The treatment that patients with knee OA are offered varies largely. There is a need for more evidence-based individualized guidance to treatment choice for knee osteoarthritis. This study will register and evaluate the course and outcome of treatment in 5,000 patients with knee osteoarthritis. The understanding of knee OA treatment will advance in three ways: Firstly, by describing the different treatment pathways that are currently being used for knee OA. Secondly, by identifying wich individual factors that may impact the outcome of the treatment course. And finally, by conducting the economic burden related to different treatment modalities.

The problem: Knee Osteoarthritis (OA), the most common form of arthritis, is a chronic, widespread disease with a steadily increasing prevalence that places a major economic burden on the society. Patients with knee OA suffer from chronic knee pain and functional disabilities. The treatment that patients with knee OA are offered varies largely, and is often not in coherence with clinical guidelines. The guidelines recommend patient education, exercise, and weight control as core interventions for all patients with knee OA and surgical intervention for end-stage knee OA. However, the timing of surgery is debated, and whether the outcome of different treatment strategies depend on individual factors such as the patients symptoms, OA severity, or prior treatment, has not been established.

The solution: This large-scale study aims at improving the understanding of which treatment should be offered to which patient with knee OA at which time. The study is an interdisciplinary, collaborative effort, including orthopaedic surgeons, physiotherapists and general practitioners from two different regions in Denmark. At least 5,000 patients with primary referrals to orthopaedic surgeons due to knee OA, at two public hospitals, will be recruited through a two-year inclusion period. Patients' prior treatment and symptom severity will be registered at the first consultation with the orthopaedic surgeon using a patient self-reported questionnaire. OA severity will be defined on plain standing knee radiographs using a well-established classification system. The course of treatment will be registered after 6 months and 2 years through a self-reported questionnaire with additional questions asking whether patients can accept their current symptom state, or, if not, whether they consider the treatment to have failed.

Societal impact and clinical implications: The understanding of knee OA treatment will advance in three ways: Firstly, our results will improve the overview by describing the different treatment pathways that are currently being used for knee OA. Secondly, predictors of good and poor treatment outcomes of different treatment modalities, or combinations hereof will be identified. And finally, the cost-effectiveness of different treatment modalities will be evaluated. These results will be used to develop a treatment algorithm to help patients with knee OA and clinicians to tailor the right treatment at the right time through shared decision-making.

Inclusion Criteria:

• primary referrals to one of two orthopedic department's outpatient clinics due to knee osteoarthritis

Exclusion Criteria:

• previous total or unicompartmental knee replacement
• osteotomy around the study knee
• unable to read or write Danish
• patients who decline to answer the questionnaire at inclusion

All patients with primary referrals to one of two public hospitals orthopedic department's outpatient clinics due to knee osteoarthritis are consecutively invited to participate. The two hospitals cover the larger part of Zealand in Denmark.

Decision in pending on whether the total database with all anonymised data will be transferred to the Danish National Archives' Collections once the study is completed.