The Genux®, A new Biomechanical Conception In Reducing Internal Knee Forces in OA

Introduction

The treatment of patients with OA is still a great challenge in orthotics. Although surgical intervention techniques have refined considerably over the years, surgery will remain to be limited in its applicability, leaving large groups of patients at the mercy of other treatment options. Orthotic intervention is one of these options. A central goal in the orthotic treatment of patients with OA is the reduction of pain levels. The theory is that the continuous loading of the degenerated cartilage at the effected condyle evokes repeated inflammatory reactions associated with pain. A knee brace can be used in order to try to unload the affected condyle.

This article discusses the way in which knee braces unload the knee. The influence of brace rigidity and length is elaborated on. A new biomechanical approach to even further reduce internal loading inside the knee is discussed. The constructive realisation of the theoretical conceptions discussed in this article in a new innovative knee brace is presented. Finally preliminary clinical results with this new brace are discussed.

How do knee braces unload the knee?

A knee brace unloads a condyle of the knee by imposing coronal plane moments to the leg. Through a three or four point support the orthotic frame limits coronal plane rotations of the leg segments. The net result of the interface forces is a coronal plane moment of force (and sometimes also an M-L force) on the knee that equilibrates the external moment of force that tries to push the knee into a valgus or varus position. Quite often the terminology used for these braces is derived from this action. With valgus bracing usually the application of a brace that pushes the knee into abduction is meant. Unfortunately this is a very confusing and in my view incorrect terminology. A valgus brace could (should?) be a brace that is used to prevent or stabilise a valgus instability, but quite often it is a brace that is used to prevent or stabilise a varus instability (by pushing back in the opposite direction). The latter is most frequently used, but least logical. A valgus is an anatomical misalignment. It shouldn’t be the goal of an orthotic device to push a joint into an anatomical misalignment. It’s therefore preferable to use abduction or adduction when joint rotations in the coronal plane are meant. Or otherwise at least use the terminology ‘anti-valgus’ or ‘anti-varus’ to avoid confusion.

As said, the orthotic frame limits the coronal plane rotations at the knee. The efficacy of the frame to do so is highly dependent on its rigidity, as thoroughly explained by Carlson (1) in his excellent study on that subject. The rigidity of the orthosis-leg combination is dependent on the rigidity of the orthotic frame itself, but also on the length of the brace. Carlson explains that this works twofold. Firstly a longer brace has longer lever arms. When loaded with the same external moment of force that will lead to lower interface forces. Lower interface forces will lead to lower soft tissue deformations and interface deflections, meaning a more rigid behaviour. Secondly, the same soft tissue deformation at an interface location further away from the centre of rotation (longer lever arms) leads to a smaller angular deflection (see Figure 1). However there’s a third positive element in a more rigid construction of the brace (not mentioned by Carlson, but worth mentioning here). When we consider the first two elements then a more rigid brace will lead to a less deflected situation in loaded conditions. Because of that the leg will remain more upright. As a result the lever arm of the floor reaction vector with respect to the knee will also be smaller (see Figure 2). This means a reduced external loading and therewith even further reduced deformations and deflections of soft tissue and brace. The conclusion of this short analysis cannot be different than stressing the importance of brace length. Longer braces simply work better. It’s really amazing that so little of this simple and effective biomechanics is seen in today’s bracing. In this paper a brace will be presented which does address the above considerations.

Figure 1. A leg with two triangles is presented. The horizontal edge of both triangles is equal in length. The two triangles furthermore share the same intersection of the other two edges. If we suppose that the horizontal edge of the triangles is a soft tissue deformation of the same magnitude, but at a different lever arm from the center of rotation of the knee (being the intersection of the other two edges of the triangle), then it’s clear that longer lever arms leads to smaller angular deflections even with the same soft tissue deformation.

A closer look at internal knee forces

The above described working principle of knee braces may be able to locally reduce the loading a an affected condyle, but does little to the ove loading levels inside the knee, apart from the effect of redression as described in Figure 2. As a result the loading inside the knee w still present fairly high peak force particularly from femur to tibia. Literature shows a wide range of tibio-femoral compression forces depending on the sort of activity Messier (2) suggests peak values of the tibio-femoral compressi during walking of up to 3.7 times body weight (BW) for an OA group which is slightly, but not significantly higher than with the control group (3.1 BW). Scott (3) mentions knee compression forces of 7.0-11.1 BW during running. Gruber (4) mentions even higher compression forces: 21 BW after a jump down from 1.5 m. Nisell (5) calculated the tibio-femoral compression force during isokinetic knee extension to be 9 BW. Taylor (6) comes up with forces of 3.1 BW for walking and 5.4 BW for stair climbing. It is clear that during normal activities tibio-femoral compression forces, that can reach peak values of several times body weight, have to be taken into account. A simple biomechanical model of the knee can easily explain where these high compression forces come from. For this a closer look at Figure 3 can help. Due to the relatively small lever arm of the quadriceps to the knee centre, very high quadriceps forces are necessary to stabilise an external flexing moment of force.

