Fundamental Biomechanical Principles
in the Orthotic Treatment
of the Knee
André BÄHLER
Introduction
Fitting a knee orthosis is particularly challenging because of the complexity of the
joint's coordinated movement. The knee
joint possesses an extremely complicated anatomical structure. For a long time, the function of the individual parts was unclear, perhaps because the knee joint differs in its basic mechanical structure from other joints,
such as the hip and shoulder.
To understand the knee joint, the complicated mechanics must be reduced to their
simplest forms. The working and contact areas of the knee's two joint surfaces are relatively small and are connected to one another by muscles, ligaments and other structures
that influence movement. The peculiar form
of the two differently shaped femoral condyles also affects movement.
A further distinction of the knee joint is its
changing rotational freedom. On the one
hand, the joint is forcibly guided in the last
part of the extension movement, yet during
flexion, it retains rotational freedom to a
greater or lesser degree. In fitting a knee
orthosis, we are mainly interested in flexion/
extension and rotation.
Flexion/Extension
The femur can be compared mechanically to
a lever in which the center of gravity is raised
as the position is changed, thereby stabilizing the body as weight is placed on the leg.
When fitting an orthosis, it is essential to
analyze the movement of the femoral shaft
and the upper-leg (as the arm of the lever).
During flexion, the dorsal form of the condyle produces a corresponding rise in the
femoral shaft, lengthening the leg (see Figure 1
).
The asymmetrically curved joint surfaces
of the femoral condyles can also be compared to ellipses, but there are differences
between the condyles of the tibia and femur.
Since the form of the tibial plateau's working
surface only partially corresponds to that of
the femoral condyle, movement is complicated. The various radii of the two femoral
condyles cause the femur to rotate externally
during flexion and internally during extension. It is generally held that it was the Weber brothers who, in 1836, described the
flexion/extension of the knee joint as a combination of swivel, glide and roll movement.
According to Menschik, Kapandji and
Mueller, recordings of the point of contact
between the femoral condyle and tibial plateau show the ratio of roll and glide changes
according to the amount of flexion. According to Mueller, the ratio of roll and glide is
1:2 at the beginning of flexion and 1:4 toward the end. This roll and glide movement
can be compared to a twisted four-jointed
chain.
Mueller convincingly described this movement as follows: When a four-bar linkage is
twisted so that two of the opposite sides
cross, a shape known as a crossed four-bar
linkage is obtained (see Figure 2
and Figure 3
). If
the two arms, A/C and B/D, are replaced by
cruciate ligaments, which are, figuratively
speaking, rigid connections, the result is the
arrangement of the knee's movement system
(see Figure 4
).
Knee movement can be traced using a
model of this four-bar linkage. This movement serves as the basis for judging the mechanical joint construction of various orthotic devices. (The transverse rotation that occurs at the end of extension or at the beginning of flexion will be disregarded for the
moment.)
The Axis of the Knee Joint
When considering the femoral condyles, it is
necessary to distinguish between a larger and
smaller curvature. If the axis, or turning
point, of the knee is set in the center of the
larger curvature, then the axis experiences
significant positional change, or displacement, during flexion. If, on the other hand,
the axis is set in the center of the smaller
curvature, axis displacement is smaller. The
result is a half-moon-shaped curve.
When fitting a knee orthosis, the aim is to
find an axis of rotation that does not change
its position for the greatest possible part of
the flexion/extension range. How to find this
point can be best explained by using a wheel
as an example. Consider an imaginary point
on the wheel other than the axis (see Figure
5
). As the wheel turns, this point rises on one
side of the axis and sinks on the other. If the
wheel now rolls up an inclined plane, set to
rise the same amount as the point sinks, then
the point remains at a constant height for a
certain part of the motion.
If this principle is transferred to the elliptically shaped femur, experiments show a center of rotation remains practically unchanged up to almost 90 degrees, i.e., it
scarcely rises or falls, it only moves backward.
Experiments performed more than 20
years ago while constructing a knee endoprosthesis resulted in a compromise axis for
the knee, which, from the front to the back,
was divided by a ratio of 6:4 in relation to the
sagittal diameter at patellar level. When fitting an orthosis, the average height above
the knee joint space was set at 25 mm.
