There is currently no sufficient evidence if there is superiority of one of the two available types (onlay type, inlay type) of patellofemoral arthroplasty. Therefore, it is the aim of this study to compare the two types in a prospective comparative setting. Patients on the waiting list for patellofemoral arthroplasty are randomized to either onlay or inlay type patellofemoral arthroplasty. The KOOS - knee score serves as the primary outcome parameter. Secondary outcome parameters are the Marx Activity Scale, the HSS patella score, the Kujala Score, radiographic loosening, progression of tibiofemoral degeneration, revision surgery and parameters of patellofemoral tracking. Inferential statistics will be performed at 1-year follow up data-sets.

Inclusion Criteria:

• patellofemoral osteoarthritis, waiting list for patellofemoral arthroplasty

Exclusion Criteria:

• systemic inflammatory arthropathy
• tibiofemoral joint degeneration IKDC grade C or worse
• severe degeneration of also the trochlea margins (inlay PFA not adequate)
• patellectomy
• previous implantation of tibiofemoral metal implants
• extension loss >5 degree
• knee flexion of less than 100°
• planned concomitant tibiofemoral procedures (eg. coronal plane osteotomies, tibiofemoral cartilage procedures, tibiofemoral ligament procedures)