9.1 Classification of Joints

Lindsay M. Biga; Staci Bronson; Sierra Dawson; Amy Harwell; Robin Hopkins; Joel Kaufmann; Mike LeMaster; Philip Matern; Katie Morrison-Graham; Kristen Oja; Devon Quick; Jon Runyeon; OpenStax

9.1 Classification of Joints

Learning Objectives

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 three structural types of joints and give an example of each
Describe the planes of movement possible in diarthrodial joints

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 the adjacent bones are strongly anchored to each other by fibrous connective tissue or cartilage, or whether the adjacent bones articulate with each other within a fluid-filled space called a joint cavity. Functional classifications describe the degree of movement available between the bones, ranging from immobile, to slightly mobile, to freely moveable joints. 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.

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 fibrous connective tissue. 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.

Functional Classification of Joints

The functional classification of joints is determined by the amount of mobility found between the adjacent bones. Joints are thus functionally classified as a synarthrosis or immobile joint, an amphiarthrosis or slightly moveable joint, or as a diarthrosis, which is a freely moveable joint (arthroun = “to fasten by a joint”). Depending on their location, fibrous joints may be functionally classified as a synarthrosis (immobile joint) or an amphiarthrosis (slightly mobile joint). 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 (plural = synarthroses). The immobile nature of these joints provide for a strong union between the articulating bones. This is important at locations where the bones provide protection for internal organs. Examples include sutures, the fibrous joints between the bones of the skull that surround and protect the brain (Figure 9.1.1), and the epiphyseal growth plate, a cartilaginous joint that unites the epiphyses and diaphysis of a growing long bone like the femur.

This image shows the lateral view of the human skeleton. The lambdoid, coronal, and squamous sutures are labeled. — ***Figure 9.1.1 – Suture Joints of Skull:*** The suture joints of the skull are an example of a synarthrosis, an immobile or essentially immobile joint.

Amphiarthrosis

An amphiarthrosis (plural = amphiarthroses) is a joint that has limited mobility. 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 (Figure 9.1.2). 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. 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. The strength of the pubic symphysis is important in conferring weight-bearing stability to the pelvis. During pregnancy, increased levels of the hormone relaxin lead to increased mobility at the pubic symphysis which allows for expansion of the pelvic cavity during childbirth.

This image shows the lateral view of the intervertebral disc located between two vertebral discs. — ***Figure 9.1.2 – Intervertebral Disc:*** An intervertebral disc unites the bodies of adjacent vertebrae within the vertebral column. Each disc allows for limited movement between the vertebrae and thus functionally forms an amphiarthrosis type of joint. Intervertebral discs are made of fibrocartilage and thereby structurally form a symphysis type of cartilaginous joint.

Diarthrosis

A freely mobile joint is classified as a diarthrosis (plural = diarthroses). This functional classification of joints describes all synovial joints of the body, which provide the majority of body movements. Most diarthrotic joints are found in the appendicular skeleton and 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, biaxial, or multiaxial joints.

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 (sometimes called polyaxial or triaxial joint). This type of diarthrotic joint allows for movement along three axes (Figure 9.1.3). The shoulder and hip joints are multiaxial joints. They allow the upper or 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. This third movement results in rotation of the limb so that its anterior surface is moved either toward or away from the midline of the body.

This image shows a multiaxial joint. The left panel shows the acetabulum of the hip bone and the head of the femur. The right panel shows a simplified ball-and-socket joint structure to illustrate the movement of the hip joint. — ***Figure 9.1.3 – Multiaxial Joint:*** A multiaxial joint, such as the hip joint, allows for three types of movement: anterior-posterior, medial-lateral, and rotational.

Chapter Review

Structural classifications of the body joints are based on how the bones are held together and articulate with each other. At fibrous joints, the adjacent bones are directly united to each other by fibrous connective tissue. Similarly, at a cartilaginous joint, the adjacent bones are united by cartilage. In contrast, at a synovial joint, the articulating bone surfaces are not directly united to each other, but come together at a fluid-filled joint cavity.

The functional classification of body joints is based on the degree of movement found at each joint. A synarthrosis is a joint that is essentially immobile. This type of joint provides for a strong connection between the adjacent bones, which serves to protect internal structures such as the brain or heart. Examples include the fibrous joints of the skull sutures and the cartilaginous epiphyseal plate. A joint that allows for limited movement is an amphiarthrosis. An example is the pubic symphysis of the pelvis, the cartilaginous joint that strongly unites the right and left hip bones of the pelvis. The cartilaginous joints in which vertebrae are united by intervertebral discs provide for small movements between the adjacent vertebrae and are also amphiarthrotic joints. Thus, based on their movement ability, some fibrous and cartilaginous joints are functionally classified as synarthroses while others are amphiarthroses.

The most common type of joint is the diarthrosis, which is a freely moveable joint. All synovial joints are functionally classified as diarthroses. A uniaxial diarthrosis, such as the elbow, is a joint that only allows for movement within a single anatomical plane. Joints that allow for movements in two planes are biaxial joints, such as the metacarpophalangeal joints of the fingers. A multiaxial joint, such as the shoulder or hip joint, allows for three planes of motions.

