N.G.A. van Leerdam, MSc, PhD
Ambroise
Enschede, The Netherlands
Introduction
Knee brace slippage is an issue as old knee bracing itself. In its mildest form it’s an annoying
side effect of the orthosis. In a more serious form it can be the cause of rejection or even worse,
the cause of adverse effects like damage to internal structures of the knee. If you use the
keywords ‘knee’, ‘brace’ and ‘slippage’ in Google, you’ll roughly get 24.000 hits. This tells you
that slippage apparently is an issue. The majority of these hits will be from commercial sites
stating that the product involved on that particular site has a solution. Despite the fact that the
majority of brace manufacturers state that slippage is not a problem with their design, provided
that the orthotist fits the orthosis properly and the patient applies the orthosis correctly, clinical
experience shows that it still exists.
In contrast to the number of times slippage is ‘addressed’ on commercial internet sites,
the amount of times the problem is mentioned in the more serious scientific literature is
astonishingly low. The same key words, ‘knee’, ‘brace’ and ‘slippage’ on the Entrez-Pubmed
search engine will only give you 5 hits. On the Google Scholar search engine you’ll get 181 hits.
In general the thus found publications just mention the existence of the problem, either as a
remark or observation, as the reason for skin irritation or damage, or as a possible cause for a
greater risk on knee injuries while wearing the brace (1, 2, 3 and 4).
Discussions in literature on the underlying reasons for brace slippage are hard to find.
This article will discuss some of the theories behind slippage in more detail. It will be shown that
the reason for slippage of knee braces is actually quite simple. Based on that a conceptual design
that prevents knee brace slippage is presented. The constructive realisation of this concept in a
totally new design of an OA brace is shown and preliminary clinical results will be discussed.
Current Theories Why Knee Braces Slip
As stated above, there’s little discussion in the literature about the theories behind knee brace
slippage. However, when asking around under orthotic professionals, everybody seems to have a
theory. The majority of these theories can be divided into four major groups:
Gravity combined with lack of friction control; Tapered shaped legs; Mismatch of instantaneous anatomical and orthotic centre of rotation; Dynamic nature of the forces and movements of the brace;
Ad 1 - Gravity cmobined with lack of friction control:
The theory is that gravity will try to pull the brace down. The brace can stay in place by means of
friction forces that will equilibrate the downwardly directed gravity force. The problem is that
it’s impossible to control friction in contact with the human skin. Although temporarily very high
coefficients of friction can exist in contact with the skin, these cannot be maintained for longer periods of time, due to perspiration, but perhaps even more important due to skin creep. As a
result we are confronted with the reality of sometimes incredibly low coefficients of friction.
This theory sounds appealing and promising. Even more so when we think that the
general solution that has been applied in the majority of currently available braces, being more
straps (for generating more and higher normal forces) and more rubbery interfaces (for creating
more and higher coefficients of friction) does not seem to have solved the problem at all. The
downside of this theory, however, is that currently available braces aren’t that heavy. As a result
you wouldn’t need very high normal forces to even with low coefficients of friction come up
with sufficient friction forces to equilibrate the gravity force.
Ad 2 - Tapered shaped legs:
The theory is that every leg is more or less tapered shaped. Orthotic devices have to obey
Newton’s laws as well. This means that when the brace pushes against the anatomy in order to
stabilise or redress the leg, the anatomy will push back with equal magnitude against the brace.
When the interface between brace and leg is somewhat tapered shaped, the normal forces from
the leg to the orthosis will not only have a horizontal component, but will have a vertical
(downwardly directed) component as well. It’s this vertical component that pushes the brace
down. See Figure 1.
The appealing part of this theory is that it is plane and
simple mechanics. Furthermore it fits nicely to clinical
observations that severely tapered shaped legs have a much
greater tendency to create problems than less tapered shaped
legs. However it does not answer the question why the problem
can be so persistent in situations in which the leg is not at all
tapered. The downward component of the normal force on the
brace will only be very small then. Even in combination with
theory 1 the practical observations of slippage show a problem
that seems so much greater than can be explained by theory 1
and 2 alone. |
 |
Figure 1. When the leg is tapered
shaped the normal force FN from
the skin to the brace can be
divided in a horizontal component
FH and a vertical component FV.
This vertical component will push
the brace down. |
Ad 3 - Mismatch of instantaneous anatomical and orthotic centre
of rotation:
The knee is a polycentric joint (5). Placing an orthotic frame on
top of the anatomy results in two rotating parallel structures. If
these structures do not have identical instantaneous centre of
rotation (ICR) pathways, then rotating the combined system will
lead to deflections at the interfaces and/or movement of one
system with respect to the other. This can lead to slippage (6).
Both Winter (7) and Blankevoort (8) showed that there can be a considerable variation in
the ICR pathway depending on walking speed (Winter) or external load configuration
(Blankevoort). Considering that, then the problem of slippage becomes rather difficult to solve,
because it would require joints that change their rotation characteristics dependent on their use.
However, Bähler (9), in his excellent comparison of both monocentric and polycentric orthotic
knee joints showed that it’s preferable to accept a constant, but small error between the two
ICRs. The monocentric joint was one of the two joints that did best in that respect and proved to result to only minimal deflections at the interfaces with the leg, which suggests that there’s
probably more than just ICR mismatch to cause brace slippage.
Ad 4 - Dynamic nature of the forces and movements of the brace:
The theory is that use of knee braces is highly dynamic by nature. Loading in and on the brace
can rapidly change both in magnitude and in sign. Particularly the latter is of importance. This
means that on the one moment a particular interface of a brace can be loaded, whereas on another
moment the opposing strap of that interface could be loaded, leaving the original interface
unloaded. Since that interface is no longer loaded, there’s no normal force (and friction force)
between that interface and the skin. This opens the door to movements of the skin with respect to
that interface. This will happen with each interface periodically, leading to a slipping brace at the
end. The brace sort of wobbles down.
