Definition of Synovial Articulations: Functional Classes, Periosteum, Cartilage, Bursas, Types and Characteristics

The synovial joints have a synovial cavity between the bones of the joint.

This cavity is filled with synovial fluid that reduces friction in the joint, allowing the articulated bones to move freely.

Functional classes of joints:

  • Synarthroses (joints with little or no movement)
  • Amphiarthroses (slightly mobile joints), and
  • Diarthroses (mobile joints or “freely mobile joints”)

Synovial joints are freely mobile joints. A formal medical term is “diarthroses” but synovial joints are commonly described as simply “mobile” or “freely mobile”.

The main joints of the appendicular skeleton (including the arms, hands, legs, feet) are synovial joints.

This type of articulation, therefore, is very important in many health sciences, not only medicine and nursing but also physiotherapy, sports science, occupational therapy, massage therapies and so on.

Some synovial joints are more complicated than others. For example a metacarpophalangeal joint (finger), above the right.

More complicated synovial joints may involve two or more bones, may include sesamoid bones such as the kneecap in the knee joint, may include bursas and may include accessory ligaments.

Bone articulation

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

Periostio

Periosteum is a hard white fibrous membrane that covers the outer surface of bones wherever its surface is not covered by cartilage. The periosteum is sometimes labeled in synovial joint diagrams to distinguish areas of bone covered by articular cartilage from the other surfaces of the bone.

Articular cartilage

The articular cartilage is hyaline cartilage (a bluish white elastic material). The functions of articular cartilage in synovial joints include:

Reduces friction: aided by its layer of synovial fluid: as the load on the joint increases the spongy cartilage absorbs water, increasing the proportion of hyaluronic acid in the lubricating film.

Absorbs shocks: cushioning effect helped by the contribution of bone immediately behind the layer of articular cartilage.

Three types of joints

  • Joint capsule
  • Fibrous capsule

The fibrous capsule is formed by dense irregular connective tissue. It joins the periosteum (layer) of the bones of the joint. The functions of the fibrous capsule in synovial joints include:

  • Flexibility (of the fibrous capsule) allows movement in the joint
  • Tensile strength (collagen fibers 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 and adipose tissue fibers.

The functions of the synovial membrane in synovial joints include:

  • Synovial fluid discharge
  • Synovial fluid

Synovial fluid functions in synovial joints include:

  • Lubricate and reduce friction in the joint.
  • Nutrient supply
  • Elimination of metabolic waste.
  • Phagocytic cells eliminate microbes and waste, due to mechanical damage by the use of the joint.
  • Meniscal cartilage = articular disc, mediscus or meniscus

Keep in mind that several different names / terms are used to refer to meniscal cartilage. Menisco and Mediscus are the singular forms, so that medisci and meniscos are the corresponding plurals.

Ligaments accessories

Intracapsular ligaments: s “Intracapsular” ignifies located inside the joint capsule.Intracapsular ligaments are, however, surrounded by folds of the synovial membrane. An example of an intracapsular ligament is one of the cruciate ligaments of the knee.

Extracapsular ligaments

It means “Extracapsular” outside the joint capsule, although an extracapsular ligament can be fused with the joint capsule. An example of an extracapsular ligament is the fibular collateral ligament of the knee joint.

Bursa or bursas (plural)

Bursas are present in some synovial joints. Where they currently lie between: muscle and bone, or tendon and bone, or ligament and bone. The function of the bursas within synovial joints is to reduce the mechancial friction between a structure for example a bone and another one for example an extracapsular ligament, during the movement of the structures in relation to the others.

Types of synovial joints

Synovial joints are classified into six different categories, based on the shape and structure of the joint. The shape of the joint affects the type of movement allowed by the joint. These joints can be described as planar, hinge, pivot, condylar, saddle or ball – articulated joints.

The six types of synovial joints allow the body to move in a variety of ways.

  1. Pivot joints allow rotation about an axis, such as between the first and second cervical of the vertebra, which allows the rotation of the head from side to side.
  2. The hinge joint of the elbow works like a door hinge.
  3. The joint between the trapezius carpus bone and the first metacarpal bone at the base of the thumb is a saddle joint.
  4. Planar (or flat), like those between the tarsal bones of the foot, allow the sliding movements between the bones.
  5. The radiocarpal joint of the wrist is a condylar joint.
  6. The joints of the hip and shoulder are the only ball – articulated joints of the body.

Flat gaskets

Flat joints have bones with articulation of flat or slightly curved surfaces. These joints allow you to plan movements; therefore, joints are sometimes referred to as slip joints.

The range of movement is limited and does not involve rotation. The flat joints are found in the bones of the carpus in the hand and the tarsal bones of the foot, as well as between the vertebrae.

Hinge joints

In the hinge joints, the end slightly rounded or fits into the slightly-hollow end of another bone. In this way, one bone moves while the other remains motionless, similar to the hinge of the door. The elbow is an example of a hinge joint. Sometimes the knee is classified as a modified hinge joint.

Pivot joints

Pivot joints consist of the rounded end of a bone that fit into a ring formed by the other bone. This structure allows rotary movement, as the rounded bone moves around its axis.

An example of a pivot joint is the articulation of the first and second vertebrae of the neck that allows the head to move forward and backward. The wrist joint that allows the palm of the hand to rotate up and down, is also a pivot joint.

Condylar joints

Condilar joints consist of one end of a bone that fits into a hollow of another similarly oval bone.

This is sometimes called an ellipsoidal joint. This type of articulation allows angular movement along two axes, as seen in the joints of the wrist and fingers, which allows the movement of both, side by side and up and down.

Saddle together

Each bone in a saddle joint resembles a chair, with concave and convex portions that fit together. They allow similar angular movements to condylar joints, but with a greater range of motion.

An example of a saddle joint is the thumb joint, which can move forward and backward, up and down; It can move more freely from the wrist or fingers.

Ball – articulated joints

They have a rounded end similar to a ball. It allows the greatest range of movement, all types of movement are possible in all directions. Examples of ball – articulated joints are the hip and shoulder joints.