N.G.A. van Leerdam, MSc, PhD
Enschede, The Netherlands
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.
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
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 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
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.
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.
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.
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
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