Hip and knee osteoarthritis is one of the leading causes of global disability. There is no cure for the disease, but research and guidelines recommends that patients with osteoarthritis, early after the diagnosis, receive individually adapted physical exercises, information about their disease and about self-management and, if needed, recommendations on weight loss as a core treatment to prevent disability and impairment in health. This core treatment improves symptoms in osteoarthritis by reducing pain and increasing function, and have the potential to delay replacement surgery. In Sweden, core treatment has since 2008, been standardized in a supported self-management programme and evaluation and follow-ups are registered in the National Quality Register BOA (Better management of patients with OsteoArthritis). The course of deterioration in pain and physical functioning among different individuals with osteoarthritis is currently difficult to predict. Factors such as socioeconomic status and comorbidity contribute to progression of the disease, but are not fully established. There is a need for early identification of individuals who have a good prognosis with slow disease development and those that should be recommended joint replacement surgery in the future. Overriding aim of the study is to increase knowledge about the population who has received education and supervised exercises as a core treatment for hip and knee osteoarthritis and been registered in the BOA Register and to identify factors that can predict long-term outcome for this population.

Study design: This is a nationwide observational cohort study using prospectively obtained individual-level register data from five main sources; the BOA Register, the Swedish Hip Arthroplasty Register (SHAR), the Swedish Knee Arthroplasty Register (SKAR), Statistics Sweden and the National Board of Health and Welfare, Sweden. By using the personal identity number (PIN) that is unique to all citizens in Sweden, data will be merged together to create a research database that will be used for answering the following research questions in the study:

1. What are the differences between the population included in the BOA Register and the general Swedish population regarding health and socioeconomic factors?
2. What factors that predicts replacement surgery can be identified at the first registration, 3 months and/or 12 months follow-up, in the BOA Register in patients with hip or knee osteoarthritis?
3. What factors that predicts outcome after replacement surgery can be identified at the first registration, 3 months and/or 12 months follow-up in the BOA Register, in patients with hip or knee osteoarthritis?

Study population:The study cohort consists of all patients with a first registration (baseline) in the BOA Register between 2008 and 2016 (approximately n=75 000). These patients have sought treatment for knee and/or hip pain in primary health care in Sweden and been referred to the standardized core treatment of education and supervised exercises after confirmed clinical and/or radiographic osteoarthritis diagnose.

A control cohort (approximately n=225 000) covering the general Swedish population, who never have been included in the BOA register, will be recruited from the Swedish population register at Statistics Sweden and matched (1:3) to each patient in the study cohort by the same year of birth, gender and residence (as for the study cohort at baseline).

Data sources: In the study the investigators will collect the following data from the five registers: from the BOA Register: data reported by patients (e.g. most affected joint, pain, symptoms, health-related quality of life and physical activity level) and physiotherapists (e.g. earlier examinations and treatments) at baseline (the first registration before the standardized core treatment), and three and 12 months after. From the SHAR and the SKAR: patient-related and procedure-related data on all hip- and knee replacement surgeries in Sweden, including patient-reported outcomes (PROM) such as joint pain, health-related quality of life and satisfaction with treatment before surgery and one year postoperatively. From Statistics Sweden: data on socioeconomic factors and vital events such as birth, death, residence and marital status. From the National Board of Health and Welfare: data on comorbidity, health care consumption and expenditures of prescribed drugs.

Inclusion Criteria: for participation in the standardized core treatment (of education and supervised exercises) and registration in the BOA Register are;

• pain from the knee or hip.
• diagnosis of osteoarthritis confirmed by a physical therapist, by medical history and a physical examination based on the clinical criteria for osteoarthritis

Exclusion Criteria: for participation in the standardized core treatment and registration in the BOA Register are;

• confirmed or suspicion of tumor, rheumatoid arthritis, sequel hip fracture, chronic pain or fibromyalgia,
• total joint replacement within the past 12 months,
• other surgery of the knee or hip joint within the past 3 months,
• not able to read or understand Swedish

The population who has received standardized core treatment of education and supervised exercises for hip and knee osteoarthritis and been registered in the BOA Register