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