Objective: To assess the relationship between knee varus–valgus motion and functional ability, and the impact of knee varus–valgus motion on the relationship between muscle strength and functional ability in patients with osteoarthritis (OA) of the knee.
Methods: Sixty-three patients with knee OA were tested. Varus–valgus motion was assessed by optoelectronic recording and three-dimensional motion analysis. Functional ability was assessed by observation, using a 100 m walking test, a Get Up and Go test, and WOMAC questionnaire. Muscle strength was measured by a computer-driven isokinetic dynamometer. Regression analyses were performed to assess the relationships between varus–valgus motion and functional ability, and to assess the impact of varus–valgus motion on the relationship between muscle strength and functional ability.
Results: In patients with high varus–valgus range of motion, muscle weakness was associated with a stronger reduction in functional ability (ie, longer walking time and Get Up and Go time) than in patients with low varus–valgus range of motion. A pronounced varus position and a difference between the left and right knees in varus–valgus position were related with reduced functional ability.
Conclusions: In patients with knee OA with high varus–valgus range of motion, muscle weakness has a stronger impact on functional ability than in patients with low varus–valgus range of motion. Patients with knee OA with more pronounced varus knees during walking show a stronger reduction in functional ability than patients with less pronounced varus knees or with valgus knees.
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In patients with knee osteoarthritis (OA), limitations in daily activities such as walking, climbing stairs and getting out of a chair are common.1–3 It has been found that patients with knee OA show reduced functional ability in the presence of varus–valgus laxity of the OA knee.4 5 Furthermore, malalignment of the knee predicted decline in functional ability.6 The terms varus and valgus refer to lateral and medial angulations of the tibia from the centre of the knee in the frontal plane.7 Recently it has been found that a varus position of the knee during mid-stance may predict reduced functional ability.8 It has also been shown that patients with knee OA use greater magnitudes of muscle activities during walking,9 10 presumably to minimise high varus–valgus motion.
During normal walking there is low varus–valgus motion of the knee.7 High varus–valgus motion of the knee may causes difficulties in carrying out physical tasks in which the knee is pivotal and therefore may predict reduced functional ability. Thus, it is hypothesised that varus–valgus motion is associated with reduced functional ability.
The relationship between functional ability and muscle strength in patients with knee OA is well established.11 It is assumed that low varus–valgus motion results in efficient use of muscle strength during walking.12 On the other hand, high varus–valgus motion may result in inefficient use of muscle strength. This implies that muscle weakness would lead to more severe functional disability in patients with high varus–valgus motion than in patients with low varus–valgus motion.
The following two hypotheses were tested in this study: (1) high varus–valgus motion is associated with reduced functional ability, and (2) in patients with high varus–valgus motion, muscle weakness is associated with a more severe reduction of functional ability than in patients with low varus–valgus motion.
PATIENTS AND METHODS
Sixty-three patients diagnosed with knee OA were included in the study. Inclusion criteria were knee OA (uni- or bilateral) according to the clinical criteria of the American College of Rheumatology,13 and age between 40 and 85 years. Exclusion criteria were: polyarthritis, presence of rheumatoid arthritis or other systemic inflammatory arthropathies, knee surgery within the last 12 months or a history of knee arthroplastic surgery, intra-articular corticosteroid injections into either knee within the previous 3 months, and/or inability to understand the Dutch language. All patients provided written informed consent. The study was approved by the human research ethics committee of the VU University Medical Center in Amsterdam.
Patients visited the laboratory twice within the same week. During the first visit, patients completed a questionnaire, muscle strength was tested and two performance tests for functional ability were carried out. The second visit consisted of a three-dimensional gait analysis. Patients were tested at a similar time of day, by the same examiner. This was always at the end of the afternoon for all patients.
A series of demographic variables were obtained including age, gender, height, weight and duration of complaints.
An Optotrak motion analysis system (model 3020, Northern Digital Inc., Waterloo, Ontario, Canada) recorded the three-dimensional position of light emitting diode markers in order to assess varus–valgus motion. Three-dimensional ground reaction force were synchroniously recorded using a 51×46.5 cm force plate (AMTI, Watertown, MA, USA). An open source Matlab software program BodyMech (http://www.bodymech.nl) was used to reconstruct the anatomical axis and, from that, three-dimensional knee motion and loading data.14 Varus–valgus knee motion resulted from decomposing knee motion using a flexion–varus–exorotation sequence.
