Finger injection. Conceptual image of an injection into a finger joint. The syringe may be used to remove a sample of synovial fluid from the joint to aid diagnosis, or to inject drugs such as anti- inflammatories to treat any disorder. There may be gout or arthrosis deformans in the joint. In gout, crystals of uric acid build-up in the joint due to a faulty metabolism. Arthrosis deformans is caused by repetitive strain injury (RSI). The articulating cartilage of the affected joint becomes worn down causing damage to the bone beneath. Both disorders result in stiffness and pain. RSI affects people whose joints are overused.