smith & Nephew, Inc., Memphis, TN) with Simplex -P polymethylmethacrylate cement (Howmedica, Inc., Rutherford, NJ) giving 4 - and 2-mm cement mantles, respectively. The force required to lever-out th e liners from the shells was measured using the protocol described by Trado nsky et al.(1) Assemblies with 3-mm mantles dissociated at an average of 32 2 +/- 47 in-lbf; however, the assemblies with 2-mm mantles would not dissoc iate before the polyethylene yielded at torques as high as 600 in-lbf. Thes e results compare favorably with the previously reported range (43 to 684 i n-lbf) for modular acetabular components. These results suggest that unders ized polyethylene liners can be cemented into well-fixed acetabular shells and expected to be stable.