The advents of minimally invasive surgery (MIS) and good 10-year results have rekindled enthusiasm for Unicompartmental Knee Replacements (UKR). Mobile, constrained and fixed bearing prostheses exist. The 2003 National Joint Register figures for England and Wales show 1914 UKRs were inserted over 8 months using 10 different prostheses of which the Oxford made up 78%. During this period 18515 Total Knee Prostheses (TKR) were implanted.
Fig.1 |
UKR is bone and ligament sparing, simulates knee kinematics better, has less severe complications , requires less exposure with easier and quicker rehabilitation and is cheaper than TKR. The 15 yr survival is similar to TKR and can be converted to a standard TKR if failed. Revision TKR surgery is unpredictable with high complication rates.
Careful patient selection and meticulous surgical technique at UKR is needed to avoid poor results and complications.
INDICATIONS
Anteromedial Osteoarthritis [1] and Avascular Necrosis are the indications for isolated medial compartment replacement( Fig 1).
Full thickness cartilage loss medially and preserved laterally, intact ACL, correctable varus, Fixed Flexion Deformity (FFD) less than 10˚ and flexion to at least 90˚ is required. Varus and valgus stress views (Fig 2) which reflect medial joint loss and lateral preservation and lateral radiographs assist in decision making with an up to 95% accuracy in a study by Keys et al in 1990 [2].Overall 25% of OA knees are suitable for UKR.
 
Fig.2 |
Young age, high activity levels and obesity are debatable contra-indications in the USA but not excluded in most European centres. A recent study has shown similar survival at 10 years in patients undergoing Oxford UKR at less or more than 55 yrs of age.
Inflammatory Arthritis and prior HTO are excluded. Goodfellow found no correlation between the state of the Patello-Femoral(PF) joint and Oxford UKR clinical outcome and disregards this [3] but others exclude full thickness wear at this site [4].
PRINCIPLES
Mobile bearing UKRs are fully congruent –( all metal and plastic surfaces maintain full articulating contact ) - and unconstrained – (can move freely ) allowing simulation of normal knee kinematics and reducing polyethylene wear to 0.01mm/year in the Oxford on 10 year retrieval studies [5].This equates to 350 years for the thinnest poly bearing to wear through. Fixed bearing UKRs and all TKR’s cannot be totally congruent and/or unconstrained and poly wear in all designs continues to cause concern in 2005 as the leading cause of failure of knee replacements. A penetration rate of 0.08mm/year for the Sled prosthesis is reported [6].
Fig.3 |
The Oxford UKR being the most commonly inserted is discussed. The polyethylene bearing being concave spherical above and flat below, can articulate perfectly between the metal spherical femoral and flat tibial components, while allowing them complete freedom to rotate and slide upon one another (Fig3).
The normal femoral articular surfaces are nearly spherical and articulate with a constant radius of curvature and this is hence simulated in the Oxford design.The bearing is unconstrained and held in place by natural ligament tension and its upper dish shape. The two articulating contact surfaces average 6cm². Retaining all the ligaments without releasing the medial collateral allows for simulation of the 4 bar linkage system and hence normal mechanics of the knee. This design is the most studied knee replacement ever and was first inserted in 1976.
In anteromedial OA intact full thickness cartilage posteriorly on the femur and tibia allows the MCL to tighten to its normal length on flexing over 90˚ and avoids contracture and consequent uncorrectable varus. This together with an intact ACL sets up well for UKR insertion which requires surgical precision.
TECHNIQUE
The instrumentation for the Oxford III design was devised for MIS and setting the prosthetic joint line at its original level together with providing an exactly equal flexion-extension gap to allow the ligaments to remain isometric and avoid bearing dislocation.
The leg is placed on a rest and allowed to hang freely. An oblique incision from mid patella to tibial tubercle 2cm medially is made. The quadriceps is not cut at all and no ligaments are touched.The PF and lateral compartments are left alone. The final components are noted in Fig 4 with the small incision size..
