Values show the magnitudes at midstance (50% of stance) as well as the magnitude and timing of the overall peak values for each variable. KAM = knee adduction moment.
Varus thrust in medial knee osteoarthritis: Quantification and effects of different gait-related interventions using a single case study
Article first published online: 28 JAN 2011
Copyright © 2011 by the American College of Rheumatology
Arthritis Care & Research
Volume 63, Issue 2, pages 293–297, February 2011
How to Cite
Hunt, M. A., Schache, A. G., Hinman, R. S. and Crossley, K. M. (2011), Varus thrust in medial knee osteoarthritis: Quantification and effects of different gait-related interventions using a single case study. Arthritis Care Res, 63: 293–297. doi: 10.1002/acr.20341
- Issue published online: 28 JAN 2011
- Article first published online: 28 JAN 2011
- Accepted manuscript online: 7 SEP 2010 01:08PM EST
- Manuscript Accepted: 24 AUG 2010
- Manuscript Received: 29 JUN 2010
Osteoarthritis (OA) is a prevalent chronic condition commonly affecting the medial knee joint compartment, and is associated with symptoms of pain and physical dysfunction. Given its high prevalence in weight-bearing joints, cartilage breakdown is generally believed to result from excessive and/or unbalanced joint loading. Knee joint loading during walking is usually quantified via the external knee adduction moment (KAM), a surrogate noninvasive indicator of medial compartment load (1).
Medial compartment knee OA is typically accompanied by varus joint malalignment, which often worsens over time due to progressive cartilage loss in the medial tibiofemoral joint. When combined with ligamentous laxity and/or neuromuscular insufficiency, some patients with varus malalignment demonstrate a varus thrust, i.e., an abrupt lateral displacement of the knee with respect to the hip and ankle, during the early stance phase of walking (2). A varus thrust is a potent risk factor for disease progression at the medial tibiofemoral joint (2) and hence, from a clinical perspective, developing interventions that can reduce varus thrust is important. However, one of the challenges associated with a varus thrust is its accurate identification and quantification. The purpose of this case report was 2-fold: first, to describe the kinematic and kinetic characteristics of a varus thrust during walking in an individual with medial compartment knee OA and varus malalignment; and second, to evaluate the immediate biomechanical effects of a variety of gait-related interventions aimed at minimizing the magnitude of a varus thrust.
A 64-year-old woman (height 1.58 meters, weight 59.3 kg) reported to a private physiotherapy clinic with longstanding bilateral medial knee pain. She reported increasing difficulty with weight-bearing tasks such as walking, especially on uneven surfaces. Previous treatment for a foot condition included the provision of custom-made orthotic devices, which incorporated medial rearfoot posting and medial arch support. Radiographs confirmed the presence of OA in both knees. There was marked medial joint space narrowing in her right knee with moderately sized tibial osteophytes both medially and laterally. Her left knee also exhibited definitive medial joint space narrowing with a moderately sized osteophyte on the medial aspect of the tibia. Lower leg alignment was measured bilaterally on the short-film radiographs using published methods and equations (3), and was determined to be 4° varus for the right leg and 2° varus for the left leg.
She presented with a stable left knee upon examination of medial–lateral laxity, although some lateral joint line tenderness was reported. In contrast, ligamentous stability tests indicated significant varus laxity in her right knee and there were reports of joint line tenderness across the entire tibiofemoral joint (medial greater than lateral). All other ligament and special tests were unremarkable. Visual observation of her gait revealed a noticeable lateral displacement of her right knee soon after initial foot contact, indicative of a varus thrust.
The patient reported a reduction of symptoms with a physiotherapy treatment focusing on thigh muscle strengthening and enhancing functional recovery. However, the subjective sensation of instability persisted, and no reductions in varus thrust were observed visually within the clinic using a variety of interventions (insoles and gait retraining). Therefore, further quantitative gait analysis using more sensitive methodologies was warranted.
Quantitative gait analysis took place in the biomechanics laboratory, Department of Mechanical Engineering, The University of Melbourne. Kinematic data were captured using a motion analysis system (Vicon, Oxford Metrics) with 9 cameras sampling at a rate of 120 Hz. Ground reaction force data were collected using 3 force plates (Advanced Mechanical Technology) sampling at a rate of 1,080 Hz. Reflective markers (14-mm diameter) were attached at specific locations on the patient's trunk, pelvis, and both lower extremities. An initial static trial was performed to calibrate relevant anatomic landmarks and establish joint centers. The hip joint center was defined as per Harrington et al (4), while the orientation of the knee flexion–extension axis was determined using a dynamic optimization approach (5). The anatomic reference frame for the trunk was based upon that proposed by Nguyen and Baker (6). The anatomic reference frames for the pelvis and lower extremities were as per Schache and Baker (7).
