Bone Composition

It is assumed that persons reading this text will already be somewhat familiar with the basic composition of bone. However, in trying to convey the effects of the physics

Milliseconds

Fig. 2. Sample force plot from femur impact at 7.5 m/s.

involved in bone fracture, we must briefly revisit the materials of which bone is composed and discuss them in terms that engineers might use.

A bone is a nonhomogeneous, composite organ consisting of several types of tissue, although osseous connective tissue is dominant. Bone occurs in two forms: a low-density tissue (0.05-1.0 g/cc) termed cancellous, trabecular, or spongy bone; and a high-density form (1.8-2.0 g/cc) referred to as compact, cortical, or hard bone (2,3). In long bones, both cancellous and compact osseous tissues are present, but their relative amounts vary by region. The epiphyses are large masses of cancellous bone covered by a thin layer of compact bone (and hyaline cartilage within the joint cavities). The diaphysis is a thickened tube of compact bone that has a thin layer of cancellous bone lining its medullary (marrow) cavity. The composition of the patella and tarsal bones is very similar to that of the epiphyses.

As with most connective tissues, the extracellular matrix is the defining feature, not the cells themselves. Water accounts for approx 25% of total bone weight. The remaining osseous connective tissue is generally described as roughly 50% organic and 50% inorganic by mass. The protein collagen accounts for about 95% of the organic extracellular matrix. Embedded in the matrix are the inorganic crystals of the mineral hydroxyapatite, Ca10 (PO4)6 (OH)2. The crystals are 50 to 100 angstroms (A; where 1 A = 10-7 mm) long and provide bone with great compressive strength, while the collagen gives bone considerable tensile strength (3-5).

Compact bone is arranged in functional units termed osteons or Haversian systems. Osteons take the form of small columns organized parallel to the long axis of a bone. Each osteon is approx 20 mm long and 200-400 |m in diameter (6). At the center of each adult osteon is a small canal roughly 70 |m in diameter that transmits vascular components. Mature bone cells, or osteocytes, are arranged in concentric rings around each canal. The organic and inorganic components occupy the areas between osteocytes, such that each cell is enclosed in a small cave termed a lacuna. Tiny canaliculi serve as conduits for nutrients and waste transport between the lacunae and blood vessels in the larger central canal system. The concentric rings of cells are spaced according to very organized patterns of collagen and hydroxyapatite crystals. This composite of organic and inorganic materials, combined with nonuniform shapes, gives bone the property of being anisotropic. In terms of traumatic impact, this means bone behaves differently when the impact arrives from different angles. For example, bone is significantly more resistant to compressive forces along its long axis than when it is struck transverse to the axis. This is a product of the tendency of the osteonal structure to follow the long axis of the bone (7). Additionally, it has been shown in cadaver studies that lower-extremity long bones offer greater resistance to anterior impacts than to medial, lateral, or posterior strikes, but there is no statistically significant difference between the breaking strength of left and right bones struck in the same plane (1).

The next section serves as an introduction to biomechanics as it relates to bone. There are many excellent texts (e.g., 8-10) devoted to biomechanics. However, the text by Lucas et al. (11) is highly recommended for anyone seeking an efficient discussion of details that cannot be covered in this single chapter. Their paperback text published in 1999 is particularly interesting and well organized. Each of the 17 chapters begins with a "Clinical Question," and the authors skillfully guide the reader (presumably an orthopedic resident) through some fairly complicated concepts that can only be introduced here.

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