Offloading strategies for knee osteoarthritis

Altering joint alignment, surgically or nonsurgically, is associated with decreased pain and improved function in patients with knee OA. Researchers are working to explain the mechanisms underlying these effects, particularly with respect to bracing.

by Yatin Kirane, D.Orth, PhD; Rebecca Zifchock, PhD; and Howard Hillstrom, PhD.

Knee osteoarthritis (OA) ranks among the most common disabling conditions in adults. Characterized by persistent joint pain and limitation of function, knee OA adversely affects one’s quality of life by impeding an individual’s ability to work and to perform recreational and daily living activities. It has significant economic and societal impact due to the direct health care costs as well as the indirect costs from morbidity and loss of productivity.

Approximately 9.3 million people in the U.S. were estimated to be suffering from knee OA in the Bone and Joint Decade’s report in 2005.¹ Radiographic signs of knee OA, with or without the presence of symptoms, have been observed in about 14.1% of men and 22.8% of women over the age of 45 years in the U.S. and European populations.² Based on the findings of two highly-regarded prospective cohort studies (Framingham and Johnston County), the prevalence of symptomatic knee OA in the American population has been estimated to be 5% in adults of 26 years of age and older, and 17% in adults over the age of 45 years.¹ The overall risk for developing knee OA in one’s lifetime has been reported to be 46% in the normal population, 57% when there is a history of knee injury, and 66% among obese individuals.³

The cascading cycle of pathological changes in OA includes biomechanical and biochemical factors that ultimately destabilize and destroy the cartilage matrix. Cartilage destruction is accompanied by degenerative changes in the underlying bone (e.g., cysts, osteophytes) as well as the surrounding soft tissues (e.g. capsular contracture, ligament laxity, muscle wasting). Several genetic, mechanical, and biochemical factors seem to play a role in the causation and progression of knee OA. Routine wear-and-tear of the joint results in primary OA in genetically susceptible persons during their later years of life. In contrast, secondary knee OA following injuries, such as medial meniscus and anterior cruciate ligament tears, is common among the younger population.⁴ In addition, obesity⁵ and lower limb malalignment⁶ (abnormal positioning or alignment) have also been associated with increased risk for knee OA.⁷

Treatment options

There is no known cure for OA. In advanced knee OA, with most of the joint cartilage worn off, the status is often referred to as “bone-on-bone” in lay person’s terms, and joint arthroplasty (replacement) may be recommended. Total knee replacement (TKR) can eliminate knee pain, realign the joint, and restore its function significantly. In cases of unicompartmental OA, when only one of the three compartments within the knee (Figure 1) is primarily affected and the other two are relatively normal, a unicondylar knee replacement (UKR) can be performed. With the ongoing improvements in implant designs, materials, and surgical techniques, TKR procedures yield excellent long-term outcomes.^8,9 For appropriately selected patients, the UKR procedures also yield favorable results.¹⁰ Nevertheless, the artificial joint components have a limited lifespan — a particular concern for relatively young and active patients, and these procedures are not without complications. Infection may necessitate prosthesis removal, while implant loosening could require revision arthroplasty. Other surgical procedures may be beneficial to delay the development or progression of OA associated with certain knee conditions. For example, early arthroscopic repair of a meniscus tear or removal of loose bodies may potentially delay joint degeneration. Realignment surgeries such as high tibial osteotomy may be recommended if there is an associated limb malalignment, and either the medial or lateral knee compartment is primarily affected with the other one being relatively normal.^11,12

Although joint replacement often yields successful outcomes, conservative treatment methods can be attempted to prolong the life of the natural joint. This is especially true for patients with less advanced OA or those who are not suitable candidates for surgery. Only a small fraction of knee OA patients absolutely require surgery, while symptoms in the majority of patients can be treated nonoperatively with reasonable success.¹³ Therefore, conservative methods that may prevent or postpone the need for arthroplasty are warranted.

Ambulatory aids (e.g. canes and crutches), injury avoidance, weight reduction, physical therapy, and exercises may benefit all knee OA patients.¹⁴ Specifically, land-based exercise regimens, focused on lower-limb muscle strengthening and/or aerobic fitness seem to reduce knee pain.¹⁵ In addition to conventional exercises focused on strengthening quadriceps and hamstring muscles, those focused on hip abductors (gluteus medius and minimus) may also be beneficial.^16,17 Aquatic exercises have also yielded favorable outcomes.^18,19 Oral supplements of glucosamine and chondroitin sulfate, especially when taken together, could effectively reduce knee pain in some patients.²⁰ Hyaluronic acid injections into the affected joint (visco-supplementation) may significantly reduce pain and improve joint function,^14,21 but their effect seems to wear off in about a year.²² Painkillers and anti-inflammatory medications (e.g. NSAIDs and COX-II inhibitors), and oral and intra-articular corticosteroids are typically used to manage acute flare-ups; however, long-term administration of these medications could have serious side effects.²³ Knee braces and other orthoses are another type of conservative therapy that play a critical role in management of knee OA, and have been shown to improve function.^24,25

Since the abovementioned nonsurgical treatment methods are not equally effective for all patients, a “cocktail” or combination of different treatment modalities is often helpful. In this regard, a physician’s clinical judgment and experience play an important role in finding the right combination for the patient, and some trial-and-error is often required.