To describe skeletal movement, body segments were considered as rigid bodies (lower leg, thigh, pelvis and trunk) with a local coordinate system defined to coincide with a set of anatomical axes.15 The limb segments were determined by anatomical landmarks: greater trochanter, medial and lateral femur condyl, medial tibia condyl, caput fibulae, lateral and medial malleolus, superior anterior and posterior iliac crest, acromion, spinal processus Th8 and xiphoid processus. A cluster of three surface infrared light emitting diodes (LEDs) were secured to six body segments (lower leg 2×, the thigh 2×, the sacrum and the spinal processus C7). The three-dimensional position of each LED was sampled with a frequency of 50 Hz. Using these LED positions, data collection of knee varus–valgus motion started when the foot reached the force plate (ie, initial contact) and continued until the foot left the force plate. These data produced a vertical ground reaction force curve and a curve presenting the varus–valgus position in time.
The ground reaction force curve presents itself as a M-shaped curve, from which the loading response phase (ie, from zero to the first peak) and mid-stance (ie, the lowest point of the M shape in between two peaks) were determined. These two parts of the ground reaction force curve were used to determine (1) the knee varus–valgus range of motion (VV-ROM), and (2) the varus–valgus position (VVP) (see fig 1).
The varus–valgus range of motion of the knee was measured from initial floor contact to the instance in which maximum ground reaction force was recorded (ie, loading response phase) (see fig 1). The movement of the knee in the varus and valgus direction was assessed. The difference between the peak excursion in the varus direction and the peak excursion in the valgus direction reflects VV-ROM (in degrees) (see fig 2). The position of the knee was measured in mid-stance. Mid-stance is the instance in which the other foot has been lifted, the body weight has been aligned over the forefoot and the knee is extended. At the start of measurement, prior to walking, the patients were standing on the platform with body weight divided over both legs (bipedal stance). During this “rest” or anatomic posture, the knee position was determined. The knee position at the lowest point of the M shape in between two peaks of the ground reaction curve was compared with the position of the knee at the beginning of the measurement to determine the mid-stance VVP. Mid-stance VVP was expressed in degrees (see fig 3).
All subjects were instructed to walk at a self-selected normal speed along an 8 m walkway. They practised until they could consistently and naturally make contact with the force plate. In order to achieve a natural gait pattern, subjects were not informed of the need to contact the force plate. The measurement of varus–valgus motion began with some steps before the force plate, to obtain a fluent walking pattern and stopped a few steps after leaving the force plate. Three acceptable trials were obtained for each knee and averaged to yield representative values of VV-ROM and mid-stance VVP. The mean in degrees for VV-ROM and mid-stance VVP of the right and left knees obtained from these three measurements was used for analysis.
Functional ability was assessed with both two standardised physical performance tests and a self-report questionnaire (Western Ontario and MacMasters Universities Osteoarthritis Index, WOMAC). As a performance-based measure of function a 100 m walking test and a Get Up and Go (GUG) test were used.16 The 100 m walking test measured the time to walk a distance of 20 m five times (100 m) along a level and unobstructed corridor. Patients were instructed to walk the distance as fast as possible. On the command “go”, patients walked along the corridor. They were instructed not to stop before crossing the finish line. A stopwatch was used to measure in seconds the time from the command “go” until subjects crossed the finish line. The examiner was standing at the finish line during the test. Patients who used canes while walking were permitted to use them during the test. All patients were wearing walking shoes.
