Tornetta Rockwood Adults 9781975137298 FINAL VERSION

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SECTION ONE • General Principles

TABLE 1-3. Relative Energy Resulting in Fracture for Different Loading Directions

BIOMECHANICS OF FRACTURES

Bone

Load Direction

Failure Energy

The previous section outlined the basic biomechanical prin- ciples affecting the musculoskeletal system. This section will describe the loads that cause fractures, the postoperative load- ing that must be supported by fracture fixation constructs and how the fixation constructs affect the risk of complications after fracture treatment. TRAUMATIC LOADS RESULTING IN FRACTURE Fractures result from loads different in both magnitude and direction than the loads normally experienced during locomo- tion or activities of daily living. 127,146,227 The fracture pattern is largely determined by the direction of the applied load, whereas the severity of the fracture is determined by the magnitude of the load. 7,15 Because of the anisotropic nature of bone, the force required to cause a fracture varies based on the location and the direction of the applied load, with tensile fractures requiring the lowest load and compression fractures requiring the greatest (Table 1-3). The surrounding tissues can also affect the frac- ture pattern and severity by absorbing energy and changing the loading direction. 62 The following section outlines typical frac- ture patterns seen in clinical practice in the context of the loads required to generate them. Transverse Fractures Transverse fractures are oriented perpendicular to the long axis of the bone. They are caused by loading of the bone in tension, which causes failure in a plane perpendicular to the direction of applied load. Transverse fractures may result from internal load- ing as in the case of avulsion fractures at the attachment of ten- dons (Fig. 1-6). 4 Examples of transverse fractures from internal loading include basilar fifth metatarsal fractures as well as some patella fractures, medial malleolus, and olecranon fractures.

Patella

Tension

3 J

Vertebral body

Compression

100 J

Tibia

Torsion

10 J

Data from Carter DR, Schwab GH. Tensile fracture of cancellous bone. Acta Orthop . 1980;51(1–6):733–741.

Transverse fractures may also occur secondary to a low energy bending force in the absence of a significant compression com- ponent. 121 In this case, tensile forces are generated in the cortex opposite the applied bending loads, resulting in a fracture per- pendicular to the long axis of the bone. Because bone is weakest in tension, transverse fractures are typically lower energy relative to more complex fractures. 44 Oblique Fractures Oblique fractures are oriented diagonally to the axis of the bone. Three loading modes can result in oblique fractures: pure axial loading, combined bending and axial loading, and combined torsion and bending. In experimental tests, pure axial compression results in a short oblique fracture because bone is strong in compression and weak in shear. Under axial compression, the bone fails along the plane of maximum shear, which is 45 degrees to the long axis of the bone. Clinically, oblique diaphyseal fractures rarely occur in pure axial loading because the weaker cancellous bone in the metaphysis fails first, such as in a tibial plafond fracture. 4 A more common etiology for an oblique fracture is a combination of bending and axial compression, which produces a short oblique fracture line. 4,62 Another mechanism that produces short oblique fractures is a

Figure 1-6.  Fracture types resulting from applied loads. Compression and bending may result in either a short oblique fracture or a butterfly fracture, depending on the loads applied.

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