What's The Difference Between Tendonitis And Tenosynovitis?

In practical terms there's not a lot of difference and the two conditions often manfiest jointly, anyway.

Tendons are fibrous tissues that join muscles ot bones.  They have a smooth covering around them (the synovial sheath).  Tendonitis is painful swelling of the tendons themselves, while tenosynovitis is painful swelling of the sheath of tissue around the tendons.  Often, it is scar tissue caused by a previous injury to the tendon that causes the tenosynovitis.

Both tendonitis and tenosynovitis tend to come about either because of joint wear (arthritis) or because of an over-use injury (RSI, or repetitive strain injury).  Although the term RSI is usually used with respect to upper-limb disorders in computer uses, athlettes and musicians (including amateur players) are also affected by RSI, and it can occur in any joint in the body.

Treatment depends a lot on the nature of the injury and requires specific individual advice.  Anti-inflammatory drugs, physiotherapy, massage and major changes in work habits may be required. Long-term recovery may be only partial.