Joints move in a manner dependent on the capsule of collagenous tissue surrounding the joint being acted on by the muscular force directed at the bones by tendons. Ligaments are a regularly arranged dense connective tissue serving this function.
Ligament Structure
The cells within ligaments are fibroblasts. Ligaments are made up of 70-80% collagen, which gives the tissue tensile strength. Elastin fibers in the extracellular matrix provide some flexibility, countering the solid nature of bone. Though ligaments are somewhat flexible, they are considered to be an inelastic tissue. Stretching of a ligament can require surgery for repair.
The extracellular fibers are arranged in the same direction, forming the so-termed regular arrangement, though in what some consider a weaving pattern. Ligaments are also avascular, in that they do not have a blood supply like skin or organs. The tissue obtains nourishment from the blood vessels in the membranes around the joint.
How Ligaments Attach to Bone
Ligaments form a capsular sac, called the joint capsule, around movable joints. The joints are also surrounded by synovial fluid, cartilage, and other connective tissues that protect the ends of the bones. Spinal ligaments extend from vertebrae to vertebrae, binding at various points of the bone to form a joint around the fibrocartilage disc. The connective tissue is able to attach to bone because bone is covered by a connective tissue membrane called the periosteum.
Ligament Function
The ligament-based joint capsule holds the bones together when the joint is flexed and limits the extension of movement, preventing, for example, 360 degree rotation. Several ligaments may be involved in any given joint. Spinal ligaments are involved in the shape and limited movement of the spine, holding the vertebrae together and enforcing posture.
Some people have hyper-flexible joints, called joint hypermobility syndrome. Sometimes referred to as being double-jointed, the ligaments do not restrain the joint movement as they should in these individuals because of a defect in collagen formation. The defect can be inherited or spontaneous, but it is generally congenital. These individuals sometimes require medical assistance and advice for protecting their joints and assessing that the collagen defect does not lead to other health problems.
Other resources:
NYU Hospital for Joint Diseases, Spine Center. Med.nyu.edu Accessed June 18, 2010.
Scanlon and Sanders. Essentials of Anatomy and Physiology, 4th edition, 2003.
Toy et al. Arterial supply to the human anterior cruciate ligament. Journal of Athletic Training, 30(2), 1995.
Rutgers University, Division of Life Sciences. Lifesci.rutgers.edu Accessed June 18, 2010.
University of Michigan College of Engineering. “BME/ME 456 Biomechanics: Structure and function of ligaments and tendons”. Engin.umich.edu Accessed April 20, 2010.
Alicia Mae Prater - Alicia received her doctorate in Experimental Pathology in 2007. She has been a freelance writer and scientific editor since 2008.
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