The general purpose of this study is to examine whether the dual mobility cup (Novae® Serf) reduces the number of hip dislocations after total hip replacement, both early dislocation (≤1 year) and late dislocation (>1 year) after primary THR compared with a conventional cup design (Trabecular MetalTM Modular Acetabular System).

Surgery with THR for arthritis and other hip disorders has long been one of the most successful orthopedic surgical treatments. The majority of the patients gain pain relief and experience improved hip function. In 2017, a total of 10,691 THRs were implanted in Denmark (annual report 2018).

Hip dislocation is a known complication after THR. It is a painful condition which may have far-reaching consequences for the patient, who may experience both physical and mental discomfort. Ultimately, multiple hip dislocations may result in reoperation.

In the literature, the incidence of hip dislocation is reported at 2-6 %, and a more recent review reports a risk of 0.5-10 %.

In Denmark, recurrent hip dislocation is the cause of 21 % of the reoperations per year according to The Danish Hip Arthroplasty Register. According to the British National Joint Registry (NJR) and the Australian register (AOANJRR), hip dislocation is the cause of 15.5 % and 21 %, respectively, of the reoperations.

The best method to avoid reoperation due to instability is to choose the correct implant for the individual patient and thereby spare the patient of having to endure a painful complication and yet another procedure. There are multiple risk factors for instability after THR, and increased use of dual mobility cups (DMC) is already documented.

Inclusion Criteria:

• Patients with hip arthritis who are amenable to surgery with THR and age ≥ 65 years referred to the Department of Orthopedic Surgery at either Section Farsø, Frederikshavn Hospital or Aalborg University Hospital. Patients who are willing to participate in the trial and who give their written, informed consent after receiving oral and written information.

Exclusion Criteria:

• Patients who do not wish to participate.
• Sequelae of Calvé-Legg-Perthes' disease, caput necrosis, epiphysiolysis.
• Proximal femoral or acetabular fracture.
• Hip dysplasia with subluxation Crowe types III and IV.
• Earlier hip bone surgery (excluding arthroscopic surgery).
• Elective bilateral surgery.
• If the patient's bone morphology is unsuitable for a cemented Exeter® femoral stem.
• Patients with no e-mail.