The patient wore standard sandals and loose-fitting clothing for testing. An initial 5 trials of normal walking at her self-selected speed were conducted and served as baseline data for comparison against intervention trials. The patient then performed a series of walking trials with the following gait-related interventions: 1) a self-selected increased amount of lateral trunk lean over the stance leg, 2) a self-selected increased amount of toeing out of the stance leg foot, 3) the insertion of a lateral shoe wedge (full-length, approximately 5° wedge made of high-density ethyl-vinyl acetate) into both sandals, and 4) the insertion of the patient's own prescribed and custom-made orthotics (described above) into both sandals. All of the trials for a given intervention were completed in the same block (5 clean force platform contacts), but the order of the interventions was randomized. A limited number of practice trials were provided for each intervention, and verbal feedback was only provided to ensure that the patient was able to perform the interventions and that walking speed was similar between trials. After each intervention, the patient used an 11-point numerical rating scale to rate her knee pain (where 0 = no pain and 10 = maximal pain) and the difficulty experienced in performing the intervention (where 0 = no difficulty and 10 = maximal difficulty/unable to perform). Postprocessing of gait data involved the calculation of knee kinematics (specifically, the varus/valgus angle) and kinetics (specifically, the KAM) (7) as well as the magnitude of the frontal plane component of the perpendicular distance (moment arm) between the ground reaction force and the knee joint center (8) for each trial. The magnitude and timing of peak values for each variable were identified, as were the values at the midpoint (50%) of the stance phase.
Dynamic frontal plane alignment curves for both extremities during normal walking are shown in Figure 1A, with discrete values found in Table 1. Knee adduction in the right leg throughout the midstance phase (40–60% of stance) of normal walking trials was relatively constant at approximately 10° of varus. In contrast, early stance was characterized by a definitive increase in knee adduction angle peaking at 14° at 20% of stance, which is consistent with the visual observance of a varus thrust. Dynamic knee adduction in the left leg was approximately 8° throughout most of the stance phase and lacked any appreciable increase during early stance that was evident in the right leg.
|Baseline gait||Modified gait|
|Normal (L)||Normal (R)||Toe out (R)||Trunk lean (R)||Orthotics (R)||Lateral wedges (R)|
|Varus angle, degrees|
|Time at peak, % stance||23||20||21||19||21||21|
|Time at peak, % stance||34||22||22||19||22||22|
|Moment arm, mm|
|Time at peak, % stance||36||43||23||23||37||26|
Knee adduction angle curves from increased trunk lean and toe-out as well as the shoe insert trials are shown in Figures 1B and C, respectively, with discrete data found in Table 1. Performance of gait modification trials involving increased lateral trunk lean or toe-out angles was quite variable; therefore, only data from the single “best” trial (defined as the largest modification from normal walking) were analyzed. While the patient exhibited self-selected toe-out and lateral trunk lean during normal walking trials of 0.5° and 2°, respectively, these were increased to a maximum of 10° of toe-out and 6° of lateral trunk lean during the respective modification trials. The varus thrust observed and quantified in the right leg during normal walking trials was evident across all gait modification and shoe insert trials. A substantial reduction in dynamic adduction angle at the knee was observed only in the trial employing increased lateral trunk lean with a peak value of 8.9°, representing a 38% reduction in knee adduction in the right leg, and approaching the amount of knee adduction of the contralateral leg (8.3°).
Kinetic data are also summarized in Table 1. When walking normally, the overall peak KAM was larger in the right knee (0.81 Nm/kg) compared to the left (0.69 Nm/kg). Despite subtle differences in magnitude throughout stance, neither the addition of lateral wedges nor orthotics changed the KAM appreciably. In contrast, walking with an increased toe-out angle reduced the KAM at 50% of stance, while walking with increased lateral trunk lean reduced the KAM and ground reaction force moment arm throughout stance. These findings are consistent with previous studies investigating the effects of these gait modifications on KAM magnitudes (9, 10). We also found that the KAM and peak adduction angle for the right knee coincided in time (approximately 20% of stance) for all walking conditions, while the peak KAM occurred later in stance (34%) than the peak adduction angle (23%) for the left knee during normal walking. These findings would suggest that the KAM in the right leg was closely related to the magnitude and timing of the varus thrust.
The interventions had a beneficial effect on right knee pain. Pain was rated 4/10 during the initial normal walking trials, which remained constant during the toe-out trials, but decreased to 3/10 with exaggerated lateral trunk lean and 2/10 with the insertion of both the lateral wedges and orthotics. In contrast, the patient reported no difficulty with normal walking or walking with the insertion of her custom-made orthotics (0/10), but some difficulty with lateral wedges (1/10) and trunk lean (2/10) and moderate difficulty with the toe-out walking trials (5/10).
Results from this case study show that a visually observed varus thrust during walking can be quantified using noninvasive 3-dimensional motion analysis techniques, and that the magnitude of such a phenomenon can potentially be reduced using simple gait-related modifications. Although these results are only from a single subject, they lend support to the use of specific gait-retraining strategies in the treatment of knee OA, particularly those with a suspected varus thrust.