Figure 2. When the leg is more upright the lever arm of the floor reaction vector to the affected joint is smaller, leading to lower external loading on the joint. A more rigid brace will be more effective to keep the leg as upright as possible

Reducing internal knee loading even further

Understanding the origin of the large internal knee forces opens a route to the reduction of the peak values for patients with OA. If we were able to reduce the peak values of the quadriceps we would be able to at the same time reduce the tibio-femoral compression force as well. Reducing the peak values of the quadriceps can be achieved by (partly) helping the quadriceps to stabilise the knee in the sagittal plane. For that we can design a knee brace that is able to generate a threshold moment of force that prevents flexion during standing and the stance phase. Johnson (7) reports that the overall normalised (height and body weight) maximum knee extension moment of force during walking for a group of OA patients is 0.09. If we were able to let the knee brace take a portion (25-30%) of that, than peak values of the quadriceps force (and therewith tibio-femoral compression force) could drop a fair bit as well. A positive secondary side effect of such a brace would be that it helps to stabilise the knee in the sagittal plane for those OA patients that suffer from reduced quadriceps control as a side effect of their condition. Literature suggests that there is a close correlation between OA and quadriceps weakness (8, 9, 10, and 11). It’s probably not a good idea to go beyond more than 25-30% of the maximum moments with the added support by the brace, because it might evoke the quadriceps to reduce (even further) in strength.

Figure 3. Free body diagram of the knee. The knee is loaded with a flexing moment of force (M). The quadriceps is fired to stabilise the knee against that load, leading to Fquad. Because the moment arm from the quadriceps tendon to the knee centre is so small, Fquad will reach considerable magnitude. As a result Fpt (the force from patella to tibia) and Ftfy (the tibio-femoral compression force) will reach values of comparable magnitude.

The Genux® OA knee joint and orthotic frame

We have designed a knee joint (see Figure 4) that is capable of applying a maximum of 4 Nm threshold moment of force for the first 10°-12° of flexion, against flexion. After that the joint moment quickly drops to (almost) zero in order to prevent undesired side effects during swing. The dimensions of the joint have remained small. The joint is mounted in a relatively long orthotic frame, forming a knee brace, named the Genux® OA. The positive effect of long lever arms has been discussed at the beginning of this paper. The long frame is constructed of light weight tubular elements. This combination leads to a very rigid construction, but also to a light weight construction. As discussed, it’s important to redress the leg as much as possible (see also Figure 2). This not only leads to an unloaded condyle at the affected side, but also to lower loading in general of the leg and brace. The Genux® OA has a system to individually fine tune the redression of brace and leg.

Preliminary clinical results

The Genux® OA is tested on 50 patients so far. Preliminary results are promising. Patients report reduced pain levels, also the ones that have previously used different types of OA braces. Further research is however necessary to fully validate the theory of reduced internal loading and the effect of that on pain levels. Some patients report a positive effect of the increased sagittal support on (the sensation of) sagittal plane stability, but this is not noted by every patient, although remarks on a general increase in support while wearing the brace are common to most users. A rigid frame allowing the orthotist to fine tune redression, combined with sagittal plane support therefore seems to enhance both (the sensation of) stability as well as reduce pain levels.

Figure 4. The Genux® OA knee brace. A relatively long orthotic frame, combined with a slim joint are responsible for the low profile of the brace. The joint is capable of developing up to 4 Nm extension moment of force during the stance phase.

References

1. Carlson, J.M..; French, J.; Knee orthoses for valgus protection, experiments on 11 designs with related analyses of orthosis length and rigidity; Clin Orthop 1989; 247: pp 175-192.

2. Messier, S.P.; DeVita, P.; Cowan, R.E.; Seay, J.; Young, H.C.; Marsh, A.P.; Do older adults with knee osteoarthritis place greater loads on the knee during gait? A preliminary study; Arch Phys Med Rehabil 2005: 86: 4: pp 701-709.

3. Scott S.H.; Winter, D.A.; Internal forces of chronic running injury sites; Med Sci Sports Exerc 1990; 22: 3: pp 357-369.

4. Gruber, K.; Legal, H.; Ruder, H.; Biomechanical analysis of locomotion patterns in the lower limb. II. Forces in joints; Z Orthop Ihre Grenzgeb 1983; 121: 2: pp 146-153 (In Germann)

5. Nissel, R.; Ericson, M.O.; Nemeth, G.; Ekholm, J.; Tibiofemoral joint forces during isokinetic knee extension; Am J Sports Med 1989; 17: 1: pp 49-54

6. Taylor, W.R.; Heller, M.O.; Bergmann, G.; Duda, G.N.; Tibio-femoral loading during human gait and stair climbing; J Orthop Res 2004; 22: 3: pp 625-632.

7. Johnson, G.R.; Ferrarin, M.; Harrington, M.; Hermens, H.; Jonker, I.; Mak, P.; Stallard, J.; Performance specification for lower limb orthotic devices; Clin Biomech 2004; 19: 7: pp 711-718.

8. Brandt, K.D.; Neuromuscular aspects of osteoarthritis: a perspective; Novartis Fund Symp 2004; 260: pp 49-58.

9. Baker, K.R.; Xu, L.; Zhang, Y.; Nevitt, M.; Niu, J.; Aliabadi, P.; Yu, W.; Quadriceps weakness and its relationship to tibiofemoral and patellofemoral knee osteoarthritis in Chinese: theBeijing osteoarthritis study; Arthritis Rheum 2004; 50: 6: pp 1815-1821.

10. Segal, N.A.; Toda, Y.; Absolute reduction in lower limb lean body mass in Japanese women with osteoarthritis; J Clin Rheumatol 2005; 11: 5: pp 245-249

11. Hall, M.C.; Mockett, S.P.; Doherty, M.; Relative impact of radiographic osteoarthritits and pain on quadriceps strength, proprioception, static postural sway and lower limb function. Ann Rheum Dis 2005; 11: 24.