subject: What are synovial joints? [print this page] What are synovial joints? What are synovial joints?
If you are currently studying, or an early practitioner of, anatomy, massage/bodywork, physical therapy, chiropractic, medicine, physiotherapy, sports therapy, or any other health-related field it's likely you'll be required to know about synovial joints. So what are synovial joints and how do they differ from other types of joints? There are a range of really great, clearly illustrated anatomy books on the market that go into specific detail, but if you want a general overview of what synovial joints are composed of and what they do read on.
A synovial joint, which is also known as a diarthrosis, contains synovial fluid which lubricates the joint; wherever your limbs are required to be particularly mobile you'll have a synovial joint. Synovial joints are the most common joints in mammals, and are the most moveable. They have a number of distinguishing features such as:
Articular cartilage (which covers the ends of the bones that form the joint)
Joint cavity (filled with lubricating synovial fluid, the joint cavity is enclosed by a double layered sleeve known as the aritcular capsule, and the external layer of the articular capsule is known as the capsular ligament, a tough, flexible, fibrous connective tissue)
Synovial fluid (the amount of fluid varies in different joints, but is always sufficient to form a thin film to reduce friction, the synovial fluid is a key component of the synovial joint. Synovial fluid nourishes the cartilage which contains no blood vessels. During injury extra fluid is produced and creates the characteristic swelling of joint we all recognise.)
Accessory ligaments - sometimes called collateral ligaments (joints are reinforced and strengthened by a number of ligaments)
Bursae (fluid filled sacs found cushioning the joint)
Tendon sheaths (same structure as bursae, wrapped around tendons subject to friction in order to protect them)
Articular discs (which act as shock absorbers)
In total there are seven types of synovial joints, which include:
Plane or gilding: movement occurs when two, generally flat or slightly curved, surfaces glide across one another. Example: the carpal bones in the wrist
Hinge: movement occurs around one axis only (think of a hinge). Example the elbow
Pivot: movement occurs around a vertical axis. Example rotation of the head from side to side
Ball and socket: the spherical end of one bone rotates within the concave socket of another. Example: shoulder joint
Condyloid: Similar to the ball and socket joint in many ways, consists of a spherical surface that fits into a matching concavity, doesn't however, rotate around a vertical axis. Example: fingers
Saddle: Similar to condyloid joints except saddle joints allow even more movement. Example: thumbs
Ellipsoid: Similar to a ball and socket joint except the surfaces are ellipsoid (egg shaped), rather than spherical, so no rotation. Example: wrist
Often synovial joints are referred to as freely moveable joints as they enable a range of movement, including abduction (movement away from the body), adduction (movement towards the body), extension (straightening out of limbs), flexion (bending of limbs at a joint) and rotation (circular motion).
Conversely a synarthrosis is a type of joint which allows a small amount, or even no movement. Suture joints and the gomphosis joint fall under this category, which include the teeth and the cranium.
