The Structure of Synovial Joints

diagram of a simple synovial joint

Above: Diagram of the structure of a synovial joint

Synovial joints have a synovial cavity between the articulating bones. This cavity is filled with synovial fluid that reduces friction at the joint, enabling the articulating bones to move freely.

The page about types of joints explains the 3 functional classes of joints:

  • synarthroses (immovable joints)
  • amphiarthroses (slightly movable joints), and
  • diarthroses (movable joints, or 'freely movable joints')

Synovial joints are freely movable joints.

A formal medical term is 'diarthroses' but synovial joints are commonly described as simply 'movable' or 'freely movable'.

Most of the main joints of the appendicular skeleton (including the arms and hands, and the legs and feet) are synovial joints. This type of joint is important in many health sciences, not just medicine and nursing but also physiotherapy, sports sciences, occupational therapy, massage therapies and so on.

The basic structure of a synovial joint is shown in the diagram on the right. The main parts of synovial joints are labelled on the synovial joint diagram and described in the table below.

Some synovial joints are more complicated than others. An example of a simple synovial joint, e.g. a metacarpophalangeal (finger) joint, is shown above-right. More complicated synovial joints can involve more than two bones, might include sesamoid bones e.g. the patella in the knee joint, might include bursae, and possibly accessory ligaments. An example of a general (more complex) synovial joint follows after the table.

Part of (general)
synovial joint
Definition / Description
Bones
Articulating bone(s)

There are at least 2 bones forming each joint. An example of a simple synovial joint is the 3rd metacarpophalangeal (MCP) joint, which is formed by two articulating bones, the 3rd metacarpal and the 3rd proximal phalanx.

Periosteum

Periosteum is a tough white fibrous membrane that covers the outer surface of bones wherever their surface is not covered by articular cartilage - see below. Periosteum is sometimes labelled on synovial joint diagrams to distinguish the areas of bone covered by articular cartilage from the other surfaces of the bone(s).

Articular cartilage

Articular cartilage is hyaline cartilage (a bluish-white, shiny ground elastic material within a matrix of chondroitin sulphate containing many fine collagen fibrils and numerous chondrocytes).
The functions of articular cartilage in synovial joints include:

  • Reduces friction - helped by its coating of synovial fluid: As the load on the joint increases the spongy cartilage absorbs water, increasing the proportion of lubricating hyaluronic acid in the lubricating film.
  • Absorbs shocks - shock absorber effect helped by contribution from the bone itself immediately behind the layer of articular cartilage.
Articular Capsule
Fibrous capsule

The fibrous capsule is formed from dense irregular connective tissue. It is attached to the periosteum (layer) of the articulating bones. The functions of the fibrous capsule in synovial joints include:

  • Flexibility (of the fibrous capsule) permits movement at the joint
  • Tensile strength (collagen fibres in the capsule) resists dislocation of the joint
Synovial cavity
Synovial membrane

The synovial membrane is a layer of loose connective tissue that includes both elastin fibres and adipose tissue.

The functions of the synovial membrane in synovial joints include:

  • Secretion of synovial fluid
Synovial fluid

The functions of synovial fluid in synovial joints include:

  • Lubricating and reducing friction in the joint
  • Supplying nutrients
  • Removing metabolic waste products (v. useful because cartilage lacks blood vessels)
  • Phagocytic cells remove microbes and debris e.g. due to mechanical damage from use of the joint
Meniscal cartilage
= articular disc, mediscus or meniscus

Note that several different words are used to refer to meniscal cartilage.

Mediscus and meniscus are the singular forms, for which medisci and menisci are the corresponding plurals.

These are pads of fibrocartilage that enable articulating bones whose shapes do not match each other exactly to fit more snugly together. This extra pad of cartilage within the synovial cavity can also contribute to the absorption any mechanical shocks to the joint.

Accessory ligaments
Intracapsular ligament

'Intracapsular' means located inside the articular capsule. Intracapsular ligaments are, however, surrounded by folds of synovial membrane.

An example of an intracapsular ligament is any of the cruciate ligaments of the knee joint.

Extracapsular ligament

'Extracapsular' means outside of the articular capsule, though an extracapsular ligament may be fused with (attached to) the articular capsule.

An example of an extracapsular ligament is the fibular collateral ligament of the knee joint.

Bursa
or Bursae (plural)

Bursae are sac-like structures are present in some synovial joints.

Where present they are located between:

  • muscle and bone, or
  • tendon and bone, or
  • ligament and bone.

The function of bursae within synovial joints is to reduce the mechancial friction between one structure e.g. a bone and another e.g. an extracapsular ligament, during movement of the structures relative to each other.


The following diagram of a general synovial joint includes accessory ligaments and a bursa:

Different Types of Synovial Joints

This page concerns the general structure of synovial joints, i.e. the list of components of synovial joints in the table above and the diagrams showing how the bones, ligaments, and soft tissues are arranged.

Synovial joints can be sub-divided into 6 types according to the shapes of the surfaces of the articulating bones and therefore the types of movement enabled by the joints. These are described on the page about types of synovial joints.

This is the end of this page about synovial joints.

See also types of joints, features on bones, the structure and functions of bones, the 206 human bones, cranial and facial bones, bones of the feet and hands and skeletal disorders.







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