The biomechanics of lateral knee bracing

Abstract

In Part I of our study on lateral knee bracing, we evaluated brace effectiveness using cadaveric knees and two commercially available braces (McDavid and Omni). The results indicated that for low-rate dynamic valgus loading, neither brace provided significant pro tection against MCL injury. Also, four potentially ad verse effects were noted. The goals in Part II were to: 1) determine the clinical significance of brace induced MCL preload; 2) define the functional character of an "ideal" brace; 3) design and validate a surrogate knee model for testing brace effectiveness; and 4) determine brace performance under impact loading using the sur rogate knee and six commercially available brace types (manufactured by DonJoy, McDavid, Mueller, Omni Sci entific, Stromgren-Scott, and Tru-Fit). Knee braces, modified to measure varus/valgus bending force, were used to determine MCL preload effects in 13 human volunteers. An anatomically correct surrogate knee model, instrumented to measure ligament/tendon ten sion and medial joint opening, was developed and validated using information from our previous cadaver studies and results of analyses on the effects of high strain rates (100% versus 1000% strain/sec) on MCL failure. Over 500 impact tests were performed on the surrogate knee in unbraced versus braced conditions. Tests were conducted for three impactor masses, two flexion angles, and free or constrained limb positions. Impact safety factors (ISF) were calculated for each test condition and brace type. An ISF of 1.50 (MCL load reduction of 30%) was considered significant. The results were: 1) brace induced MCL preload in vivo was negated by joint compressive forces; 2) the "ideal" brace should increase the lateral force at MCL injury by 80%; 3) at a 1000% strain/sec strain rate, MCL failure force was increased by 28%; and 4) on the average, only one brace exceeded the minimum ISF (DonJoy, average ISF = 1 .51 ).