Inclusion Criteria:  Subject must provide written informed consent Subject must be aged 18 years or older at the time of surgery Subject is listed for a primary unicompartmental knee replacement (UKR) The indication for the UKR is primary arthritis of the knee joint involving one compartment The subject agrees to give informed consent to participate in the study including consenting to any incidental findings being reported to their General Practioner (GP) The subject agrees to consent to and to follow the study visit schedule The subject plans to be available throughout the 1-year post-operative period Routine radiographic assessment is possible  Exclusion Criteria:  The subject undergoes a robotic-assisted UKR on the index joint as a revision for a previously failed surgery, or there is a need for complex implants, or any other implant than a standard unicompartmental knee arthroplasty (UKA) Subject is listed for a simultaneous bilateral UKR Subject does not understand written or spoken English used in the informed consent form Subject has active infection or sepsis Subject is to have a Smith+Nephew Journey II UNI implanted by non-robotic conventional instrumentation Subject is not available throughout the 1-year post-operative period Subject, in the opinion of the investigator, has a neuromuscular disorder that prohibits control of the index joint Subject is undergoing the surgery as a private patient Patients who, in the opinion of the clinical staff, do not have capacity to give consent Women who are pregnant Subjects with a medical or physical condition that, in the opinion of the investigator, would preclude safe subject participation in the study