In 1975, Prof. Manfred Nietert published
the "Untersuchungen zur Kinematik des
menschlichen Kniegelenkes im Hinblick auf
ihre Approximation in der Prothetik."
These studies show it is relatively easy to find
the compromise axis (see Figure 6
). In contrast to our tests, Prof. Nietert set the axis at
19.7mm ± 3mm above the knee space, approximately 5 mm lower than we did. Follow-up studies have shown his conclusions to
be correct.
On X-ray pictures, the compromise axis
lies in the middle of the intercondylar notch,
approximately at the proximal insertion of
the posterior cruciate ligament. The motion
of the femoral condyles over the tibial plateau can be seen with the help of X-rays. As
could be seen with the four-bar linkage, the
femoral condyles glide over the tibial plateau
posteriorly. If the movement is followed using the assumed axis, it can be seen from the
X-rays that the axis moves down toward the
back-a fact that poses mechanical problems
if this movement is to be reproduced exactly.
It can be seen that the movement of the knee
can be copied only by a mechanical joint that
reproduces the positional change described
by the femoral condyle over the tibial plateau.
Analysis and Comparison of Joints
Eight styles of joint construction are available. They are
- a child's joint
- a joint made of V2A alloy
- Otto Bock's joint with a 16-mm backward displacement of the axis
- Otto Bock's joint with a 22-mm backward displacement of the axis
- a dual-axis joint
- a four-bar linkage joint (our construction)
- a physiological joint (commercially
available)
- a four-bar linkage joint (commercially
available)
Although the results are of a theoretical
nature and can hardly be demonstrated with
such precision in practice, they are still significant.
The orthotic joints were judged on the
following criteria:
- Position and positional change of the
axis during flexion and extension.
- Concurrence or divergence between
the longitudinal axis of the upper and
lower leg and the direction of the orthotic
joint.
- Measurement of the difference in length
between the upper leg and the orthotic joint
in flexion and extension.
The following aids were used for evaluation: X-rays of the knee joint taken with the
aid of a mechanical system to secure position, plaster models and practical trials with
a tightly fitting knee device that was fitted
with various orthotic joints.
To better understand movement, orthotic
joints were used, but the movement can be
transferred without limitation to all knee
orthoses. The upper and lower parts of the
orthotic joints were positioned on the X-ray
so the orthotic joint produced the maximum
congruence between the zero, or extended
position, and the knee joint's flexed position
of 90 degrees. This allowed deviations in the
movement between the orthotic joint and
the knee to be kept to a minimum (see Figure
7
). This method was chosen because, when
fitting an orthosis, attention is focused not
on the specific actions of the joint, but rather
on the resulting behavior of the thigh and
leg.
At the beginning of the tests, it became
immediately clear that it was impossible to
work from the fixed assumed compromise
axis by moving it forward or down according
to the orthotic joint's backward displacement. The differences between the movement of the orthotic joint and the knee joint
were too great. However, it became apparent that the more the mechanical joint axis
was moved backward, the lower the axis had
to be set to keep the arm of the joint parallel
to the thigh from 0 to 90 degrees.
This parallel displacement can best be
seen in Figure 8
and Figure 9
. The backward displacement of the knee axis causes the upper
part of the orthotic joint to create a longer,
slightly different path compared to the hinge
joint. At the same time, a parallel upward
displacement of the upper part of the orthotic joint occurs. The amount of upward displacement is equal to the backward displacement of the axis, calculated from the center
point of a normal hinge-joint.
Results of Evaluation I
(Figures 10 & 11
, Figures 12 & 13
, Figures 14 & 15
, Figures 16 & 17
)
Positioning the Axis
The results show that the axes of the orthotic
joints must be moved downward about the
same amount that the orthotic joint axis is
displaced posteriorally. One exception to
this is the dual-axis joint, which achieves optimal harmony with the thigh if the axis is
displaced slightly forward. A precise joint
axis could not be determined for those joints
with changing centers of rotation.
Parallel Displacement
A comparison of the parallel displacement of
the various joints shows that single-axis
joints with a 16- or 22-mm backward displacement of the axis were better. The best
orthotic joints were those with a backward
displacement of 22 mm and dual-axis joints.
