BONE ARCHITECTURE

The architecture of the proximal femur beautifully illustrates the general principle that the external form and shape of bone as an organ and the internal organization of bone as a tissue are well adapted to the forces placed upon them. There are dynamic internal forces as well as static and dynamic external forces on bone. The internal forces are created by muscle contraction; the external forces, by Earth’s ubiquitous gravitational field and by the dynamic compressive forces of weight bearing. The upper half of Plate 2-36 depicts the bony trabeculae of the proximal femur aligned along the lines of stress according to Wolff’s law. This intersecting network of trabeculae is the biologic response to the sum of internal and external physical forces on that region of the skeleton. These trabecular networks form arches that disperse and distribute loads corresponding to the principal tensile and compressive lines of force. Reduced weight bearing resulting from disuse or immobilization leads to a progressive thinning and eventual loss of these trabecular networks; those experiencing the greatest reduction in weight-bearing loads are resorbed first. A similar pattern is seen in all weight-bearing bones, including the loss of trabecular networks in the axial skeleton, especially in the vertebral bodies, which are largely composed of weight-bearing trabecular bone.

Mechanical forces also play a significant role in the external shape of bone. For example, the applied dynamic force of contraction of the gluteal muscles influences both the size and shape of the greater trochanter. If these muscles are paralyzed during skeletal development (as in certain types of poliomyelitis or in meningomyelocele), the greater trochanter does not attain its normal size and shape.

BONE REMODELING

Wolff’s law is also demonstrated by the straightening of a malunion of a long bone. With time, growth, and weight bearing, a malunion that has an angulation of as much as 30 degrees will straighten completely, at least in the infant and young child (lower half of Plate 2-36). This phenomenon would seem contrary to mechanical loading principles predicting that an angulated structure repetitively bent should eventually fatigue and fail. However, the exact opposite happens and the bone straightens with growth.

What is the explanation of this phenomenon? Some biologic or physical signal must arise from the concave side of the bone at the site of the malunion, inducing the osteoblasts there to lay down additional bone tissue, and a corresponding signal must arise from the convex side of the malunion, stimulating the osteoclasts there to remove bone tissue. The working hypothesis to explain this phenomenon lies in bone’s principal function—namely, to bear repetitive loads—whereby biophysical signals direct bone formation and resorption. Various investigations of this hypothesis identified the nature of signals in stressed bone and those in viable nonstressed bone. These studies determined that two types of electrical action potentials are present in bone: stress-generated, or strain-related, potentials and bioelectric, or standing, potentials.