Knee osteoarthritis (OA) is a significant problem in the aging population, causing pain, impaired mobility, and decreased quality of life. Conservative treatment methods are necessary to reduce rapidly increasing rates of knee joint surgery. Recommended strategies include weight loss and knee bracing to unload knee joint forces. Although weight loss can be beneficial for joint unloading, knee OA patients often find it difficult to lose weight or exercise due to knee pain, and not all patients are overweight. Uni-compartment offloader braces can redistribute joint forces away from one tibiofemoral (TF) compartment, however, <5% of patients have uni-compartmental TFOA, while isolated patellofemoral or multi-compartmental OA are much more common. By absorbing body weight and assisting the knee extension moment using a spring-loaded hinge, sufficiently powerful knee-extension-assist (KEA) braces could be useful for unloading the whole knee. This paper describes the design of a spring-loaded tri-compartment unloader (TCU) knee brace intended to provide unloading in all three knee compartments while weight-bearing, measures and compares the force output of the TCU against the only published and commercially available KEA brace, and calculates the static unloading capacity of each device. The TCU and KEA braces delivered maximum assistive moments equivalent to reducing body weight by 45 and 6 lbs, respectively. The paper concludes that sufficiently powerful spring-loaded knee braces show promise in a new class of multi-compartment unloader knee orthoses, capable of providing a clinically meaningful unloading effect across all three knee compartments.