Supine Knee Positioning Does Not Interfere with Mobile-Bearing Unicompartmental Knee Arthroplasty Performance

The Food and Drug Administration has only approved mobile-bearing unicompartmental knee arthroplasty (MB-UKA) to be performed with a hanging leg holder. The purpose of this study is to evaluate the impact of a supine knee position on MB-UKA performance.In total, 16 cadavers were randomized so that either the right or left knee was placed in the flexed or supine positions. One board-certified orthopaedic surgeon and three adult reconstruction fellows that attended the required Oxford partial knee instructional course performed four operations in each position. The primary outcome was final knee balance. Secondary outcomes included procedure duration, timing of individual surgical steps, implant sizes, range of motion, implant alignment, and fracture. A Students t-test was used to examine differences between positions with significance set at p < 0.05. Secondary analyses using two one-sided tests were conducted to explore equivalence between the two positions.There was no significant difference in mean final balance between supine (1.7 mm ± standard deviation [SD] = 1.5 mm) and flexed (1.3 ± 1.3 mm) positions (p = 0.390). There were also no significant differences between positions for procedure time (p = 0.497), tibia coronal alignment (p = 0.614), tibial slope (p = 0.194), femoral component sagittal alignment (p = 0.091), and fractures (n = 0). Exploratory equivalence analyses indicated that the positions were equivalent for final balance (p = 0.002).MB-UKA performed in the supine position is not significantly different from the flexed position in terms of ligament balance, overall procedure time, and radiographic appearance. These initial safety data warrant further clinical investigations and support the expansion of the surgical technique to include performing MB-UKAs in the supine position.