Unicompartmental osteoarthritis in the absence of any significant sagittal joint contracture hypothetically allows an unloader knee brace to provide a counterforce and reduce the forces through the arthritic compartment.⁷ The candidate for knee bracing is one with pain caused by mild to severe knee OA who is willing to tolerate an external brace. Most manufacturers recommend that coronal deformity not exceed 10°, and that patients with a flexion contracture of >10° may not be ideal candidates for bracing.² Coronal deformity does not need to necessarily correctable as there is evidence that bracing works by load sharing or limiting muscle contraction as opposed to altering the coronal angle.⁸ Patients with ligamentous incompetence in the direction of the brace moment (e.g., medial collateral ligament instability for a valgus unloader brace) should be avoided, and patients with peripheral vascular disease or skin lesions should be assessed carefully to minimize the risk of complications.⁹ Patellofemoral disease is not a contraindication to unloader bracing, though severe bicompartmental osteoarthritis may limit the benefit of this modality.¹⁰ Timing and duration of brace use lacks consensus; mild OA may only benefit from bracing during high impact activities, while severe arthritis can benefit with a longer duration of bracing, e.g., during activities of daily living.¹

Knee brace designs are fabricated in a range of configurations, with various degrees of angular moments generated and supportive struts utilized to provide a buttress against the arthritic knee deformity (Fig. 23-1). These braces apply mechanical leverage through the use of upright struts or bars, hinges, and cuffs. The factors that increase the brace leverage include the length of bars, the number of fixation points (with four points being superior to three for leverage), the fit of the brace, and the brace material properties.¹¹ Application points applied over subcutaneous bone, such as the anteromedial tibia, provides greater leverage than areas with larger amounts of soft-tissue coverage.

In the most common scenario, a valgus-producing force is utilized in medial compartment osteoarthritis using a three- or four-point bending moment, with the goal of relieving the arthritic medial compartment and shifting the mechanical axis laterally (Fig. 23-2). Designs can include a single or dual upright support, along a hinge mechanism that allows for knee motion. Hinge mechanisms can range from a basic mechanism permitting sagittal motion to polycentric designs aimed at providing more natural knee motion in greater than one plane.¹² A displacement mechanism for achieving angular correction is used in an unloading brace and can be accomplished by pads, straps, or a more elaborate mechanism for fine-tuning the degree of valgus moment created.

Several prior investigations have assessed the magnitude of radiographic deformity correction with a valgus-producing brace. In one early study, 15 patients with unicompartmental OA were assessed with fluoroscopy to determine the degree of coronal alignment and condylar separation at heel strike while using a treadmill.¹³ These subjects were then fitted with an unloader brace, and the fluoroscopic examination was repeated. The authors determined that the brace generated a small difference in the amount of mean condylar separation distance (1.2 mm) and mean condylar separation angle (2.2°), though there were three subjects that did not have any measurable difference in either parameter. A subsequent study proposed that if a medial unloader brace did indeed
shift the mechanical axis laterally, a valgus-producing brace should increase that bone density of lateral compartment.¹⁴ Utilizing dual-energy bone mineral density testing, they studied a series of patients with medial compartment osteoarthritis before and 3 months after wearing an unloader brace. There was a statistically significant increase in the lateral compartment bone density, whereas the lateral compartment in the unaffected contralateral limb did not show a significant increase in bone density.

Advances in dynamic three-dimensional imaging have allowed the ability to determine continuous changes in knee compartment space with an accuracy of submillimeter distances.¹⁵ Previous studies were limited in their ability to determine changes in joint space in two planes at discrete point of the gait cycles (e.g., heel strike, stance, etc.). In a series of 10 patients with varus knee OA, Nagai and colleagues studied the medial compartment dynamic joint space (DJS) using continuous biplane radiography during gait with and without unloader bracing, in addition to determining changes in the ground reactive force (GRF) throughout the gait cycle.¹⁶ The subjects had worn the unloader brace for 2 weeks prior to the radiographic study. Subjective improvement with the brace was determined with questionnaires. A small but statistically significant difference was found in medial compartment DJS with brace use (0.3 mm, P = .005), though there was no difference in GRF. The patients also reported an improvement in pain with brace use.

Radiographic-based investigations have demonstrated modest though discernible radiographic effects of unloader bracing in the arthritic knee. Biomechanically, the postulated effect of unloader bracing on shifting the mechanical axis toward neutral alignment appears to have some supporting evidence, though its relevance clinically is debatable.

Prior investigators have reasoned that if unloader bracing changes the static mechanical alignment of the limb, then an unloader brace should also improve the kinematics of gait in the arthritic knee. In addition to correcting the coronal deformity of the arthritic knee, the external coronal moment is an important contributor to force distribution across the knee.² This moment, or torque, is created when the foot contacts the ground during the stance phase of gait and the ground reaction force vector falls medial (varus knee) or lateral (valgus knee)¹⁷ (Fig. 23-3). Also described as the knee adduction moment, this force is optimally counteracted with an unloader brace in addition to correction of the coronal deformity.¹⁸

Small series have demonstrated changes in the varus moment created during gait with an unloader brace. In one investigation, five subjects with medial compartment osteoarthritis were fitted with a custom valgus-loading knee brace.⁸ Three-dimensional analysis was coupled with force plate information to calculate the forces at the knee. The varus moment was found to be significantly reduced in the subjects during the early stance phase of gait when wearing an unloader brace. In another study using three-dimensional gait analysis, the authors attempted to quantify the decrease in the varus moment of 11 arthritic patients with and without an unloader brace. The authors determined that valgus bracing reduced the net moment around the knee by 13% and the medial compartment load of the knee by an average of 11%.¹⁹

More recently, Ramsey and colleagues coupled gait analysis studies with electromyography to determine the contribution of reduced muscle forces in the braced knee.²⁰ The authors hypothesized that rather than correcting the varus moment of the arthritic knee, unloader bracing mediated relief by stabilizing the joint and reducing muscle contractions and joint compression around the knee. The authors performed both gait analysis and dynamic EMG with the subjects wearing no brace, wearing the brace in neutral alignment, or wearing the brace in 4° of valgus (Fig. 23-4). When wearing the brace in neutral, patients experienced greater subjective knee stability and function than when wearing the brace in an unloader setting. Notably, the varus knee moments and muscle co-contraction were similar or improved when wearing a neutral brace in comparison to the brace in the valgus unloader mode, and the study concluded the benefits of bracing may emanate from a reduction in muscle contraction rather than a mechanical correction of the varus moment.