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. Author manuscript; available in PMC: 2011 Nov 16.
Published in final edited form as: J Bone Joint Surg Am. 2007 Nov;89(11):2398–2407. doi: 10.2106/JBJS.F.01136

A MECHANICAL HYPOTHESIS FOR THE EFFECTIVENESS OF KNEE BRACING FOR MEDIAL COMPARTMENT KNEE OSTEOARTHRITIS

Dan K Ramsey 1, Kristin Briem 1, Michael J Axe 1, Lynn Snyder-Mackler 1
PMCID: PMC3217466  NIHMSID: NIHMS324547  PMID: 17974881

Abstract

Background

Evidence that knee braces used for the treatment of osteoarthritis mediate pain relief and improve function by unloading the joint (increased joint separation) remains inconclusive. Alternatively, valgus braces may mediate pain relief by mechanically stabilizing the joint and reducing muscle co-contractions and joint compression. This study therefore sought to examine the degree to which unloader knee braces control knee instability and influence muscle co-contractions during gait.

Methods

Sixteen subjects with radiographic evidence of medial compartment knee osteoarthritis and malalignment were recruited and fitted with a custom Generation II Unloader Brace. Gait analysis was performed with the knee unbraced and with the brace in neutral alignment and 4° valgus. A two week washout period separated brace conditions. Muscle co-contraction indices were derived for agonist and antagonist muscle pairings. Pain, instability and functional status were assessed using self-report questionnaires. Repeated-measures ANOVA’s, correlations and regression analysis were used for statistical analysis.

Results

The scores for pain, function and stability were worst when the knee was unsupported (baseline and washout). At baseline, 9 of 16 subjects reported knee instability, of which 5 complained it affected activities of daily living. Poor knee stability was significantly correlated with decreased activities of daily living, quality of life, global knee function and higher pain and symptoms. Knee function and stability scored highest with the neutral brace compared to the valgus brace. Vastus lateralis-lateral hamstring and vastus medialis-medial hamstring muscle co-contractions were significantly reduced as a result of bracing. Subjects with greater varus alignment exhibited greater decreases in vastus lateralis-lateral hamstrings co-contraction.

Conclusion

Neutral alignment performed as well or better than valgus alignment, in reducing pain, disability, muscle co-contraction, and knee adduction excursions. Pain relief may result from diminished muscle co-contractions rather than so called medial compartment unloading.

INTRODUCTION

Knee osteoarthritis (OA) is the most common cause of functional disability among Americans and the medial compartment is most often affected 1,2. To correct varus alignment, so called unloader braces that provide an opposing valgus force are a common non-operative intervention. In theory, braces apply an external three-point bending force to the knee and attenuate load on the medial compartment 3–5. Evidence for joint unloading 3,6–9 and effectiveness in decreasing pain and improved function 3,7,10–15 have been reported.

Frontal plane joint laxity and mediolateral instability exacerbate functional decline and disease progression 16–18. Varus-valgus knee laxity is exacerbated by meniscal and articular cartilage degeneration that decreases the distance between the tibiofemoral surfaces 18–21. Frontal plane laxity appears to be localized to the medial compartment 17. Greater laxity raises the likelihood of episodes of knee instability 22. Many patients report functional instability 17,22,23, defined as the patient’s perception of the knee “shifting, buckling and giving way during activities of daily living” 24.

Joint laxity and mediolateral instability necessitate increased muscle activity and co-activation of antagonistic muscles to stabilize the knee 17,18,23,25. Persons who are functionally unstable often stiffen their knee which involves a reduction in knee flexion excursion and increased muscular co-contraction 26–30. Increased co-contraction, while stabilizing the knee, increases joint contact pressures that could exacerbate joint destruction 17,23,25,31.

The aim of this study was to examine the degree to which valgus unloader knee braces control instability and influence muscle co-contraction during gait. Our first hypothesis was that frontal plane joint laxity and functional instability would be controlled mechanically via the brace, whether set in the patient’s normal varus alignment or at 4° of valgus (relative to normal). Secondly, pain relief may be mediated by decreased muscle co-contraction, rather than mechanically unloading (increased joint separation) from an opposing valgus force. Thirdly, improved self reported knee function and levels of pain would be similar for both brace conditions.

