Exploring Factors Linked to Worsening Over Time in People With Early-stage Knee Osteoarthritis and Those With Prior Proven Acute Knee Injury.

The goal of this observational, non-interventional cohort study is to identify tests or markers in blood or knee fluid, or any genetic risk factors that might contribute to worsening or improvement of knee osteoarthritis. Males and females aged ≥18 with early-stage knee osteoarthritis, prior proven acute knee injury, or degenerative meniscal tear will be recruited at Imperial College Healthcare NHS Trust. Participants with prior proven acute knee injury must be aged ≤ 50. A single knee per participant will be followed throughout the study; this will be referred to as ''affected knee'', identified at baseline. The study involves: 1. Samples collection: * From individuals recruited in-person: blood or buccal (cheek) swab and/or knee joint (synovial) fluid - this will be collected during routine NHS visit, if individuals undergo scheduled knee surgery or knee aspiration procedure (Knee aspiration is a procedure to remove excess fluid from the knee joint for diagnosis or treatment). * From individuals recruited remotely: saliva sample. 2. Clinical data collection at baseline: Participants will complete study Core Questionnaire Pack. These are questionnaires asking about participants' knee pain, function, and activity level. 3. Clinical data collection at yearly remote visits, through years 1-5: Participants will complete study Follow-Up Questionnaire Pack, asking about participants' knee pain, function, and activity level - same as at baseline, but additionally participants will be asked about any knee surgeries on the affected knee, since the last study visit. 4. Imaging data collection at baseline and at Year 2 or Year 3 follow-up visit: If at baseline visit X-ray imaging is available as a part of usual NHS care, then participants will only be invited to attend one X-ray appointment at Year 2 or Year 3 follow-up visit. The study expect to recruit up to 200 participants with approximately 100 participants having early-stage knee osteoarthritis or degenerative meniscal tear and the remaining participants having prior proven acute knee injury. Individuals will be able join the study for between 2-3 years. Follow up visits will last up to 5 years.

Inclusion Criteria: ALL participants must meet the following criteria: * Male or female, aged ≥18 years of age AND at least one of the following (a-d): 1. A definite diagnosis of knee osteoarthritis , evidenced by consistent knee symptoms and imaging findings, commensurate with non-advanced KL grade changes (KL 0-2 where an X-ray grade is available) 2. A symptomatic degenerative meniscal tear with or without definite imaging findings suggesting knee osteoarthritis 3. Probable early-stage symptomatic knee osteoarthritis without definite imaging changes, fulfilling NICE diagnostic criteria for osteoarthritis: * Age 45 or over * Activity-related knee pain. * Either no morning stiffness or that lasts \