Unicondylar knee arthroplasty: what’s new? (2013)

Unicondylar knee arthroplasty (UKA) has gained increasing interest over the last decade. Unicondylar knee arthroplasty implantation is extremely demanding due to the fact that the prosthesis needs to be integrated in the natural anatomy of the knee in order to preserve the integrity of the other two compartments and all related ligaments. This ensures the integrity of the natural knee kinematic. Biomechanical studies using robotic technology have shown that knee kinematic does not change significantly after medial UKA as reported in the issue [5]. Similar findings have been published by others using fluoroscopy [1, 3]. Normal AP translation but with less knee rotation has been observed in patients during deep flexion after medial UKA in comparison with the natural knee. Other tasks such as treadmill gait and stair stepping have also been studied. The preservation of both cruciate ligaments contributes to a significant functional advantage. This could explain why patients, following UKA, demonstrate higher positive postoperative clinical outcomes than after total knee arthroplasty [15]. However, the achievement of a perfect component placement, allowing knee kinematics as close as possible to normal, remains a huge challenge. There is an ongoing discussion about the ideal choice of patient for UKA. Isolated medial or lateral osteoarthritis, a good range of motion, stability of the collateral and cruciate ligaments, and no significant deformity used to be the primary requirements for UKA. Long-standing weight-bearing X-rays will provide the information about leg alignment and degree of OA. Stress X-rays might be helpful for the assessment of the contralateral compartment. Two papers of the current issue deal with clinical aspects that might be relevant for patient selection for UKA [6, 14]. One focuses on the impact of patients’ pain location on the clinical and functional outcome, whilst the other evaluates the impact of anterior instability on the survival rate following UKA. A retrospective analysis showed that patellofemoral osteoarthritis does not appear to be a contraindication for UKA [4]. The same group has published a paper in the current issue about the preoperative pain location as a poor predictor of outcome following UKA. Pure medial or anterior knee pain was compared with generalized knee pain. No difference after one and 5 years was observed between the groups [14]. The finding may cause confusion in terms of patient selection for UKA. Unfortunately, the question about the more appropriate indications and predictors for UKA remains unanswered. This study is very interesting because it shows the difficulty of interpretation of pain in osteoarthritis. The suprapatellar pouch, the Hoffa fat pad, the medial and lateral retinacular, and the cruciate ligaments are the most sensitive structures in the knee [8]. No pain sensation was found during palpation of the femoral condyle and the tibia plateau, the area of cartilage degeneration. This might partially explain why it is sometimes difficult to localize the pain in an osteoarthritic knee. Pain in osteoarthritis also involves numerous interactive pathways including biological, psychological and social factors [13]. R. Becker (&) Department of Orthopaedics and Traumatology, City Hospital Brandenburg, Hochstrasse 26, 14770 Brandenburg, Havel, Germany e-mail: roland_becker@yahoo.de