Development of a New Method to Determine Knee Varus/Valgus
Jonathan Singer, B.Sc. and D. Gordon E. Robertson, Ph.D., FSCB
School of Human Kinetics, University of Ottawa, Ontario, Canada
Introduction
Excessive knee varus/valgus is known to alter joint loading
patterns, placing unusual stresses on the articular surfaces,
joint capsule and ligaments. Such stresses can be the result
of either a chronic condition, such as genu varum or valgum
(Kerrigan et al., 2002), or an acute injury, which may occur
during physical activity (Besier et al., 2001).
Three-dimensional videography and inverse dynamics
have been extensively used to quantify the varus/valgus
moment at the knee. Such analyses can often be time
consuming for the clinician, who must assess many
patients, and difficult for the researcher, who may wish to
set up equipment to collect data in remote locations.
Methods
Data from two subjects, performing a number of walking and
crossover cutting trials, were collected with the participant
wearing the instrumented brace. To ensure consistency
among the cutting trials, the angle of cut was monitored,
effectively falling between 30 and 60 degrees from the original
direction of motion. Force platform data, using one Kistler
force platform, were collected simultaneously with lateralsupport strain gauge data for the stance phase of the left leg
during one stride. All data were sampled at 240 Hz for the
duration of stance and swing phases.
Purpose
To develop a new method to determine the external knee
varus/valgus moment. The proposed method employs strain
gauges configured to measure the bending moment applied
to the lateral support of a knee brace.
Methods
Four strain gauges–two active and two Poisson–were wired
in a Wheatstone bridge and applied to the distal aspect of
the lateral support of a functional knee brace, such that the
bending moment applied to the support could be measured.
To determine the sensitivity of the lateral support, the most
distal aspect was securely clamped in a vice and a range of
known forces were applied to the support at a point just
below the hinge. Applied moment of force values were
correlated with the lateral support gauge output voltages.
The sensitivity of the lateral support gauges, in both varus
and valgus directions, was determined by the slope of the
least squares regression line.
Figure 1. External bending moment of force applied to the
lateral support (top) and corresponding ground reaction forces
(bottom) during walking (left) and crossover cutting (right).
Results and Discussion
Peak varus moment of force values for the walking and
crossover cut condition were 0.59 N·m and 0.67 N·m,
respectively. Although the peak varus moment applied to the
brace was larger in the crossover cut condition, it was not
substantially so. This may be due to the relatively low
coefficient of static friction (μ = 0.52) between the shoe sole
and the force platform, barring the participant from sufficiently
planting the foot and performing a vigorous cut. As the
participants were aware of the need to perform a crossover
cut, the small difference between the two applied moments
may also be caused by postural adjustments made in
preparation for the cut, in attempt to protect the knee
structures (Besier et al., 2001). From this, future work entails
comparing applied moments to internal knee moments and
collecting data in vivo to account for concerns due to friction
and anticipatory mechanisms.
References
Allard, P. et al. (1997). Three-dimensional Analysis of Human
Locomotion. Toronto:Wiley.
Besier, T.R. et al. (2001). Med Sci Sport Exer, 33:1176-81.
Kerrigan, D.C. et al. (2002). Arch Phys Med Rehab, 83:889-93.
Figure 2. Instrumented lateral support.
Biomechanics Laboratory
Results and Discussion
Figure 1 illustrates the moment of force pattern that was
applied to the brace during one stride, beginning and ending
at heel-strike. It is interesting to note that in the walking
condition, the moment applied to the brace lacks the initial
peak that has been reported to occur within the initial 15-30%
of the stance phase (Allard et al., 1997). This may be due to
the fact that the internal valgus moment of force effectively
controls the varus motion of the leg during weight
acceptance, resulting in a smaller applied moment to the
brace. The second varus peak, occurring just prior to toe-off,
corresponds with the internal valgus moment that typically
occurs during 60-85% of stance (Allard et al., 1997). The
moment applied to the brace in the crossover cut appears to
be representative of that condition, whereby a greater
moment is applied over the duration of stance.