Review Questions

For further instruction on using H5P activities, please visit Appendix D.

Critical Thinking Questions

Define how joints are classified based on function. Describe and give an example for each functional type of joint.

Reveal

Functional classification of joints is based on the degree of mobility exhibited by the joint. A synarthrosis is an immobile or nearly immobile joint. An example is the epiphyseal plate or the joints between the skull bones surrounding the brain. An amphiarthrosis is a slightly moveable joint, such as the pubic symphysis or an intervertebral cartilaginous joint. A diarthrosis is a freely moveable joint. These are subdivided into three categories. A uniaxial diarthrosis allows movement within a single anatomical plane or axis of motion. The elbow joint is an example. A biaxial diarthrosis, such as the metacarpophalangeal joint, allows for movement along two planes or axes. The hip and shoulder joints are examples of a multiaxial diarthrosis. These allow movements along three planes or axes.

Explain how degree of mobility is related to joint strength.

Reveal

Joint mobility is inversely related to joint strength. A synarthrosis, which is an immobile joint, serves to strongly connect bones thus protecting internal organs such as the heart or brain. A slightly moveable amphiarthrosis provides for small movements while maintaining stability between adjacent bones as in the vertebral column. The freedom of movement provided by a diarthrosis can allow for large movements, such as is seen with most joints of the limbs. However, these joints are the most frequently injured due to their looser articulations at the joint cavity.

Glossary

amphiarthrosis: slightly mobile joint
articulation: joint of the body (where two bone surfaces meet)
biaxial joint: type of diarthrosis; a joint that allows for movements within two planes (two axes)
cartilaginous joint: joint at which the bones are united by hyaline cartilage (synchondrosis) or fibrocartilage (symphysis)
diarthrosis: freely mobile joint
fibrous joint: joint where the articulating areas of the adjacent bones are connected by fibrous connective tissue
joint: site at which two or more bones or bone and cartilage come together (articulate)
joint cavity: space enclosed by the articular capsule of a synovial joint that is filled with synovial fluid and contains the articulating surfaces of the adjacent bones
multiaxial joint: type of diarthrosis; a joint that allows for movements within three planes (three axes)
synarthrosis: immobile or nearly immobile joint
synovial joint: joint at which the articulating surfaces of the bones are located within a joint cavity formed by an articular capsule
uniaxial joint: type of diarthrosis; joint that allows for motion within only one plane (one axis)

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Anatomy & Physiology 2e Copyright © 2025 by Lindsay M. Biga, Staci Bronson, Sierra Dawson, Amy Harwell, Robin Hopkins, Joel Kaufmann, Mike LeMaster, Philip Matern, Katie Morrison-Graham, Kristen Oja, Devon Quick, Jon Runyeon, and OpenStax is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License, except where otherwise noted.

Functional Category	Structural Category	Subtype	Description
Synarthrosis (immobile)	Fibrous	Suture	A fibrous joint that connects the bones of the skull (except the mandible). The fibrous connective tissue found at a suture strongly unites the adjacent skull bones and helps to protect the brain and form the face.
	Fibrous	Gomphosis	A narrow fibrous joint that anchors the root of a tooth into its bony socket within the maxillary bone (upper jaw) or mandible bone (lower jaw) of the skull.
	Cartilaginous	Synchondrosis	A cartilaginous joint where bones are joined together by hyaline cartilage, or where bone is united to hyaline cartilage. A synchondrosis may be temporary or permanent.
Amphiarthrosis (slightly mobile)	Fibrous	Syndesmosis	A fibrous joint in which two parallel bones are united to each other by fibrous connective tissue. The gap between the bones may be narrow, with the bones joined by ligaments, or the gap may be wide and filled by a broad sheet of connective tissue called an interosseous membrane.
Amphiarthrosis (slightly mobile)	Cartilaginous	Symphysis	A cartilaginous joint where the bones are joined by fibrocartilage. Fibrocartilage contains numerous bundles of thick collagen fibers, thus giving it a much greater ability to resist pulling and bending forces when compared with hyaline cartilage.
Diarthrosis (mobile)	Synovial	Pivot	A pivot joint is where a rounded portion of a bone is enclosed within a ring formed partially by the articulation with another bone and partially by a ligament.
		Hinge	A hinge joint is where the convex end of one bone articulates with the concave end of the adjoining bone.
		Condyloid	A condyloid joint (ellipsoid joint) is where the shallow depression at the end of one bone articulates with a rounded structure from an adjacent bone or bones.
		Saddle	A saddle joint is where both of the articulating surfaces for the bones have a saddle shape, which is concave in one direction and convex in the other.
		Plane	A plane joint (gliding joint) is where the articulating surfaces of the bones are flat or slightly curved and of approximately the same size, which allows the bones to slide against each other.
		Ball-and-socket	A ball-and-socket joint is where the rounded head (the ball) of one bone fits into the concave articulation (the socket) of the adjacent bone.