MATERIALS AND METHODS

Subjects

Sixteen subjects (54.9 ± 8.8 yrs, body mass index (BMI) 31.1 ± 4.2 kg/m2) with genu varum and medial compartment knee OA were referred from a local orthopedic practice. Diagnosis was based on clinical history, a physical examination, and joint space narrowing as observed from standing posteroanterior radiographs with the knee flexed 30° 32–34. Personal and radiographic data of the study subjects are presented in Table 1. Joint space width, the weight-bearing line (center of the femoral head to the center of the ankle mortise) and joint laxity from stress radiographs were measured by a single examiner.17,35–38

Table 1.

Subjects’ personal and radiographic data depicting joint alignment, joint space and laxity.

Subjects Age (Yrs) BMI (kg/m2) Knee alignment Joint space Joint laxity
WBL (%) Mech axis (°) Medial (mm) Lateral (mm) Medial (mm) Lateral (mm)
1 57 27.2 0.084 170 3.3 4.9 5.4 4.8
2 61 37.3 0.012 170 1.8 7.1 4.7 2.8
3 70 28.8 0.309 177 1.1 5.7 4.4 2.9
4 47 32.0 0.198 177 0.4 6.4 3.8 1.8
5 61 30.4 0.257 175 0.5 8.8 7.4 6.4
6 55 28.7 0.068 170 0.5 7.9 7.1 5.4
7 73 28.9 0.536 178 1.1 5.8 3.3 1.8
8 53 26.0 0.139 172 0.6 8.0 7.5 1.1
9 57 25.2 0.242 175 2.2 4.5 6.5 3.9
10 58 35.2 0.148 174 0.5 7.6 6.1 2.2
11 47 38.2 −0.043 156 0.0 7.3 6.1 1.5
12 51 37.5 0.173 174 5.5 7.9 4.2 3.7
13 40 29.6 0.053 170 4.0 9.6 2.7 3.7
14 51 32.8 0.088 171 0.9 7.5 5.4 2.8
15 55 32.4 0.255 175 3.3 7.4 3.5 4.4
16 43 26.9 0.091 171 0.3 5.8 7.7 5.1
n = 16 54.9 (8.8) 31.1 (4.2) 0.163 (0.139) 172.2 (5.1) 1.6 (1.6) 7.0 (1.4) 5.4 (1.6) 3.4 (1.5)
Significance p < 0.001 p = 0.001
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BMI: Body mass index

WBL: Weight bearing line

Mech axis: Mechanical Axis

Physical activity had been curtailed because of knee pain in all patients. No one received physical therapy prior to the study. Persons with a history of ligament deficiency or reconstruction, cardiovascular disease, diabetes, neurological impairment, impaired balance, rheumatoid arthritis, total knee replacement in either knee, orthopedic problems in the hips, ankles or spine, or a body mass index (BMI) ≥ 40.0 were excluded. Intra-articular corticosteroid and hyaluronic acid injections were not administered within three months of the study. We did not control for the use of oral anti-inflammatory medications. The study was approved by the Human Subjects Review Board and all subjects provided informed consent to participate in the study.

Motion Analysis

Subjects underwent 3-dimensional lower extremity gait analysis with simultaneous surface electromyographic (EMG) measurement on three separate occasions, (knee unbraced, neutral brace alignment, and 4° valgus correction). Kinematic data were collected at 120Hz using a passive 6-camera system (VICON 512, Oxford Metrics, UK) and ground reaction force data recorded at 1800 Hz from a Bertec force platform (Bertec Corp, Worthington, OH). Motion and kinetic recordings were synchronized for simultaneous collection.

