Dynamic traction should be used for the treatment of selected intra-articular fractures of the finger [Hardy 2004]. A dynamic traction splint involves the use of static tension while allowing joint movement. Traction provides tension to the ligaments, which incorporates the use of ligamentotaxis to bring boney fragments back into anatomic alignment. With constant traction applied throughout the range of motion, the splint allows the fracture to heal while maintaining adequate glide of surrounding soft tissues. Mobilization splints may also be used following a severe burn to assist in wound healing. The use of such splinting during wound healing will facilitate proper collagen alignment and scar formation.

Biomechanics of Dynamic Splinting

Anatomic Considerations

To fabricate a dynamic splint accurately, a therapist must understand the principles of hand biomechanics and must know in what ways the application of external force affects normal hand function [Fess 2002]. Knowledge of complex mathematical calculations is not required for a therapist to have a basic understanding of the biomechanics of dynamic splinting.

The goal of a mobilization splint is to restore a joint’s normal range of motion, and to minimize the effects of inflammation and scar tissue. Application of an external force to healing joint structures raises the following questions. During what stage in the healing process should a splint be used to apply force? How much force should the splint apply? Where should the force be applied? The therapist should exercise caution when applying force to an injured joint until the inflammation and pain are under control. Mild inflammation is acceptable, but edema should not fluctuate significantly. A mobilization splint applied too early after injury might result in increased inflammation and decreased motion.

Soft-tissue structures respond to prolonged stress by changing or reforming. This activity is called creep and results from the application of prolonged force [Brand and Hollister 1993b]. Soft tissue responds to excessive force with increased pain and a reintroduction of the inflammatory process [Fess and McCollum 1998]. By applying controlled stress to the tissue over a prolonged period of time, the therapist can create tension gentle enough to allow creep without tissue injury. Provided it remains within elastic limits, the stress from a mobilization splint can positively affect the gradual realignment of collagen fibers (resulting in increased tensile strength of the tissue) without causing microscopic tearing of the tissue. The ability to alter collagen formation is greatest during the proliferative stage of wound healing but continues to a lesser degree for several months during scar maturation [Colditz 2002].

Torque and Mechanical Advantage

To provide the greatest benefit from a mobilizing splint, the therapist must understand relevant theories of physics. Mechanical advantage involves the consideration of various forces applied by the splint base and the dynamic portion of the splint. As seen inFigure 11-2, Fa refers to the applied force and Fr refers to the resistance force. Fm is determined by the sum of the opposing forces (Fa+ Fr) [Fess 1995]. Mechanical advantage is defined as [Brand 2002]:

• Length of the lever arm of the applied force (la)

• Length of the lever arm of the applied resistance (lr)

By adjusting the length of the splint base or the length of the outrigger, the mechanical advantage can be altered (Figure 11-2) [Smith et al. 1996]. The goal of a splint is to maintain a mechanical advantage of between 2/1 and 5/1, meaning that the lever arm of the applied force is at least twice as long as the lever arm of the applied resistance [Brand 2002]. A splint with a greater mechanical advantage will be more comfortable and durable [Fess 1995]. A mobilizing MCP flexion splint that is forearm and hand based rather than just hand based will disperse pressures in addition to providing a greater mechanical advantage due to the longer lever areas of the applied force.

Figure 11-2 Mechanical advantage is determined by the ratio of the lever arm length (la) of the applied force (Fa) to the lever arm (lr) of the applied resistance (Fr). Splint A has a better mechanical advantage than splint B.

Torque is defined as the effect of force on the rotational movement of a point [Fess et al. 2005]. The amount of torque is calculated by multiplying the applied force by the length of the moment arm (Figure 11-3). A correlation exists between the distance from a pivot point and the amount of force required. To achieve the same results, a force applied close to the pivot point (i.e., short moment arm) must be greater than the force applied on a longer moment arm. This force is called torque because it acts on the rotational movement of a joint.

Figure 11-3 The 2-inch moment arm produces 24 inch ounces of torque. The 3-inch moment arm produces 36 inch ounces of torque.

In practical terms, the therapist should place the force as far as possible from the mobilized joint without affecting other joints [Brand and Hollister 1993b]. A forearm-based dynamic wrist extension splint should be constructed so that its mobilizing force is on the most distal aspect of the palm, while not affecting MCP movement. An exception to placing the force as far from the mobilized joint as possible occurs when rheumatoid arthritis is involved. If the joint is unstable, a force applied too far from the joint will result in tilt rather than a gliding motion of the joint (Figure 11-4) [Hollister and Giurintano 1993]. Therefore, when splinting a hand with rheumatoid arthritis the force should be applied as close to the mobilizing joint as possible.

Figure 11-4 A force applied too far from an unstable joint will result in “tilt” (A) rather than glide (B). (From Hunter JM, Mackin EJ, Callahan AD (eds.) (1995). Rehabilitation of the Hand, Fourth Edition. St. Louis: Mosby p. 1586.)

Application of Force

In dynamic splinting, the therapist applies force to a joint or finger through the application of nail hooks, finger loops, or a palmar bar. When applying force to increase passive joint range of motion, the therapist must keep the direction of pull at a 90-degree angle to the axis of the joint and perpendicular to the axes of rotation [Cannon et al. 1985]. As range of motion increases, the therapist must adjust the outrigger to maintain the 90-degree angle (Figure 11-5) [Fess et al. 2005]. The outrigger should not pull the finger or hand toward ulnar or radial deviation.

Figure 11-5 The line of tension must be maintained at 90 degrees from the long axis of the bone.

