Anatomy and Physiology: Types of Joints
Types of Joints
Any place where two bones meet is called an articulation (or joint). It might help you to remember that a person is articulate when she/he can string words together (making joints!) well. Don't forget that bones are used for more than movement, so the range of motion is going to vary a great deal, with the least amount of motion in those bones that are mainly for protection. There are two basic ways to classify joints (as seen in the following table): by virtue of structure and by virtue of function. (There's that structure and function duality again!)
|Classification of Joints|
|Fibrous||No joint cavity, held by fibrous connective tissue||Skull sutures, tooth and mandible|
|Cartilaginous||No joint cavity, held by cartilage||Pubic symphysis, epiphyseal plate|
|Synovial||Joint cavity, bones are surrounded by articular capsule, and often accessory ligaments||Knee, elbow, phalanges, and so on|
|Synarthrosis||Immovable joint||Skull sutures, tooth and mandible, epiphyseal plate|
|Amphiarthrosis||Slightly movable joint||Pubic symphysis, tibia-fibula (distal end)|
|Diarthrosis||Freely movable* joint||Knee, elbow, phalanges, and so on|
*Diarthrosis joints are “freely movable,” because they can move freely, but only within a limited range of motion based on the type of diarthrosis.
I Can't Move!
We have all learned so well that bones help us to move (which explains why some texts refer to a musculoskeletal system) that we forget how often bones are used for simple structural support and protection. One very useful way to accomplish these functions is to be immobile. There are three types of immobile joints in the body. Sutures in the skull are one example; their principal function is to protect the brain. These fibrous joints, which form as the skull develops, resemble the meandering pattern of a river, and their pattern is unique in every skull. Remember the fontanels from the last section? Fontanels, which are basically preossified sutures, enable the skull to be flexible during delivery.
A gomphosis is a peg-and-socket joint. This type of joint makes perfect sense when you consider your teeth. The root of each tooth is the peg shape, and in both the inferior surface of the maxilla and the superior surface of the mandible, there is a line of sockets for each of the teeth. At the tip of the root there is a periodontal ligament that connects the root to the jaw, which is what makes pulling teeth so difficult.
Lastly, a synchondrosis is an example of a joint where you don't think a joint would be, simply because it is within the bone. As I mentioned in The Bones, at the epiphysis of every long bone is an epiphyseal plate of hyaline cartilage where the bone growth occurs, at least until adulthood when it ossifies (turns to bone), at which point it is called a synostosis.
A Little Wiggle Room
There are two types of slightly movable joints (amphiarthrosis): syndesmosis and symphysis. A syndesmosis is similar to a suture, complete with the fibrous connective tissue, but it is more flexible. Such a joint is useful if the body needs to link two bones, but allow a little flexibility. A perfect example can be found between the tibia and the fibula. The proximal joint involves only those two bones, but the distal end of each bone also articulates with the talus, which is part of the ankle. The joint between the tibia and fibula thus needs to be both rigid and flexible.
The other amphiarthrosis is called the symphysis, and it is characterized by a broad, flat piece of fibrocartilage, which both cushions the joint and allows some movement. There are two examples of this in the body: the intervertebral disks and the pubic symphysis. The intervertebral disks wear down and flatten over time, which is one of the reasons why people get shorter when they age! The pubic symphysis is the only place where the two pelvic bones articulate (the other articulations are with the femur and the sacrum). This joint is loosened by the hormone relaxin during the ninth month of pregnancy, which eases delivery.
With all these terms ending in -arthrosis, is it any wonder that an inflammation of a joint is called arthritis? There are many forms of arthritis, with a wide variety of causes. Perhaps the most debilitating is osteoarthritis, in which the cartilaginous parts of the joint actually ossify (turn into bone), thus permanently eliminating movement in that joint.
A diarthrosis is also called a synovial joint, named after the structures that make them so freely movable. The defining characteristic of a synovial joint is what is called the synovial joint cavity, filled with synovial fluid, which separates the bones in the joint. This cavity, and the structures in and around it, are more complex than other joints, but they are necessary in terms of providing so much motion.
Have you ever heard people crack their knuckles and wondered what that gross sound was? By pulling on a knuckle you increase the volume of the cavity, thus decreasing the pressure (see Boyle's law in The Digestive System); this can cause some of the synovial fluid to evaporate, and thus make a bubble. When the pressure in the capsule exceeds that in the bubble, the bubble implodes—thus the pop! Any small bubbles left need to fully dissolve before the joint can be cracked again (23 to 30 minutes).
At the end of each articulating bone is a cap of hyaline cartilage called articular cartilage, which serves to protect the bones, but does not connect the two bones. You might remember having seen it the last time you ate a chicken leg. Around the joint as a whole is an articular capsule, which encases the synovial joint cavity. The articular capsule has two layers: a tough outer layer called the fibrous capsule and an inner synovial membrane.
The fibrous capsule, which is made of dense, irregular connective tissue, has two distinct functions. One is to provide enough flexibility to allow movement, and the other is to provide enough strength to prevent dislocating the joint. The greater the flexibility in a joint, the easier it is to dislocate the joint. The ball-and-socket joint allows the greatest range of motion, but the shoulder has far more flexibility than the hip. This is presumably related to our evolutionary past, in terms of needing the pelvis to provide adequate support for our bipedal nature, and a holdout of our ancestors' heavy use of brachiation (arm over arm swinging—think monkey bars). As such, it is understandable that the shoulders get dislocated far more often than the hips.
The inner synovial membrane is a bit of a mish-mash, with areolar connective tissue, adipose tissue, and some epithelial tissue with the cells uncharacteristically widely spaced. This membrane secretes a viscous synovial fluid the consistency of raw egg white that lubricates the joint. Wear and tear on the joint is also cleaned up via macrophages in the fluid. Lastly, since cartilage is avascular, the fluid aids in the distribution of nutrients to the cartilage around the joint.
Most synovial joints also have accessory ligaments. Ligaments outside the capsule are called extracapsular ligaments, with those inside the capsule called intracapsular ligaments. A joint with a lot of use will also have a pad of fibrocartilage called an articular disk, which is also called a meniscus. All of this motion might also cause a lot of friction against the muscle and skin. An extra measure of protection are the bursa, which are similar to articular capsules, complete with fluid, but without the bones. When these capsules get inflamed, that condition is known as bursitis.
Excerpted from The Complete Idiot's Guide to Anatomy and Physiology © 2004 by Michael J. Vieira Lazaroff. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.