REALLY?

When you get a joint replacement, like a knee or hip, how exactly is the muscle attached to the artificial implant?

Ordinarily, “no muscle attaches to any hip or knee implant,” said Dr. Mathias P. Bostrom, a joint replacement surgeon who is director of the orthopedic residency program at the Hospital for Special Surgery in New York. In general, he said, the only human tissue that adheres to an implanted joint is bone, which does it quite well.

The link is made either with cement or directly to the bone. When the connection is direct, the bone-producing cells grow new material that forms a bond with the implant, a process called osseointegration.

“A lot of knee implants use a kind of cement, really more of a grout, that fixes to the bone itself,” Bostrom said. “It is an acrylic, similar to Plexiglas, called PMMA, for polymethyl methacrylate.”

In the case of a tumor prosthesis for a bone like the femur - in which a lot of bone is removed and large segments are replaced with metal - there may be some linkage of muscle and tendon to implant, he said.

In most joint implants, however, the muscle is still attached to the bone via the tendon, and the surgeon especially tries to avoid detaching any muscle insertion in the hip or knee.

“We want the tendon’s attachment to the bone to be intact,” Bostrom said.