Sagittal kinematics of mobile unicompartmental knee replacement in anterior cruciate ligament deficient knees

• Anterior cruciate ligament deficient knees after unicompartmental knee replacement
• Deficient and intact patient groups compared during step-up and lunge exercises
• Deficient knee femurs were more posterior on tibia at 30–40° of flexion.
• The deficient ligament group took longer to perform the activities.
• Mobile unicompartmental knee replacement in a deficient knee not advised

BackgroundThere is a greater risk of tibial component loosening when mobile unicompartmental knee replacement is performed in anterior cruciate ligament deficient knees. We previously reported on a cohort of anterior cruciate ligament deficient patients (n = 46) who had undergone surgery, but no difference was found in implant survivorship at a mean 5-year follow-up. The purpose of this study was to examine the kinematic behaviour of a subcohort of these patients.MethodsThe kinematic behaviour of anterior cruciate deficient knees (n = 16) after mobile unicompartmental knee replacement was compared to matched intact knees (n = 16). Sagittal plane knee fluoroscopy was taken while patients performed step-up and forward lunge exercises. The patellar tendon angle, knee flexion angle and implant position was calculated for each video frame.FindingsThe patellar tendon angle was 5° lower in the deficient group, indicating greater anterior tibial translation compared to the intact group between 30 and 40° of flexion. Large variability, particularly from 40–60° of flexion, was observed in the bearing position of the deficient group, which may represent different coping mechanisms. The deficient group took 38% longer to perform the exercises.InterpretationKinematic differences were found between the deficient and intact knees after mobile unicompartmental knee replacement; but these kinematic changes do not seem to affect the medium-term clinical outcome. Whether these altered knee kinematics will have a clinical impact is as yet undetermined, but more long-term outcome data is required before mobile unicompartmental knee replacement can be recommended for an anterior cruciate ligament deficient patient.