The Assessment of Patient Specific Instrumentation for Unicompartmental Knee Replacement

The aim of this prospective study is to compare the accuracy of implantation and functional outcome of mobile bearing medial Unicompartmental Knee Replacements implanted with and without patient specific instrumentation by experienced Unicompartmental Knee Replacement surgeons.

This single-centre parallel-design trial was conducted between March 2012 and March 2014 at the Nuffield Orthopaedic Centre, Oxford. Four expert knee surgeons performed all of the procedures in this study. Patients who were being placed on the waiting list for a medial Unicompartmental Knee Replacement, and met the entry criteria for the trial, were asked to see if they would be willing to receive further information about participation in the study. They were provided with a study information leaflet that they could read in their own time. A member of the research team subsequently contacted the patients in order to determine if they would agree to take part in the study and enrolled them onto the study.

Patients were randomized to either Patient Specific Instrumentation, or Conventional Instrumentation group. There were 23 patients (23 knees) in the Patient Specific Instrumentation group and 22 patients (22 knees) in the Conventional Instrumentation group. Randomization was performed using sealed opaque envelopes. Concealment from operating surgeons and patients was not possible owing to the surgeon needing to confirm the Patient Specific Instrumentation plans, and the patients undergoing pre-operative MRI scans. Patient Specific Instrumentation group patients underwent an MRI scan using the protocol outlined by the Patient Specific Instrumentation manufacturers to plan development of the Patient Specific Instrumentation guides. The preliminary plan indicating prosthesis size, positioning, alignment, and proposed bone resection levels was reviewed by the surgeons who accepted the default preoperative plans unless gross errors were present. The patient specific cutting blocks were then manufactured and sent for sterilization.

Operative Technique:

All patients received a mobile bearing medial Oxford unicompartmental knee replacement via a minimally invasive approach under general anaesthetic, and high thigh tourniquet. Intra-operatively, the bone cuts were made through the Patient Specific Instrumentation guides without the use of any intra or extra-medullary instrumentation on the femoral side but with a tibial extramedullary guide for some cases. The subsequent milling process and all soft tissue balancing was performed manually in the standard fashion.

Post-operatively, the need to have a blood transfusion, and the change haemoglobin levels were also recorded. Oxygen saturation levels over the first 24 hours post surgery were also recorded. Screened anteroposterior and lateral post-operative radiographs were performed prior to discharge. Patients attended the physiotherapy ward discharge clinic at six weeks. A further clinical review was performed at 12 months at which point Oxford Knee Scores were recorded.

The primary outcome measure was radiological assessment of component positioning. (See Outcome measures section for more detail)

Target sample size:

The sample size was calculated from a previous study that used similar radiological assessments to compare unicompartmental knee replacements performed using conventional instrumentation, with those performed using computer navigation. In this study, the standard deviation of the tibia varus/valgus angle for the control group was 3.6°. Assuming a minimum clinically important difference of 3°, the standard mean difference would be 0.8. Hence with a power of 0.8 and significance level of 0.05, a total sample size of 44 patients (22 in each group) was required.

Inclusion criteria

• Both cruciate ligaments functionally intact
• Full thickness cartilage in the lateral compartment
• Correctable intra-articular varus deformity
• Full thickness cartilage loss in the medial compartment

Exclusion criteria

• Contra-indication for MRI
• All forms of inflammatory arthritis