TIP OF THE MONTH: February/March 2014
The Laws of Material Concentration

by David Gallant BSN, RN, OTC

The application of casts and splints is affected not only by the products that are used but more often how the process is influenced by the physical laws of nature. This article addresses several of those factors and my view as to how they apply to the stabilization of injuries.

As brilliant as he was, Galileo Galilei was not the inventor of gravity. In the early 1600’s, he did observe that “gravity is the attractive force between objects (1).” The ease of splint application is aided by acknowledging the effects of gravity on what we do and methods that we may employ to cope with those effects.

In Figure A, the splint on the supinated hand is being held perpendicular with the hand preventing the splint from moving any further. Now, if the hand is pronated, as in Figure B, without the resistant force of the assistant’s supporting hand, the splint is headed for the floor. To make work easier for the applicator, in Figure C, the arm and splint are brought into the same gravitational line. Hence, securing the splint and any other accompanying splint may be accomplished without adverse wrinkling. That is assuming that the applicator will not prematurely release the grasp of the splint. Unfortunately, if the applicator lets go, the splint is destined to end up on the floor in a crumpled heap.

Left to right: Figure A, Figure B, Figure C.

The notion of the Force of Friction may remedy the disposition of that splint. Galileo reasoned that, “Whenever two surfaces move or try to move across each other a force called friction is created.” Friction, which is dependent upon the texture of both surfaces, always opposes the motion or attempted motion across the surfaces. Friction is also dependant on the amount of contact that pushing the two surfaces together. With two smooth surfaces, such as the skin and the stockinet covering the fiberglass or plaster, there is little friction between the two surfaces causing cast to fall. Now, what would happen if the skin was covered with a textured surface, such as cast padding, instead of stockinet? The wet plaster is placed directly against the padding, and the friction between the two surfaces is so great that if the application is held perpendicular to the floor, the splint will usually stay in place. The application of a smooth padded surface, such as Webril, will achieve the impossible: the adherence of wet plaster to the underside of an extremity being held parallel to the floor.

It is a popular fable that while sitting under an apple tree, Sir Isaac Newton was inspired by a knock on the head from a falling apple. In his book, Principia, he described his theory of universal gravitation and his three laws of motion, which to some degree affect the level of trauma that we see daily (2). It is his familiar third Law “For every action there is an equal and opposite reaction”, that affects what we do when attempting to stabilize a fractured bone.

The Laws of Material Concentration (LMC)
Knowledge of the laws of nature will prepare those who must stabilize extremities with the techniques to provide safe and affective management. To that end, I propose the Laws of Material Concentration (LMC) to help to explain specific issues associated with the stabilization of extremities.

The First LMC
The shape of the human body sets the ground rules for stabilization of extremities. Hence, any useful, practical stabilizing device that is applied to the extremity cannot be absolutely flat. Not only must it have some curvature and angle, it must be secured circumferential to the body by something, whether it is adhesive tape, strips of gauze, cast padding, or Velcro for the device to remain in place.

The body is essentially a fluid sac filled with an internal tubular supporting framework. If this internal support loses its structural integrity, the body immediately begins the lengthy process of self-repair. This osteogenesis is supported by internal fixation (whether it be intramedullary rods, plates and screws, or a combination of the two) and external support of splinting, casting, or bracing. Thus, due to the shape of the body, any exterior support must be circumferential and tubular by design.

As seen in Figure D, a straight tube may correctly align a bone on one axis, but it does little to address rotation or angular deformity unless it is purposely bent and angled. Extremities are most often best stabilized in a neutral position, which does not mean absolutely straight (4).

Left to right: Figure D and Figure E

If a one dimensional, lateral view is shown of a bent tube (Figure E), one will note that the tube is made up of two parallel angles separated by the width of the body and the thickness of the applied support. The point or peak at which the lines of each angle meet is called the Vertex (3). Splints and casts will all have an Anterior and Posterior Vertex and are referred to as “inside or outside vertex,” relative to the extremity in need of stabilization. For example, the elbow will have an Anterior Inside Vertex (AIV) and a Posterior Outside Vertex (POV), whereas a knee will usually have an Anterior Outside Vertex (AOV) and a Posterior Inside Vertex (PIV). Thus, the First LMC states that: “Any useful, practical extremity stabilizing device cannot be absolutely flat.”

