Modular knee prostheses have been used since the 1980's. Their goal was to offer an alternative to reconstructing large bony defects without using bone grafting. Initially, they were used for reconstruction after resection of tumours about the knee. Their success encouraged their use in the treatment of some failed total knee arthroplasties, with large bony defects and severe ligamentous instability. We have implanted 7 modular knee prostheses between July 2006 and January 2009. There were 4 tumoral cases, 1 case of desarthrodhesis after a Campanacci procedure and two cases of failed total knee arthroplasties. We have achieved encouraging results using these implants with all but one patient surviving at two years after surgery. All the implants were considered to function normally regarding range-of motion, muscle strength and gait. The surgical and medical management of patients with bone sarcomas has advanced greatly during the last 20 years, improving their overall survival. Thus, the surgeons are provided with increased abilities to perform limb-sparing or joint-mobility sparing surgery.