Total hip prostheses using cervico-metaphyseal modularity were designed to better replicate the geometry of the native extra-medullary femur. However, they are associated with numerous complications including corrosion, disassembly, pseudotumours and, most notably, fractures of the modular neck. All reported cases of modular neck fractures occurred with titanium components (Ti-6Al-4V). To prevent this weakness, manufacturers developed modular necks made of cobalt-chromium (Co-Cr). We report a fracture of a long, 8° varus, Co-Cr modular neck connected to a 36-mm short (-3.5mm) femoral head. The fracture occurred 22 months post-implantation in a woman who had a low level of physical activity and a body mass index of 28.7 kg/m(2). To our knowledge, this case is the first reported instance of Co-Cr modular neck fracture. It may challenge the wisdom of further developing this modularity design, as our patient had none of the known risk factors for modular neck fracture. In addition, cases of pseudotumour have been reported with Co-Cr modular necks subjected to fretting corrosion, which contributed to the fracture in our patient.

The quality of life of operated patients after TKA obviously would be considered as the main priority, which was better obtained by a "regular design" in our study. Hence "high flexion" cannot be considered as an absolute target when choosing a model for total knee arthroplasty.

Level IV, retrospective study.

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