This study will compare functional and clinical outcomes of partial knee joint replacement using the Persona Partial Knee (PPK) and a standard care total knee replacement. Reported benefits of unicompartmental knee arthroplasty (UKA) compared to total knee arthroplasty (TKA) are less invasive surgery, less post-operative pain, quicker recovery, shorter inpatient stay, preservation of the anterior cruciate ligament (ACL), better knee function and lower risk of complications. Also, the PPK more closely matches the anatomy of knee and has more sizing options. These aim to allow the surgeon to reconstruct the knee to better match the native knee and achieve natural function.

Little data exists which investigates biomechanical function of patients after UKA or comparisons between UKA and TKA and none investigating the PPK. Available data focuses on level walking which is not particularly challenging for these patients and is unlikely to highlight biomechanical differences between UKA and TKA. The investigators will investigate biomechanical differences between UKA and TKA during more challenging activities. These will mirror activities of everyday life: sitting into/rising from a chair, stepping, walking on a slope.

The investigators will investigate level walking as all studies of this nature report these data. Unlike TKA, UKA retains the ACL. The soft tissues of the knee play a role in balance control and the investigators will assess this with single leg balancing. Biomechanical differences will be assessed using 3D motion analysis before and after surgery (6 and 12 months). Comparisons will be made within and between groups and between the patient groups and existing healthy data. Radiographic and patient reported outcomes (satisfaction, pain and functional abilities) will be investigated.

Fifty patients will be recruited and randomised to undergo TKA or UKA. Only patients who are suitable for UKA will be recruited. Data for 25 healthy, matched controls from the investigator's existing healthy control database will be extracted.

All patients identified as possible participants for the study will be given a "consent to be approached for research" form on arrival at the clinic by the unit co-ordinator. On this, patients can indicate their willingness or otherwise to have the study explained to them. The completed form will be given to their consultant prior to their appointment. Patients who have indicated willingness, and who are deemed suitable for the study following the consultation, will have the study introduced to them by their consultant, who will then ask a member of the research team to fully explain the study. These patients will be given a copy of the Participant Information Sheet (PIS) to take away.

At a later date (at least a day after receiving the PIS), these patients will be contacted by telephone to ask for verbal consent. Patients will also have the opportunity to ask questions about the study. Verbal consent will not be binding, but is necessary to give the research team time to arrange pre-operative study interventions. Written consent will be taken when the patient arrives for the pre-operative biomechanical testing session, once the researcher is happy that the patient is fully informed. Consent will be sought by the principal investigator or one of his delegates.

Standard care for knee arthroplasty patients at the GJNH has them completing the Oxford Knee Score (OKS) and the EQ-5D pre-operatively and at six weeks post-operatively. In addition, satisfaction is measured on a five point Likert scale six weeks post-operatively. Study participants will also be asked to complete these three measures at six and 12 months post-operatively. Two additional questionnaires will be used in the study. The UCLA Activity Score will be used pre-operatively and at six weeks, six months and 12 months post-operatively. The Forgotten Joint Score (FJS) will be used at the three post-operative time points (six week, six months and 12 months).

At 12 months post-operatively, study participants will have two radiographs taken (short AP and lateral views) which are additional to the standard care radiography protocol. The six and 12 month post-operative visits are additional to standard care and, as such, participants will be offered the reimbursement of reasonable travelling costs. Biomechanical assessment using clinical movement analysis (CMA) in the movement analysis laboratory (MAL) at the GJNH will be carried out on three occasions (pre-operatively and at six and 12 months post-operatively). The MAL is a purpose built facility with restricted access for privacy. Changing facilities are provided within the laboratory for participants to change from their outdoor clothes into a T-shirt and shorts for the testing. They will also be asked to perform the testing barefoot. At this point a health care support worker will apply a bandage to the operated knee covering the wound to maintain the blinding of the person collecting the biomechanical data. During these assessments, participants will have a number of small reflective markers attached to their body using toupé tape. The movements of these markers will be tracked by a number of infrared cameras connected to a computer. The data collected in this manner can generate dynamic angles of the joints during the study tasks. In addition, force plates embedded in the floor will measure forces generated during the study tasks which can be converted to forces at each of the joints. During the assessment, participants will be asked to perform a number of activities of daily living; level walking, incline and decline walking, stand-to-sit-to-stand, step ascent and descent and single leg balance. Participants will be asked to perform each task a minimum of three times to ensure that three representative samples of each task are obtained; except for the single leg balance, which will be performed once for each limb. Pre-operative testing will determine baseline function while post-operative assessment will highlight any changes in function.

Following recruitment, participants will be randomised to receive the PPK unicondylar knee implant or the total kneeimplant routinely used in the GJNH. Surgery will be performed as per the surgeons' standard practice using the implant the participant was randomised to receive. After their 12 months follow-up, the participants' involvement in the study will be over.

Inclusion Criteria:

• Suitable for medial unicompartmental knee arthroplasty
• Aged between 18 and 75 years inclusive
• Taller than 1.5m in height (the 10th percentile for the population)
• Functionally intact anterior cruciate ligament
• Able to give informed consent
• Able to return to GJNH for follow-up
• BMI under 40

Exclusion Criteria:

• Inflammatory joint disease e.g. rheumatoid arthritis
• Previous hip procedure in the previous twelve months
• Previous knee surgery other than arthroscopy and medial menisectomy
• Previous ankle surgery
• Symptomatic foot, hip or spinal pathology
• Any medical condition limiting normal ambulation e.g. stroke, Charcot-Marie-Tooth disease, severe lymphedema, advanced cardiac disease
• Unable to read and understand English and follow verbal and visual instructions unless they can bring a relative or friend to all study assessments to act as a translator