MECHANICAL INJURY

"Force or mechanical energy is that which changes the state of rest or uniform motion of matter. When a force applied to any part of the body results in a harmful disturbance in function and or structure, a mechanical injury is said to have been sustained." 

1. Injuries related to sports participation can be caused by external forces directed on the body or can occur internally within the body.

2. To understand sports injuries there must be a knowledge of tissue susceptibility to trauma and the mechanical forces involved.

Tissue Properties

Tissues have relative abilities to resist a particular load. The stronger the tissue, the greater magnitude of load it can withstand. Strength pressure, or power, is often used to imply a force. A force can be described as a push or pull. 

Tissue properties are described according to engineering terminology:

1. A load can be a singular or group of outside or internal forces acting on the body.

2. The resistance to a load is called a mechanical stress, and the internal response is a deformation, or change in dimensions. Deformation is also defined as a mechanical strain.

3. All human tissue is viscoelastic; it has both viscous and elastic properties, allowing for deformation. Tissue such as bone is brittle and has much fewer viscoelastic properties when compared to softer tissue. Tissue also is anisotropic, responding with greater or lesser strength depending on the direction of the load that is being applied.

4. When tissue is deformed to the extent that its elasticity is almost fully exceeded a yield point has been reached.

5. When the yield point has been exceeded mechanical failure occurs, resulting in tissue damage.

There are three primary tissue stresses leading to sports injuries: tension, compression, and shear.

Tension is that force that pulls or stretches tissue. 

• Stretching beyond the yield point leads to rupturing of soft tissue or fracturing of a bone.

• Examples of stretching injuries are sprains, strains, and avulsion fractures.

Compression is a force that, with enough energy, crushes tissue.

• When the force can no longer be absorbed, injury occurs.

• Where there is constant submaximum compression over a period of time, the contacted tissue can develop abnormal "wear." 

• Arthritic changes, fractures, and contusions are commonly caused by compression force.

Shearing is a force that moves across the parallel organization of the tissue.

• Injury occurs once shearing has exceeded the inherent strength of a tissue.

• Shearing stress can result in skin injuries such as blisters, rips of the hands, abrasions, or vertebral disks injuries

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SOFT TISSUE TRAUMA

SKIN INJURIES

Generally, trauma that happens to the skin is visually exposed and is categorized as a skin wound. It is defined as a break in the continuity of the soft parts of body structures caused by a trauma to these tissues.

Anatomical Characteristics

The skin, or integument, is the external covering of the body. It represents the body's largest organ system and essentially consists of two layers:  the epidermis and the dermis. Because of the soft, pliable nature of skin, it can be easily traumatized.

Injurious Mechanical Forces

Numerous mechanical forces can adversely affect the skin's integrity. These forces are friction or rubbing, scraping, compression or pressure, tearing, cutting, and penetrating.

Wound Classification

Wounds are classified according to the mechanical force that causes them (Table 6-1).
                                                                                                 
Friction Blisters

• Continuous rubbing over the surface of the skin cause a collection of fluid below or within the epidermal layer called a blister.

Abrasions

• Abrasions are common conditions in which the skin is scraped against a rough surface. The epidermis and dermis are worn away, exposing numerous blood capillaries.

Skin Bruise

• When a blow compresses or crushes the skin surface and produces bleeding under the skin, the condition is identified as a bruise, or contusion.

Laceration

• A laceration is a wound in which the flesh has been irregularly torn.

Skin Avulsion

• Skin that is tom by the same mechanism as a laceration to the extent that tissue is completely ripped from its source is considered an avulsion injury.

Incision

• An incision wound is one in which the skin has been sharply cut.

Puncture Wound

• Puncture wounds, as the name implies, are penetrations of the skin by a sharp object.