Alan R. Cross

Bone Structure

Bone is a remarkably complex tissue. Although unique features are noted among species, bone broadly conserves its general structure and shape. The femur, for instance, is usually recognizable as a femur, regardless of the species of origin. Common ancestral origins likely account for this conservation of shape, although subtle differences develop over time as the result of evolutionary pressures. The functional demands placed on the skeleton also contribute to the shape of its bones. The presence of light but strong bones liberates often scarce energy resources for other metabolic uses. Although the metabolic requirement of bone is relatively low in skeletally mature organisms, the weight of the skeleton poses a greater metabolic concern. All but the largest organisms need metabolically efficient locomotion to escape predators or pursue prey. Carrying around as little bone weight as possible provides a survival advantage. According to Wolff’s law, bone optimizes itself by forming in areas of high stress and not in areas of little stress, resulting in a bone shaped for greatest strength with the least weight.

The ultrastructure of bone contributes to its functional behavior as well. Bone tissue is viscoelastic: The strength of bone depends upon the rate at which it is loaded. Bone is stronger when loaded rapidly than when loaded slowly. This property likely has evolutionary origins as well. Bone fracture tends to result from impact with high-loading-rate forces. In another illustration of metabolic optimization, viscoelasticity allows bone to be strong enough to sustain sudden impact, such as falling, without so much mass to impair normal locomotion. A cortical bone property related to its ultrastructure is anisotropy. Because the mechanical properties of bone strongly depend on the direction of loading, long bones generally better resist forces placed along the long axis than those across the axis. This makes evolutionary sense as well in that long bones are usually oriented perpendicular to the forces of gravity.

Biomechanical Concepts

A thorough understanding of these unique properties and behaviors of bone tissue is essential for achieving favorable results when attempting to manipulate bone healing, as are some basic biomechanical definitions and concepts.

Fracture Biomechanics

Bones are subjected to many and combined forces during normal function. When the magnitude of the sum of these forces exceeds the ultimate strength of the bone, a fracture occurs. It is conceptually useful to identify these forces by their orientation to the loaded bone (Figure 41-5).

Axial forces act parallel to the long axis of the bone. Further divided, tensile forces tend to lengthen the bone and compressive forces tend to shorten it uniformly. Shear forces are difficult to conceptualize with respect to an entire bone but are common within bone. Shear forces act parallel or tangential to the face of a material (Figure 41-6).

Torsional forces act to twist the bone about its long axis. They create shear stress in the bone (tension and compression in oblique planes), which increase with distance from the center axis.

Bending forces act to make the bone convex on one side and concave on the opposite side. Bending forces are often referred to as moments. A moment is a tendency for a force to twist or rotate an object and is expressed in units of torque. Bending loads may be applied in different ways. Pure bending results from applying an equal and opposite bending moment at each end of the bone. This causes a uniform bending moment across the length of the bone (Figure 41-7, A). As such, a fracture could occur at any point along the bone. This bending situation rarely occurs in the appendicular skeleton. Cantilever bending results from fixing the bone at one end and applying a transverse load at the other. This causes a bending moment that is highest at the fixation point, and from there it linearly decreases (Figure 41-7, B). For example, when a soccer player kicks a ball, the proximal tibia becomes the fixation point, and the transverse load is applied to the distal tibia through the ankle joint. In cantilever bending, the fracture will occur at the site of fixation. Three-point bending results from an equal load applied at each end and an opposite load applied at some point between the ends. This causes a bending moment that is highest at the site of load application and decreases linearly with distance from that site (Figure 41-7, C). In this case, the soccer player, standing on the field, gets kicked in the tibia by another player. In three-point bending, the fracture will occur at the site of load application. Four-point bending results from loads applied at each end and two opposite loads applied at points between the ends. This causes a bending moment that increases from one end to the first inner load, is constant to the next inner load, and decreases to the other end load (Figure 41-7, D). This loading condition is rare in the appendicular skeleton as well.