Joint centers were defined and rigid thermoplastic shells affixed with four orthogonal markers were attached to the thigh and shank with an elastic wrap to minimize movement artifacts. A marker triad placed on the sacrum and two additional markers on the subject’s heel counter along with the marker on the 5th metatarsal head were used to track pelvis and foot movement.

EMG activity was recorded concurrently at 1800 Hz using a 16-channel system (Motion Lab Systems, Baton Rouge, LA) and bandpass filtered between 20–1000 Hz prior to sampling. Pre-amplified surface electrodes (20mm inter-electrode distance, 12mm disk diameter) were placed over the medial and lateral hamstrings, vastus medialis and lateralis, and medial and lateral heads of the gastrocnemius muscles 39. Maximal voluntary isometric contractions (MVIC) were performed to ensure correct electrode placement. An MVIC and a resting baseline were recorded for each muscle for normalization.

Brace

Patients performed the first of three gait analyses (Test 1). Custom knee braces (Generation II Unloader Select, Generation II USA, Inc., Bothell, Washington) were then manufactured based on measurements taken at the initial assessment. Braces were factory-set in 4° of valgus, relative to the varus alignment measured at the time of fitting. To set braces in neutral, the proximal and distal hinges were loosened ½ turn each to remove the valgus load. The same physical therapist confirmed proper brace fit and instructed subjects on brace application. Persons wore the brace throughout the day for two weeks before returning for the second gait analysis (Test 2). Following test 2, no brace was worn for two weeks (washout period). After the washout period braces were reset to the original 4°of valgus setting, relative to the varus alignment measured at the time of fitting. Patients then wore the brace for an additional two weeks before the final gait analysis (Test 3). Tension in the dynamic force strap was standardized using a compact digital triple scale, accurate to within ±0.05 kg, to ensure consistent strap tension between the neutral and valgus brace test sessions.

Pain and Functional Status Measurement

Pain and functional status were assessed during each of the brace conditions and washout period using the Knee injury and Osteoarthritis Outcome Score (KOOS) 40–43. Instability was assessed using the question: “To what degree does giving way, buckling or shifting of the knee affect your level of daily activity?” taken from the Knee Outcome Survey-Activities of Daily Living Scale. Reliability and responsiveness of the questionnaire and this particular question for assessing instability in individuals with osteoarthritis (OA) has been assessed and reported by others 22,24.

Data Management and Processing

Marker trajectories were low-pass filtered (Butterworth 4th order, phase lag) at 6 Hz using custom-written software (LabView 7, National Instruments Corporation, Austin, TX). Three-dimensional joint kinematics were calculated using the Euler angle sequence and referenced to the coordinate system from a standing calibration taken prior to motion recordings (Visual 3D, C-Motion, Rockville, MD). Stance was normalized to 100 data points and averaged across 10 trials for each subject and brace condition.

Raw EMG were low pass filtered at 350 Hz, full wave rectified and filtered a second time with a phase corrected 8th order, 20Hz-low pass Butterworth filter to generate the linear envelope (LabView 7, National Instruments, Austin, TX). Linear envelopes were normalized to peak EMG recorded during MVIC’s. Co-contraction indices (simultaneous antagonist muscle activation) were derived for the following muscle pairs: vastus medialis-medial hamstring, vastus medialis-medial gastrocnemius, vastus lateralis-lateral hamstring, and vastus lateralis-lateral gastrocnemius muscles. Muscle responses were analyzed from 100msec prior to initial contact (to account for electromechanical delay 44) to the first peak knee adduction moment. This interval was normalized to 100 data points. Co-contraction indices for each pair were derived using the procedure of Rudolph et al. 45.

Statistical Methods

Repeated-measures analysis of variance (ANOVA) was performed with post hoc pairwise comparisons (Least Significant Difference) for pain, function, instability, kinematic variables, and muscle co-contraction indices. Pearson product moment correlations were used to assess salient relationships among parametric variables. Spearman rank correlations were performed to assess the relationship between instability with KOOS subscores, for each period. Linear regression analysis was used test the effect of knee alignment on muscle co-contraction for each brace condition. Statistical significance was set at p < 0.05 except for muscle co-contraction indices. Significance was set at p < 0.1 in an effort to avoid a type I error given the highly variable nature of EMG data 46. Using a general linear model for repeated measures for within-subjects effects, we obtained an observed power greater than 0.8, indicating good statistical level. All statistical analyses were performed using SPSS 14 (SPSS inc., Chicago, IL).