The GUG test was performed as described by Hurley et al.17 To perform the test, subjects were seated on a standard height chair with armrests. On the command “go” subjects stood up without help of their arms and walked along a level, unobstructed corridor as fast as possible. A stopwatch was used to measure the length of time it took for the subject to stand and walk 15 m. Patients who used canes while walking were permitted to use them during the test. All patients were wearing walking shoes. A longer time to complete the GUG test represents reduced functional ability. The intraclass correlation coefficient (ICC) for the intratester reliability is 0.98 and the ICC for the intertester reliability is 0.98.17
The Dutch version of WOMAC was used to assess self-reported functional ability.18 WOMAC is a disease-specific measure of pain, stiffness and physical function for individuals with knee OA. WOMAC (with a possible range of 0–96) includes five items related to pain, two items related to stiffness and 17 items related to physical function (PF). Each item is scored on a five-point Likert scale. Reliability and validity of WOMAC have been established.18 Higher scores on WOMAC represent greater reduction in functional ability. The ICC for Dutch WOMAC-PF is 0.92.18
Muscle strength was assessed for flexion and extension of the knee using an isokinetic dynamometer (EnKnee; Enraf-Nonius, Rotterdam, the Netherlands). Quadriceps and hamstring strength were measured isokinetically at 60°/s.
All patients were assessed according to a previously described device and protocol.19 The mean in Nm/kg body weight for quadriceps and hamstring strength of the right and left maximum voluntary contraction obtained from three measurements was used for analysis. The mean of the right and left knee were averaged to obtain a measure for total muscle strength around the knee at the patient level.11 20
Radiography and skeletal alignment
Radiographs of the knee were scored in a blinded fashion by an experienced radiologist using the grading scales proposed by Kellgren and Lawrence (K/L).21 22 Weight-bearing, anteroposterior radiographs of the knee joints were obtained following the Buckland–Wright protocol.23 Skeletal alignment was assessed by a goniometer. In the frontal plane the angle between the thigh and shank was measured in degrees, with the axis of the arm of the goniometer at the transversal axis of the knee. The measurement was carried out in a non-weight-bearing position, with the knee extended.
Multilevel (linear mixed-model) analysis was applied for varus–valgus motion (VV-ROM and mid-stance VVP) to analyse the dependency between left and right knees of the same patients.24 In this way two levels were distinguished: between-patients and between-knees within patients. As functional ability (ie, walking ability and WOMAC-PF score) was specific to patients, varus–valgus motion were averaged across right and left knees for analyses involving functional ability.
First, Pearson correlation coefficients were computed to establish the bivariate relationships between varus–valgus motion and functional ability. Second, a regression analysis was used to assess the relationship between varus–valgus motion, muscle strength and functional ability. An interaction variable between VV-ROM and muscle strength was added to the regression analysis, to assess the role of VV-ROM as a modifier of the relationship between muscle strength and functional ability. To adjust for the dependency of the left and right knees for VV-ROM, the mean of both knee measurements and the difference between both knee measurements were added to the regression analyses. This procedure controls for the independent contribution to the regression model of the left and right knee data of VV-ROM. When the difference between the two knees had a significant effect in regression analyses, the difference was included into the final model. The same regression analysis was performed with mid-stance VVP, muscle strength and their interaction as independent variables. The independent contribution to the regression model of the left and right knee data of mid-stance VVP was controlled by the same procedure as for VV-ROM. The variables VV-ROM, mid-stance VVP and muscle strength were centred around the mean.25 Centring allows for a meaningful interpretation of main effects when interaction is present in the model. Other independent variables in the analyses comprised age, gender, duration of complaints and current pain.
Results were considered statistically significant if p<0.05. All analyses were performed using SPSS version 14.0 software (Chicago, IL, USA).
Characteristics of the study sample are listed in table 1. Mean VV-ROM in the loading response phase of a step was 3.24° (SD 1.47°). In left knees, the VV-ROM was 3.49° (1.72°) and in right knees 2.98° (1.74°), with a Pearson correlation coefficient of r = 0.44 (p<0.001) between VV-ROM of the left and right knee. Mean mid-stance VVP in the mid-stance phase of a step was 2.22° (1.65°). In left knees, the mid-stance VVP was 3.02° (1.79°) and in right knees 1.37° (2.72°), with no significant Pearson correlation coefficient (r = 0.05; p = 0.685) between mid-stance VVP of the left and right knee. At mid-stance 105 knees showed a varus position, 19 a valgus position and two were neutral.