POST OPERATIVE PROTOCOL
Local anaesthetic is injected prior to closure. A short bandage is applied. Flexion and SLR exercises are started within hours and full weight bearing encouraged. Drainage is optional and no splint is used. Analgesia should allow rapid rehabilitation. This has been shown to be at least twice as quick as that of TKR [7] or UKR using a standard length incision [8]with discharge within 1 to 3 days although some centres advocate day case surgery. In Norwich 80% of their cases go home on the same day but this requires tremendous team work and avoidance of spinal anaesthetics.
RESULTS
Recent publications of fixed bearing UKRs demonstrated a 95% 10-year survival with the Marmor prosthesis [9] and 92% 11-year survival in patients under 60 years with the Miller-Galante UKR system [10].
Oxford UKR results from Svard in Sweden [11] in 431 knees have a 95% 10-year survival with 97% of these having no PF pain. Their knee score data at 10 years demonstrated over 90% of patients had very good function and pain relief. In the 17 years covered by their study only one bearing was revised for wear related problems with a size 3 bearing fracturing at 5 years.
Goodfellow̉̉s original cases as the designer are reported as 98% survival at 10-years [12]. Kumar et al had a 85% 10-year survival [13] and the Swedish Knee Register a 7-year 85% survival but with a wide variation between centres reflecting a difference in technique and selection [14]. Robertsson reported centres that did less than 23 a year had an 80% 7-year survival but 93% if more were done [15]. Keys with a similar survival however showed that doing 8 cases a year on average achieved a 100% survival [16]. In a longer term review of his patients at Macclesfield and Cape Town since 1990, the survival in 260 patients in Keys study showed a 98% survival at 10 years with no failures due to infection or poly wear. 2 bearing dislocations were simply reinserted but 2 failed due to lateral compartment subsequent wear and 2 aseptic loosening. These have been presented at various International Knee meetings including that held at Macclesfield in 2005.
A comparative randomised study of fixed and mobile bearings in Bristol showed more complications in the Oxford group [17]. Emerson in a non randomised comparative study demonstrated better survivorship in the mobile group at 11 years [18] but in Milan a randomised trial at 5 years showed no difference [19].
Weale demonstrated normalpreservation of the lateral compartment on screened radiographs at 5 years [20].
Price Radiologically demonstrated equal accuracy of implantation of the Oxford UKR through a MIS ie reduced visibility approach (Fig 5) compared to a more extensive one in experienced Surgeons [7] but less so in inexperienced hands as confirmed by Fisher et a [21].
COMPLICATIONS
Mobile bearing subluxation or dislocation, lateral compartment arthrosis, aseptic loosening, persistent pain and sepsis are the most common causes of failure and revision. Tibial fracture, component subsidence or severe malposition, recurrent haemarthrosis and inflammatory arthrosis are also recorded as causing failure.
Revision to a standard TKR with minimal technical difficulty in the Oxford UKR [22], possibly due to initial minimal bone resection, is not replicated in fixed bearing revisions [23]. In a study from Macclesfield, Oswestry and Svards Swedish patients, amoung 1000 Oxford UKR’s 46 revisions were studied over a 15 year period. Over 90 % were converted to standard primary Total Knee Replacements without difficulty allowing these patients ‘ 3 steps to heaven’ rather than the usual 2 ( primary and revision THR) !
CONCLUSION
The advent of MIS may lead to increased complications in this technically demanding procedure particularly in inexperienced hands and caution is urged to temper the recent enthusiasm for UKRs especially with the introduction of a number of new untried prostheses to Orthopaedics.
However if done properly with good knowledge of indications and principles then excellent 15 year survival is possible with much quicker and easier recovery with significant savings to the NHS. It is up to those of us with significant experience of the technique to teach other Surgeons and indeed with the Oxford , as with Charnley’s philosophy , we do not recommend its insertion unless an advanced instructional course is attended.
 
Fig 5 ; post op x-ray of Oxford UKR |
REFERENCES
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