MAIN FUNCTIONAL REQUIREMENT: Allow relative movement between the tibia and the femur bones as is required for walking, running, jumping, etc.
DESIGN PARAMETER: Knee joint (hinge joint)
The joint where the femur and tibia meet must be able to flex, extend and rotate while withstanding large amounts of force exerted on it and relative sliding between components during vigorous activity
The parts of the knee fit into 5 basic categories as follows:
Between components of the knee, each force exerted in one direction must be counter-balanced by a force in the opposite direction. Muscles, ligaments, tendons and bones combined are able to do this as shown in the following diagrams. They can be analyzed roughly like a mechanical linkage.
Ligaments, bones and muscles can only withstand so much force. Unfortunately, this means they can be strained, broken, and knocked out of place by a force greater than those encountered in normal activities or by repeated use in a high impact way. That limit varies from person to person depending on the size and strength of each individual's muscles, tendons, bones and ligaments. Furthermore, bone behavior, like that of any structure, depends on the geometry or shape of the structure and the properties of the material from which it is composed. For bone, it is difficult to estimate yield points because it is not a uniform material. Stress is the load distributed over a unit area while strain is the amount of elongation of a unit length. When enough stress is applied to a bone, it fractures. This point is called the ultimate stress.
The properties listed here are for single-load-to-failure tests. Remember, however, that in vivo (i.e. in an actual person) bone tissue must withstand repetitive loading. Thus, the fatigue properties of bone tissue are also important. Fatigue is a process by which a material fails due to cyclic loading at levels below that necessary to cause failure in a single load.
Mechanical Properties of Adult Human Bone Tissue are as follows:
Ultimate tensile stress 133 Mpa
Ultimate compressive stress 193 Mpa
Maximum shear stress 68 MPa
MPa=10^6 Pascals=1.45x10^3 psi.
The properties of bone tissue are dependent on other factors besides load weight. For example, age has a significant effect on the materials properties of bone tissue. Human bone shows a decrease in elastic modulus, yield stress, and ultimate stress of about 2 percent per decade during adult life.
Burstein, Albert H. Ph.D. and Timothy M. Wright, Ph.D. Surgery of the Musculoskeletal System, Vol. 1. Churchill Livingstone, New York: 1983, p. 1:251-1:293.
Gray, Henry. Grays Anatomy. Running Press, Philadelphia: 1974.
Jones, Bruce, M.D. Interview on 1/28/99.
Klein, Kenneth and Alfred Tria, Jr. An Illustrated Guide to the Knee. Churchill Livingston, New York: 1992.
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