Modular Head Mismatch in THA

Modular femoral heads have been used successfully since the mid-1980s in total hip arthroplasty. The use of metallic modular junctions presents a unique set of advantages and problems for use in total hip arthroplasty (THA). The separation of the head from the stem by a Morse taper has provided many benefits on the precision and balancing the reconstructed joint. Historically few complications have been reported for the modular Morse taper connection between the femoral head and trunnion of the stem in metal-on-polyethylene bearings. However, the risks or concerns are a little harder to identify and deal with. Certainly corrosion, and fatigue failure are the two most prevalent concerns but now the specifics of fretting wear and corrosive wear increasing particulate debris and the potential biological response is having an impact on the design and potential longevity of the reconstructed hip. This paper is dealing with a simpler consequence of head/stem modularity. Modular head mismatch to the socket bearing articulation. Two patients by two different surgeons at two different hospitals underwent cementless THA. Both patients were female and both presented with degenerative changes to the hip articulation. Both patients underwent hip replacement via a direct anterior approach using a standard hemispherical porous coated shell. One patient had a ceramic on ceramic bearing and the other had a ceramic head on a polyethylene liner. Both patients had a 32 mm inside diameter liner implanted and both had a 36 mm ceramic femoral head. The ceramic on ceramic mismatch was not recognized until the second office visit at eight weeks. The ceramic poly mismatch was not recognized until first office visit at six weeks. Both underwent correction surgery. These two cases demonstrate human mistakes can be made and steps need to be established to prevent future mistakes of this nature.