The downside of this theory is that it can be easily tricked by prestressing the straps of a
brace to ensure constantly loaded interfaces. It should be noted however that prestressing levels
should at least match maximum loading levels at the opposing interfaces to make prestressing
work.
None of the above theories is capable of fully explaining why the problem still exists. Even if we
consider a combination of all four, it’s difficult to match the disastrous clinical results with all
the efforts done in modern braces to prevent slippage. Maybe it’s therefore nice to throw in a
fifth theory.
A New Theory Why Knee Braces Slip
A knee brace is attached to the skin, but is designed to follow the skeletal structure. A closer look
on what the skin does during knee flexion shows the importance of that seemingly trivial remark.
In Figure 2 it can be seen that a flexed knee requires considerable soft tissue (skin) migration.
The required ‘compression’ at the posterior
side is of little influence to bracing. There
are enough options for that. More interesting
is the appearance of the ‘gap’ at the anterior
side. This of coarse doesn’t happen in
reality. In reality the patella slides gently
over the distal part of the femur and the skin
stays covered on top of the patella. It can,
however, only do so by means of
considerable skin stretching and migration.
Particularly the skin at the anterior side of
the upper leg is active in that. In every
flexion movement this skin is stretched and
pulled downwards. This has great
consequences for bracing. If we were to
position a knee brace on the leg of Figure 2
and made sure that it is stuck to the skin
quite nicely by means of lots of rubbery
interfaces at the anterior shells, then, with
every flexion movement, the skin would try to pull the brace down. The shape of the leg and gravity will help. At the end the skin will
succeed in pulling the brace down. This brings us to the paradox of knee bracing:
If the brace can’t move it slips
Figure 2. Flexing the knee requires skin migration. In the left
image a leg is presented. The dotted lines mark the center of
rotation (5) at the mid-patellar position, 2/3rd from anterior. In
the right image the same leg I clipped into an above knee
section and a below knee section and rotated around the center
of rotation. It can be clearly seen that this will lead to lots of
soft tissue ‘compression’ at the posterior side of the leg, but
also to considerable skin stretching and migration at the
anterior side of the leg.
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The Solution That Will Prevent Slippage
Knowing the problem makes finding a solution always much easier. In order to prevent a brace
to slip down we should facilitate skin migration at the anterior side of the upper leg. In Figure 3
the magnitude of skin
migration is shown when
we actually do that. For this
a brace has been designed
that allows skin migration
at the anterior interfaces of
the upper leg by means of a
triple set of rollers.
Particularly at the interface
just above the knee the
magnitude of the migration
is quite considerable. For
the most proximal anterior
interface the skin migration
is less impressive, but still
large enough to take into account. Looking at the pictures of Figure 3 it’s not difficult to predict
what will happen if a brace is stuck to the skin at these interfaces.
Figure 3. Magnitude of the skin migration while flexing the knee. In the left image
an extended knee is presented. On the leg marker lines are drawn. Each line is 1
cm apart. A knee brace is fitted to the leg. The brace is designed to allow the skin
to migrate underneath the brace at the anterior side of the upper leg by means of
rollers. In the right image the same knee is given in a flexed position. It is clear
that the skin has stretched and migrated underneath the brace for roughly 4 cm.
 |
A New Brace That Doesn't Slip Down
Based on the above findings a new brace has been designed that successfully prevents slippage.
This brace, called the Genux® OA, allows skin migration at the anterior interfaces of the upper
leg. It consists of a light weight tubular steel frame with triple half circular arcs at the anterior
interface locations of the upper leg. These arcs are
covered with a large array of thin plastic rollers,
see Figure 4. Skin can easily migrate underneath
these interfaces. The brace is suspended to an
elastic band of a calf strap. The elastic band
allows for micro migration of the entire brace
with respect to the anatomy, compensating for the
result of any mismatch between the ICR of brace
and knee. The elastic band furthermore pulls the
calf strap down on the anterior side only. As a
result the strap angles down and therewith locks
itself to the skin. The magnitude of the force in
the elastic band may vary, but will never be zero.
Because of this the forces that keep the strap in
place are constantly maintained. The solution to
the paradox of knee bracing is an orthosis that moves down (slightly) but also up again. The net result is that the brace stays in place.
Figure 4. The Genux® OA knee brace. Roller interfaces
at the anterior side allow the skin to migrate. The brace
itself is suspended by means of an elastic band attached
to a calf strap.
 |
Clinical Results with the Genux® OA
The Genux® OA has been tested in a clinical setting for over 50 patients. Within the design of
this new orthosis several new innovations have been realized. Focusing on just the anti-slip
system of the orthosis, it can be concluded that slippage of the brace has not been a problem in
these 50 patients. General comment on the roller interfaces of patients is, perhaps surprisingly,
that they are actually quite comfortable, particularly in warm conditions. The patient using this
new orthosis longest has been using it for over 3 years now.
Concluding Remarks
The Genux® OA offers an interesting new approach to solving the problem of slippage of knee
braces. Although the orthosis has been designed for use in patients with OA, the anti-slip system
is applicable in orthoses for other pathologies as well. It is important to understand that the
implications of the discussed theory behind brace slippage, with a central focus on allowing skin
migration at particular locations stretch beyond knee bracing. It is hoped that designers of
orthotic devices think about the presented theory in this article and experiment with different
fitting concepts, because there’s still a lot to win in comfort levels for users of orthotic devices.
Genux® is a trademark of Ambroise, The Netherlands
References
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