A joint is the point where two or more bones meet. There are three main types of joints;
• Fibrous (immovable)
• Cartilaginous (partially movable)
• Synovial (freely movable)
Fibrous (synarthrodial): This type of joint is held together by tough fibrous material. This often permits no movement. For example, the joints between the skull bones (sutures), are completely immovable; the teeth are held to their bony sockets by a periodontal ligament and the radioulnar and tibiofibular joints.
In this type of joints there is a piece of cartilage between the bones which hold the bones together and makes a joint. Cartilaginous joints are further divided into the following sub-types:
1. Primary cartilaginous joints: (synchondrosis):
In this sub-type the bones are united by a plate of hyaline cartilage so that the joint is immovable and strong. These joints are temporary in nature because after a certain age the cartilaginous plate is replaced by the bone.
For example, the joint between the epiphyses and diaphysis of a growing long bone.
2. Secondary cartilaginous joints: (symphysis):
These are also known as fibro-cartilaginous joints. There articular surface is covered by a thin layer of hyaline cartilage and the bones are united by fibro-cartilage. These joints are permanent and
persist throughout the life of an individual. They permit limited movements as a result of a compressible pad of cartilage in them. The thickness of the fibro-cartilage in these joints is directly related to the range of movement the joint offers.
For example, the symphysis pubis and intervertebral joints between the vertebral bodies.
Synovial joints are by far the most common classification of joint within the human body. They are highly movable and all have a synovial capsule (collagenous structure) surrounding the entire joint, a synovial membrane (the inner layer of the capsule) which secretes synovial fluid (a lubricating liquid) and cartilage known as hyaline cartilage which pads the ends of the articulating bones. There are six types of synovial joints which are classified by the shape of the joint and the movement available.
Synovial joints are freely movable joints. Most of the joints in the body are of the synovial type. The following are the main characteristics of a synovial joint:
• The ends of the bones are covered with a layer of smooth hyaline cartilage, called articular cartilage in the joint regions. This reduces friction at the point.
• The joint is completely enclosed by a bag-like capsular ligament which holds the joint together and helps to contain the synovial fluid.
• The capsular ligament is lined with a synovial membrane. This membrane secretes synovial fluid into the synovial cavity and acts as a seal, waterproofing the joint. The synovial fluid lubricates the joint.
• In addition to the capsule, the bones are also attached and held together by strong, tough ligaments made of dense connective tissue. These ligaments prevent dislocation during normal movement.
• The articulating surfaces of adjacent bones are reciprocally shaped.
Types of Synovial Joints
There are many different classes of synovial joints in the body. They are classified according the range of movement possible or to the shape of the articulating parts of the bones involved.
• Ball and socket joints:
These joints have the freest range of motion of any joint in the body – they are the only joints that can move in a full circle and rotate around their axis. However, the drawback to the ball and socket joint is that its free range of motion makes it more susceptible to dislocation than less mobile joints.
Examples are the shoulder and hip.
• Gliding joints
The articular surfaces are flat and allow for the bones to glide over one another, allowing sliding or twisting without any circular movement.
This happens in the carpals in the wrist and the tarsals in the ankle.
• Hinge joints
These joints, such as the elbow and knee, limit movement in only one direction so that the angle between bones can increase or decrease at the joint. The limited motion at hinge joints provides for more strength and reinforcement from the bones, muscles, and ligaments that make up the joint.
• Pivot joints:
Rounded or conical surfaces of one bone fit into a ring of one or tendon allowing rotation.
An example is the joint between the axis and atlas in the neck.
• Condyloid joints:
An oval shaped condyle fits into an elliptical cavity of another allowing angular motion but not rotation. This occurs between the metacarpals (bones in the palm of the hand) and phalanges (fingers) and between the metatarsals (foot bones excluding heel) and phalanges (toes).
• Saddle joints:
These joints, such as the one between the first metacarpal and trapezium bone, permit 360 degree motion by allowing the bones to pivot along two axes.