My father had some kind of acid injected into both his knees. They didn’t think it was going to take, but after the third treatment, he was back to square dancing and bowling. He says he couldn’t be better. What kind of acid can reboot the knees like that and how does it work?

You may be referring to hyaluronic acid, a substance that is present in the fluid structure outside, around, and between cells. It is a thick substance that is sometimes referred to as a goo molecule. It’s found in the synovial fluid that lines and lubricates the joints. It is also the protective coating around each cartilage cell.

Hyaluronic acid has the ability to suck up and water needed to cushion joints from the shear stresses and compression they are subjected to. It seems to have many roles. Besides remaining elastic under high shear forces, it also makes it possible for the joint to withstand the heat that develops within the joint even with low shear stress.

Hyaluronic acid can store mechanical energy for release later when needed. It bathes the cartilage cells with fluid and keeps them nourished. It even has antiinflammatory properties to reduce joint inflammation and an ability to reduce pain — or at least the perception of pain.

Although the treatment works well for some people, it doesn’t have the same effect on everyone. Some patients obtain no relief whatsoever from their painful symptoms. Up to one third of the patients treated with viscosupplementation experience some type of negative side effect. Reports of temporary effects such as pain, warmth, swelling at the injection site have been noted.

It’s not clear yet which patients are the best candidates for this treatment. Your father was lucky to be one who responded well. More time and larger studies will eventually help us sort out who can benefit from this treatment and who should try something else.