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Maintenance and Adaptation of Bone Tissue: the importance of mechanical stimuli
Chris R Jacobs, PhD; Gary S Beaupré, PhD; Dennis R Carter, PhD
The skeletal system serves a number of important functions, however, its primary purpose is a structural one. In particular, large forces are supported by the bones of the lower limb during ordinary activities such as rising from a chair, walking, or climbing stairs. The magnitude of the forces at the hip joint frequently exceeds three times body weight. The bones of the lower limb must be capable of withstanding the repeated application of these loads as many as 10,000 times each day for an active individual.
Experimental studies have shown that the integrity and maintenance of the skeletal system is critically dependent upon sufficient levels of daily mechanical stimulation. Skeletal disuse from prolonged bedrest or cast immobilization can cause significant bone loss. Bone loss in astronauts during space flight can be avoided only with intensive exercise regimens. Alternatively, an increase in bone mass can result from increased mechanical demands as observed in the dominant arm of professional tennis players.
A relationship between mechanical stimuli and bone adaptation was suggested more than 100 years ago. Several mathematical theories for bone adaptation have been developed in recent years. Our research group has developed a comprehensive theory for bone adaptation that can be used to simulate bone changes during development, growth, adaptation and aging. Because of its importance to locomotion and total joint replacement, we have focused on the proximal femur to assess the accuracy of our theoretical models and computational simulations. Figure 1 shows a radiograph of a slice from the proximal femur. Key architectural features visible in this slice include thick columns of compact bone in the shaft of the femur and an intricate pattern of cancellous bone within the femoral head and neck.
Figure 1. This radiograph of a coronal section taken from the proximal
femur reveals some of the structure of cancellous bone. Note that some regions
are made up of highly organized trabecular bone while other regions are much
less oriented.
The results of a computer simulation showing the predicted distribution of bone density within the proximal femur are shown in Figure 2. This distribution of bone density bears a strong similarity to the actual distribution in Figure 1. Displaying the results in the form of a density distribution (Figure 2), however, does not illustrate the highly directional (anisotropic) material properties of bone. By adopting ideas from other engineering disciplines (continuum damage mechanics) we have modified our bone remodeling theory making it possible to predict the anisotropy of the bone material, as well as the density distribution. The anisotropic orientations at select points are shown in Figure 3. The predicted orientations bear a strong similarity to the fine-scale, cancellous bone patterns seen in Figure 1.
Figure 2. The density distribution predicted with the anisotropic remodeling formulation. |
Figure 3. The associated predicted stiffness tensor was used to prepare this plot of the normal stiffness as a function of direction for selected elements. |
The anisotropic extension of our bone remodeling theory, therefore, represents a significant advance in our ability to simulate bone adaptation in response to changes in loading or the presence of prosthetic implants. These kinds of simulations should make it possible to improve implant design leading to longer lasting prosthetic joint replacements.
Republished from the 1994 Rehabilitation R&D Center Progress Report. For
current information about this project, contact
Gary S Beaupré.
