Knee osteoarthritis is a common condition that happens when the cartilage that cushions the knee joint starts to wear down. Botulinum is a toxin made by the bacterium Clostridium botulinum. Because it can shut down nerve cells, doctors can use it to ease muscle spasms. Some doctors are trying botulinum to help treat joint pain. The theory is that it might permanently deaden nerves and offer relief. There’s not enough evidence to know if it works.