7.2: Classification of Joints

By the end of this section, you will be able to:

• Distinguish between the functional and structural classifications for joints
• Describe the three functional types of joints and give an example of each
• Describe the structural features of fibrous joints and of cartilaginous joints
• Distinguish between a suture, syndesmosis, and gomphosis
• Give an example of each type of fibrous joint and each type of cartilaginous joint
• Distinguish between a synchondrosis and symphysis

A joint, also called an articulation, is any place where adjacent bones or bone and cartilage come together (articulate with each other) to form a connection. Joints are classified both structurally and functionally. Structural classifications of joints take into account whether there is a space (joint cavity) between the bones or whether the gap is fully filled by either dense connective tissue or cartilage. Functional classifications describe the degree of movement available between the bones, ranging from immobile, to slightly mobile, to freely moveable joints.

Structural Classification of Joints

The structural classification of joints is based on whether the articulating surfaces of the adjacent bones are directly connected by fibrous connective tissue or cartilage, or whether the articulating surfaces contact each other within a fluid-filled joint cavity. These differences serve to divide the joints of the body into three structural classifications.

• A fibrous joint is where the adjacent bones are united by dense regular connective tissue.
  • A suture is an example of a fibrous joint found in the skull.
• At a cartilaginous joint, the bones are joined by hyaline cartilage or fibrocartilage.
• At a synovial joint, the articulating surfaces of the bones are not directly connected, but instead come into contact with each other within a joint cavity that is filled with a lubricating fluid. Synovial joints allow for free movement between the bones and are the most common joints of the body. These three structural classifications will be discussed in further detail in the following sections of this chapter.

Suture

All the bones of the skull, except for the mandible, are joined to each other by a fibrous joint called a suture. The dense regular connective tissue found at a suture (“to bind or sew”) strongly unites the adjacent skull bones and thus helps to protect the brain and form the face. In adults, the skull bones are closely opposed and fibrous connective tissue fills the narrow gap between the bones. The suture is frequently convoluted, forming a tight union that prevents most movement between the bones. (Figure \(\PageIndex{1}\)) Thus, skull sutures are functionally classified as a synarthrosis, although some sutures may allow for slight movements between the cranial bones.

(Figure \(\PageIndex{1}\)): Suture. The sutures of the skull are very narrow areas of dense regular connective tissue that are immovable. (Image credit: "Fibrous Joint - Suture" by Jennifer Lange is licensed under CC BY-NC-SA 4.0, based on originals from Anatomy Standard (skull) and BlueLink (illustration).)

Functional Classification of Joints

The functional classification of joints is determined by the amount of mobility found between the adjacent bones. The amount of movement available at a particular joint of the body is related to the functional requirements for that joint. Thus immobile or slightly moveable joints serve to protect internal organs, give stability to the body, and allow for limited body movement. In contrast, freely moveable joints allow for much more extensive movements of the body and limbs. Joints are thus functionally classified as

• a synarthrosis or immobile joint,
• an amphiarthrosis or slightly moveable joint, or
• a diarthrosis, which is a freely moveable joint.

Depending on their location, fibrous joints may be functionally classified as a synarthrosis or an amphiarthrosis. Cartilaginous joints are also functionally classified as either a synarthrosis or an amphiarthrosis joint. All synovial joints are functionally classified as a diarthrosis joint.

Synarthrosis

An immobile or nearly immobile joint is called a synarthrosis. The immobile nature of these joints provides for a strong union between the articulating bones. This is important at locations where the bones provide protection for internal organs. Examples include sutures, such as the coronal, squamous, and lambdoid sutures, which are the fibrous joints between the bones of the skull, and the manubriosternal joint (Figure \(\PageIndex{8}\)) that unites the manubrium and body of the sternum for protection of the heart.

Figure \(\PageIndex{8}\): Synarthrosis - Manubriosternal Joint. The joint between the manubrium and the body of the sternum is an example of a synarthrosis, an immobile or essentially immobile joint. This joint is a synchondrosis that may ossify late in life. (Image credit: "Synarthrosis - Manubriosternal Joint" by Jennifer Lange is licensed under CC BY-NC-SA 4.0, modification of image from Anatomy Standard under CC BY-NC 4.0.)

Amphiarthrosis

An amphiarthrosis is a joint that has limited mobility but slight movement is available in many directions. An example of this type of joint is the cartilaginous joint that unites the bodies of adjacent vertebrae. Filling the gap between the vertebrae is a thick pad of fibrocartilage called an intervertebral disc. Each intervertebral disc strongly unites the vertebrae but still allows for a limited amount of movement between them. However, the small movements available between adjacent vertebrae can sum together along the length of the vertebral column to provide for large ranges of body movements.

Another example of an amphiarthrosis is the pubic symphysis of the pelvis (Figure \(\PageIndex{9}\)). This is a cartilaginous joint in which the pubic regions of the right and left hip bones are strongly anchored to each other by fibrocartilage. This joint normally has very little mobility because the strength of the pubic symphysis is important in conferring weight-bearing stability to the pelvis. During childbirth this joint loosens so the pelvis can shift to adjust to the fetus passing through the pelvic outlet. Following birth, the joint should tighten again.

Figure \(\PageIndex{9}\): Amphiarthrosis - Pubic Symphysis. The fibrocartilage joining the right and left symphyseal surfaces allows for limited movement between the right and left os coxae and thus functionally forms an amphiarthrosis type of joint. (Image credit: "Amphiarthrosis - Pubic Symphysis" by Jennifer Lange is licensed under CC BY-NC-SA 4.0, modification of image from Anatomy Standard.)

Diarthrosis

A freely mobile joint is classified as a diarthrosis. These types of joints include all synovial joints (structural classification) of the body, which provide the majority of body movements. Most diarthrotic joints are found in the appendicular skeleton and thus give the limbs a wide range of motion. These joints are divided into three categories, based on the number of axes of motion provided by each. An axis in anatomy is described as the movements in reference to the three anatomical planes: transverse, frontal, and sagittal. Thus, diarthroses are classified as uniaxial (for movement in one plane), biaxial (for movement in two planes), or multiaxial joints (for movement in all three anatomical planes).

• A uniaxial joint only allows for a motion in a single plane (around a single axis). The elbow joint, which only allows for bending or straightening, is an example of a uniaxial joint.
• A biaxial joint allows for motions within two planes. An example of a biaxial joint is a metacarpophalangeal joint (knuckle joint) of the hand. The joint allows for movement along one axis to produce bending or straightening of the finger, and movement along a second axis, which allows for spreading of the fingers away from each other and bringing them together.
• A joint that allows for the several directions of movement is called a multiaxial joint (polyaxial or triaxial joint). This type of diarthrotic joint allows for movement along three axes (Figure \(\PageIndex{10}\)). The hip joint, where the head of the femur articulates with the acetabulum of the hip bone, is a multiaxial joint. This allows the lower limb to move in an anterior-posterior direction and a medial-lateral direction. In addition, the limb can also be rotated around its long axis.

Figure \(\PageIndex{10}\): Diarthrosis - Multiaxial Joint. A multiaxial joint, such as the hip joint, allows for movements in each of the three planes, moving: anterior-posterior, medial-lateral, and rotational. (Image credit: "Diarthrosis - Multiaxial Joint" by Jennifer Lange is licensed under CC BY-NC-SA 4.0, modification of image from Anatomy Standard.)

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Contributors and Attributions

• OpenStax Anatomy & Physiology (CC BY 4.0). Access for free at https://openstax.org/books/anatomy-and-physiology