Introduction to Clinically Oriented Anatomy
ــــــJoints
By: Hasan Arafat
Joints are unions between two or more bones or rigid parts of the skeleton
• They exhibit a variety of forms and functions
• They are classified according to the type of the material by which the articulating bones are united
• They can be also classified according to mobility
Three general types of joints are recognized: fibrous, cartilaginous and synovial
• These types differ in the manner of material by which each joint is made
• Freely movable synovial joints are the most common type
• Most of our movements happen at synovial joints
• However, the other types perform a number of very important functions
Synovial joints
• In Synovial joints, the articulating are united by a joint capsule
• The joint capsule is composed of an outer fibrous layer lined by serous synovial membrane
• The joint capsule encloses an articular cavity
Synovial joints (continued)
• The joint cavity is a potential space that contains a small amount of lubricating synovial fluid
• The lubricating synovial fluid is secreted by the synovial membrane that lines the fibrous layer of the joint capsule
Synovial joints (continued)
• Inside the capsule, articular cartilage covers the articulating surfaces of the bones
• All other internal surfaces are covered by synovial membranes
• The periosteum investing the participating bones external to the joint blends with the fibrous layer of the capsule
Fibrous joints
• The articulating bones are united b fibrous tissue
• The amount of movement at this joint depends on the length of the fibers uniting the articulating surfaces
• The sutures of the cranium are examples
Fibrous joints (continued)
• Fibrous joints can be subdivided into 2 subtypes
• A syndesmosis type unites the bones with a sheet of fibrous tissue, either a ligament or fibrous membrane
• This type is partially movable
• The interosseous membrane between the radius and ulna is an example
Dentatoalveolar syndesmosis: a fibrous joint in which a peg-like process fits into a socket articulation between the root of the tooth and the alveolar process of the jaw
• It’s also called gomphosis or socket
• Mobility at this joint indicates a pathological state, affecting the supporting tissue of the tooth
• So, it’s normally immobile
• However, microscopic movements here is related to proprioception
Cartilaginous joints
• They are united by hyaline cartilage or fibrocartilage
• Can be divided into 2 types
• These types are primary cartilaginous joints, or synchondroses, and secondary cartilaginous joints, or symphyses
Cartilaginous joints (continued)
• In synchondroses, the bones are joined by hyaline cartilage
• Hyaline cartilage permits slight bending during early life
• Primary cartilaginous joints are usually temporary unions
• E.g. the growth plate
Cartilaginous joints (continued)
• Symphyses are strong, slightly movable joints
• They are united by fibrocartilage
• E.g. intervertebral discs
• They provide strength and shock absorption as well as considerable flexibility to the vertebral column
Features of Synovial joints
• They the most common type
• They provide free movement
• They are joints of locomotion
• Typical for limbs joints
Features of Synovial joints (cont)
• They are reinforced by accessory ligaments
• Accessory ligaments are either separate (extrinsic) or thickening portions of joint capsule (intrinsic)
Features of Synovial joints (cont)
• They also contain fibrocartilaginous articular discs
• They are also called meniscus
• They are found where articulating surfaces are incongruous
Classification of Synovial joints
• Six major types of synovial joints are classified according to the shape of articulating surfaces and the type of movement they permit
• These types are plane joints, hinge joints, saddle joints, condyloid joints and ball-and-socket joints
1. Plane joints
• They are nonaxial
• They permit gliding or sliding movements in the plane of the articular surfaces
• The opposed surfaces are flat or almost flat
• Movement is limited by their tight joint capsules
1. Plane joints (cont)
• They are numerous and are nearly always small
• E.g. the acromioclavicular joint
2. Hinge joints
• Permit flexion and extension only
• They are uniaxial
• Movements at these joints happen in the sagittal plane around a transverse axis
• Their joint capsules are thin
2. Hinge joints (cont)
• The joint capsule lax anteriorly and posteriorly where movement occures
• The bones are joined by strong laterally placed collateral ligaments
• The elbow is an example
Saddle joints
• Permit abduction, adduction, flexion and extension
• Biaxial joints, movements occur in sagittal and frontal planes
• Circumduction is also available
• E.g. carpometacarpal joint at the thumb
Condyloid joints
• Permit flexion, extension, adduction and abduction
• Biaxial Joints
• The movement at the sagittal is greater than the frontal
• More restricted circumduction
• E.g. metacarbophalangeal joints
Ball and socket joints
• Multiple axes and planes
• Allow flexion, extension, abduction, adduction, medial and lateral rotation and circumduction
• Spheroidal surface of one bone moves within the socket of another
• E.g. hip joint
Pivot joints
• Permit rotation around a central axis
• Uniaxial joints
• The rounded process of bone rotates within a sleeve or ring
• E.g. the median atlantoaxial joint
• The atlas rotates around the dens of the axis during rotation of the head
Synovial joints
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