The Second LMC
There will be more wrapped casting material found at the inside vertex. Because of this concentration of material, an increase in tissue pressure is directly proportional to the increase in the joint angle.

Without making any alterations in the natural flow of wrapping any amount of material (cast padding, fiberglass, elastic wrap etc.) will always concentrate at the peak of the inside vertex.

Example 1: Elastic bandages wrapped around a knee will concentrate all of their tension on the soft tissue of the popliteal region (PIV) as the knee angle is increased. The result is the creation of deep wrinkles which may be filled by hard material which will ultimately cut into tissue.

Example 2: Elastic bandages wrapped around an elbow will concentrate all of their tension on the soft tissue of the antecubital region (AIV) as the elbow angle is increased. The result is the same as in Example 1 and seen in figure F.

Figure F

The first step in resolving this problem is acknowledging the occurrence of this phenomenon. For the next step, I recommend distributing the pressure away from the acute angle by applying a disorganized clump of padding into the antecubital region and securing it with circumferential padding. This technique creates a gentle slope of padding rather than an acute angle and avoids the aforementioned cutting effect.

The Third LMC
There will be less wrapped casting material found at the outside vertex. The decrease of protective padding is directly proportional to the increase in the joint angle.
Without making any alterations in the natural flow of wrapping any amount of material (e.g. cast padding, fiberglass, elastic wrap, etc.) will always be the thinnest at the base of the angle.

Example 1: Cast padding wrapped around the foot and ankle will concentrate all of their bulk on the dorsum which will result in very little padding on the heel (POV) and will become thinner as the angle of the ankle is increased as in Figure G. The result is the creation of deep wrinkles on the dorsum (AIV) which may be filled by hard material which will ultimately cut into tissue and insufficient padding on the heel which will result in pain and ultimately a pressure ulcer. See Figure G.

Figure G

Example 2: Cast padding wrapped around an elbow will concentrate all of its bulk on the soft tissue of the antecubital region, as the elbow angle is increased, and will result in thinner padding on the posterior elbow (POV) and subsequent pain and potential tissue damage if left unaddressed.

Example 3: A short leg cast is applied in neutral with ¼” of fiberglass on the anterior ankle and visible stockinet on the heel. Again, the first step in resolving this problem is acknowledging the occurrence of this phenomenon. In addition to the circumferential padding, extra padding, such as a star pad, placed over the bony prominences of the posterior elbow, is the recommended method for avoiding excessive cast pressure. Casting the heel area is improved with using a fan-folding technique or applying a circle-rolled, “x number-ply” splint to reinforce the heel. This same reinforcing maneuver should be used at the posterior elbow in the long arm cast and in a hip spica cast, the central region of the gluteus maximus (POV).

The Fourth LMC or “The Intern’s Triangle”
Complications involving the first three Laws are exacerbated by physics, biomechanics, product design, and inexperience. “The Intern’s Triangle” is a disparaging term, which refers to the area of cast or splint that is 180 degrees away from the line of vision of an inexperienced practitioner. Due to lack of proper experience, the act of wrapping casting material quite often results in either lack of padding, or any cast material for that matter, in the elbow, heel, patella, lateral buttocks (hip spica cast), or base of thumb. The results are ill padded casts which are structurally unsound.

An alternate example of the Law of Material Concentration is to view the anterior / posterior contour of the slightly flexed leg as a series of peaks and valleys. As material is wrapped around the leg, the valleys will fill up whereas the peaks will be sparsely covered. Examination of the same leg in the prone position will reveal that the peak of the heel will be padded sparsely while the valley of the popliteal region will be full.

With experience and being aware of the effects of the Laws of Material Concentration, the application of material will be done in a proper manner and the stabilization of the extremities will be uncompromised by iatrogenic creations.

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About the Author


David Gallant BSN, RN, OTC resides in Westbrook, Maine, is employed by Orthopedic Trauma and Fracture Care in Portland, Maine, and has been a member of NAOT since 1983.