RESULTS

Instability

Data from all 16 subjects are presented. During the baseline assessment (Test 1), nine of 16 subjects reported the presence of knee instability, 5 of which indicated that it affected their ability to perform activities of daily living (Table 2). After 2 weeks of wearing the brace in neutral (Test 2), 1 of 16 subjects reported instability affected daily activity. Eight subjects complained that instability affected daily activity during the washout period and 6 with the valgus brace setting.

Table 2.

Response to the knee outcome survey-activities of daily living scale (KOS-ADLS) question of giving way, buckling, or shifting of the knee

Subjects
Baseline Neutral Washout Valgus
I do not have giving way, buckling, or shifting of the knee 7 7 6 8
I have the symptom but it does not affect my activity 4 8 1 2
The symptom affects my activity slightly 2 1 7 5
The symptom affects my activity moderately 2 1 1
The symptom affects my activity severely 1
The symptom prevents me from all daily activity
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Self Report Questionnaires

Differences were found between conditions for all five subscales of the Knee Osteoarthritis Outcome Scores (KOOS). Pain, symptoms and activities of daily living were significantly lower (worse) during the baseline and washout periods compared with the neutral brace setting (Figure 1). Based on the numbers, no significant differences were evident between the washout and valgus condition or between the two braced conditions for pain and activities of daily living scores, but scores for symptoms were worse in the valgus than in the neutral condition (p=0.049). Based on the numbers, there were no differences in pain, symptoms and activities of daily living scores between the baseline and washout conditions. Sport and recreation, and quality of life scores significantly improved from baseline as a result of both bracing conditions. With the numbers available, scores were no different between bracing conditions and between the unbraced and washout period except for sport and recreation where it remained significantly higher during the washout period. Knee instability was significantly correlated to self-report ratings of pain, symptoms, activities of daily living and quality of life (Table 3). Those complaining of instability had increased pain and symptoms with lower activities of daily living knee function and quality of life.

Figure 1.

Figure 1

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Change in Knee injury and Osteoarthritis Outcome Scores (KOOS) induced by brace setting.

Table 3.

Correlation between knee instability and pain, symptom, activity of daily living (ADL), and quality of life (QOL) for each bracing condition.

Instability Pain Symptom ADL QOL
Baseline Correlation Coefficient .506(*) .593(**) .470(*) .716(**)
Sig. (1-tailed) .023 .008 .033 .001
Neutral Correlation Coefficient .557(*) .264 .517(*) .444(*)
Sig. (1-tailed) .012 .162 .020 .043
Washout Correlation Coefficient .395 .463(*) .472(*) .452(*)
Sig. (1-tailed) .073 .041 .038 .045
Valgus Correlation Coefficient .461(*) .304 .474(*) .690(**)
Sig. (1-tailed) .036 .126 .032 .002
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*

Significant at the 0.05 level (1-tailed).

**

Significant at the 0.01 level (1-tailed).

Muscle Co-Contraction

The level of vastus medialis-medial hamstring (VMMH) muscle co-contraction was significantly reduced with the 4° valgus setting (p=0.068) (Figure 2). Vastus lateralis-lateral hamstrings (VLLH) co-contraction was significantly reduced from baseline in both neutral and valgus conditions (p=0.014 and p=0.023, respectively). At baseline measurement, frontal plane knee angle strongly predicted VLLH co-contraction (p=0.005; R2=.438) and VMMH co-contraction (p=0.066; R2=.221), where greater co-contraction was seen in subjects with more varus angulation. The magnitude of change in VLLH co-contraction seen in both of the braced conditions was observed to be related to degree of baseline varus angulation. Subjects in more varus alignment at baseline showed a greater decrease in VLLH co-contraction, both in the neutral brace (p<0.001; r= .801, R2=.642) and the valgus setting (p<0.001; r=.772; R2=.596) (Figure 3). A similar relationship was seen between degree of varus alignment and the decrease in VMMH co-contraction measures from baseline to both neutral brace and valgus brace conditions (p=0.07; r=.464; R2=.215 and p=0.06; r=.464; R2=.23, respectively).

