Date: January 28, 2001; added to interim archive on December 11, 2002.

(This document ties in with my three-part write-up on functional bracing, the most-recent version of which can be found at http://factotem.org/library/database/Interim-Archive/kbr8a.shtml.)

Dear Mark,

In response to your posting, here is my write-up on osteoarthritis (also known as "unloader") knee braces.

The document below ties in with my three-part write-up on functional knee bracing. Because many osteoarthritis knee braces are, in addition to forcing the knee sideways, intended to serve as functional knee braces (given that many osteoarthritic knees harbour at least some ligament-injury history), and also because many custom-made osteoarthritis braces share numerous similarities with their non-OA functional siblings, most the biomechanical and engineering aspects of functional braces also apply to OA braces.

Feel free to e-mail me if you have any further questions.

Yours truly, Michael Frind.

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Osteoarthritis Knee Braces (January 2001)

One should always keep in mind that the goal behind OA bracing does not involve curing the root problem. By forcing the knee sideways (in such a way as to transfer all the weight to the undamaged side of the side), the brace simply provides relief from the pain of unicompartmental osteoarthritis. Yet, by taking the damaged compartment out of weight-bearing service, the brace can limit further damage to that portion of the joint. Assuming the undamaged compartment remains uninjured, the brace can provide good pain relief (and inflammation relief, along with correction of certain types of deformities) for a number of years. (Of course, for anyone who has arthritis in both sides of the knee, an OA brace would not provide any pain relief.) If the patient is reasonably gentle with the knee (this is important because concentrating the load on half the knee effectively doubles the pressure in the intact compartment), it can be expected that the undamaged compartment will tend to remain healthy for quite some time. (Because knee conditions vary greatly, some people will find an OA brace to be of no benefit at all, while others will find it an idea short-term solution. And, still others will find it a good medium-to-longer-term solution.)

The osteoarthritic compartment of the knee tends to exhibit a slight amount of natural separation (of the damaged compartment) when the knee is not weight-bearing. This separation is lost when the knee is in the weight-bearing position - hence the knee is painful only when bearing weight. So, an OA brace merely has to restore the joint separation. In a knee with medial-compartment OA, the brace does this by exerting an inwards force at the joint line, along with a simultaneous outwards force at the proximal and distal medial ends.

I should note that the goal of an OA brace is, in some ways, quite similar to that of an osteotomy (e.g. high tibial osteotomy). In an osteotomy, a wedge of bone is removed in order to "correct" the knee - that is, to cause all (or most) of the loading to be transferred to the undamaged half of the joint, or to correct a varus or valgus deformity (i.e. bow-leggedness or knock-kneedness, respectively). The OA brace, however, has one major advantage: it provides instant relief, without the pain and rehabilitation that any major surgery entails. (Of course, the counterpoint is that the benefit of the brace is only present whenever the device is strapped to the leg.)

Osteoarthritis braces can be either off-the-shelf or custom-made. Custom-made models provide the closest fit, and are made to be very comfortable (and thus ideal for all-day wear); however, some off-the-shelf OA braces are also amenable to all-day wear. If you want a custom-made hardshell model, note that adjustability will usually be somewhat limited (although most custom models permit the clinician/orthotist to make modifications in the field). Given that the purpose of an OA brace is to gently force the knee sideways, and given that the knee will be forced sideways almost continually (while bearing weight), it follows that with the passage of time, a need to adjust the orthosis might arise. Some braces provide this adjustability through swappable condyle pads; to obtain more unloading effect, just put in a thicker pad. (In the case of medial-compartment arthritis, the brace will be forcing the knee inwards, and so it will apply a force on the outside of the knee. This is termed a valgus-directed force.) Other braces accomplish this with some sort of bendable frame, or with screw/cam arrangements at the hinges.

There are several anatomy-related aspects germane to OA bracing. The knee tends to be more prone to OA in its medial compartment; part of the reason behind this has to do with the medial compartment being small than the lateral one. As things work out, the knee can tolerate about 10-12 degrees of lateral-compartment unloading, yet only about 7 degrees of medial unloading. One must also remember that unloading the medial compartment entails forcing the leg towards knock-kneedness. For people with normal knee alignment (which involves only a small amount of clearance between the knees, as measured when the person is standing erect with feet together), this could become problematic over time. For people who are bow-legged, this corrective force would be very desirable.

