INTRODUCTION: Hip arthroscopy has become a standard procedure in the treatment of hip joint pain not related to osteoarthritis or dysplasia in the young and active patient. There has been increasing focus on the contribution of the hip capsule to function and on stability following hip arthroscopy. It has been suggested that capsular closure after hip arthroscopy may prevent microinstability and macroinstability of the hip joint and reduce revision rate. However, it remains unknown whether capsular closure should be performed as a standard procedure when performing hip arthroscopies, especially in patients without additional risk factors for instability such as hypermobility or dysplasia of the hip. We hypothesised that capsular closure will lead to a superior outcome in hip arthroscopy for femoroacetabular impingement syndrome (FAIS) compared with non-capsular closure.

METHODS AND ANALYSIS: In this randomised controlled, multicentre trial, 200 patients scheduled for hip arthroscopy for FAIS will be cluster randomised into one of two groups (group I: hip arthroscopy without capsular closure, group II: hip arthroscopy combined with capsular closure). Inclusion criteria are: age between 18 years and 50 years and FAIS according to the Warwick agreement. Exclusion criteria are: previous hip surgery in either hip, previous conditions of Legg-Calvé-Perthes or slipped capital femoral epiphysis, malignant disease, recent hip or pelvic fractures, arthritis, Ehlers-Danlos or Marfan disease, recent (within 6 weeks) application of intra-articular corticosteroids, language problems of any kind, and radiological signs of osteoarthritis, acetabular dysplasia or acetabular retroversion. Surgery will be performed in Denmark at four centres by four surgeons, all performing an interportal capsulotomy and closure with at least two absorbable sutures. Patients in both groups, who are blinded for the intervention, will receive the same standardised rehabilitation programme. As primary outcome scores, HAGOS (sport) will be used with HAGOS (symptoms, pain, function in daily living, participation in physical activities and hip and/or groin-related quality of life), Hip Sports Activity Scale, short validated version of the International Hip Outcome Tool, EQ-5D, Visual Analogue Scale for pain, complications and reoperation rate as secondary outcome tools. Using HAGOS (sport) as primary outcome parameter the power analysis required a minimum of 84 individuals per group. Together with a clinical examination performed by the patient's surgeon 1 year after surgery, patient reported outcome measures will be completed preoperatively, as well as at 3 months, 1 year, 2 years and 5 years postoperatively. In addition, adverse effects will be recorded.

ETHICS AND DISSEMINATION: The study is approved by the Central Denmark Region Committee on Biomedical research ethics. The results of this study will be presented at national and international congresses and published in peer-reviewed journals.

TRIAL REGISTRATION NUMBER: NCT03158454; Pre-results.