The investigators intend to establish a national cohort including all persons in the ages from 15-20 years old with patellar instability (PI) or a prior knee injury (ACL-ruptur or meniscus damage) . The cohort will be nicknamed "The Faroese Knee Cohort".

The overall aim is to investigate two groups.

1. The patella instability group, in which we intend to investigate the following.

   • Prevalence of patellar dislocation and trochlear dysplasia in the Faroe Islands.
   • Risk factors for patellar dislocations-
   • Heredity of trochlear dysplasia.
   • If there is a specific gene responsible for the development of trochlear dysplasia.
   • The development of retropatellar artrhosis, its onset and its impact on quality of life and function,

2. Knee injury group (ACL-ruptur or meniscus injury)

   • The prevalence of ACL-rupture and/or meniscus injury in this specific group in the Faroe Islands.
   • The impact on quality of life and function.

The Faroese Knee Cohort aims to consist of all persons in the Faroe Island between 15-20 years old with patellar instability* (PI) or a knee injury.

Special focus will be on PI, Patella dislocation (PD) and Trochlear Dysplasia (TD), a condition where the femoral trochlea has a dysplastic shape and lacks the lateral support of the patella. The diagnosis will be set by MRI of the knees and is in this study defined as the presence of one of the following: Crossing sign on a true lateral x-ray, lateral trochlear inclination angle < 11 degrees or a trochlear depth < 3 mm.

The hypothesis is that abnormal values in the following measures will be associated with increased risk of patellar dislocation and worse patient reported outcome: Caton-Deschamps index, Lateral Trochlear Inclination, Trochlear Facet Asymmetry, Trochlear Depth, Tuberostias Tibia - Trochlear Groove distance, Patello-trochlear index and Tuberositas Tibia - Posterior Cruciate Ligament distance.

Furthermore, we expect pivoting sports to be a behavioral risk factor.

The primary endpoint is measured by the number of patellar dislocations or near-dislocation the patient experiences (daily, weekly, monthly, yearly).

The secondary endpoint is the patient reported outcome measures.

*Definition: Patellar instability (PI) is the overall term encompassing both 1) recurrent patella dislocations and 2) recurrent episodes of a clear sensation of the kneecap riding on the outer border/aspect of the knee before popping back in track. The episodes must be a severe complaint for the patient. PI usually follows after a first time patellar dislocation.

Inclusion Criteria:

• Age between 15-20 years at the start of patient enrolment.
• The feeling of patellar instability, or patellar dislocation/near-dislocation.
• The person has to be from the Faroe Islands.
• All persons under the age of 18 must get written consent by their parents

Exclusion Criteria:

• Terminal illness
• Mental disabilities and illness, meaning that the patient cannot independently answer the PROM´s and cooperate to the clinical investigation.

All patients must either live or be born in the Faroe Islands, as this is a national cohort study. They will be invited to join the study if the live up to the inclusion criteria.