Joints
The structure of a joint allows for its movement. Hinge joints are structurally different than ball and socket joints; therefore their movements and risk of injury are quite different. A protected joint that has limited mobility, supporting ligaments and bony structures is less likely to be injured than a highly mobile joint.
The ball and socket joint is made for motion and movement. It is not confined by rigid structural bones or ligaments. Its movement and flexibility allows us to move the shoulder in every direction, but it also exposes the joint to increased risk of injury because of its unsupported positions.
Highly mobile joints are created at a loss of stability and protective support structures.
Joints can be classified based on their level of movement:
Synarthrotic joints – almost no movement, such as at the skull.
Amphiarthrotic joints – minimal or limited movement, such as at the distal tibia and fibula.
Diarthrotic joints – freely movable joints.
Joints can also be classified based on their structure:
Fibrous joints are composed of a thin layer of dense connective tissue between bones that are very close together. In between flat bones of the skull they form a suture. Sutures can produce almost no movement or very little movement.
Cartilaginous joints – Hyaline or fibrocartilage connects bones together. This is common in the vertebrae, and where intervertebral discs made of fibrocartilage absorb shock between the vertebrae during movement. Cartilaginous discs allow limited movement (amphiarthrotic).
Synovial joints allow free movement (diarthrotic) and are the most common in the body. They are more complex than cartilaginous or fibrous joints. A basic synovial joint is formed with a surrounding joint capsule; this ligamentous structure provides support. Underneath the joint capsule is the synovial membrane, which secretes a lubricating synovial fluid. The synovial fluid decreases friction and damage between the ends of the bones. The ends of long bones are covered with articular cartilage (hyaline cartilage).
Synovial joints can have shock absorbing fibrocartilage, called meniscus. They can also have fluid filled sacs called bursa, which are common between tendons and bones. Bursa reduces friction and decreases damage as tendons slide over bony structures.
Questions:
Which joint allows for the least amount of movement? ____________________________________
Which joint allows for the most amount of movement? ___________________________________
Which joint means slightly movable? __________________________________________________
The intervertebral disc forms this type of joint? __________________________________________
Which is the most common type of joint?_______________________________________________
What is the purpose of a bursa?_______________________________________________________
What function does synovial fluid provide?_____________________________________________
What would happen to a joint if it did not produce synovial fluid?___________________________
What covers the ends of long bones in a synovial joint?____________________________________
What type of cartilage is this?________________________________________________________
What function does knee meniscus provide?_____________________________________________
Synovial joints are classified on their shape and movements:
1. Ball-and-socket-joint (spheroidal joint)
A ball articulates with a cup-shaped cavity. These are highly mobile joints, allowing for the greatest range of motion possible. These joints are capable of rotation around a central axis. Think of your shoulder and hip joint.
2. Hinge joint
Allows for movement in one plane. A convex surface fits into the concave surface of the adjacent bone. Think of a door hinge, which rocks in one plane of movement. It is located between the humerus and ulna, and in between the phalanges.
3. Pivot joint (trochoid joint)
One bone rotates within a ring. The ring can be formed by bone, ligament or a combination of both. This allows for rotation around a central access. An example of this joint is between the Atlas and the dens of the axis; or between the head of the radius and ulna.
4. Plane joints (gliding joints)
Two slightly curved adjacent bones allow for a gliding motion across the joints. The joints of the wrist and ankles are common gliding joints. In addition, the articular processes between the superior articular process and the inferior articular process allows for the sliding and twisting movements of the vertebrae. The sacroiliac joint and the joints formed by ribs 2-7 connecting to the sternum are also gliding joints.
5. Condylar (ellipsoidal joint)
The joints between the metacarpals and phalanges are condylar joints. A condyle of one bone fits into an elliptical cavity of another. This allows for movements in all planes, except for rotational movements.
6. Saddle joint
This joint allows for the motion of your thumb. The trapezium and first metacarpal have both concave and convex regions, which allows for an incredible amount of movement in all planes.
Direction of Movement
Joint movement is produced from contraction of skeletal muscle; when muscle contracts one end and moves towards the other producing movement. The fixed end is called the muscle origin and the movable end is called the muscle insertion. The insertion typically moves towards the origin when describing movements.
Flexion – movement decreases the angle between the bones, such as bending the knee or elbow.
Extension – angle between the bones increases, such as straightening the knee or elbow.
Dorsiflexion – ankle movement that moves foot closer to the tibia, such as walking on your heels.
Plantar flexion – ankle movement that moves foot further from the tibia, such as walking on your toes.
Inversion – ankle movement that turns the plantar surface towards the midline.
Eversion – ankle movement that turns the plantar surface towards the lateral.
Adduction – moving a part toward the midline of the body. Such as bringing arm towards the side in the lateral plane.
Abduction – moving a part away from the midline of the body, such as raising the arm away from the side of the body in the lateral plane.
Rotation – rotating around a central axis, such as looking over your left and right shoulders.
Circumduction – movement of one end in a circular path while the other is in a fixed location, such as moving your elbow in a big circle while the shoulder is stable.
Pronation – from the anatomical position, turning the Palm backwards or downwards.
Supination – Turning the palm forward or returning to anatomical position.
Protraction – moving a part forward, such as jutting the jaw forward.
Retraction – moving a part backwards, such as pulling the jaw or scapula backwards.Hyperextension – term used to describe extension movement beyond a normal plane, resulting in injury. Such as hyperextending a knee or elbow.