Optimising the outcomes of the Oxford unicompartmental knee replacement

Thesis

The two main surgical options for knee arthritis are total knee replacement (TKR) and unicompartmental knee replacement (UKR). UKR has advantages over TKR but has higher revision rates. As a result, UKR only forms 10% of all primary knee replacements. The broad aims of this thesis were to determine; the long term implant survival of the Oxford UKR and to assess current and develop new evidence based strategies to reduce revision rates.

A systematic review investigating the long term outcomes of 8,658 Oxford UKRs found the 10 year implant survival was 93% with leading revision causes including aseptic loosening, pain and bearing dislocation. There was a lack of cementless studies.

A large prospective cementless Oxford UKR cohort study (n=1000) in Oxford allowed the assessment of long term clinical outcomes using various endpoints; reoperation, revision and revision to TKR. The respective 10 year survivals were 96.6%, 97.5% and 98.9%.

A retrospective observational study from the National Joint Registry (NJR) was used to compare the long term outcomes of 14,814 matched cemented and cementless UKRs. The cementless group has significantly reduced revision rates (HR=0.76) given rates of revision for aseptic loosening, pain and lysis decreased. However the revision rate for peri-prosthetic tibial fracture increased. These results were mirrored by a systematic review comparing fixation types for all UKR types (n=10,736).

Other retrospective observational studies from the NJR were used to investigate the effect on revision rates of (1) increasing surgeon caseload, (2) Microplasty (new guiding surgical instrumentation) and (3) adding a second femoral peg on revision rates. All three were associated with lower revision rates.

A radiostereometric study of polyethylene wear was undertaken to determine why an increased rate of bearing fracture has been observed. The wear rate for the current bearings are 57 μm/year which is substantially higher than in previous studies. Although moulded bearings were found to have less wear than machined, no factors studied including recent changes to the bearing resin or manufacture accounted for the increased wear.

An experimental study found that new wider surgical saw blades halved the force required to seat the tibial component whilst maintaining fixation achieved. This should reduce the risk of periprosthetic fractures and is now used clinically.

This thesis has characterised the performance of the Oxford UKR and has highlighted strategies to lower UKR revision rates, to levels similar of TKR. If this is achieved the only real disadvantage of UKR compared to TKR will have been addressed.

Authors: Mohammad, H

• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: Clinical outcomes; Unicompartmental knee replacement
• Subjects: Orthopaedics