Joints 7 and 8, or the physiological joints,
received the lowest marks because they exhibit parallel displacement at 90 degrees
which, despite various positional changes,
cannot be eliminated.
The orthotic joints with a backward displacement of 16 mm and the dual-axis joints
take first place in shortening or lengthening
the joint between 0 and 90 degrees flexion.
Again, joint no. 7 with the changing centers
of rotation performed poorly, and no. 8 cannot compete with the best.
Joint no. 4, with a backward displacement
of 22 mm, is not very satisfactory with regards to shortening, as the shortening at 90
degrees is already 16 mm, although it is reduced to 12 mm in the extreme position. It
can be assumed, however, that the parallel
displacement is of greater importance than
the lengthening/shortening since it causes incongruence between the orthotic joint and
the knee. Lengthening/shortening has a
pumping effect on the orthotic joint, thus
causing it to slip. However, tests show that in
practice, a shortening/lengthening of approximately 10 mm can be ignored.
Incongruence Between the Thigh
and the Joint
If orthotic joints were fitted according to the
compromise-axis, then most joints would exhibit an incongruence between the thigh and
the joint. Experimental positioning showed
that no congruence could be achieved in
joints with changing centers of rotation.
These experiments demonstrated that the
following demands must be met if a mechanical joint is to reproduce the movement of the
knee:
- During flexion to 90 degrees, the upper
part of the joint must move upward to the
same degree as the femur moves upward
over the elliptical form of the condyles.
- After 90 degrees flexion, the down and
backward displacement of the femoral condyle over the tibial plateau must be complete.
- The joint construction design must
lengthen during flexion.
Each knee orthosis exhibits its own specific behavior, determined by joint construction (design).
Evaluation II
The number of joints studied in the first evaluation was limited. Therefore, after the results of the evaluation were published, new
joints were submitted for testing. The second evaluation did not lead to any significant new findings, but it was possible to enlarge the list of available knee joints. It is
imperative to point out that the construction
of the knee joint itself represents only one
aspect of the fitting of the orthosis, a fact that
is underestimated at present.
The influence of the muscle forces on the
knee are great and very difficult to describe
or quantify. The individual morphological
differences are also great and exert a strong
influence on treatment.
Findings
To summarize evaluations I and II: There
are four mechanical orthotic joints that are
capable of fulfilling the above-mentioned
demands:
- Single-axis joints with posteriorly displaced axes, approximately 16 mm, for average-sized patients. The axis of the joint relative to the compromise axis must be set correspondingly lower.
- Dual-axis joints. The distance between
the two parts of the joint at flexion corresponds to the specific form of the distal femur. This type of joint could be substantially
improved by setting the upper part of the
joint approximately 7.8 mm further forward.
(Distance of femoral elevation at flexion-
see appropriate reference in text.)
- Joints with four-bar linkage only function when the relative positions of the axes
correspond to the model of the cruciate ligaments. Each deviation from this causes a
change in the condition of movement, resulting in a lack of correspondence to actual
knee movement.
- Physiological joints or joints with changing axes of rotation. Although this joint performed poorly in tests, the results are based
only on those joints available to us. If the
construction is selected such that the resultant behavior of the movement really corresponds to the kinematics of the knee, then
there can be no objections to its use. However, in practice, our tests show this is difficult
to achieve.
With the mechanical joints available at
present, the knee joint is well-provided for
from a technical-orthopaedic point of view,
providing the respective mechanical joints
are used correctly. The actual individual fitting of the device is at least as difficult as
imitating the knee joint with a mechanical
joint and demands equally high standards.
Orthosis Construction
Having seen, compared and judged the
joints, this report would be incomplete if
orthosis construction were not given equal
consideration. The influence of a joint on a
knee orthosis, which is securely fixed to the
thigh and leg, is far greater than that of an
identical joint on a device with elastic bands
that only partly surround the leg.
Due to its bony structure, the anterior part
of the shank has a strongly defined shape,
whereas the posterior portion is muscular.
