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HUMAN KNEE
QUESTIONS OR COMMENTS
cavic_alexis.jpg (2820 bytes) AUTHOR: Alexis Cavic
E-MAIL: alexis@MIT.EDU
COURSE: Undeclared
CLASS/YEAR: 1

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


GEOMETRY/STRUCTURE:

Knee ExtendedKnee Bent
Front View

The parts of the knee fit into 5 basic categories as follows:

  1. Bones –support knee and provide a rigid structure for the joint
    1. femur(large thigh bone)
    2. tibia (larger of two bones in lower leg)
    3. fibula(smaller of two bones in lower leg) – main function is actually to create a cradle with the tibia at the ankle in which the talus can slide freely
    4. patella (knee cap)-small triangular bone that rides on the knee joint and allows bones to slide smoothly on each other
  2. Ligaments- keep bones aligned properly and limit rotational movement
    1. anterior cruciate ligament (ACL) – prevents femur from sliding backward on the tibia
    2. posterior cruciate ligament (PCL)- prevents femur from sliding forward on the tibia
  3. Meniscus – c-shaped tissue made of cartilage, between tibia and femur, helps protect the joint. Its three main functions are.
    1. stability-acts as secondary stabilizers as opposed to ligaments like ACL and PCL
    2. lubrication and nutrition- acts as a spacer between femur and tibia so helps minimize friction and allows diffusion of normal joint fluid and its nutrients into tissue surrounding joint called articular cartilage
    3. acts as a shock absorber - meniscus lowers the stress applied to the articular cartilage
  4. Muscles- move the joint by expanding and contracting opposing muscles. As one muscle extends, the opposing muscle flexes to allow bending. The two muscles mainly responsible for knee movement are as follows:
    1. hamstrings (muscles covering the back of the knee)
    2. quadriceps (muscles across the front of the knee)
  5. Bursa or snyovial capsule- surrounds the joint and secretes a liquid called synovial fluid (like raw egg white) – nourishes the joint surfaces and acts as a lubricant

DOMINANT PHYSICS:

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.

Note: These are not proper free-body diagrams, but are simplified for explanation. The direction of the forces changes depending on the duty being performed and the position of the knee. This is possible because of the flexibility of ligaments, tendons and muscles.

Diagram of Forces In The Knee (Front View)

 

Note: These are not proper free-body diagrams, but are simplified for explanation. The direction of the forces changes depending on the duty being performed and the position of the knee. This is possible because of the flexibility of ligaments, tendons and muscles.

Diagram of Forces In The Knee (Front View - Flexed Knee)

 

Note: These are not proper free-body diagrams, but are simplified for explanation. The direction of the forces changes depending on the duty being performed and the position of the knee. This is possible because of the flexibility of ligaments, tendons and muscles.

Diagram of Forces In The Knee (Lateral View)

LIMITING PHYSICS:

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

 

Pa=N/M^2

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. 


PLOTS/GRAPHS/TABLES:

None Submitted


REFERENCES/MORE INFORMATION:

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. Gray’s 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.

http://www.scoi.com/kneeanat.htm

http://www.arthroscopy.com/sp04001.htm


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