Figure 2.

Figure 2

Figure 2

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Muscle co-contraction during gait. Co-contraction values calculated from 100 ms prior to initial contact through peak knee adduction moment for a) VMMH (vastus medialis–medial hamstrings), VMMG (vastus medialis–medial gastrocnemius), b) VLLH (vastus lateralis–lateral hamstrings), and VLLG (vastus lateralis–lateral gastrocnemius). Values represent mean and standard deviation.

Figure 3.

Figure 3

Figure 3

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Scatter plots depicting the association between the change in a) VLLH (vastus lateralis–lateral hamstrings) and b) VMMH (vastus medialis–medial hamstrings) muscle co-contractions for the neutral and valgus brace settings relative to the degree of baseline varus alignment. Best-fit linear regression lines are superimposed on the data.

Knee adduction excursions were significantly reduced as a result of bracing (Figure 4), with excursions (from heel-strike to first peak adduction moment) were lowest at 4° valgus correction. The magnitude of knee flexion excursion and medial joint space narrowing were strongly correlated, those with substantial narrowing exhibited greater knee stiffening, demonstrated by lower knee flexion excursions during weight acceptance (p<0.001, r = −0.776). Peak knee flexion angle and flexion excursions remained unchanged from the unbraced conditions.

Figure 4.

Figure 4

Figure 4

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Knee flexion a) and adduction excursions b) during stance phase of gait. Bracing reduced knee flexion and adduction excursions during weight acceptance, from initial contact to peak knee flexion.

DISCUSSION

Self reported knee pain and functional disability in people with medial knee osteoarthritis (OA) were significantly reduced in both the neutral and unloading brace conditions. Muscle co-contraction and knee adduction excursions were also lower when the knees were braced. Our hypothesis that bracing in neutral would afford the same benefits to patients with medial knee OA and varus malalignment as a brace with a 4° valgus correction was supported by the results of this study.

All patients had medial joint laxity, the majority complained of knee instability and 30% said it affected their activities of daily living. Poor knee stability was significantly correlated with decreased activities of daily living, quality of life, global knee function and higher pain and symptoms. Ultimately, functional stability was accomplished by compensatory neuromuscular adaptations, demonstrated by the higher levels of vastus medialis-medial hamstrings (VMMH) and vastus lateralis-lateral hamstrings (VLLH) muscle co-contractions. As expected functional knee stability improved with the neutral brace setting, with only 1 reporting that instability affected daily activity. Functional knee instability worsened during the washout period with 8 patients saying instability affected their activities of daily living. The fact that more individuals complained of functional instability during the washout period after having worn the brace in neutral suggests that they may have been unstable at the outset, without realizing it. The neutral brace setting also resulted in the highest overall improvements in pain and knee function score. The equivalent baseline and washout scores clearly illustrate that the 2 week washout period was sufficient, a duration often reported in the literature 47–50.

At baseline when the knee was unsupported, subjects demonstrated significantly greater VMMH (p=0.068) and VLLH (p=0.014) co-contraction during weight-acceptance, which may be an attempt to stiffen the knee via increased joint compression. Work in our lab has shown that greater VMMH co-contraction in varus aligned knees, coupled with the larger medial load (adduction moment) appears to be a response to stabilize the medial compartment through increased compression 17. This strategy is contra-intuitive because increased co-contraction, while stabilizing the knee, increases joint contact pressures that could exacerbate joint destruction and pain 17,23,25,31. The findings that VLLH co-contractions were greater in magnitude than VMMH confirm earlier studies, and may represent an attempt to redistribute the load laterally as others have speculated 51,52. Both bracing conditions led to a significant overall lowering of antagonist muscle co-contractions on both the medial and lateral sides of the joint. This may result in decreased joint compression.

