Biomechanical Validation of Medial Pie Crusting for TKA Soft Tissue Balancing

Introduction: Soft tissue balancing is a part of every total knee arthroplasty (TKA) surgery. Traditionally, balancing the varus knee has been approached by releasing portions of the medial soft tissue sleeve in a sub-periosteal nature off of the proximal tibia, but this may lead to undue laxity or residual pain about the area the release was performed. More recently, “pie crusting” of the medial soft tissue sleeve has been used to balance the varus knee without compromising the structural integrity of the ligament. This technique may provide advantages over a sub-periosteal release by targeting only medial tight bands that can be palpated with the capsule distracted in 90 degrees of flexion and full extension. This study aims to biomechanically validate the pie crusting technique of the medial soft tissue sleeve and compare the results to those of standard medial releases that have been previously reported. Methods Six cadaveric TKA specimens were retrieved through the Medical Education and Research Institute (Memphis TN). For each specimen tested, the skin and muscle tissue was removed, and the femur and tibia were cut transversely 180 mm from the joint center. Specimens were fixed in extension in a custom knee testing platform (Little Rock AR) and subjected to a 10 Nm varus and valgus torque. The angle at which these moments occurred was recorded, and each test was repeated for 0, 30, 60, and 90 degrees of flexion. After tests were performed on TKA specimens, a fellowship trained orthopedic surgeon performed “pie crusting,” making alternating stab patterns with a number 11 scalpel blade along the anterior half of the superficial medial collateral ligament (SMCL) or posterior half of the SMCL including the posterior oblique ligament (POL). Three specimens had the anterior capsule pie crusted first and three had the posterior pie crusting performed first, followed by complete pie crusting. After two stages of pie crusting, the medial soft tissue sleeve was released off of the proximal tibia in a sub-periosteal fashion for comparison. Laxity was defined as the angles at which valgus torque equaled 10 Nm. Any increase or decrease in laxity was referenced to the normal TKA laxity. Results: In full extension, average laxity using complete pie crusting and complete standard ligament release were similar in all flexion angles tested. Pie crusting the anterior half of the medial capsular structures resulted in increased valgus laxity of 3.9 ± 1.6 degrees in 90 degrees of flexion compared to 0.2 ± 2 degrees in full extension. After posterior pie crusting of the medial capsular structures, valgus laxity increased in full extension by 1.8 ± 0.9 degrees with little difference in 90 degrees of flexion. Discussion: A standard medial collateral ligament release did not cause significantly greater overall valgus laxity compared to pie crusting at any of the flexion angles tested. As previously reported, the anterior half of the SMCL affected laxity in flexion, while the posterior SMCL and POL affected laxity in extension. This knowledge could enable surgeons to make more informed choices when deciding which TKA laxity balancing techniques to use.