This difference in tissue structure can give
rise to a certain amount of play between the
device and the lower-leg. Because the thigh
is muscular on all sides, this makes it difficult
to find an exact point of reference for fixing
the device since only the medial and lateral
femoral-condyles are fixed points.
Practical triaals have shown that joint construction and support must form a harmonious unit so movement of the mechanical
joint can be transferred to the leg to control
and support knee movement.
Automatic Axial Rotation
So far, the discussion has centered on flexion
and extension, but active and automatic axial rotation pose further problems for the
technical-orthopaedic treatment of the joint.
According to Lanzwachsmut, the active rotation reaches 40 degrees external rotation
and 30 degrees internal rotation, depending
on position.
At automatic axial rotation, which has
been accurately described by Menschik and
Mueller, during the last 20 degrees of extension, the shank rotates approximately 15 degrees outward relative to the thigh, resulting
in an abduction of the shank relative to the
thigh. In other words, an increased knock-kneed position results. Observations of this
phenomenon show this abduction differs
among patients and is most evident in slim
people.
This automatic axial rotation poses problems when fitting a knee orthosis. If the
knee's movement is guided exactly, then the
abduction that occurs naturally without the
device is opposed and suppressed. Therefore, a well-fitting comprehensive orthosis
can exert an undesired pull on the anterior
cruciate ligament. The pseudo-joint movement between the knee and orthosis causes
the device to slip during automatic axial rotation. Often, this cannot be avoided, despite a
good fit.
After syndesmopexy (knee surgery), particular care should be taken in the choice of
construction. At the Wilhelm Schulthess
Clinic in Zurich, there is a preference after
surgery for devices that prevent extreme
knee movement and give additional support
and stability to the knee, without limiting its
finer movements. These functional demands
on the orthosis can be met in the choice of
the construction and materials.
Aims Of and Demands On Orthoses
The range of indications for orthotic devices
is wide. It would be beyond the framework
of this study to attempt to examine all possibilities. For practical purposes, the devices
can be divided into three categories:
- Prophylactic or support devices that
support the injured knee or weak ligaments.
- Rehabilitation devices used after surgery.
- Functional devices that stabilize and
support the deformed joint.
Since many devices are similar in operation, categorizing and using devices according to diagnosis and aims is logical, but this
practice is insufficient. The relationship between the indication and the function of the
device is very important. One must determine what is required: A device that guides
the movement as closely as possible, thereby
limiting any free play of the joint? Or should
the device stop all extreme movements such
as hyperextension, flexion or lateral deviation? Should the device give the knee stability and support without fundamentally restricting movement? Setting precise objectives makes it possible to construct the device so all demands are met.
A ready-made orthosis can never attain
the fit and stability of a device made according to definite principles and individually fitted. In making a plaster model, we benefit
most from the rules of prosthetics. The evaluations here have been made in an attempt
to show and weigh the influence of various
mechanical joint constructions available on
knee movement.
Summary
Some guiding principles in the fitting of a
knee orthosis are:
- The aims of the technical-orthopaedic
treatment of the knee must be clear to determine device function.
- The same function can be achieved by
different devices.
- Only a joint that reproduces the movement of the femur relative to the tibia-i.e.,
where at 90 degrees the actual axis of the
joint moves upward and then backward and
down-deserves the name physiological
joint.
- A single-axis joint with sufficient backward displacement or a dual-axis joint can
be used for most orthotic corrections closed
by elastic bands, without detrimental effect
on the joint. Joints with four-bar linkage
and physiological joints offer optimal treatment if they are truly capable of reproducing the kinematics of the patient's knee
joint.
- The greater the play between the thigh
and leg cuffs, the less important the joint
construction.
- The automatic axial rotation tends to be
limited by a tightly fitting cuff. This exerts
extra pressure on various ligaments, e.g., the
anterior cruciate ligament.
"Fundamental Biomechanical Principles in the Orthotic Treatment of the Knee" originally appeared in the February 1989 Orthopädie-Technik. It is reprinted with permission of the publisher.
ANDRé BÄHLER is with Orthopädie Bähler,
Rehabilitations und orthopädische Hilfsmittel,
Kreuzstrasse 4-6, 8008 Zü;rich, Switzerland.
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