By Elizabeth A. Murray, Mount St. Joseph University
As we begin our discussion of classifying joints and the movements they allow, we need to first recognize that not all joints move. A joint is simply what we call in anatomy an articulation. That’s any place in the body where bone meets bone. So there are many, many joints that link the approximately 206 bones in the body.
Any joint in the body can be classified in two ways: One way is based on the joint’s function—specifically, how much it moves; the second approach classifies joints by their structure in terms of their connective tissue linkages.
Functionally, joints are classified as synarthrotic, amphiarthrotic, or diarthrotic joints by their degree of movement—since arth- is the medical term for “joint”.
Synarthrotic joints do not move at all, amphiarthrotic joints provide a bit of limited movement, while diarthrotic joints are the typical articulations we think of, because they provide free movement in at least two directions.
In this functional scheme, the prefix syn- means “united” or “with”, like in the words synthesis or synchronous. Joints like a growth plate in a child’s bone, a tooth in a dental socket, or the cranial sutures—which are the joints between bones of the skull—would all be examples of synarthrotic joints. None of those places, where bone articulates with bone—or tooth root articulates with bone—none are joints that allow movement.
Amphi- means “both” or “around”—like in amphibians, most of which can live on land and in water, or the word amphitheater, meaning “theater in the round”.
With regard to joints of the body, amphiarthrotic joints are bone-to-bone linkages that allow a fairly limited degree of motion. Examples would include joints between two adjacent vertebrae where the intervertebral disks link two bones of the spine, or the pubic symphysis that unites the two pelvic bones anteriorly.
Di- is a prefix that means “two”, because diarthrotic joints are freely movable in at least two directions. This functional classification includes all the stereotypical joints people think of—like the shoulder, hip, elbow, knee, and joints within the fingers or the toes.
These are the most complex joints in the body as well, and so they are also the most prone to injury and painful dysfunction. The diarthrotic joints are a primary focus of health professions and other fields, including orthopedic medicine, physical and occupational therapy, fitness coaching, and athletic training.
What Holds Synarthrotic and Amphiarthrotic Joints Together?
We’ve defined synarthrotic, amphiarthrotic, and diarthrotic as the three functional classifications. Now, let’s talk about the three structural classifications to which joints can also be assigned.
Joints that have little or no movement—those are the amphiarthrotic and synarthrotic joints—are structurally classified either as fibrous joints or cartilaginous joints, based on the type of connective tissue that links the facing surfaces of the two bones that form the joint. Facing surfaces means their adjacent, articulating ends.
This article comes directly from content in the video series How We Move: The Gross Anatomy of Motion. Watch it now, on Wondrium.
In fibrous joints, the two bones are united—edge to edge—by some type of dense fibrous connective tissue. These tissue types are loaded with collagen fibers, which is why they are said to be dense. To go back to some earlier examples, a tooth is held in a dental socket by fibrous connective tissue that forms the periodontal ligament.
The sutures between the bones of the skull are knitted together by what is known as dense irregular fibrous connective tissue. This is because the bundles of collagen travel in multiple, or irregular, orientations to help resist force or movement in many directions. In both of these examples—a tooth in its socket or the sutures of the skull—the joints are structurally fibrous because dense connective tissues join the adjacent bones, and they are functionally classified as synarthrotic, since both are articulations that do not move.
The second structural classification is that of cartilaginous joints. In these, the two bones are united together, end to end, by some type of cartilage—a firm, white, flexible tissue. The growth plate in a child’s bone is an example of a cartilaginous joint. One in which the shaft—remember that’s called the diaphysis—is united to the bony ends, the epiphyses, by plates of cartilage—specifically a variety of cartilage known as hyaline cartilage.
A growth plate is definitely a joint between bones—or really, within a bone—that should not move. In fact, if the growth plate is disrupted by injury, the result can be devastating to the growth of that bone. Unless it’s medically treated, damage to the growth plate can cause premature union of the shaft and the adjacent end that can permanently stunt the growth of that bone.
With regard to the intervertebral disks of the spine, or the pubic symphysis of the pelvis, the connecting tissue between the faces of the adjacent bones is a pad of fibrocartilage, which contains more collagen fibers, so is stronger than hyaline cartilage.
But the intervertebral disks and the pubic symphysis still allow for a slight degree of movement, so they are functionally amphiarthrotic joints, and structurally, cartilaginous joints—since fibrocartilage is a type of cartilage. There’s a little bit of movement between the bodies of any two adjacent vertebrae, which aids the flexibility of the spine, and a little bit of movement at the pubic symphysis, such as when we walk.
Common Questions about Classifying Joints and the Movements They Allow
Classifying joints by their function specifically means classifying them based on how much they move. In this regard, joints are classified as synarthrotic, amphiarthrotic, or diarthrotic; going from least movement to most.
Classifying joints by their function leads to three categories; synarthrotic, like cranial sutures; amphiarthrotic, like the pubic symphysis; and diarthrotic, like joints within the fingers.
One way we can classify joints is by the connective tissue that holds them together. For example, synarthrotic and amphiarthrotic which are classified by their function can also be classified as fibrous or cartilaginous joints based on their connective tissue.