Study subjects will participate in the following steps: (1) Initial diagnostic visit with Orthopaedic Surgeon, (2) Blood collection as part of PRP therapy, (3) Ultrasound-guided PRP therapy administered by Stanford radiologist, and (4) Blood collection for the study. Steps (1) through (3) are all part of the standard of care, which the patient will have chosen independently of the study. In step (4), blood samples will be collected by venipuncture at various time points and assayed for growth factor concentrations. Less than 10 mL (2 teaspoons) of blood will be collected at each of seven time points over five consecutive days. In order to minimize inconvenience to the patient we will meet patients at a location of their choosing, such as the patient's home or office, for blood collection on days 2-5. Participants will be compensated after all seven blood samples have been donated.