Types and Function of Joints

Structural Classification

• Fibrous or Fixed Joints: In fibrous joints, the articulating bones are bound very tightly together by fibrous connective tissue and there is no synovial space. These joints are either immobile or allow just a very little amount of movement. There are three varieties of fibrous joints:
• Sutures: For instance, the joints between the skull's bones.
• Syndesmosis: The interosseous ligament, a fibrous tissue that connects bones, acts as: ulnar radio joint
• Gomphosis: This particular joint exists between a tooth's alveolus bone and the tooth itself.
• Cartilaginous Joints: The lack of a synovial cavity in cartilaginous joints prevents much movement. Here, cartilage tightly connects the articulating bones.
• Synchondrosis: It is a cartilaginous junction where hyaline cartilage connects the bones. These joints are transient main cartilaginous joints that are entirely replaced by bone to become bony joints. Examples include the joint between the first rib and the manubrium of the sterum, and the joint between the epiphysis and diaphysis of growing long bones.
• Symphasis: Hyaline cartilage covers the ends of the articulating bone in these joints, which are connected by a disc of fibrocartilage. Examples include the manubrio sternal joint and pubic symphysis.
• Synovial or Freely Movable Joints: The synovial joints' bones are separated from one another by a joint cavity that houses a lubricant known as synovial fluid. The existence of a synovial cavity between the articulating bones is one of a synovial joint's distinctive features. A joint can move freely thanks to the synovial cavity. Albumin and hyaluronic acid are abundant in synovial fluid, giving it a sticky, slippery feel. Additionally, it has phagocytes that constantly clear away material left over by the deterioration of the joint cartilage. In young, healthy joints, a layer of hyaline articular cartilage around 2 mm thick covers the surfaces of the bones that are next to one another. Synovial joints almost have friction-free movement thanks to the cartilages and synovial fluid.

Types of Synovial Joints

Despite having a similar structure, the articulating surfaces of different synovial joints have different shapes. As a result, various movements like adduction, abduction, circumduction, rotation, etc., may take place. Six categories of synovial joints are categorized according to the type of movement.

• Ball and socket joints: Spherical articular surfaces have a globular head that fits into a cup-shaped socket. They have a lot of movement. The hip joint and shoulder are two examples. The head of the femur fits into the acetabulum of the pelvic bone, and the head of the humerus fits into the glenoid cavity of the capsules. The bones' shape permits a variety of movements—referred to as multiaxial—in various directions. For instance, flexion, extension, abduction, and adduction. Multiaxial joints are what these are.
•  Hinge joints: One bone has a convex surface that fits into the concave depression of the other bone at the hinge joint. Hinge joints are referred to as monoaxial because they can only move in one axis at a right angle to the bones involved. For instance, interphalangeal joints, the elbow, and the ankle.
•  Condylar (Bicondylar) joint: Two distinct condyles (convex male surface) fit into reciprocally concave female surfaces as articular surfaces. They move in both directions. Knee joints, left and right jaw joints, and temporomandibular joints are a few examples of condyloid joints.
•  Saddle joint: The bone's articular surface is saddle-shaped, meaning it is concave in one joint and convex in the other. First carpometacarpal joint, sternoclavicular joint, incudomalleolar joint, and the joint between the femur and patella are a few examples.
• Pivot joint: These joints allow one bone to rotate on its longitudinal axis in relation to the other thanks to a projection on one bone that fits into a ring-shaped ligament on the other. An example of a pivot joint is the atlanto-axial joint, which allows the head to turn side to side and allows the atlas to rotate around its axis.
• Ellipsoid Joint: The convex oval-shaped projection of one bone fits into the oval-shaped depression of another bone to form an ellipsoid joint. They have the ability to move in two directions (biaxially), such as up and down and side to side. Wrist joints, metacarpophalangeal joints, and atlanto-occipital joints are a few examples.
• Gliding (Plane) joint: These joints' articular surfaces are mostly flat or barely concave and convex. The adjacent bones only move very slightly against one another. The sternoclavicular joint, the acromioclavicular joint, and the joints between the carpal and tarsal bones are examples.

According to Degree of Mobility

• Synarthrosis (immovable), like fibrous joints.
• Amphiarthrosis (slightly movable), like cartilaginous joint
• Diarthrosis (freely movable), like synovial joints.

Regional classification

• Skull type: Immovable
• Vertebral type: Slightly movable
• Limb type: Freely movable

According to number of articulating bones

• Simple joints: Only two bones are articulated. E.g. interphalangeal joint
• Compound joint: More than two articulate within one capsule. E.g. elbow joint, wrist joint.
• Complex joint: When joint cavity is divided by an intra-articular disc, e.g. Temporomandibulajoint and sternoclavicular joint

Types of Movements (Range of Motion)

• The events that can take place at synovial joints are described by anatomists and physical therapists using specific terminology. The terms describe the ways in which movements take place or how moving parts relate to one another.

Movement at Synovial Joints

• Angular movement: The angle between the articulating bones can change during this type of movement. It can be categorized as follows:

  • Flexion: Angle between the articulating bones should be reduced.
  • Extension: Angle the articulating bones at a greater angle.
  • Abduction: Is a movement away from the body's median (midline) plane.
  • Adduction: Is a movement that is directed at the body's median plane (midline).
  • Circumduction: Movement of the body part's distal end in a circular motion that involves continuous flexion and extension. For instance, rotating the hand at the wrist joint.

• Rotation: There is a rotational movement or twisting of a body part. The long axis of the bone rotates. Medial (internal) rotation is the movement of an limb bone when the anterior surface is turned toward the midline. Rotation or twisting to the side of the body's median plane is referred to as lateral rotation.

• Special movement: Only certain joints experience specific movements.

• Elevation: Is it possible to move a body part in an upward motion while closing the mouth at the temporomandibular joint?

• Depression: Is the movement of a body part downward, such as when the mouth is opened to depress the mandibles.

• Protraction: Is the anterior movement of a body part in the transverse plane.

• Retraction: Is the return of a stretched-out body part to its anatomical position.