This project aims to assess two different techniques used to position the knee replacement implant during surgery. The patients will be randomised to receive either the following:

1. The established Navigation System (gold standard)
2. The new Prophecy Technique (validated by the Navigation System)

The study hypothesis is there will be no difference between the gold standard Navigation system and the Prophecy technique in relation to the placement of the knee implant.

Knee joint pain is often the cause of reduced joint and limb function. Total knee joint replacement is a procedure which aims to relieve pain and help to improve mobility. There has been continued development and improvement in knee implant design and the instruments required during surgery to assist with the accurate placement of the implant. These developments have reduced surgery time and assisted in the accurate positioning of the knee implant, which leads to better long term outcomes.

Navigation system is a computerised system currently used intraoperatively to position the Advance knee implant. The Prophecy technique is a pre-operative alignment technique. Patient undergo a MRI or CT scan which is used to manufacture personalised cutting blocks which are then used intraoperatively to position the knee implant.

Inclusion Criteria:

1. Patients requiring a total Knee Arthroplasty as determined jointly by the surgeon and the patient.
2. Over 65 years of age at time of surgery.
3. Patients with the Varus or Valgus Osteoarthritis
4. Patients who understand the conditions of the study and are willing to participate for the length of the prescribed term of follow-up.
5. Patients who are capable of, and have given, informed consent to their participation in the study.
6. The individual does not have an active infection within the affected joint.
7. The individual has not had a previous total knee replacement or knee fusion of the affected knee joint.
8. The individual is skeletally mature.
9. The individual is not pregnant.
10. The individual is not a prisoner.
11. The individual has no plans to relocate to another geographic area before the completion of the study.

Exclusion Criteria:

1. Previous surgery requiring implanting a device.
2. Knee deformity is >30° anatomic varus (as measured on an AP radiograph taken with the patient standing)
3. Knee deformity is >30° anatomic valgus (as measured on an AP radiograph taken with the patient standing) >20° knee flexion contracture (as measured by investigator examination)
4. Those patients requiring bone grafting or any other procedure that would extend the operative time beyond that of just replacing the knee should be excluded.
5. The individual is classified as morbidly obese (>40 BMI).
6. The individual is physically or mentally compromised (i.e., currently being treated for a psychiatric disorder, senile dementia, Alzheimer's disease, presence of alcohol or substance abuse), and is unwilling or unable to comply with postoperative scheduled clinical and radiographic evaluation and rehabilitation.
7. The individual has a neuromuscular or neuro-sensory deficiency which limits the ability to evaluate the safety and effectiveness of the device.