In functional knee braces (and particularly if used in sports), having a brace which fits well implies that the appliance's hinge motion matches that of the knee as closely as possible. This issue is relevant to osteoarthritis bracing as well. With nearly all braces, the device's lower half is more firmly clamped to the leg than the upper half. (From the perspective of biomechanics, this makes sense because the tibial crest provides a good bony-reference anchoring point; also, the bulk of the gastrocnemius group provides a practical point for the brace to anchor itself onto, thus explaining why nearly all braces require the strap immediately below the knee to be tightest.) Because it is impossible to absolutely perfectly match the knee's natural motion, this means that the brace's upper half will necessarily slide slightly with respect to the thigh. If the brace has an anatomically-correct hinge motion, then such pistoning will be imperceptible (and perhaps even nonexistent) - but if the brace has only a simple pin hinge (or geared polycentric hinge), and especially if the user is very active, or wears the brace for many hours each day, then it could prove annoying. For some reason, only the braces of Innovation Sports, Townsend Design, and Flextech (as well as the lesser-known firms EMT, Exotec, and Biedermann/Montana) have natural-motion hinges. (Technically, the human knee's motion involves more than mere front-to-back motion. There is also the "screw-home mechanism" which involves tibial rotation. However, the closest and most-appropriate hinge design involves a sagittal-plane rotation with slight translation in this plane; this is what these manufacturers use. (Generation 2 has an odd design, which will be discussed later.)

Designing any knee brace requires an understanding of the interrelationship between skin-contact area and leverage. The longer the brace frame, the less skin surface area must be covered in order to achieve the same effect. (This explains why surgery-rehab braces are so long - maximum protection against hyperextension and sideways forcing, but very cumbersome to wear.) As things work out, there is a practical limit to how long functional and osteoarthritis braces can be made - this is why for a given leg length, all braces models are quite similar in length. To enable the brace to compress soft tissue more effectively (yet without making the frame longer), manufacturers therefore expand the brace shell.

Because an OA brace is designed to force the knee sideways, it will tend to be most comfortable if the contact-interface surface area is greatest. This means that the brace's side members will have to cover a considerable amount of skin. The greater the surface area, the less concern about pressure points there will be; the drawback is that the user might find the brace to be sweaty during hot weather.

A rigid-frame brace, with interchangeable padding, can only apply limited amounts of sideways forcing to the knee. If the brace contains two hinges assemblies (one on each side of the knee), and if increasing amounts of condylar padding are inserted on one side, then at some point the other side of the knee will impinge on the other side of the brace. Also, merely inserting condylar padding on one side will, by virtue of how it alters the knee's alignment (i.e. valgus-varus angle, or angle in the frontal/coronal plane), affect the fit of the upper and lower halves of the brace shell. If the brace is off-the-shelf, then this might not be much of a problem (given that a close fit is rarely attained with off-the-shelf bracing anyway), but if it is custom-made, then the appliance's comfort might be affected. (Fortunately, brace manufacturers address this issue by providing warranties on the fit of custom-made braces. Furthermore, many of the custom-made braces can are field-modifiable.)

An example of an off-the-shelf OA brace which depends on interchangeable padding is the Omni Align. This composite-frame brace has minimal contact area between the lateral frame components and the skin. (The Omni brace's frame is such that the top half is made of two uprights composed of flexible neoprene.) Adding extra condylar padding to one side would not result in appreciable impingement of the knee on the opposite side. Unfortunately, the Align's design is weak from a structural-engineering viewpoint. Frame failure is to be expected.) This brace is not likely to be a good choice, although it would be adequate for people whose OA is only beginning. For someone with more serious OA, or who requires deformity correction, a brace with a stronger frame would constitute a much better choice.

There are some braces which have hinges containing sideways-forcing adjustments directly within the hinge. (Again, the disadvantage of such an arrangement is that, if successive adjustments must be made, there is the possibility of the knee's other condyle eventually impinging on the inside of the other hinge.) This type of arrangement entails using a screw-type device, which allows the condyle pad to be moved inwards simply by tightening a screw. In this design category, a viable design is the German-designed Ultratech. (The manufacturer of this brace is Biedermann GmbH; however, this brace is produced here under licence by Montana Medical Brace. The Ultratech is better-known in Europe than in North America.) Like many of the custom-made functional (i.e. for ligament-instability use) braces, this model is made from cast, and should fit reliably. Its slightly-flexible frame allows for some accommodation of leg-shape changes, and

Another sideways-forcing brace (albeit an off-the-shelf one) containing a flexible frame is DonJoy's new OAdjuster. This brace entails an aluminum frame with an usual clamp-screw assembly incorporated into the medial arms of the frame. One drawback with the Oadjuster is that its mechanical reliability is still unproven. The frame assembly is an example of a high-stress design, and failure of the assembly might be anticipated.

