Classifying Joints

Classification of Joints in the Human Body

A joint is defined as a connection between two bones in the skeletal system. Joints can be classified by the type of the tissue present (fibrous, cartilaginous or synovial), or by the degree of movement permitted (synarthrosis, amphiarthrosis or diarthrosis). In this article, we shall look at the classification of joints in the human body.

Classification by type of tissue

• Fibrous – bones connected by fibrous tissue.
• Cartilaginous – bones connected by cartilage.
• Synovial – articulating surfaces enclosed within fluid-filled joint capsule.

Classification by degree of movement

• Synarthrosis – immovable.
• Amphiarthrosis – slightly moveable.
• Diarthrosis – freely moveable.

Fibrous Joints

A fibrous joint is where the bones are bound by a tough, fibrous tissue. These are typically joints that require strength and stability over range of movement. Fibrous joints can be further sub-classified into sutures, gomphoses and syndesmoses.

Sutures are immovable joints (synarthrosis), and are only found between the flat, plate-like bones of the skull. There is limited movement until about 20 years of age, after which they become fixed and immobile. They are most important in birth, as at that stage the joints are not fused, allowing deformation of the skull as it passes through the birth canal.

Gomphoses are also immovable joints. They are found where the teeth articulate with their sockets in the maxilla (upper teeth) or the mandible (lower teeth). The tooth is bound into its socket by the strong periodontal ligament.

Syndesmoses are slightly movable joints (amphiarthroses). They are comprised of bones held together by an interosseous membrane. The middle radioulnar joint and middle tibiofibular joint are examples of a syndesmosis joint.

Cartilaginous

In a cartilaginous joint, the bones are united by fibrocartilage or hyaline cartilage. There are two main types- synchondroses (primary cartilaginous) and symphyses (secondary cartilaginous).

In a synchondrosis, the bones are connected by hyaline cartilage. These joints are immovable (synarthrosis). An example of a synchondrosis is the joint between the diaphysis and epiphysis of a growing long bone.

Symphysial joints are where the bones are united by a layer of fibrocartilage. They are slightly movable (amphiarthrosis). Examples include the pubic symphysis, and the joints between vertebral bodies.

Synovial

A synovial joint is defined by the presence of a fluid-filled joint cavity contained within a fibrous capsule. They are freely movable (diarthrosis) and are the most common type of joint found in the body. Synovial joints can be sub-classified into several different types, depending on the shape of their articular surfaces and the movements permitted.

• Hinge – permits movement in one plane – usually flexion and extension. E.g. elbow joint, ankle joint, knee joint.
• Saddle – named due to its resemblance to a saddle on a horse’s back. It is characterised by opposing articular surfaces with a reciprocal concave-convex shape. E.g. carpometacarpal joints.
• Plane – the articular surfaces are relatively flat, allowing the bones to glide over one another. E.g. acromioclavicular joint, subtalar joint.
• Pivot – allows for rotation only. It is formed by a central bony pivot, which is surrounded by a bony-ligamentous ring E.g. proximal and distal radioulnar joints, atlantoaxial joint.
• Condyloid – contains a convex surface which articulates with a concave elliptical cavity. They are also known as ellipsoid joints. E.g. wrist joint, metacarpophalangeal joint, metatarsophalangeal joint.
• Ball and Socket – where the ball-shaped surface of one rounded bone fits into the cup-like depression of another bone. It permits free movement in numerous axes. E.g. hip joint, shoulder joint.

In human anatomy, joints of the skeletal system are generally classified as either fibrous, cartilaginous, or synovial joints. Fibrous joints connect bones together in a non-movable manner, with the bones held together by a toughened, fibrous tissue such as tendons, ligaments, and capsules. Cartilaginous joints are characterized by the presence of cartilage between the bones, which allows for some movement, though it is usually minimal. Finally, synovial joints are the most commonly found type of joint in the human body and involve the presence of a fluid-filled joint capsule, allowing for considerable range of mobility in the joint.

