An example of this type of joint is the cartilaginous joint that unites the bodies of adjacent vertebrae. Filling the gap between the vertebrae is a thick pad of fibrocartilage called an intervertebral disc Figure 9. Each intervertebral disc strongly unites the vertebrae but still allows for a limited amount of movement between them. However, the small movements available between adjacent vertebrae can sum together along the length of the vertebral column to provide for large ranges of body movements.
Another example of an amphiarthrosis is the pubic symphysis of the pelvis. This is a cartilaginous joint in which the pubic regions of the right and left hip bones are strongly anchored to each other by fibrocartilage. This joint normally has very little mobility. The strength of the pubic symphysis is important in conferring weight-bearing stability to the pelvis. During pregnancy, increased levels of the hormone relaxin lead to increased mobility at the pubic symphysis which allows for expansion of the pelvic cavity during childbirth.
This functional classification of joints describes all synovial joints of the body, which provide the majority of body movements. Most diarthrotic joints are found in the appendicular skeleton and give the limbs a wide range of motion. These joints are divided into three categories, based on the number of axes of motion provided by each. An axis in anatomy is described as the movements in reference to the three anatomical planes: transverse, frontal, and sagittal.
Thus, diarthroses are classified as uniaxial, biaxial, or multiaxial joints. A uniaxial joint only allows for a motion in a single plane around a single axis. The elbow joint, which only allows for bending or straightening, is an example of a uniaxial joint.
A biaxial joint allows for motions within two planes. An example of a biaxial joint is a metacarpophalangeal joint knuckle joint of the hand. The joint allows for movement along one axis to produce bending or straightening of the finger, and movement along a second axis, which allows for spreading of the fingers away from each other and bringing them together.
A joint that allows for the several directions of movement is called a multiaxial joint sometimes called polyaxial or triaxial joint. This type of diarthrotic joint allows for movement along three axes Figure 9. The shoulder and hip joints are multiaxial joints. They allow the upper or lower limb to move in an anterior-posterior direction and a medial-lateral direction.
In addition, the limb can also be rotated around its long axis. This third movement results in rotation of the limb so that its anterior surface is moved either toward or away from the midline of the body.
Structural classifications of the body joints are based on how the bones are held together and articulate with each other. At fibrous joints, the adjacent bones are directly united to each other by fibrous connective tissue. Similarly, at a cartilaginous joint, the adjacent bones are united by cartilage.
In contrast, at a synovial joint, the articulating bone surfaces are not directly united to each other, but come together at a fluid-filled joint cavity. For physiological reasons, women have a greater thickness of the fibrocartilaginous disc, allowing more mobility of the pelvic bones and thereby providing a larger pelvic diameter needed for childbirth.
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These ligaments cling to the fibrocartilaginous disk to the point that fibers intermix with it. Two such ligaments are the superior and inferior , these being the ligaments that provide the most stability; the posterior and anterior ligaments are weaker. The strong and thicker superior ligament is reinforced by the tendons of the rectus abdominis , obliques externus , gracilis and thigh adductors muscles. The inferior ligament in the pubic arch is known as the arcuate pubic ligament.
At the time of birth the symphysis pubis is mm in width, with thick cartilaginous end-plates. By mid-adolescence the adult size is achieved. During adulthood the end-plates decrease in width to a thinner layer. Degeneration of the symphysis pubis accompanies aging and postpartum. Women have a greater thickness of this pubic disc which allows more mobility of the pelvic bones, hence providing a greater diameter of pelvic cavity during childbirth.
Fibrocartilage is composed of small chained bundles of thick clearly defined type I collagenfibers. This fibrous connective tissue bundles have cartilage cells between them, these cells to a certain extent resemble tendon cells. The collagenous fibers are usually place in an orderly arrangement parallel to tension on the tissue. Glycosaminoglycans are long unbranched polysaccharides relatively complex carbohydrates consisting of a repeating disaccharide unit.
Disaccharide a sugar a carbohydrate is composed of two monosaccharides. The two monosaccharides are bonded via a dehydration reaction that leads to the loss of a molecule of water. Fibrocartilage does not have a surrounding perichondrium. Perichondrium surrounds the cartilage of developing bone, it has a layer of dense irregular connective tissue and functions in the growth and repair of cartilage.
Hyaline cartilage is the white shiny gristle at the end of long bones. This cartilage has very poor healing potential, and efforts to get it to repair itself frequently end up with a similar, but poorer fibrocartilage.
During pregnancy in the human, hormones such as relaxin remodells this cartilage allowing the pelvic bones to be more flexible for delivery. The non-pregnant gap of the symphysis pubis is mm but in pregnancy there will be an increase of at least mm, therefore, it is considered that a total width of up to 9mm between the two bones is normal for a pregnant woman. The symphysis pubis separates to some degree during the birthing process in some women this separation can become a diastasis of the symphysis pubis.
The diastasis could be the result of a rapid birth [2] or a forceps delivery [3] or maybe even prenatal. A diastasis of the symphysis pubis is a symptom of pelvic girdle pain PGP. PGP has a reported rate of occurrence of 1 in 37 women [5]. Historically, when labor failed to progress because of the head of the fetus was too large, the mother's symphysis pubis was cut and the fetus' skull was crushed.
In remote, isolated tropical areas women presenting with a large baby and small pelvis travelled for great distances for medical help; a symphysiotomy has been suggested [6]. This practise was carried out in Europe before the introduction of the Caesarean Section. It is still considered as an option in Third World countries [7]. Metabolic disease, such as renal osteodystrophy , produces widening, while ochronosis results in calcific deposits in the symphysis.
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