Proximal tibial morphology and risk of posterior tibial cortex impingement in patients with AA-sized Oxford unicompartmental knee arthroplasty tibial implants

Background: In cases of Oxford unicompartmental knee arthroplasty (UKA), an increase in anteroposterior and medial-lateral length is usually disproportional when comparing AA and A-sized tibial components. Asynchronous increments may cause tibial keel impingement leading to complications. Methods: Radiographic measurements were performed in five patients with AA-sized tibial implants. The posterior cortex of proximal tibia had two angles recorded as M1 and M2. The minimum distance between the tibial component keel and outer margin of the posterior tibial cortex (mDKC) was measured, and the correlation between the preoperative posterior slope angle (PSA), M1, and mDKC was analyzed. Results: All patients showed an acceptable component positioning. Only one patient had an mDKC of < 4 mm that fulfilled the criteria for the posterior tibial cortex at risk. The patient had an increased PSA and M1 compared to other patients. A negative correlation was found between preoperative PSA and mDKC (r = - 0.935, p = 0.0193); and M1 and mDKC (r = - 0.969, p = 0.0032). However, no stem tip pain, periprosthetic fracture, or component loosening were observed. Conclusions: The distance between the tibial keel and posterior tibial cortex was reduced in AA-sized patients with a large PSA andM1; therefore, the risk of the tibial cortex injury should be considered.