Although the use of PRP injections to treat knee arthritis is a fairly recent innovation, PRP has been used for decades to treat a wide variety of conditions such as tendonitis and to speed healing after surgery to repair a torn ACL. One published study randomized arthritic knees to receive either a placebo injection or PRP injections. Six weeks after the injections, the PRP group had less pain, less stiffness, and improved knee function, and these results were maintained for at least six months. Another published study followed patients with MRI after receiving PRP injections. Normally, around five percent of the cartilage degrades per year in an arthritic knee, but in the knees given PRP injections the cartilage remained the same, with no progression, for up to a year after the injection.