A Multi-Center Study Assessing the Safety and Efficacy of the LEGION Medial Stabilized (MS) Insert in Patients Undergoing a Total Knee Arthroplasty (TKA)

Inclusion Criteria: 1. A). Prospective Subjects: Subject needing primary Total Knee Arthroplasty (TKA) due to degenerative joint disease (primary diagnosis of osteoarthritis), post-traumatic arthritis or inflammatory arthritis. OR B). Retro-prospective Subjects: Subject has undergone primary TKA using the investigational product in the past 12 months to repair degenerative joint disease, post-traumatic arthritis or inflammatory arthritis and all the following conditions have been met: * Pre-operative Knee Injury and Osteoarthritis Outcome Score for Joint Replacement (KOOS JR) has been obtained * Post-operative radiographs have been obtained, or these can be collected prospectively in window per schedule of events * 12-month post-operative KOOS JR have been obtained, or these can be collected prospectively in window per schedule of events 2. Subject's treating clinician has decided the study device is suitable for the subjects TKA procedure and the study device is used in line with the applicable Instructions For Use (IFU) (listed in section 6). 3. Subject agrees to consent and follow the prospective study visit schedule up to 10 years post-surgery (as defined in the study protocol and informed consent form) by signing the Independent Review Board (IRB)/Independent Ethics Committee (IEC) approved consent form. 4. Subject is able to read, understand and communicate responses to Patient Reported Outcome Measure (PROM). 5. Subject is 18-80 years old at the time of consent (inclusive). Exclusion Criteria: 1. Subject received revision TKA on the contralateral knee for a previously failed TKA, or Unicondylar Knee Arthroplasty (UKA). 2. Subject has a Body Mass Index (BMI) ≥ 40 at pre-operative visit. 3. Subject has ipsilateral hip arthritis resulting in flexion contracture of the hip joint. 4. At the time of surgery, subject has one or more of the following arthroplasties that are not fully healed and/or well-functioning, in the opinion of the Investigator: Ipsilateral or contralateral primary total hip arthroplasty or hip resurfacing arthroplasty; contralateral primary TKA or UKA. 5. Subject has a condition that may interfere with the TKA survival or outcome (e.g., Paget's or Charcot's disease, vascular insufficiency, lupus, muscular atrophy, uncontrolled diabetes, moderate to severe renal insufficiency or neuromuscular disease). 6. Subject has a known allergy to one or more of the components of the study device. 7. Any subject with hardware present in ipsilateral distal femur or proximal tibia. 8. Any subject that meets the definition of a Vulnerable Subject per ISO 14155 Section 3.55. 9. Subject is entered in another drug, biologic, or device study or has been treated with an investigational product in the past 30 days (30 days from operation/dosing). 10. Subject, in the opinion of the Investigator, has an emotional or neurological condition that would pre-empt their ability or willingness to participate in the study, ability to consent or complete the PROMs, including mental illness, drug or alcohol abuse. 11. In the opinion of the Investigator, or site staff, Subject is at risk for lost to follow-up, or failure to return for scheduled visits. 12. Women who are pregnant, nursing, or of child-bearing potential who are not utilizing highly effective birth control measures at the time of screening or the time of surgery. 13. Subjects who have participated previously in this clinical trial and who have been withdrawn. 14. Subject does not meet the indication or is contraindicated for TKA according to specific Smith+Nephew LEGION Knee System IFU.