Principles of the Oxford® (Zimmer Biomet) Unicompartmental Knee Arthroplasty (OUKA)

The Oxford® (Zimmer Biomet) Unicompartmental Knee Arthroplasty (OUKA) is a unique design and philosophy that enables rolling/sliding biomechanics via its mobile polyethylene insert. It is designed to restore normal ligament tension, based on tension at rest on the medial collateral ligament. Oxford knee replacement is based on anterointernal knee OA (osteoarthritis) with a functionally intact ACL and conserved cartilage thickness. Ligament release should never be performed, and the proper balance between flexion and extension is created by removing bone from the distal femur. It is important to restore the correct joint space height in anteromedial knee OA to avoid damaging the distal insertion of the MCL or placing the implant in a more fragile area of bone. Tibial cut and positioning of the tibial implant are important for restoring the joint space. Calibration spoons and G-clamp are used to ensure the level of tibial cut height, which can be adjusted by the thickness of the spoon. A G-clamp is used to adjust the positioning height of the tibial implant, with a 3-mm insert in short patients to reduce the height of the tibial cut and limit bone resection. The sagittal tibial cut is done flush with the ACL and includes a small part of the medial tibial spine. The operator must be careful not to damage the posterior tibial cortex. The tibial component should be balanced against the posterior and medial tibial cortices and not overlap forward. A centromedullary rod should be introduced into the femoral shaft to make holes and receive fixation contact points. The femoral component should be positioned at 10° flexion and 7° varus. The positioning guide must be adjusted manually to position it at the centre of the femoral condyle, and two 4- and 6-mm holes are drilled to position the posterior femoral section guide. Measurement of the difference between the reference space in flexion and the space in extension is more reliable in anteromedial knee OA. Trial spacers are used to measure the residual space between the femoral and tibial components. The spigots determine the depth of milling and quantity of bone removed from the distal femur. If the difference is equal to 1, a larger-size spigot should be used. It is important to perform successive millings step-by-step without skipping steps to avoid over-milling the distal condyle. Precautions should be taken when dissecting osteochondritis, with a 0 spigot milling up to the stop, retesting with trial components, excising posterior osteophytes, and finalising the tibia preparation with a reciprocating “toothbrush” saw blade. The Oxford knee replacement is a mobile polyethylene insert that pushes back the physiological space existing in flexion to the space in extension, giving a good ligament balance and physiological joint biomechanics.