A linear mixed model analysis established variance in VV-ROM scores between patients and between knees within patients resulting in an ICC of 0.42. This means that 42% of the variance in VV-ROM score occurs between patients and 58% occurs between knees within patients. A linear mixed model analysis of mid-stance VVP established an ICC of 0.19. This means that 19% of the variance in mid-stance VVP score occurs between patients and 81% occurs between knees within patients.
No correlation was found (r = −0.019; p = 0.831) between the mid-stance VVP and the skeletal alignment measured by goniometer.
Bivariate relationships between varus–valgus range of motion, mid-stance varus–valgus position and functional ability
VV-ROM was not significantly correlated with reduced functional ability (walking time r = 0.24; p = 0.060 and GUG time r = 0.13; p = 0.332). However, a small correlation was found with WOMAC-PF (r = 0.26; p = 0.043). Mid-stance VVP was correlated with reduced functional ability (walking time r = 0.27; p = 0.034 and WOMAC r = 0.30; p = 0.017). However, no correlation was found with GUG time (r = 0.18; p = 0.169).
Multivariate relationships between varus–valgus range of motion, muscle strength and functional ability
To analyse the relationship between functional ability, VV-ROM and total muscle strength, a multiple regression model was constructed: functional ability = b0+b1×VV-ROM+b2×muscle strength+b3×VV-ROM×muscle strength (table 2). The difference between the left and right data of the variable VV-ROM in the loading response phase did not add to the regression model. For that reason only the variable representing the mean score for VV-ROM at the patient level was used in the analyses of the data in the loading response phase. The model explaining the total variation of walking time in the loading response phase was as follows (see table 2): walking time = 96.61+3.96×VV-ROM– 53.94×muscle strength– 14.21×VV-ROM×muscle strength (F = 19.04, p<0.001, R2 = 0.50, n = 63). This means that 50% of the total variation of walking time is explained by VV-ROM, muscle strength and their interaction. The interaction between VV-ROM and muscle strength (b = −14.21, p = 0.020) was significantly associated with walking time. In the presence of high VV-ROM, muscle weakness was associated with an enhanced reduction of functional ability.
The model explaining the total variation of the GUG time and WOMAC-PF score are presented in table 2. For the GUG time the results show the same trend as the results obtained with walking time (p = 0.067). This means that muscle weakness is associated with a higher GUG time in the presence of an increased VV-ROM. Muscle weakness was associated with both GUG time and WOMAC-PF score.
To visualise the interaction between VV-ROM and muscle strength in the loading response phase, VV-ROM was dichotomised into low VV-ROM and high VV-ROM using the median-split method (fig 4).
Multivariate relationships between mid-stance varus–valgus position, muscle strength and functional ability
The difference between left and right knee mid-stance VVP data contributed to the variance in walking time and GUG time (see table 3), but not to the variance in WOMAC-PF score. Therefore, the difference between left and right VVP data (in mid-stance) was added to the regression model. The main effect of mid-stance VVP was significant for walking time, GUG time and WOMAC-PF score. This means that patients with an increased mid-stance VVP (ie, more varus position) have a stronger reduction in functional ability, than patients with low mid-stance VVP (ie, less varus or more valgus position). Thus, “bowing out” of the knee is related to reduced functional ability. The interaction between mid-stance VVP and muscle strength was not significant and did not contribute to the variance in walking time, GUG time and WOMAC-PF score.
All analyses were repeated in a more extensive model, with the demographic variables from table 1 as controlling variables (age, gender, disease duration and current pain). The results of those analyses were consistent with the results reported here.
This study shows that varus–valgus motion of the knee is related to functional ability in patients with knee OA. It was found that in patients with knee OA muscle weakness has a stronger impact on functional ability when the knee shows a high VV-ROM. It was also found that a pronounced varus position in mid-stance is associated with reduced functional ability. Finally, a left–right difference in mid-stance position of the knee is associated with a reduction in functional ability.
