Lateral Unicompartmental Knee Arthroplasty

Lateral UKA is rare due to the low incidence of genu valgum deformity and better long-term tolerance of lateral osteoarthritis. Alternatives include varus osteotomies and total knee replacement. Anatomical and kinematic characteristics of the external knee compartment make it more challenging to perform than medial UKA. The biomechanics and kinematics of lateral UKA differ between the medial and lateral tibiofemoral compartments, and the screw-home mechanism is key to knee stability in normal gait. Indications and preoperative assessment are based on anatomical and radiological criteria. Preoperative, clinical, and X-ray assessment should seek to determine the origin of the valgus deformity to differentiate six situations: lateral femoral condyle dysplasia, post-traumatic valgus, post-meniscectomy pain syndrome, avascular osteonecrosis, valgus secondary to coxofemoral disorder, and valgus secondary to congenital tibial deformity. Femoral condyle hypoplasia is the most common cause of OA, and post-traumatic or post-meniscectomy OA and osteonecrosis can be treated with lateral UKA. Surgical technique involves a lateral parapatellar approach and lateral facet and patellar osteophytes resected. It is important to release the peripheral capsule around the tibial plateau and osteophytes minimally to ensure good ligament tension. Osteophytes in the intercondylar notch and lateral femoral condyles should be conserved. Contact point between anterior part of femoral condyle and anterior part of tibial plateau should be noted. Sagittal tibial resection should be performed close to the tibial spine mass, followed by distal femoral resection to compensate for hypoplasia and wear. Posterior femoral resection should be minimal to compensate for posterior condylar offset. Rotation of the resection guide is essential to avoid excess internal rotation of the implant in flexion. Size of the resection guide is a compromise between an anatomical position and the long axis of impact perpendicular to the tibial plateau. Positioning the implants is a compromise between maximum tibial coverage and 15–20° internal rotation. The lateral UKA procedure is a resurfacing procedure to correct only intra-articular wear while leaving the extra-articular deformity intact. It involves cemented and placed tibial implants in complete flexion and internal rotation to increase exposure of the lateral compartment. UKA has good results with mean and long-term survival greater than 90%, but OA progression in the medial tibiofemoral compartment is the main cause of failure. Cemented implants with a “resection” technique have excellent long-term survival. Lateral unicompartmental knee arthroplasty is an effective procedure for isolated damage in the lateral compartment with good long-term results similar to those with medial UKA.