Offloading strategies for knee osteoarthritis.
Brace therapy plays an important role in conservative treatment of knee OA. Several types of braces are commercially available from multiple manufacturers; however, “unloading” braces are becoming increasingly popular.
Excerpt from Offloading strategies for knee osteoarthritis, Yatin Kirane D.Orth PhD, Rebecca Zifchock PhD and Howard Hillstrom PhD. The Motion Analysis Laboratory at the Hospital for Special Surgery in New York City. Lower Extremity Review September 2010
Newer, lightweight brace designs incorporate components such as carbon composite shells, single or double uprights, pivot and geared polycentric hinges, and cleverly positioned dynamic straps.
Similar to surgical realignment, the concept of “unloading” the affected compartment using a brace is also aimed at correcting the mechanical axis deviation. The unloading braces are specifically designed to apply a corrective torque (moment of force) to a varus or valgus deformity associated with uni-compartmental knee OA. They are intended to dynamically off-load the damaged compartment via a “three-point bending” mechanism. As depicted in Figure 4, force F1 is being applied by the dynamic strap, while forces F2 and F3 are being applied by the thigh and leg cuffs, respectively.
The effect of unloading braces on self-reported knee pain and function have been evaluated by several researchers using different scores and surveys, such as WOMAC (Western Ontario & McMaster Universities), KOOS (Knee injury and Osteoarthritis Outcome Score), SF-36 (Short Form) survey, and Hospital for Special Surgery (HSS) knee scores. Significant reduction in knee pain and functional improvement have been reported with the use of an unloading brace for six months. Kirkley et al reported an unloading knee brace to be more effective than a neoprene sleeve in patients with varus knee OA. Brouwer et al reviewed five studies and concluded that both an unloading brace and a wedged insole may have small beneficial effects, in the form of increased walking distance, and reduction in knee pain and the need for painkillers. However, they observed low adherence to brace and insole treatment beyond one year. Similar observations were reported in a subsequent multi-center randomized control trial as well.
In an in vivo study on the effectiveness of unloading braces, Dennis et al recorded separation between the femoral and tibial condyles using dynamic fluoroscopy as their subjects walked on a treadmill. They reported maximum condylar separation ranging from 0.4 to 3.4 mm at heel strike while wearing a brace. However, the condylar separation varied widely across subjects as well as brace manufacturers.
Pollo et al observed that the dynamic strap of an unloading brace shares the load at the knee joint. The authors calculated the stress in the medial knee compartment using a mathematical model, and reported that the joint stress decreased with the use of an unloading brace. Nevertheless, it is important to note that since the forces in a natural knee joint can not be directly measured in a person, they have to be estimated based on mathematical or computer models, and considerable assumptions and limitations are associated with model-based techniques.
In 2007, Ramsey et al observed decreased co-contraction of important thigh and leg muscles (vasti and hamstrings) when wearing an unloading brace. Therefore, they suggested that pain relief may be resulting from improvement in knee stability, and consequent reduction in muscle co-contraction and compressive forces across the joint rather than direct compartment off-loading.
In general, braces, regardless of type, tend to slide, rotate, and lose their alignment with the joint center, which makes them less effective than when they are positioned correctly. A placebo effect is often suspected in patients’ satisfaction with their brace. Thus, in spite of reports of reduced knee pain and improved function in several studies, the mechanical effects of unloading knee braces are not clear. It is not known if unloading braces can apply the corrective torque consistently and reliably across the gait cycle. Furthermore, the long term effectiveness of these braces beyond six months has not been adequately evaluated.
Analysis of biomechanical effectiveness of valgus-inducing knee brace for osteoarthritis of knee,Thomas Schmalz, PhD, Elmar Knopf MD, Heiko Drewitz CPO, Siegmar Blumentritt PhD. J Rehabil Res Dev. 2010;47(5):419-29