The results of the present study suggest that high VV-ROM is associated with inefficient use of muscle strength in the loading response phase. Patients with knee OA show greater magnitudes of muscle activities during walking.9 10 We presumed that low VV-ROM results in the efficient use of muscle strength during walking. Low VV-ROM is a condition for functional ability. Conversely, in the presence of increased VV-ROM and muscle weakness patients are at risk of being disabled. Our results also suggest that a pronounced varus position is associated with reduced functional ability, independent from the influence of muscle strength. Therefore, in the presence of a pronounced varus position of the knee, patients are at risk of developing reduced functional ability. These results are in agreement with the study of Chang et al.8
The results for VV-ROM are different from the results for mid-stance VVP. The differences in findings between VV-ROM and mid-stance VVP may be explained by the different phases of the gait cycle in which the data were collected. Forces at the knee are the highest in the first phase of the gait cycle (ie, loading-response phase).26–28 During the loading-response phase, the knee is flexed.7 With the knee in flexion, forces at the knee are primarily absorbed by muscle actions.26–28 In mid-stance the knee is extended.29 With the knee in extension, forces at the knee are primarily absorbed by the passive restraint of the knee and not by high muscle action.7 The difference in knee position (ie, flexed or extended) may explain the differences in findings between VV-ROM and mid-stance VVP.
Mid-stance VVP was used according to Chang et al.8 In that study mid-stance was chosen to assess the varus position (ie, thrust) of the knee. In mid-stance full body weight is on one leg and at that moment of the gait cycle the knee is most vulnerable to malalignment.8 Mid-stance VVP was established relative to the patients’ posture at rest, rather than relative to position of neutral alignment. This might explain the absence of a correlation between mid-stance VVP and skeletal alignment.
It should be noted that the analysis showed a high variance in mid-stance VVP between left and right knees within patients. To take into account the high variance between knees within patients, the difference between left and right mid-stance VVP was included in regression analyses. This difference was significantly related to functional ability, indicating that walking ability is more limited in patients with pronounced asymmetric varus–valgus knees than in patients with symmetric knees. Varus knees and asymmetric knees may lead to a greater demand on compensating mechanisms in the knee joint stabilisation process, which may ultimately lead to a stronger reduction in functional ability.
It has been stated that an adequate gait pattern contains little or no movement of the knee in the frontal plane due to sufficient passive restraint of the knee.7 12 The passive restraint of the knee is measured as the laxity of the knee joint. Knee joint varus–valgus laxity is measured statically in an unloaded situation,4 5 whereas varus–valgus motion was measured dynamically in a loaded situation. No relationship was found between joint laxity and varus–valgus motion (results not presented). Previously, we have found that joint laxity affects functional ability.19 Our present findings suggest that varus–valgus motion and joint laxity independently affect functional ability.
This study has strengths and limitations. We assessed functional ability with both a questionnaire and performance-measures. We controlled for the dependency of left and right knee data within patients by using multilevel analysis. A limitation of our study is the lack of measuring compensating mechanisms responsible for maintaining walking ability, such as muscle co-contractions,10 trunk movements,7 29 movements of the hip and ankle,30 reduced walking speed29 31–34 and the compensating movements of the knee in the sagittal (flexion-extension) and transversal (internal and external rotation) plane.35 These compensating mechanisms were not taken into consideration. Future research could examine the effect of different compensating mechanisms, particularly the effect of walking speed on varus–valgus motion of the OA knee in relation to functional ability.
Our results may have implications for exercise therapy directed toward increasing muscle strength to improve functional ability. The presence of high VV-ROM may influence the efficiency of the muscles in the loading response phase. Therefore, exercise therapy may entail specific exercises with the aim of reducing high VV-ROM of the OA knee. Specific treatments that address both muscle strength and the reduction of VV-ROM should be developed and tested because they may improve functional ability. To reduce the high varus position of the knee in mid-stance a different strategy should be considered. It is speculated that a change in varus position by a lateral wedged insole may influence the relationship with functional ability.36 Research on the effect of these strategies is warranted and requires further investigation.
In conclusion, in patients with knee OA the (bivariate) relationship between varus–valgus motion and functional ability was absent or weak. In patients with knee OA with a high VV-ROM, muscle weakness has a stronger impact on functional ability than in patients with low varus–valgus range of motion. Furthermore, patients with knee OA with more pronounced varus knees during walking show a stronger reduction in functional ability than patients with less pronounced varus or with valgus knees.
We thank K Fiedler and T Baanders for expert assistance in the preparation of the manuscript.
Competing interests: None.