The presence of a varus thrust has received little attention in the OA literature, but its importance has been shown in a study by Chang et al (2), who reported that individuals with a varus thrust (67 of 401 knees) experienced a 4-fold increase in the odds of medial tibiofemoral OA progression over an 18-month period. Despite the importance of the varus thrust, the paucity of literature on this subject is likely due to the difficulty in accurately determining its presence and quantifying it. Indeed, the exact methods by which Chang et al identified the varus thrust were not described in sufficient detail to replicate. Another study (11) used an unidirectional accelerometer placed on the tibial tuberosity to quantify medial and lateral accelerations of the tibia during the loading phase of gait, although they did not report how much acceleration was indicative of a varus thrust.
Using noninvasive 3-dimensional motion analysis, we were able to measure a definitive change in the frontal plane knee kinematics corresponding to a short-term, rapid increase in knee adduction angle occurring at approximately 20% of stance that we suggest is characteristic of the observed varus thrust (Figure 2). Further evidence for this premise is that despite only a 2° difference in static (measured from radiographs) and dynamic (measured during gait analysis and observed through most of stance) lower leg alignment between extremities, our subject exhibited more than 6° greater dynamic varus alignment (knee adduction angle) in the knee determined to have a varus thrust at a time consistent with our visual observations. Therefore, we are confident that we were able to detect and quantify a varus thrust in this patient.
We chose our 4 gait-related interventions based on their hypothesized biomechanical effects on lower leg alignment and knee joint loading. Laterally rotating the foot and lower leg during the stance phase (i.e., toeing out) theoretically makes it more difficult to experience increased knee adduction while maintaining a forward progression. In our maximal toe-out trial, we observed a 2° decrease in peak knee adduction angle, which is consistent with this theory. A large reduction in the knee adduction angle (approximately 6°) when the patient increased their amount of lateral trunk lean was also apparent. Movement of the center of mass toward the stance leg with lateral trunk lean changes the loading environment within the knee such that the moment arm tending to displace the knee laterally becomes smaller. Previous research has shown that lateral wedges cause a small lateral shift in the center of pressure location of the ground reaction force (12). This potentially decreases the knee joint moment arm and thus may reduce the knee adduction moment (8). Conversely, medial arch supports, which are a common feature of most customized orthotics, can shift the center of pressure medially, thereby increasing the adduction moment (13).
Although not a primary aim of this study, we also calculated the KAM to assess the effects of a varus thrust on knee joint load. Our study noted that a larger KAM magnitude was observed in the knee exhibiting the varus thrust. Although this may be partially attributable to the larger amount of static varus malalignment in this leg, the timing of the peak KAM coincided with the varus thrust (at 20% of stance). Implementation of the toe-out gait pattern reduced the KAM in late stance, supporting previous experimental (10) and cross-sectional studies (14). Similarly, the exaggerated trunk lean gait pattern reduced the KAM magnitude throughout stance, which also supports previous studies (9, 15). Consistent with the kinematic data, the addition of the shoe wedges and orthotics appeared to have no influence on the KAM. Although most biomechanical studies demonstrate a significant mean reduction in KAM with lateral wedges, this effect is not consistent across all people with knee OA, and it is possible that one or more features of the sandals used in this study (such as their lack of heel counter) may have mitigated any beneficial effect of lateral wedges on the KAM. There has been no research to date evaluating the biomechanical effects of customized insoles on knee load in people with knee OA; therefore, our finding of a lack of effect is novel and cannot be compared to others.
Our findings are promising and support previous studies examining gait modifications, but the limitations in generalizing data from a single subject cannot be overstated. In particular, although our measurements were consistent with the expected biomechanical characteristics of a suspected varus thrust, we cannot draw firm conclusions on the characteristics of a varus thrust based on these limited data. Second, although the gait modifications, in particular the lateral trunk lean pattern, successfully altered frontal plane knee kinematics and kinetics, the outcomes were variable. This is likely due to difficulties with training the patient, based on her subjective reports as well as the limited time available during our testing session. Methods to optimize the learning process must be identified before these gait-retraining strategies can be recommended for clinical use. Finally, we used laboratory-based instrumentation to quantify the magnitude and presence of a varus thrust. This type of equipment is not easily accessible for most clinicians. However, such methodology enables the assessment of interventions, including those used in the present study, with potential to reduce varus thrust magnitudes and possibly minimize knee OA progression. Given the burden of this disease and the limited number of known effective interventions for slowing disease progression, identification of known risk factors represents an important clinical and research objective. To this end, future studies that aim to reduce the presence and/or magnitude of a varus thrust are warranted.
All authors were involved in drafting the article or revising it critically for important intellectual content, and all authors approved the final version to be submitted for publication. Dr. Hunt had full access to all of the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.
Study conception and design. Hunt, Crossley.
Acquisition of data. Hunt, Schache, Crossley.
Analysis and interpretation of data. Hunt, Schache, Hinman, Crossley.