The junction between two bones forms joints or an articulation. Joints permit varying degrees of movements. Joints are classified into three groups: immovable, slightly movable and freely movable.
Arthroscopy is the science concerned with the study of joints or articulations. The articulations of the body are classified into three areas:
a. Fibrous joints: in these joints, the bones are held together by fibro-cartilagenous tissue and the joints have no cavities.
b. Cartilagenous joints: In cartilaginous joints, the bones are held together by cartilage. These bone also lack joint cavities.
c. Synovial joints: Synovial joints have cartilages covering the articulating surfaces and ligaments that keep the bones together. These joints have fluid-filled joint cavities.
Immovable joints are called synathrosis joints. The bones that form immovable joints are tightly joined and are separated by a thin layer of fibrous connective tissue. E.g Skull bones except the mandibles are joined by immovable joints called sutures (they resemble stitches). There are three types of sutures:
· Serrate suture: is characterized by jagged interlocking articulations.
· Lap suture: the edge of one bone overlap another articulating bone (e.g squamous suture between the temporal and parietal bone).
· Plane suture: the edges of the articulating bones are fairly smooth (eg. Median palantine suture where maxillae articulate to form the hard palate).
SLIGHTLY MOVABLE JOINTS
Slightly movable joints are called amphiarthrosis joints. Bone forming slightly movable joints. The bones forming slightly movable joints are separated by a layer of cartilage or fibrous connective tissues. Eg. The joints formed by adjacent vertebrae contain intervertebral disc formed of cartilage. This arrangement in the vertebrae allow for limited movement. Other examples include the symphysis pubis and sacroiliac joints.
FREELY MOVABLE JOINTS
Most articulations in the body are produced by freely movable joints. These joints are called diarthrosis joints. The ends of the bones forming these joints are bound together by an articular capsule formed out of ligaments. A synovial membane lines the interior of the capsule and secretes synovial fluid that lubricates the joint. The ends of the bones forming the joints are covered with articular cartilage which protects the bones and reduces friction. Freely movable joints are classified into groups based on their structure and types of movement.
a. Gliding Joint
Gliding joints occur between small bones that slide over one another. They occur between carpal bones, tarsal bones and between the clavicle and scapula.
b. Condyloid Joints
Condyloid joints allow movement in two planes: side to side or back and forth. The joints between carpals and bones of the forearm (ulna and radius) and metacarpals and first phalanges are examples.
c. Hinge Joint
Hinge joint allow movement in one plane only (similar to door hinge). The elbow, knee and joints between phalanges are all hinge joints.
d. Saddle joint
Saddle joints occur where the ends of each bone are saddle-shaped; convex in one direction and concave in the other. Movement is side to side and back and forth. This type of joint is found between the trapezium (a carpal bone) and the metacarpal bone of the thumb.
e. Pivot joint
Pivot joints allow rotational movement in a single plane. The rotation of the atlas on the axis is an example of a pivot joint.
f. Ball and Sucket joint
Ball and sucket joint occur in bones where the rounded head fits into the sucket or cavity of another bone. These bones allow the greatest degree of movement. Movement may be rotational or in any plane. The shoulder and hip joints are ball and sucket joints.
MOVEMENT IN SYNOVIAL JOINTS
Two types of movements are discussed: Angular and Circular movements.
1. Angular joints: increase or decr the joint angle produced by articulating bone. Four types of angular movements are : flexion, extension, abduction and adduction.
· Flexion: movement that decr the joint angle on an anterior-posterior plane. Ex. Bending the elbow or knee
· Extension: the reverse of flexion; the angle joint is increased; returns the body to anatomical position. The angle formed is 180º an exception is the ankle joint which forms 90º.
· Abduction: Movement of body part away from the main axis of the body or away from the midsagittal plane in a lateral direction. Examples:. Moving the arm inwards, away from the body or spreading the fingers apart.
· Adduction: The opposite of abduction is the movement of a body part towards the main axis of the body.
2. Circular Movement
· Rotation: movement of a bone around its own axis e.g turning the head from side to side and twisting at the waist. Supination is a specialized rotation of the forearm that results in the palm being turned forward. Pronation is the opposite of supination. Movement of the forearm results in the palm being directed backwards. Applied to the foot pronation involves eversion (movement outward) and inversion (movement inward).
· Circumduction: movement of body part resulting in the formation or tracing of cone-shaped airspace. Ex. The shoulder, wrist, trunk, hip and ankle joints