Dmso safety for knee pain?

Horses yes. Dmso has been used for years to reduce the joint pain of horses. There is also considerable evidence going back decades that DMSO may help human joint pain but its saftey profile is still the issue. The drug is absorbed quite well through the skin and could cause side effects systemically. My recommendation is do not use it.
Caution. Dmso has been used on animals for years. It is a solvent that allows absorption through the skin. The fda recently approved a topical DMSO based anti-inflammatory called PENNSAID for use on knee pain. Off label use is not recommended.
Do not use. The safety profile of DMSO in people has not been established, and its use should be avoided.
Caution. Due to side-effects in humans, DMSO is not considered a safe treatment but is still being used. I would not use it.