Unicompartmental knee OA

Unicompartmental knee OA poses unique challenges since the mechanics and treatment vary depending on which compartment is affected. For example, OA of the medial compartment may be associated with a varus knee deformity (Figure 2), while OA of the lateral compartment may occur with a valgus deformity. These deformities are thought to cause disproportionate compartment loading and degeneration,^6,26,27 especially among the overweight population.⁷ Deformity correction (realignment) may reduce stress in the overloaded compartment, and prolong the life of the natural knee joint.

The philosophy of knee realignment is based upon the concept of shifting the mechanical axis – a line between the centers of the femoral head and ankle joint – which is often referred to as the weight-bearing axis. In normal knee alignment, the mechanical axis passes approximately through the center of the knee joint (Figure 3); while it is shifted medially (inward) in a varus deformity (Figure 2), and laterally (outward) in a valgus deformity. During a surgical realignment procedure, such as high tibial osteotomy, surgeons aim to shift the mechanical axis towards the knee joint center. This is achieved by making a cut in the upper part of the tibia, and either removing a precise bone wedge from the convex side of the deformity or inserting a wedge-shaped bone graft on the concave side. Alternatively, the correction can also be done by gradual separation of the cut bone surfaces using an external fixator (Ilizarov technique).²⁸

Brace therapy in unicompartmental OA

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. 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 unicompartmental knee OA. They are intended to dynamically off-load the damaged compartment via a “three-point bending” mechanism.²⁹ 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.^30-33 Kirkley et al reported an unloading knee brace to be more effective than a neoprene sleeve in patients with varus knee OA.³⁰ 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.³⁴ Similar observations were reported in a subsequent multi-center randomized control trial as well.³⁵

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.³⁶

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.³⁷ 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.³⁸

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.

OA research at the LRMAL

The Leon Root, MD Motion Analysis Laboratory (LRMAL) at the Hospital for Special Surgery (HSS) in New York is well-equipped to study human movement using high-tech research tools, such as a 3D motion capture system, force platforms, plantar pressure platforms, Biodex muscle-testing unit, and electromyography. The LRMAL researchers, in collaboration with the physicians at HSS, have been working on several research projects to fully comprehend the complex relationships between joint structure, pain and function, and patho-mechanisms in OA. In an ongoing study, the mechanical effects of unloading knee braces are being examined. A second study has been designed to examine the mechanical changes (walking kinetics, kinematics, and energy efficiency) in children undergoing surgical realignment of a varus knee deformity, such as Blount’s disease. Likewise, a third study has been developed to evaluate the above outcome measures in the adult population with varus knee OA.

In the unloading brace study, we measure knee pain and function using visual analog scale (VAS) and KOOS scores, knee structure (joint space on X-ray images), OA biomarkers in the joint fluid, and walking mechanics (kinetics and kinematics) using 3D gait/ motion analysis. The testing is done before, during and after one year of therapy with the Unloader One brace from Össur Americas. The Unloader One brace features a lightweight custom design that incorporates dual dynamic straps for the application of the corrective torque/moment. In 3D gait analysis, reflective markers are fixed on bony prominences of a patient’s body, and marker motion is recorded using infra-red cameras while patient performs tasks such as walking (Figure 5) or climbing up and down the stairs. Various kinetic (e.g. forces) and kinematic (e.g. joint angles and accelerations) measurements are dynamically recorded during testing. In medial knee OA, the knee adduction moment (the tendency of the forces at the knee to cause varus movement) has been associated with knee alignment and OA severity.^25,39 Although abnormal knee alignment has been associated with aberrant biomechanical function and progression of OA,^6,25-27 the exact relationships between knee alignment, pain, function and mechanics (kinetics and kinematics) are poorly understood.
We are expecting significant reduction of knee pain, and improvement of knee function, structure and chemical make-up of the knee joint fluid at successive follow-up visits, compared to baseline levels. Based on our preliminary results on seven subjects with moderately severe OA, knee pain during stair ascent/descent and walking at a comfortable speed were significantly reduced at one week, and four and 12 months. Difficulty in performing daily living and recreational activities, as well as other knee symptoms, were significantly decreased at four months and maintained at 12 months. However, the radiographic parameters were not statistically different between baseline and 12 month follow-up visits.^40-44 We are currently analyzing the correlation between the subjective and objective measurements in order to better quantify the efficacy of brace therapy. We are also seeking additional subject recruitment.

In summary, unloading braces may be a valid option for selected patients with unicompartmental knee OA, although their efficacy in other types of knee OA remains to be documented. There is no “ideal” brace for knee OA – the one that would be lightweight, comfortable and unobtrusive, fit firmly, not slip or rotate, maintain its alignment with the joint’s natural center of rotation throughout the gait cycle, allow normal knee motion without restricting rotations, impart adjustable amounts of correction for varus/valgus deformities, and adjust for inter-personal variability in body mass index, lower limb and knee joint structure. There is a need for more versatile custom braces that can be prescribed for different types of arthritis. Better insights into normal and abnormal joint mechanics will continue to play a critical role in designing better braces and other orthoses. Improved orthotic therapy and other conservative treatment modalities will hopefully be more effective in the near future for prolonging the life of the natural knee joint.

Yatin Kirane, D.Orth, PhD, is a postdoctoral fellow, Rebecca Zifchock, PhD, is an instructor and Howard Hillstrom, PhD, is the director at the Motion Analysis Laboratory at the Hospital for Special Surgery in New York City.

Note: For more information about the ongoing projects at the Leon Root, MD, Motion Analysis Laboratory at the Hospital for Special Surgery, please contact Yatin Kirane at kiraney@hss.edu.

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