The aim is to compare QOL and hip specific outcome measures in patients with a single or recurrent episode(s) of THA dislocation and patients with a THA without any dislocation.

Knowledge about the patient's own perception of both general health and more hip-related issues after experiencing a hip dislocation is lacking (19). Hip surgeons need this information in order to incorporate it with other objective aspects, when advising patients with their first hip dislocation. Due to the large study population, this study will provide solid information of what to expect after a hip dislocation and be the largest study ever conducted within this topic. Depending on the results, this study may change the path we choose to advise for our patients after single or recurrent dislocations.

A matched case-control study involving the share of THA patients with at least 1 episode of hip dislocation.

The patients had undergone primary surgery in the 5-year period from 01.01.2010 - 31.12.2014 and subsequent dislocation was evaluated in a 2-year follow-up period. Given a 3-5% approximated incidence of dislocation, 1100-1800 cases will have been identified.

Patients are subdivided in groups depending on the number of dislocations and one group containing THA revisions. In order to be included, a time frame of at least 0,5 year since the last episode of dislocation or surgery must have passed.

A matched control group of patients with primary THA without any event of dislocation are located from the original 5-year period of primary surgery. They will be matched with the group of patients with dislocation, regarding to:

1. Age (+/- 5 years)
2. Gender
3. Date of surgery and hospital

The study will be conducted in accordance with the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) guidelines.

After 01.01.2019, all patients included will be receiving 3 questionnaires (a generic - EQ-5D, a hip-specific - HOOS, and an activity scale - UCLA) by e-boks (a mandatory secure email system couplet to the patient's CPR number). Some elderly are exempt and will instead receive the questionnaires by regular mail and a pre-labeled return envelope is attached. Reminders are sent after 2 months.

Inclusion Criteria:

• patients with one or more events of hip dislocation after a primary THA
• controls with a primary THA without events of hip dislocation

Exclusion Criteria:

-

The patients had undergone primary surgery in the 5-year period from 01.01.2010 - 31.12.2014 and subsequent dislocation was evaluated in a 2-year follow-up period. Cases are found in a previously described study (Protocol-ID 3-3013-2128/1). 1100-1800 cases will be identified, anticipated.

A matched control group of patients with primary THA without any event of dislocation are located from the original 5-year period of primary surgery. They will be matched with the group of patients with dislocation, regarding to:

1. Age (+/- 5 years)
2. Gender
3. Date of surgery and hospital

Data not to be shared.