Unicompartmental knee arthroplasties (UKA) have been created to treat primary unicompartmental knee arthritis. They are indicated in case of lower-limb deformity due to unicompartmental arthritis, but not to treat unicompartmental arthritis due to extra-articular deformity. Early failures are do to ligamentar problems or to surgical technical errors. Late failures as wear or loosening are undoubtely delayed by adequate surgical indication and technique. Indeed the 10 years PUC survival is 90%. Patient age is not a selection criteria because both young (50-60 years old) and old (> 80 years old) patients may benefit of a UKA. UKA allows a young patient to continue low-impact sports and octagenarians will benefit of the lower morbidity of UKA surgery.