Question

why joints differ in their degree of mobility.

Answer 1

A joint, also called an articulation, is any place where adjacent bones or bone and cartilage articulate with each other to form a connection.

Joints are classified both structurally and functionally.

Functional classifications describe the degree of movement available between the bones, ranging from immobile, to slightly mobile, to freely moveable joints.

The amount of movement amount of movement available at a particular joint of the body is related to the functional requirements for that joint .

Thus immobile or slightly moveable joints serve to protect internal organs, give stability to the body, and allow for limited body movement.

In contrast, freely moveable joints allow for much more extensive movements of the body and limbs.

**Functional Classification of Joints:-

The functional classification of joints is determined by the amount of mobility found between the adjacent bones. Joints are thus functionally classified as a Synarthrosis or immobile joint, an Amphiarthrosis or slightly moveable joint, or as a diarthrosis, which is a freely moveable joint .

•Synarthrosis:-

An immobile or nearly immobile joint is called a synarthrosis. The immobile nature of these joints provide for a strong union between the articulating bones . This is important at locations where the bones provide protection for internal organs.  Examples include sutures of skull protecting the brain and the manubriosternal joint ,that unites the manubrium and body of the sternum for protection of the heart.

•Amphiarthrosis:-

An amphiarthrosis is a joint that has limited mobility. 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.

Another example of an amphiarthrosis is the pubic symphysis of the pelvis. This joint normally has very little mobility. The strength of the pubic symphysis is important in conferring weight bearing stability to the pelvis.

•Diarthrosis:-

A freely mobile joint  is classified as a diarthrosis. These types of joints include all synovial joints of the body, which provide the majority of body movements. Most diarthrotic joints are found in the appendicular skeleton and thus 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 (for movement in one plane), biaxial (for movement in two planes), or multiaxial joints (for movement in all three anatomical planes).

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 (polyaxial or triaxial joint). This type of diarthrotic joint allows for movement along three axes The shoulder and hip joints are multiaxial joints.