The data are clear that the average age of the recreational diver is rising, and that the number of total knee replacements (TKR) per year are steadily increasing and more common with age. Surely there is a meaningful overlap of these sets.
What should the diver considering having, or already having undergone, TKR be aware of?
Let's open with some cheery news, which includes that TKR generally is a very successful procedure yielding excellent pain reduction and increased stability. And, a study published in the "Mayo Clinic Proceedings" reported that over 75% of orthopedic surgeons recommended resumption of scuba diving following operative recovery from TKA. Moreover, the USA-based multinational Knee Society and the august Chester Knee Clinic in the UK and many other knee centers list scuba as a recommended sport for patients who have fully recovered from TKR surgery.
According to DAN, the greatest concern for any type of exercise after knee replacement surgery is for the surgical site and bone to be completely healed. Complete healing generally means that muscle strength, range of motion and knee stability are within normal limits. Once your doctor has released you for full activity, and you are able to perform exercise and daily living activities with full weight bearing without pain, swelling, stiffness, or other worrisome issues, return to scuba can be contemplated.
As regards speed of healing, this depends upon the specifics of the individual case, but usually you can get a leg up on healing rate by making sure that the muscles supporting the knee are in top shape prior to surgery and by being very diligent in following the prescribed rehabilitation that always follows TKR.
Whether or not scarring and surgical changes would tend to be a site for nitrogen accumulation due to altered blood supply is debatable, but the published research to date suggests it's at most a minor worry. Still, it would be prudent to dive conservatively by not diving as deep, as long or as often when you return to diving. And course EAN dived to an air table can't hurt.
None of the knee joint prostheses of which I am aware would be affected by changes in ambient pressure since they are solid and contain no air spaces. Done properly, the replacement should leave no "voids" in involved tissue or bone. In any event, the diver undergoing this procedure should make his concerns known to the orthopedic surgeon and get the facts verified.
As with any type of orthopedic injury and returning to diving, there is no guarantee that you will not have another injury to the knee. Therefore, it would be a good practice to be careful entering and exiting the boat/shore, and to don and doff gear in the water. By all means avoid climbing boat ladders or steep shore exits with heavy gear on.
Finally, scuba-related stress on the knee can be reduced by finning using a short flutter kick and wearing light, flexible fins. Split type fins or those with shorter, broader blades often are recommended by divers who have undergone TKR.