The field of patellofemoral disorders and accordant therapeutical interventions is a high turnover research field. It is important to verify the clinical outcome with appropriate measuring tools. There is evidence that the Kujala score is a reliable and widely used measuring instrument of patellofemoral disorders. Indeed, the Kujala score was already used in many studies investigating patellofemoral disorders - also in german speaking patient groups. Unfortunately, we are not aware of a validated german version of the Kujala score. Other relevant knee outcome scores were already successfully translated into german language and validated by previous investigators.

So, it is the aim of our study to cross-culturally adapt and then to validate the Kujala score for use in German-speaking individuals with patellofemoral instability. It is hypothesized that the german version of the Kujala score shows:

• High divergent construct validity as determined by significant differences between patients and controls (hypothesis 1)
• High convergent construct validity as determined by significant correlations with other relevant scoring systems (hypothesis 2)
• High reliability (hypothesis 3)
• High responsiveness (hypothesis 4)

Inclusion Criteria:

• german speaking
• MPFL-reconstruction planed

Exclusion Criteria:

Patients with patellofemoral instability & scheduled MPFL-reconstruction