Valgus unloader braces apply an external valgus (abduction) moment to the knee 3–5. Radiographic evaluations have shown a 1.4° lateral shift of the femoral-tibial angle with bracing 7, as well as a 2.2° change in condylar separation angle and a 1.2 mm increase in condylar separation at heel strike 8. One study however found no significant changes in femoral-tibial angle or joint space, based on the numbers 9. Intuitively, unloader braces should reduce the knee varus angle and attenuate loads transmitted to the medial compartment. For knees in genu varum, the external varus (adduction) moment depends on mechanical alignment of the knee and the frontal plane location of the ground reaction force vector relative to the knee when the foot contacts the ground during stance. Thus, the ground reaction force vector would be expected to shift laterally and the overall external varus moments reduced. Using instrumented unloader braces to measure the valgus moment exerted by the brace, net varus moments have shown to be significantly reduced at 20% and 25% during the stance phase of gait 53 and by an average of 13% 3.

The data suggest that pain relief from OA bracing may be result of reduced muscle co-contractions, mediated by the brace mechanically stabilizing the knee. This study included just 16 subjects. While small, the relatively consistent findings in the sample across the wide range of BMI’s and severity of medial compartment knee OA increase the generalizability of these results to those for whom bracing is typically prescribed. The observed power of this study is more than 80%, thus increasing the sample size may actually result in a study that is overpowered for the variables of interest. With more subjects, however, perhaps other relationships with secondary variables would emerge in the regression analysis to help determine how changes in pain and function are predicted by muscle co-contraction. Quantifying the explained variance of these relationships will produce a better understanding of these mechanisms.

Brace order was not randomized. This was intentional because the basis for our protocol was to examine the subtle influences of bracing on symptomatic pain relief and neuromuscular function between the neutral and valgus brace settings. Conventional wisdom suggests the 4° valgus setting would produce the greater decrease in symptoms. Going from valgus to a neutral setting might have masked benefits of wearing the brace in the patient’s normal alignment. Subjects served as their own controls with analysis focusing on the differences between brace conditions. As the data suggest, the two week washout period between the neutral and valgus force conditions was sufficient, therefore it is unlikely that order affected the clinical interpretations of the data.

The benefits of this study lie with the potential that the positive effects of the neutrally aligned brace may influence increased use of osteoarthritic braces as a method to reduce pain and improve function. Bracing is a cost-effective treatment intervention, 54 although greater access in response to clinical need appears appropriate. Among a group of patients with OA who were questioned about their health status, use of medications, various non surgical treatment modalities and use of health care resources, only 11% were informed of bracing and only 12% of those informed tried them 55. Their efficacy in maintaining higher levels of activities has been shown 56 and compliance for long term use are encouraging 4,15,56.

Our results could impact the treatment of patients with symptomatic medial knee. With annual sales of OA knee braces in the United States numbering well over 125,000 (Frost & Sullivan: US Orthopedic Braces and Support Market, 2004), absent the need to induce the valgus correction, a wider range of individual body types could be braced. Reductions of patient morbidity for this widespread chronic condition could have positive impact on healthcare costs and economic productivity of the affected individuals.

Acknowledgments

This publication was made possible by Grant Number P20-RR16458 from the National Center for Research Resources (NCRR), a component of the National Institutes of Health (NIH). Its contents are the sole responsibility of the authors and do not necessarily represent the official views of the NCRR or NIH. Funding was also provided by the NIH T32HD007490 and R01AR048212. No benefits were received or will be received from a commercial party related directly or indirectly to the subject of this article. Braces ordered from the manufacturer by the Physical Therapy Clinic were paid with the grants that funded this study.

Funding provided by the National Institute of Health (P20RR016458, T32HD007490, R01HD037985, R01AR048212)

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