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Mechanical Principles of Hand Splinting

by Aubrey Bailey

Hand splints are commonly used to immobilize the hand and fingers after injury. Custom splints are made from low-temperature thermoplastic materials by trained health care professionals. Improper splinting can make hand injuries worse, or even lead to deformities. Mechanical principles must be followed when splinting the hand to ensure that the forces and stresses are correct.

Overview

There are four types of hand splints: static, dynamic, serial static and static progressive. Static splints hold the hand in one position and are typically used during the immobilization phase of healing to decrease pain and prevent movement. Dynamic splints are used to improve motion -- or mobilize tissue in the hand. Serial static and static progressive splints hold structures in a stretched position. As flexibility increases, these splints are remolded to stretch the tissues in the new position.

First-Class Lever

Hand splints are first-class levers -- like a seesaw. The axis of motion -- fulcrum -- is located between the load on one end of the lever arm, and the resistance on the opposite end. For example, a dynamic wrist extension splint covers approximately half the length of the forearm and continues up into the palm. The fulcrum is the wrist joint -- where the motion will occur. The splint is secured with straps to the forearm -- one end of the lever arm. The other end of the lever arm is in the palm, pushing the wrist backward into extension. Hand splints aim to position the fulcrum at the joint where motion is desired, with two pressure points as far from the fulcrum as possible to maximize leverage.

Force

Dynamic hand splints use an external force to stretch tight structures. The angle and amount of force applied by a hand splint directly impact its effectiveness. Forces that are too high can increase pain and inflammation, and cause excess scar formation. In the hand, force must be applied at a 90 degree angle -- perpendicular -- to the finger being stretched. Soft slings wrap around the finger, and string is attached to pull the finger into a stretched position. Metal outriggers are frequently used to direct the line of pull for hand splints. As joint motion improves, the line of pull must to be adjusted to maintain a 90 degree pull.

Stress

Stress is defined as the amount of force applied per unit of area. Force applied at a 90 degree angle is considered to be normal stress. Force applied at the incorrect angle produces shear stress, leading to skin breakdown. Several steps are taken to reduce shear stress with hand splinting. Edges of the splint are flared or rounded to keep them from cutting into the surrounding skin. Wide straps are used to secure the splint to the arm to prevent excess movement or friction. A long forearm base is used to evenly distribute pressure over a larger area, reducing the risk of skin breakdown from high-pressure areas.

About the Author

Aubrey Bailey has been writing health-related articles since 2009. Her articles have appeared in ADVANCE for Physical Therapy & Rehab Medicine. She holds a Bachelor of Science in physical therapy and Bachelor of Arts in psychology from the University at Buffalo, as well as a post-professional Doctor of Physical Therapy from Utica College. Dr. Bailey is also a certified hand therapist.

Photo Credits

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