What does it mean when they say the new wrist replacements are "fourth generation." I think I understand it but I thought I'd better ask to know for sure.
Total wrist arthroplasty (TWA) (wrist replacement) has been around for over 100 years. Over the last 40 years, the implants (prostheses) have been changed and improved through four generations of products. The result is a prosthetic that is longer lasting with fewer surgical and postoperative complications. When severe arthritis has destroyed the wrist joint, an artificial joint gives the joint a new surface, which lets it move smoothly without causing pain. Increased strength and improved motion makes it possible to once again perform daily activities of living with greater ease and ability. But the early implants (first generation) were not as successful as hoped for. There were problems with the implants sinking down into the bone (called subsidence). The implants were made of one piece, so the force placed on them during movement caused them to crack and shift. And patients often developed reactions to the implants. The second batch of implants (second-generation) were made in more of a ball and socket design with separate component parts (no longer one piece). At first, patients got good pain relief and they were very happy with the results. But over time, the implant started to loosen, the joint deformed, and dislocation was not uncommon. That led to the third-generation implants. These implants attempted to balance the soft tissues around the joint and were therefore more stable. The design allowed surgeons to put the implant in place without removing so much of the old bone. Cement and screws were used to keep the prosthesis from loosening. The stem on the wrist side was longer, another feature aimed at reducing implant loosening. But component loosening was still a problem. Half of the patients ended up needing a second (revision) surgery. The implants that did survive only lasted five-to-seven years. Today's fourth generation implants have a porous surface to allow bone to grow in and around it. This is different from previous implants that always required cement to hold them in place. Cementless implants mean less bone destruction and improved durability of the implant. The newer systems are made of cobalt chrome, titanium, and polyethylene (plastic). Only two (titanium) screws are used to help stabilize the implant. Until recently (third-generation wrist arthroplasty), the complication rate was still much higher for arthroplasty (21 per cent) than for fusion (13 per cent). Long-term studies of fourth-generation implants are not available yet. Early reports (after three to five years) show improvement in pain with good satisfaction rates (95 per cent or more of the patients were happy with results). Complications such as infection, soft tissue imbalance, and implant loosening and dislocation are much improved with the new fourth-generation prostheses. There are still times when wrist fusion is considered a better treatment option but this determination is made on a case-by-case basis.