Mechanics of Orthoses
Basic principles of soft tissue mechanics, hand biomechanics, and materials properties are relevant to hand therapy clinical practice and provide a background for understanding the principles of orthotics fabrication. Dr. Paul Brand pioneered the idea of a biomedical engineer as a valued member of the hand surgery and therapy team. He championed this concept in hand centers in both India and Louisiana. He established the New Life Center in India, and the Rehabilitation Research Laboratory at the U.S. Public Health Service National Hansen’s Disease Hospital, in Carville, Louisiana. Brand and the Rehabilitation Research Laboratory he created have been responsible for many of the therapy techniques used in hand clinics today. Brand is known to some as the father of hand therapy and hand therapy centers. A dedication to him can be found in the first edition of this textbook. His work, “The Forces of Dynamic Splinting: Ten Questions Before Applying a Splint to a Hand,” is revised and updated in this chapter, closely following his original descriptions of techniques that continue to be as relevant today as when he first wrote of them.
Fabrication of an orthosis for the hand and upper extremity is a science as well as an art. The science of orthoses has advanced significantly, requiring comprehensive knowledge of biomechanics. By its very presence on the hand, wrist, and/or forearm, an orthosis must do no harm. It is inhibiting the free movement and use of the hand and is justified only if the benefit of the orthosis compensates for the potential harm of restriction.
A mobilization orthosis, commonly referred to as a dynamic splint, is one that achieves its effect through movement and force. It is a form of controlled mobilization. It may use forces generated by the patient’s own muscles or externally imposed by rubber bands or springs. Whenever passive movement is used, there is a danger that an eager surgeon or therapist will use too much force. The unintended result may be that the patient has pain and edema, and a short-term gain in motion is followed by long-term stiffness. Active exercise and work-related movements are controlled by the patient, who usually keeps within the limits of pain, which is protective of overstress of tissues. During active movement, hand reflexes and coordination, which are essential to normal hand function, are maintained or restored. Any orthosis should allow or enhance other movement where needed and restrict only what is necessary. The orthosis should only be used for the duration needed to achieve its goal.
Orthoses are needed when exercise and positioning are not enough to achieve satisfactory results and movement must be assisted, replaced, or restricted. Sometimes an orthosis may be worn only at night to enable full active movement during the day. It may be critical that an orthosis be worn during the day to control active movement when the hand is used, whereas if an orthosis is worn for correction of tissue contractures, it may be needed for 24 hours. We first must realize that the orthosis may have the same bad results as manipulation unless surgeons and therapists utilize stern discipline to ensure that the forces they impose are well controlled. Too little force may do no good; too much may do harm. So, how much is just right?
Management by Objectives
We have to measure and calculate. It is not enough to say, “rubber band traction” or “not too much force” or “be gentle.” In terms of force on the hand, we must use a unit of measure, e.g., grams of force. Then we can begin to compare results. Most books and articles about orthoses are concerned with design, not measurement of force. Paul Brand identified 10 basic questions that each therapist considers with each orthosis application after defining the objective of the mobilization orthosis for the specific joint(s) to be mobilized or modified. The 10 questions to ask are:
How much force?
Through what surface?
For how long?
To what structure?
By what leverage?
Against what reaction?
For what purpose?
Measured by what scale?
Avoiding what harm?
Warned by what signs?
How Much Force?
In most dynamic mobilization hand orthoses, the force is provided by rubber bands or steel springs. Some companies have springs that are calibrated for tension, specifically for use with dynamic hand orthoses. Rubber bands must be replaced more frequently, because rubber changes its length-tension curve with age and with constant stretch. Therapists or orthotists should purchase their rubber bands in batches that appear uniform and then test them for tension before use. A simple stress-strain diagram may be quickly prepared for each batch to be used as a guide for future use. A weight or series of weights can be used to extend the band to a measurable distance for comparison of strengths. One is hung from the rubber band, using a paper clip, while the other end is suspended from a rod. The length the rubber band extends with the weight can be measured with a standard ruler or read on a sheet of graph paper divided into different units. For a simplified measurement, one weight (e.g., 300 g) can be used to find similar bands that extend to equal length with the weight applied. By using standard 100-g, 200-g, 300-g, 400-g, and 500-g weights and measuring the lengths of the bands at these weights, one can understand the elastic properties of the rubber bands. The length of the band as it changes with each of those five weights hooked one by one onto the rubber band can be graphed as a curve of its stress versus strain. Figure 123-1 shows the curve for one of our rubber bands. If a dynamic/mobilization orthosis requires a pull of 200 g from an outrigger 6 cm from the finger, a quick glance across our graphs will show which band we should use and how far we should stretch it. If one is using a spring steel wire, this may be calibrated in advance with a similar stress-strain curve to select and match the elastic capacity of springs.
