A successful operation, for which there's a large unmet need

Hip replacements were developed in the 1960s and within a decade had revolutionised the treatment of hip arthritis in elderly patients. This success heightened interest in other joints, particularly the knee. With no constraint on the development and use of surgical implants there was a short period of divergent evolution, and a great variety of knee replacements were produced. Natural selection soon occurred and many designs failed rapidly—hinged knees caused particular problems. Thus, in the 1970s and early 1980s knee replacement was widely considered to be a poor operation. Some designs were successful, however: these attempt to resurface the joint and reproduce normal knee anatomy with a low friction joint. The remaining knee ligaments provide stability, allowing some rotational movement and good function. Total knee replacements have subsequently undergone a period of convergent evolution, and most implants …