Breg produces a Tradition Counterforce, which also contains a screw-type sideways-forcing arrangement. The Counterforce has the same frame as the regular Tradition; this means that it has the disadvantage of minimal skin-contact area - and therefore a high potential for pressure-point problems and migration. Another drawback is that the Counterforce, despite being labelled "Custom" is not a truly-custom-made brace; despite this, its price is in the custom-brace realm. (In fact, all of Breg's "custom-made" braces are actually just bent-upon-request off-the-shelf.)

The OA braces from Generation 2 (either the Canadian division or the American division) are sold under the name "Unloader". This custom-made brace was, in fact, the first brace made specifically for the OA market. The design incorporates a semi-flexible shell and a single hinge, with a strap on the side opposite the hinge. This arrangement enables the user to increase the sideways forcing merely by tightening the diagonally-spiralling "Dynamic Force Strap". For medial-compartment osteoarthritis, the hinge would be placed on the inside of the knee, with the strap on the outside. In addition to being cheap to fabricate, the single-hinge-plus-strap design has the advantage of convenience and good accommodation of leg-size variation. As a full-shell design, it has the advantage of being able to compress the leg's soft tissues from all sides. G2's design has the serious drawback of twisting on the leg each time the limb is extended; this torsional forcing would tend to make the brace prone to migrating down the leg. This makes Generation 2's braces poorly suited to active sports.

Note that the Generation 2 hinge is unusual in that it moves in several planes simultaneously. The original intent of this design was to mimic the tibia's slight internal rotation as the knee flexes. (This assumes that the brace is used for lateral-compartment OA, i.e. with the hinge located on the outside of the knee.) The Generation 2 hinge, however, greatly exaggerates this motion. (This, in fact, is one reason why Generation 2's design is poorly-suited to athletic-type functional-brace use.) The multi-plane ("poli-axial") G2 hinge brings another disadvantage: if used for medial-compartment OA (thus with hinge on medial side), it would tend to force the knee to move in a sense opposite to the normal tibial rotation and screw-home mechanism of the joint. (Osteoarthritis happens to be more common in the knee's medial compartment.) Because of the abnormal kinematics of the Generation 2 brace, I strongly suggest that the OA patient who is specifically prescribed a G2 brace discuss these technical details with his/her doctor. I think the G2 design is best reserved for cases in which the doctor has specific reasons for indicating the use of this design of brace. (One more problem with using a G2 brace for someone with medial-compartment OA: the G2 hinge would be on the medial side of the brace; being a very bulky hinge, your need to wear bilateral bracing could present a problem with regards to tripping.)

Granted, while the Generation 2 Unloader design is rather old-fashioned (given that it has been on the market for quite a while), it is very much time-proven. Many people have found it helpful with regards to pain relief (and Generation 2 does not hesitate to produce considerable numbers of case studies and reports) - although it must be kept in mind that such pain relief is a function of the fact that the brace forces sideways (as any OA brace does), not the fact that it is a Generation 2 design. I think the Generation 2 design is satisfactory for someone with lateral-compartment OA who is not into sports more demanding than golf, but I would not recommend it for medial-compartment OA.

The DonJoy Monarch and the Lenox Hill OA are also of the single-hinge design, albeit these ones depend on the padded hinge to force the knee sideways (as opposed to a strap forcing the knee towards the hinge). This means that with the DonJoy and Lenox Hill single-hinged OA braces (if used for medial-compartment osteoarthritis), the hinge would be located on the outside of the knee. Although the design is likely to be suitable for quite a few situations, there are several engineering concerns. Firstly, the design is structurally weak. Secondly, as the knee flexes and extends, the hinge (which is responsible for applying all of the sideways forcing) will have a tendency to cause the entire brace to twist on the limb. The could cause migration. I am not convinced that the DonJoy/Lenox Hill design is well-thought-out.