In most dynamic/mobilization orthoses, there will be a range of movement that will result in lengthening and shortening of the rubber band or spring. For example, in Harold Kleinert’s method following primary suture of a severed flexor tendon, he used a rubber band from the fingernail to a wristband. The purpose of this is to hold the finger flexed when it is at rest and to allow the patient to extend the finger against the tension of the rubber band by using the extensor muscles. The rubber band should be strong enough to pull the finger back into flexion without the use of the flexor muscle–tendon unit. For such a purpose, use a rubber band that is 5 cm long at rest (no tension) that would have a tension of 200 to 300 g at 15-cm length.
Long, thin bands elongate more than short, thick bands. Rubber bands should be selected that will allow the strength (tension) and elongation (length) needed for the particular orthosis being used. At the time force is applied, we can check the tension of the rubber band with a strain gauge. A Haldex gauge is often used, as it has been found to be accurate and repeatable by rehabilitation research engineers ( Fig. 123-2 ). The hand is placed in the orthosis with the rubber band in the position where it will apply traction to the intended finger or joint segment. For determining the actual tension of a band on an orthosis, the length of the rubber band is first measured when it is under tension while in place on the patient . Then the rubber band cuff is slipped off the finger, and the rubber band or spring is again elongated in the same direction and to the same distance. The rubber band can be extended the same distance by pulling it with the arm of the strain gauge at its tip. When the rubber band is extended, any standard ruler can measure length as the force is read on the strain gauge. The force read is the tension being produced by the band or spring through the same range that it will act on the finger. In applying the arm of the strain gauge to the rubber band, care should be taken to keep the arm of the gauge perpendicular to the rubber band being measured, as angulations of the arm to the band can make measurements less precise.
In many cases, the question, “How much force?” has more than one answer. If a finger is to be flexed and extended while the patient wears the orthosis, tension in both positions must be measured at the ends of their arc of movement. For example, rubber band traction may measure 250 g when the finger is extended and 50 g when the finger is flexed. To many surgeons and therapists such figures will be meaningless, because they have not used them before and do not know what 100 g feels like. One can check a tension that “feels right” on the hand and then measure how much tension is there by using a spring scale. If this seems complicated, a little experience can go a long way to create understanding of what tension is optimal and establish the ability to measure the forces of traction.
Through What Surface?
Every applied force may be presumed to act on the bones or on the musculoskeletal system. However, it has to act through the surface of the body. (Often, the limits on the amount of force we can use are set more by what the skin can tolerate than by what the joint can accept.) Most forces in a hand orthosis are applied through a sling around a finger, and most are comfortable at the time they are applied. However, as time passes they may become uncomfortable or painful because of ischemia from pressure. This is probably the most common cause of patients discarding an orthosis or becoming “uncooperative.”
Both the ischemia and the pain are caused by pressure, not just by force . Pressure is force divided by area. A given force may be safe if the sling is wide enough, and unsafe if it is too narrow. A good general rule is that if a force is to be applied continuously, it should not result in a continual resting pressure of more than 50 g/cm 2 , about the same as 35 mm Hg, which is within the margin of safety for long-term pressure on soft tissues. Note that 35 mm Hg is higher than actual capillary pressure, but capillaries in normal tissue can withstand higher pressures. Thus, a 200-g pull needs a 4-cm 2 area of sling. All of the 4-cm 2 area should apply equal pressure to the finger. When in doubt, apply a cuff with given traction for a short period of time and observe the results. If the skin tissue underlying the cuff is red and stays red for longer than 15 minutes, then the tissue has been deprived of blood and the pressure is too high. Pressure is relatively unimportant if it is intermittent. Sustained pressure that reduces blood flow to the skin is what causes discomfort or pain, causes tissue damage, and may lead to skin necrosis.
A special danger from pressure occurs when a sling becomes tilted. Perhaps the finger changes its angle because it is responding to the treatment or simply because the whole orthosis is loose. Now the same applied force acts on only the edge of the sling. The actual effective surface of the sling may now be less than a quarter of what it was, so the pressure is multiplied by four. The slings should be checked during phases of finger opening and closing, especially in the resting position.
The problem of a tilting sling is so serious and so common that Brand never used a sling that could tilt. He covered the area of skin that would be under the sling with an equal area of plaster of Paris, forming a shell covering half the circumference of the digit. When that was set, the force could be applied by a nylon thread passed around the middle of the plaster shell and secured by a small strip of plaster added at just the apex of the curve. Now, if the direction of the pull changes, it does not tilt the plaster shell through which it is applied ( Fig. 123-3 ).