Chapter 1

### Biomechanics

A fracture occurs when the stress applied exceeds the plastic strain of the bone and goes beyond its yield point. A number of factors influence fracture patterns. These include the magnitude of force, its duration and direction, and the rate at which it acts. When a bone is subjected to repeated stresses, the bone may ultimately fracture even though the magnitude of one individual stress is much lower than the ultimate tensile strength of the bone. The strength of a bone is related directly to its density, which is reduced by osteoporosis or any condition in which the osseous structure is changed, thus lowering its resistance to stress.

### Terminology

Fractures can be described in a number of ways. No one system of classification is all-encompassing, and physicians dealing with fractures on a day-to-day basis must be aware of the terminology to better understand and convey information to colleagues. It should be noted that to adequately describe a fracture, at least two perpendicular radiographic views should be obtained.

#### Direction of Fracture Lines

• Transverse: A transverse fracture runs perpendicular to the bone (Fig. 1–1A).
• Oblique: An oblique fracture runs across the bone at an angle of 45 to 60 degree (Fig. 1–1B). These fractures are due to compression and flexure at the fracture site.
• Spiral: A spiral fracture can sometimes be misdiagnosed as an oblique fracture; however, on closer study, a “corkscrew” appearance of the fracture is noted (Fig. 1–1C). It is a highly unstable fracture that is prone to poor healing. Spiral fractures are due to a torsional force. In children, it may be a clue to potential abuse.
• Comminuted: A comminuted fracture is any fracture in which there are more than two fragments (Fig. 1–1D). Other examples of comminuted fractures are the segmental and butterfly fractures (Fig. 1–1E and 1–1F).
• Impacted: An impacted fracture is one where the fractured ends are compressed together. These fractures are usually very stable (Fig. 1–1G).

###### Figure 1–1.

The classification of fractures. Segmental and butterfly fractures are specific types of comminuted fractures.

#### Anatomic Location

• In a long bone, fractures are categorized as being in either the proximal, middle, or distal portions of the bone.
• If the fracture extends into the joint space it is described as intraarticular. Fractures that do not involve the joint are extraarticular.
• Other anatomic terms used to describe the location of a fracture are head, neck, shaft, and base (e.g., metacarpal and metatarsal fractures).
• In pediatrics, fractures are described in relation to the growth plate (physis). Fractures that occur between the joint and the growth plate are epiphyseal fractures. Fractures of the diaphysis refer to the shaft of the bone. The zone of growth ...

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