When excessive force is applied to the skin for a prolonged period, tissue damage can result. The amount of pressure the skin can tolerate dictates the maximum tolerable force. As a general rule, the amount of acceptable pressure or force per unit area is 50 g/cm² [Brand and Hollister 1993a]. As the area over which force is applied becomes larger, the pressure per unit area becomes less. A leather sling with a skin contact area on a finger of approximately 4 cm² should provide a maximum pressure of 200 g [Fess at al. 2005]. A smaller sling with less skin contact area concentrates the pressure and is less tolerable.

Skin grafts, immature scar tissue, and fragile skin of older persons have less tolerance for sling pressure. The person’s tolerance ultimately determines the amount of force. The person should report the sensation of a gentle stretch, not pain [Fess 2002]. To avoid harm, the therapist should monitor the splint for the first 20 to 30 minutes of wear and at every treatment session thereafter. The person must also be educated on the importance of monitoring the splint for signs of pressure areas and skin breakdown, as well as how to don and doff the splint properly.

Features of a Mobilization Splint

Two features unique to mobilization splinting are the use of an outrigger and the application of force. The outrigger is a projection from the splint base the therapist uses to position a mobilizing force. The outrigger material depends on the amount and position of the desired force. If the outrigger and attachment to the base are not secure, the direction of the mobilizing force may change—reducing the effectiveness of the splint [Colditz 1983].

An outrigger can be a high or low profile (Figure 11-6). Each type has advantages and disadvantages. With a significant change in range of motion, the high-profile outrigger will result in slightly less deviation from the 90-degree angle of pull than the low-profile outrigger. Clients should be seen in the clinic frequently enough that increases in range of motion can be accommodated by regular adjustments to the outrigger, thus maintaining the 90-degree angle of pull [Austin et al. 2004]. It should be noted that a high-profile outrigger, on the other hand, is bulky and may decrease the person’s compliance with wearing the splint. A low-profile outrigger requires adjustments more frequently but is more aesthetically pleasing and less cumbersome.

Figure 11-6 A low-profile outrigger (left) versus a high-profile outrigger (right).

Various materials might be used for outriggers. The therapist may roll a thermoplastic material that has a high level of self-adherence to form a thick, tubular outrigger. This material offers easy adjustment by reshaping the plastic. However, a thermoplastic material outrigger may make the splint more cumbersome and thus reduce the person’s compliance with the wearing schedule. There are also commercially available low-temperature tubes that are easily formed and less cumbersome.

A therapist can form an outrigger from -inch wire rod. This diameter is thick enough to provide stability yet pliable enough to manipulate with pliers. Construction of an outrigger using a wire rod requires precise shaping, a skill that necessitates practice. Commercial adjustable wire outrigger kits are also available. Although the use of a commercial kit may increase the material cost of the splint, the application of the adjustable components is easier and may thus reduce splintmaking time. The therapist should bear in mind that charges for fabrication time are the most costly part of the splint.

The therapist can use various methods for applying dynamic force to a joint. Finger loops from strong pliable material are usually best because of the increased conformability to the shape of the finger [Fess et al. 2005]. The therapist can supply force by using rubber bands, springs, or elastic thread. Although rubber bands are more readily available and easy to adjust, springs offer more consistent tension throughout the range. A long rubber band stretched over the maximum length of the splint provides more constant tension than a short rubber band [Brand and Hollister 1993a]. Elastic thread is the easiest to apply and adjust. A nonstretchable string or outrigger line is necessary to connect the finger loop to the source of the force (Figure 11-7). The choice is usually based on clinical experience and preference.

Figure 11-7 The therapist uses nonstretchable nylon string to attach finger loops to the source of tension.

Another method of applying force is through static progressive tension. Rather than providing the variable tension of a dynamic splint, a static tension splint uses nonelastic tension to provide a constant force. An advantage of properly applied static tension is that tissue is not stretched beyond the elastic limit [Schultz-Johnson 2002]. In place of the rubber band or spring (as used on a dynamic tension splint) the therapist may use a Velcro tab, turnbuckle, or commercially available static progressive components to apply the force (Figure 11-8). Tension is increased by gradually moving the Velcro tab more proximally on the splint base or adjusting the turnbuckle. The force is static rather than dynamic but is readily adjustable by the person throughout the wearing time (Figure 11-9). Because the person has control over the amount of applied tension, the static progressive splint is more tolerable to wear than a dynamic tension splint [Schultz-Johnson 2002].

Figure 11-8 A turnbuckle can be easily adjusted to provide static tension.

Figure 11-9 The person may adjust Velcro tabs used for static progressive tension.

In determining whether to apply dynamic or static tension, the therapist must identify the “end feel” of a joint. End feel is assessed by passively moving a joint to its maximal end range. A joint with a soft or springy end feel indicates immature scar tissue. A joint with a hard end feel indicates a more mature scar tissue or long-standing contracture. A splint with static or dynamic tension is appropriate for a joint with a soft end feel, whereas a joint with a hard end feel will respond only to static tension. Regardless of the end feel, static tension will increase passive range of motion faster than dynamic tension for any joint [Schultz-Johnson 1996].

Another determinant in selecting the type of tension to be used with mobilization splinting is the stage of tissue healing. As seen in Figure 11-10, different types of splints are more appropriate at various stages of healing. The acute stage is primarily characterized as the initial inflammatory stage. The proliferative stage occurs after the initial inflammation subsides and tissues are in the early stages of reorganization. The chronic stage is attained when the cells have realigned and the joint response to stress is a hard end feel [Colditz 1995].