The aims of this study were to analyze the range of cartilage damage and patellofemoral joint (PFJ) degeneration in degenerative osteoarthritis (OA) and determine the duration for the positive effects of platelet-rich plasma (PRP) injection. This study included 65 patients suffering from OA that were treated with intra-articular PRP injection. The patients were prospectively evaluated at 1, 3, 6, 9, and 12 months after the procedure using a visual analogue scale (VAS) score and an International Knee Documentation Committee (IKDC) score. Clinical improvement in the average VAS score from 7.4 before the procedure to 4.2 at 6 months post-procedure had been reported, but the symptoms tended to deteriorate to 5.0 1 year after injection. The IKDC score also showed statistical significance (P < 0.05). Patients reported relapsed pain 8.8 months after the procedure. Developing degeneration according to the Kellgren-Lawrence grade reduced the clinical effects of PRP (P < 0.05) and also accelerated the time for feeling relapsed pain (P < 0.05). There was a statistically significant negative correlation between patient age and the PRP potential in the VAS score (slope = 0.1667) and IKDC score (slope = 1.3333). The presence of PFJ degeneration is expected to produce a worse outcome (P < 0.05). While intra-articular PRP injection can be used for the treatment of early OA, increasing age, and developing degeneration result in a decreased potential for PRP injection therapy.