Functional Brace Principles, from Bledsoe Brace Systems :
Functional braces are not magic! They have their strengths and capabilities as well as their limitations.
In order to understand what functional braces can and cannot do on the leg, it is first necessary to understand the working principles of functional braces. The two factors that enhance control of any external device are leverage and surface contact area. The factor that provides the greatest limit to control is soft tissue deflection. The stretching and movement of skin and muscle around the knee joint produces large tissue translations that bracing must accommodate. The thickening and thinning of muscle groups during flexion and extension causes the position of the bones inside the muscle mass to vary. Finally, the shape of the leg changes considerably as these motions take place. Braces must be able to accommodate these changes to remain comfortable as well as supportive.
The hinge system utilized on functional braces is important. The large range of knee motion which a functional brace must withstand (0-140 degrees) requires a hinge that tracks closely with knee motion. This means using a polycentric or eccentric cam type hinge. The hinge must be capable of proper placement on the leg based on the design of the hinge arms, shells, and straps. Just as important, is the ability to maintain the hinge in the proper place during activity through an appropriate suspension mechanism. Since functional braces tend to be shorter than their postoperative counterparts, the only place to provide adequate suspension is the slightly smaller circumference of the proximal calf muscles. The hinge should be placed in a slightly superior and posterior position during application. This allows for the effect of gravity forcing the brace lower, the forced migration produced by the cone shape of the leg, and the forced migration produced by the translating skin and muscles of the distal posterior thigh. To optimize suspension, the proximal posterior calf strap should be slightly lower than the anterior strap, and must be free pivoting to provide a locking motion on the calf muscles as the brace is forced distally. The best brace design in the world is of little use if the brace will not remain in place.
Each end (thigh and calf) of every hinged brace is a three-point lever system. These two levers share a common third point at the hinge that does not contact the leg. The remaining points on the arms of the brace where the brace straps attach to the leg form a four-point force system. Every brace ever made with a hinge is a four point brace. Maximizing length or leverage is important to brace control. The market continues to ask for shorter braces. In fact, braces need to be longer to gain better control. This is particularly true on those portions of the lever arms or shells that compress into a lot of soft tissue. Surface contact area must be maximized on these portions of the leg to increase control. The point on the leg where the least compression occurs, is the anterior tibia. There is very little soft tissue to deflect. Therefore, the overwhelming majority of brace manufacturers have chosen to use a pre-tibial shell as a center fixation point.
As long as the anterior tibial shell of a brace is held firmly in contact with the front of the tibia, it will move as the tibia moves. For instance, if the tibia subluxes anteriorly (as with a missing ACL), it will carry the tibial shell of the brace with it. This leaves only the posterior distal thigh strap and the proximal anterior shell of the brace to indent into the skin, fat, and muscles in an attempt to limit the subluxation motion. Most tibial subluxation occurs as the leg is rapidly extended in preparation for foot strike in maneuvers such as stopping, running downhill, landing from a jump, or moving laterally. These are open kinetic chain maneuvers that involve quadriceps contraction before foot strike. Due to the slow reflex arc of the hamstring muscles after loss of the mechanoreceptors that were present in the original ACL, the hamstring muscles are slow to react. This permits the posterior distal thigh strap of the brace to compress easily into the flaccid hamstring muscles resulting in very little resistance to anterior subluxation. This previous example demonstrates why most braces offer little resistance to anterior tibial movement.
Simply tightening the straps of a brace does not eliminate soft tissue compression, translation, and rotation. Strap tension is limited by patient comfort and blood circulation. Obviously, those patients with more soft tissue will experience less control from any external device. It is easy to brace the patient that is 6 ft. tall and weighs 95 pounds. It is almost impossible to brace the patient that is 5 ft. tall at 250 pounds. It is difficult to control the position of the bones and joints through the Jello-like soft tissues of the leg. This places an upper limit on the required strength and stiffness of a functional brace. Once this limit is reached no additional control is gained by making the brace stronger or stiffer. Even a one inch thick solid stainless steel cylinder cast would be limited by these same soft tissue limits.
Functional braces are divided into three categories: Passive Braces, Static Counter Force Braces, and Dynamic Braces. These categories are defined based upon how the brace provides resistance to abnormal movement, and whether force pre-loaded against a pathological movement is static (i.e. fixed strap force) or dynamic (muscle powered to work against a pathological movement). The particular design of the brace forces it into one category or another.
Passive braces are those which are not capable of being adjusted to provide a counter shear force against a specific pathological movement (e.g. anterior tibial subluxation). They can be recognized by their design. They usually have bars or shells anteriorly on the leg with straps opposing the shells to attach the brace to the leg. This strap opposing shell configuration prevents them from being adjusted to push the femur forward and the tibia back or vice versa. Tension in the straps only creates circumferential pressure without causing shear force. Therefore, they simply sit on the leg and act as a buttress that the leg can run into when abnormal motions occur. Soft tissue greatly limits their effectiveness. They provide sufficient resistance to movement for most medial or lateral collateral ligament injuries but are mechanically ineffective at preventing anterior or posterior tibial subluxation. Their primary role on the leg is therefore proprioceptive.
Static counter force braces have been designed to permit pre-compressing the soft tissue on only the anterior or posterior half of the leg with static strap tension. Since the distal posterior thigh strap is unopposed by a shell, it is possible to provide more pressure in an anterior direction than is achievable with a passive brace. Static counter force braces usually provide enough tissue pre-load in one direction to provide increased resistance against tibial subluxation for the reconstructed ACL or for ACL deficient patients with mild instability, and they function the same as passive braces against medial or lateral bending of the knee. While the amount of tibial subluxation permitted by a static counter force brace is usually less than passive braces, the tension of the straps and pre-compression of the soft tissue is again limited by blood circulation and patient comfort. Pre-bending a brace in a varus or valgus direction to a greater extent than the normal varus or valgus bend angle of the leg causes it to be a type of static counter force brace. In this case the static counter force is in a medial or lateral direction. These braces are sometimes used for uni-compartmental osteoarthritis, high tibial osteotomies, or collateral ligament conservative treatment.
In our next blog we will examine the 3rd new category of bracing, pioneered by Bledsoe Brace Systems.
This will explain why Happy Brace has chosen to focus on putting our patients in this newest type of technology.
Ask your brace provider to explain the difference to you. They should know the difference !
