Osteoarthritis (OA) is one of the leading causes of pain and disability worldwide, with more than 300 million people currently affected by the disease. The hip joint is clinically one of the most affected locations of OA. Each year, more than 40 million prevalent cases and more than 2 million incident cases of hip OA are reported worldwide. The global prevalence and incidence of hip OA are expected to increase considerably in the upcoming decades, as a result of the aging population and an increasing prevalence of risk factors such as obesity and sedentary lifestyle. Since the population with hip OA is highly heterogeneous, individualized care pathways are needed instead of the current one-size-fits-all approach in clinical management. The identification of distinct subgroups within the population with hip OA might offer a foundation for individualized care pathways with the potential improvement of clinical outcomes. Therefore, the first aim of this longitudinal prospective cohort study is to identify clinical phenotypes in persons with hip osteoarthritis based on a set of contributing biopsychosocial variables, and to compare these phenotypes in terms of pain and disability before and after total hip arthroplasty (THA).

Correspondingly with the increasing prevalence and incidence of hip OA, the mean utilization rate of hip implants per 100.000 inhabitants in Organisation for Economic Co-operation and Development (OECD) countries is expected to increase from 145 in 2010 to 275 in 2050. Annually, more than 30.000 THA's are performed in Belgium. This number is growing considerably, especially in younger, working-age patients. Total hip arthroplasty (THA) is a cost-effective procedure in patients with end-stage hip OA. However, previous studies have reported that 8% of patients are dissatisfied with the outcome, and up to 23% of patients report long-term pain after THA. Furthermore, over 30% of patients report activity limitations and about 25% of patients report participation restrictions after THA. An important step towards understanding and improving outcomes after THA is prognostic factor research. Therefore, the second aim of this clinical study is to identify prognostic factors for outcomes of pain and disability after THA in persons with hip OA.

In this prospective longitudinal cohort study, 200 persons undergoing total hip arthroplasty for hip OA will be followed one-year post-surgery. Participants are recruited at the orthopaedic department of 'Ziekenhuis Oost-Limburg' in Genk and at 'de heuppraktijk' in Herselt. Participants will be assessed on a set of biopsychosocial variables, including pain-related fear-avoidance behavior, perceived injustice, psychopathology, traumatic experiences, social support, and self-efficacy. Furthermore, the somatosensory function will be assessed with thermal quantitative sensory testing (QST) and muscle strength will be assessed with a handheld dynamometer (MicroFet 2). Outcome measures include the Hip Disability and Osteoarthritis Outcome Score (HOOS), the OARSI recommended minimum core set of performance-based outcome measures (PBMs), the Patient-Specific Functional Scale (PSFS), Pain intensity (NPRS), Global Perceived Effect (GPE), and satisfaction (NRS). All these measurements will be performed before surgery, as well as 6 weeks, 3 months, and 12 months after surgery. Pain-related fear-avoidance behavior, perceived injustice, anxiety, depression, and pain will be assessed in the early postoperative phase.