Nearly every brace manufacturer has an entry into the growing activity-oriented OA-brace market. In fact, many of the custom ACL sports braces (such as the Innovation Sports CTi2, Townsend Premier, Townsend Air, DonJoy Defiance, and Flextech F1) are available in OA versions: the manufacturer simply builds the brace deliberately slightly crooked, so that the desired sideways forcing is achieved. The problem is that if this "correction factor" proves to be either too tight or too great, then the entire brace must be sent back to the manufacturer for modifications or refabrication. Although the manufacturers make such modifications free of charge, the extra time involved (i.e. shipping back and forth) can translate into inconvenience for the patient. However, the benefit of a custom-made OA-and-functional brace is that it will provide a snug and comfortable fit, and will be suitable for use during gentle (and even moderately-knee-strenuous athletic activities. (If fact, if the wearer is involved in very active sports, then a custom-made sports-type OA brace would be best, because it would be the most comfortable to wear, and would provide top-notch protection against hyperextension and lateral impacts as well as the necessary sideways forcing for the unicompartmental OA.) Such a brace would also be well-suited to continual daily wear. The manufacturer will stand behind a custom-made OA brace: if the user finds it uncomfortable to wear continually, then the company will go to extreme lengths until the person is satisfied.

Particularly with continually-worn OA bracing, comfort is, of course, absolutely essential. This explains why the OA versions of the open-shell custom-made braces have expanded contact areas. (Because pressure is defined as force divided by area, increasing the area reduces the pressure.) Examples of these modified open-shell braces are the Townsend Premier Reliever (custom-made) and Townsend Graphite System Reliever (semi-custom-made via heat-moulding, from an off-the-shelf kit; a good choice inasmuch as off-the-shelf braces are concerned). DonJoy's Defiance lacks expanded contact areas, and would be likely to produce pressure-point problems.

Full-shell custom-made braces (such as the CTi2 or Townsend Air) already have sufficient skin-surface contact area, and thus are well-suited to being manufactured as OA bracing. A lateral-OA-version CTi2 will have the same general appearance as a regular CTi2, although a medial-OA-version CTi2 can (depending upon what is requested) have either the same appearance as the regular CTi2 (in which case the user would depend on the uppermost and lowermost straps for the corrective forcing) or be reversed (i.e. with the upper and lower arms extending medially instead of laterally). The CTi2 has a unique advantage: it is the lightest full-shell brace on the market, and weighs no more than any of the open-shelled braces. An additional advantage of this model is that it has all the features and options (i.e. ACL cable/XCL strap, twist-inhibiting alpine-ski-boot attachment) that the regular CTi2 has.

Dual-hinged OA braces have a very significant advantage over all single-hinged OA braces (whether custom-made or off-the-shelf): excellent protection against lateral forcing (i.e. other than that which the brace normally applies to the osteoarthritic knee) and hyperextension. Such protection is extremely beneficial for any knee which has any type of ligament-injury history. (Keep in mind that injury via twisting is still possible, with any brace worn in isolation.)

Within the realm of off-the-shelf OA bracing, there is one model whose structure has the unique feature of valgus-varus adjustability; this model I rank as the best of the off-the-shelf OA braces. The Innovation Sports OAsys has a very-slightly-flexible shell with a telescoping medial upright. The variable-length medial upright member enables the brace to instantly accommodate any knee valgus-varus angle, from knock-kneed (genu valgum) to bow-legged (genu varum). (The arrangement makes sense from a structural-engineering perspective, because the hinge planes remain parallel; the flexing occurs in the upper half of the brace.) The adjustable valgus-varus angle feature allows the user to quickly alter the "correction factor", without having to send the brace away, and also without the concern of the knee impinging on the inside of the other hinge. Because this design allows the brace to match the knee's valgus-varus angle very closely, the comfort obtained should be almost as good as with a custom-made brace. Of course, the problem of fitting a disproportionately-sized or unusually-shaped leg remains, and for this reason a custom-made full-shell OA-and-functional brace is overall likely still the best choice.

So, there are lots of suitable bracing options for unicompartment-osteoarthritis pain relief. Because all the OA braces work in the same fundamental way (i.e. force the knee sideways so as to unload the damaged compartment), and because OA braces are intended to be worn semi-continuously, it is suggested to pay particular attention to factors such as ergonomics (i.e. user-friendliness, ease of donning, ease of use, ease of maintenance [including user-replaceable strapping], adaptability and versatility, quality of fit) and durability (i.e. lifetime frame/hinge warranty; this tends to exist only with custom-made braces). For people who are involved in contact-containing activities or who are very active, or who need the best possible support, or who have pre-existing deformities (such as bowleggedness or knock-kneedness), a custom-made brace with hinges on both sides of the knee would be most suitable.

I wish you well.

Yours truly, Michael Frind.