Fibrous joints can be further subdivided into sutures, gomphoses, and syndesmoses. Sutures, found in the skull, are immovable joints and play an important role during birth to permit skull deformation as it passes through the birth canal. Gomphoses are immovable joints found between teeth and their sockets. Syndesmoses involve the connection of bones by an interosseous membrane and are slightly movable. Cartilaginous joints are further divided into synchondroses and symphyses. Synchondroses, featuring hyaline cartilage, are immovable joints found in growing long bones, while symphyses feature fibrocartilage and can allow for limited movement. Finally, synovial joints can be divided into various sub-types depending on their respective shapes and movements, including hinge, saddle, plane, pivot, condyloid, and ball-and-socket joints.

Types of Joints

Joints are classified into three distinct types, fibrous, cartilaginous, and synovial. Each of these three primary types of joints are then further sub-categorized into several different types, each of which serve a specific purpose and functional movement within the body.

Fibrous Joints

The primary purpose of fibrous joints is to provide strength and stability over range of movement. Fibrous joints can then be further sub-classified into three distinct types: sutures, gomphoses, and syndesmoses.

Sutures

Sutures are immovable joints, known as synarthrosis, and are only found between the flat, plate-like bones of the skull. There is very limited movement in these joints until about the age of twenty, at which point the joints become permanently fixed and immobile. This is of particular importance during the birth process, as the joints at that stage are not yet fused, allowing for deformation of the skull as it passes through the birth canal.

Gomphoses

Gomphoses are also immovable joints; however, these joints are found in slightly different places than sutures. Specifically, gomphoses are present where the teeth articulate with their sockets in the maxilla (upper teeth) or the mandible (lower teeth). These joints are held in place by the strong periodontal ligament.

Syndesmoses

Syndesmoses are slightly movable joints, referred to as amphiarthroses. They are comprised of bones held together by an interosseous membrane. Examples of syndesmoses joints include the middle radioulnar joint and middle tibiofibular joint.

Cartilaginous Joints

In cartilaginous joints, the bones are united by fibrocartilage or hyaline cartilage. There are two main types of cartilaginous joints, each with its own purpose and function: synchondroses (primary cartilaginous) and symphyses (secondary cartilaginous).

Synchondroses

Synchondroses are connected by hyaline cartilage, and are immovable joints (synarthrosis). A good example of a synchondrosis is the joint between the diaphysis and epiphysis of a growing long bone.

Symphyses

Symphysial joints have the bones united by a layer of fibrocartilage, and they are slightly movable (amphiarthrosis). Examples of symphysial joints include the pubic symphysis, as well as the joints between the vertebral bodies.

Synovial Joints

Synovial joints are defined by the presence of a fluid-filled joint cavity contained within a fibrous capsule. These joints are freely movable (diarthrosis) and are the most common type of joint found in the body. Synovial joints can be further sub-classified into several distinct types, depending on the shape of their articular surfaces and the movements permitted.

• Hinge Joints: These permit movement in one plane, typically flexion and extension, and can be found in joints such as the elbow, ankle, and knee joint.
• Saddle Joints: Named due to their resemblance to a saddle on a horse's back, saddle joints have opposing articular surfaces with a reciprocal concave-convex shape. An example of a saddle joint can be found in the carpometacarpal joints.
• Plane Joints: Plane joints have relatively flat articular surfaces, allowing the bones to glide over one another. An example of this type of joint can be found in the acromioclavicular joint and subtalar joint.
• Pivot Joints: Pivot joints permit rotation only, and are formed by a central bony pivot surrounded by a bony-ligamentous ring. Examples include the proximal and distal radioulnar joints, as well as the atlantoaxial joint.
• Condyloid Joints: Also called ellipsoid joints, condyloid joints contain a convex surface which articulates with a concave elliptical cavity. These joints can be found in the wrist joint, metacarpophalangeal joint, and metatarsophalangeal joint.
• Ball and Socket Joints: Characterized by the ball-shaped surface of one rounded bone fitting into the cup-like depression of another bone, ball and socket joints permit free movement in numerous axes. Examples of such joints include the hip and shoulder joint.

In conclusion, joints are classified into three primary types, each of which is further sub-classified into several distinct types. Those three primary types are fibrous, cartilaginous, and synovial. Ultimately, it is the purpose of these joints to provide strength and stability over range of movement in the human body.