Joints: Structure, Function & Classification

Instructor: Cheryl Rosenfeld

Cheryl has taught veterinary and medical student for over 20 years and has a DVM and PhD degree in reproductive biology.

Joints are areas where bones meet up. In some cases, bones should interlock to prevent any movement. However, other bones need to move relative to one another. This lesson describes the different joints and their functions. Updated: 02/01/2021

Bone Joints

Joints, also called articulations, are areas where one bone contacts another bone. However, the range of motion between joints varies depending on the region of the body and the type of articulating bones. They include the sutures of the skull that show little movement, which helps protect the brain and sense organs, such as the eyes. In contrast, joints like the knees, shoulders, and elbows allow for extensive movement in various directions, as can be attested by those who do various sports that involve the arms and legs. These latter joints include joint (synovial) fluid to aid in lubrication and reduce friction as the two articulating bones glide past one another. While joints generally involve bone to bone, one exception is teeth insertion into the maxilla or mandible (jaw bone). This type of fibrous joint is termed gomphosis.

Structural Classification of Joints

Structural classification of joints is dependent upon the type of tissue that bridges the two bones. Based on this classification scheme, there are four types of joints: fibrous, cartilaginous, facet, and synovial joints.

Fibrous

In fibrous joints, the two bones are connected by dense regular connective tissue. The best example of this type is the sutures of the skull and the teeth insertion into the jaw.

Cartilaginous

In a cartilaginous joint, cartilage spans the gap between the two bones. Within this category, it can be classified as primary cartilaginous with hyaline cartilage bridging the area, or secondary cartilaginous (symphyses) with hyaline cartilage covering the bones, but fibrocartilage connecting them. Examples of primary cartilaginous are the growth plate (physis), spine, or ribs. Examples of secondary cartilaginous include intervertebral disks and pubic symphyses. These types of joints allow for more movement than fibrous, but less than synovial joints.

Facet

A facet joint is between two articular processes of two vertebrae in the spinal column.

Synovial

A synovial (diarthrotic) joint is a type of joint where the two bones are not action adjoined by any specific tissue. Instead, the space between the edges of the bone, which are lined by articular (hyaline) cartilage, is occupied by a synovial cavity that is lined by a synovial membrane. All synovial joints thus have three main structures:

  • A synovial cavity contains synovial fluid that is produced by the synovial cells of the inner synovial membrane
  • A synovial membrane that is divided into the outer fibrous membrane and the inner synovial membrane that is lined by synovial cells. These synovial cells (fibroblasts) produce synovial fluid, which is similar to plasma, but contains high amounts of hyaluronic acid.
  • Articular cartilage that is hyaline cartilage, but lacks a perichondrium. Thus, this cartilage can only regenerate via interstitial growth, which is slower than appositional growth.

The knee joint (as well as some other synovial joints) also has some other features.

  • Articular disks or menisci are comprised of fibrocartilage and are situated on the appositional surfaces of the joint.
  • Articular fat pads, such as the infrapatellar fat pad in the knee joint, act as further cushioning for the articular cartilage.
  • Ligaments in the joint may be intracapsular or extracapsular, such as the anterior and posterior cruciate ligaments that help hold the bones in place during movement. The anterior cruciate ligament prevents the tibia from moving ahead of the femur.
  • Bursae are sac or cyst-like structures that contain fluid of similar composition to synovial fluid. These act to reduce frictional forces from being generated in some joints, such as in the shoulder and knee.
  • Tendons connect to muscles around the joint.

This picture shows a side profile of the skull with each of the suture lined delineated.
suture

This is a frontal picture of the skull with the suture lines designated.
suture

Functional Classification of Joints

Functional classification of joints is based on the range of movement. Based on this criterion, there are three types of joints: synarthrosis

Synarthrosis

Synarthrosis is typified by a joint where there is little to no mobility. This type of joint is seen in fibrous joints, including sutures of the skull.

Amphiarthrosis

Amphiarthrosis is characterized by a joint showing slight mobility. This category includes both types of cartilaginous joints, such as the intervertebral disk region.

Synovial

A synovial (diarthrotic) joint has an excellent range of motion. Based on the range of motion and movement, synovial joints can be further classified as:

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