Triaxial Electrogoniometer for Knee Motion
R. Mulligan, N. Alexiades, C. Navickas, J. C. Cao Yao
Dept. of Biomedical Engineering, Chemistry and Biological Sciences
Stevens Institute of Technology
Hoboken, New Jersey 07030
​ Abstract— There are approximately 1,000,000 total knee
replacements conducted annually in the United States. The
electrogoniometer provides a unique alternative to current
postoperative kinematic analysis techniques through a
wearable device that dynamically measures the rotation of the
knee in three dimensions during walking. This will replace
plastic goniometers that measure ROM statically, and also
reduce the need for expensive, off-site gait analysis per patient
post-operative rehabilitation, saving time, energy, and money.
I.
INTRODUCTION
Knee pain is one of the most prominent causes of
discomfort in an aging population. Total knee replacement
surgeries cost about $30,000 and the rehabilitation process
takes at least 6 weeks and costs about $1500, depending on
the physical therapy program and insurance coverage [1]. If
complications occur during the recovery process, a revision
surgery would be required to remedy any problems adding
to the cost of treatment.
Currently, the gold standard device for easily and
cheaply measuring the range of motion (ROM) of the knee
is a plastic goniometer. This device measures static range of
motion measurements while the patient lays in supine and
prone positions. A major pitfall of this device is its inability
to account for the changes in kinematics due to the kinetics
of gait. If additional clinical information is required, the
gold standard for accurate and in-depth analysis of the joint
is through gait analysis using motion capture technology.
While this method is accurate, it is expensive for the patient,
costing upwards of $200 per visit, and requires complex and
expensive hardware and software to calculate the desired
measurements [1]. The innovative design of KNE3-D
allows for intra-office monitoring of knee kinematic
measurements that are accurate and clinically relevant. The
device is easy to use, requires minimal training, and
produces immediate results.
II.
MATERIALS AND METHODS
A. Design Parameters
Due to the increasing need for knee replacements, the
device will be essential for collecting normal walking data
from the patient allowing for an in depth comparison
between pre and postoperative knee joint kinematics. The
use of the device The normal ROM for the knee is 0° to
140° in flexion/extension, -6° to 6° in varus/valgus, and -10°
to 30° in internal/external rotation. To accommodate these
ranges of motion the device is capable of rotating 180° in
each plane. The rotary potentiometers are capable of
accurately measuring a range of 270°.
B. Prototype Design
The
electrogoniometer
utilizes
three
rotary
potentiometers as depicted in green in Fig. 1. Each
potentiometer will measure the rotations in the three axes of
flexion/extension, varus/valgus, and internal/external
rotation to determine the rotations about the knee center.
Fig. 1. Axial Alignment of Potentiometers
C. Hardware
In order to determine the angular displacement from the
change in resistance of the potentiometers, a wheatstone
bridge will be constructed. The voltage change across the
wheatstone bridge will be measured using a data acquisition
system (DATAQ, DATAQ Instruments, Inc., Akron, OH)
that connects to a computer where software (WinDAQ,
DATAQ Instruments, Inc., Akron, OH) converts the voltage
output to the respective angle measurements using prior
calibration parameters.
clinician
may
observe
flexion-extension
while
simultaneously observing internal/external rotation and
varus/valgus angles.
D. Device Fixation
The patient will have the device secured to their leg by
straps for the superior and inferior components. The inferior
portion is secured to the patient using velcro strapping
around the lower leg. The goniometer is then aligned with
the knee and secured in position by strapping the superior
component to the thigh. The potentiometers will be aligned
prior to fixation to set an initial reading of zero from all
sensors.
III.
RESULTS
A. Anticipated Results
The wheatstone bridges will output a voltage reading
which will be converted to an angle through a calibration
curve. Using these values, a plot of angle versus time will
be created in order to provide kinematic data. The
flexion/extension plot will show a curve between 0° and
60°, and the other two plots will show a curve with the same
period (gait cycle) however with much lower extremes
around ​+​10°, the typical values for normal gait. These
graphs will be useful in diagnosing knee problems and also
keeping track of recovery in a more complete method than
traditional goniometers (expected waveforms shown in Fig.
2).
IV.
DISCUSSION
The triaxial electrogoniometer can be worn during
normal walking motion and allow for the physician to
monitor the knee kinematics in real time under normal
loading. This provides an advantage for therapists who can
subsequently tailor customized rehabilitation programs
based on kinematic functional data. Furthermore,
rehabilitation regimens can be adjusted according to patient
progress. With respect to postoperative implantation,
another advantage of the device is that is could aid in early
detection of implant loosening as evidenced by altered
kinematic patterns, Conclusion
KNE3-D is a low-cost inter-office electronic load
bearing kinematic knee goniometer providing 3D range of
motion during nonoperative or postoperative rehabilitation.
The device is targeted for therapeutic caregivers, insurance
companies, and athletic trainers of sports teams to further
optimize rehabilitation time.
ACKNOWLEDGEMENT
We would like to offer our most sincere appreciation to
our advisors, Antonio Valdevit, Ph.D. and Dr. John Capo,
M.D., for their assistance, guidance, and encouragement
throughout this project.
REFERENCES
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Fig. 2: Typical knee angles during gait cycle
The design possesses advantages over the traditional
goniometric technique. The data provided will be dynamic,
load-bearing, and accessible in-office for the physician.
Traditional methods do not record load bearing or dynamic
data, and the use of such data is limited. Traditional plastic
goniometers are accurate, but can be adversely affected by
poor user alignment upon the knee. Traditional goniometers
are affected by the fact that they are static and, only measure
one kinematic plane. The KNE3-D goniometer will provide
dynamic data under load bearing conditions ​and output
graphically the angles on a computer screen where the
[1]
BlueCross Blue Shield, “Cost Estimator” 25 Jan. 2017.
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Chao, E. Y. (1980). Justification of triaxial goniometer for
the measurement of joint rotation. ​Journal of Biomechanics, ​13(12),
989-1006. doi:10.1016/0021-9290(80)90044-5
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Lavernia, Carlos, MD, et al. "The Increasing Financial
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The-Increasing-Financial-Burden-of-Knee-Revision-in-the-United-St
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