You have observed one of the perceived inconsistencies between the various branches of the FDA. However, there is in fact a good explanation. Regarding usability, most implants are implanted in accordance with surgical technique guides, the manufacturer often provides cadaver labs for surgeons that are new to the devices and most of the devices are implanted using similar techniques. Since orthopedic surgeons are trained in a variety of implant fixation methods, usability would only be required if the technique was novel and/or novel instruments were being used for alignment and cutting guides. With regards to biocompatibility, the FDA has no question that the materials are biocompatible. The only question is whether residuals remain on the parts from the manufacturing process. I have seen new companies demonstrate there are no residuals remaining on the parts, while most established companies provide a biocompatibility certification statement that indicates the devices are identical in biocompatibility to other devices that the company already has on the market.