Skeletal System – Joints

Blausen.com staff (2014). “Medical gallery of Blausen Medical 2014”. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. (Own work) [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons

A joint occurs whenever two bones articulate or make contact with one another in some manner. The nature of these connections can vary significantly which leads to classification of joints in various ways. In this article we will discuss joint classifications and then cover synovial joints in some detail.

Joint Classifications

In this section we will discuss the various general classifications for joints.

Joints Grouped by Range of Motion

A joint is anywhere in the body where two bones connect in some manner. Some bones simply interlock with another bone making the joint immovable. These immovable joints are referred to as Synarthrosis joints. The sutures between bones in your scull are the most common examples of these types of joints.

Amphiarthrosis joints provide limited movement. The bones usually have some interconnecting fibrocartilage that holds the joint together, but the joint is not particularly flexible. The joints between the two pelvic bones and bones in the spinal column represent this type of joint.

Diarthrosis joints are joints that have a full range of motion. The knee, hip, wrist, fingers, and elbows all represent examples of this type of joint.

Joints Grouped by Structure

Joints can also be classified by how they are structured. Suture joints are immovable joints in which the fibrocartilage that separated the joints has turned to bone, solidifying the two bones into a single bone structure.

Gomphoses are immovable joints that do not contain sutures. The joints between your teeth and mandible represent this type of joint.

A Syndesmosis joint involves two bones that are held in place with one or more ligaments. These joints have limited (Amphiarthrosis) movement. The distal joint between the tibia and fibula is an example of such a joint.

Cartilaginous Joints

Joints where two bones are connective via cartilage are called cartilaginous joints. An immovable joint (such as the one between the manubrium and the first rib) is a synchrondosis joint. When joints have an intervening fibrous pad that allows some limited movement they are called symphysis joints.

Synovial Joint Types

By OpenStax College [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons

When you think of joints in the human body you are likely to think of joints such as the elbow, shoulder, wrist or knee. This are certainly some of the most commonly used joints in the body, but they are far from the only joints or the only types of joints found in our bodies. We will cover each joint type found in the human body in the sections below.

Synovial joints have articular cartilage between the two bones that comprise the joint. Synaptic fluid is used to lubricate the joint. These joints also commonly have multiple ligaments that help to limit motion so the joint is not damaged. The various types of synovial joints are discussed next.

Pivot Point

A pivot joint is comprised of one bone that has an end with a roughly cylindrical shape. This cylinder fits inside another bone (or ligament) that forms a ring that encircles the cylindrical bone. The cylindrical end is then allowed to rotate within the encircling ring. The joint between the radius and ulna bones comprises a pivot joint. There is also a pivot joint at the top of the spinal column that allows the head to rotate. These are not the only joints that can rotate, but they are the only joints with this type of ring/cylindrical shape that support rotation.

Hinge Joint

The structure of a hinge joint allows it to move in only a single plane. Your elbow is an example of a hinge joint. While it may seem your elbow can move in multiple directions, in reality the elbow can only undergo flexure or extension. It is only in combination with the shoulder joint that the elbow can appear to move in multiple directions.

The fingers are another common example of a hinge joint. The joints between finger bones cannot rotate or move to the side. They can only extend and flex.

Saddle Joint

A saddle joint is a joint allowing movement in two different planes. The joint can be moved in extension/flexion as well as abduction/adduction. It can also be moved in circumflexion, or in a circle motion. The only joint in the human body of this type is the joint between the metacarpal of the thumb and the trapezium bone in the wrist. This is in part what makes the thumb such a usual part of the human hand.

Plane Joint

Plane or gliding joints move against one another in a slide fashion along a single plane. Bones in the ankle, wrist, and vertebral column comprise most of the plane joints in the body.

Ball and Socket Joint

A ball and socket joint is a joint where the rounded end of one bone fits into a cup or socket in an adjacent bone. This provides a large degree of freedom of movement. The hip and shoulder joints are both ball and socket joints. The joint is relatively strong, but is subject to injuries where the ball is pulled out of the socket. A dislocated shoulder is a common example of this type of injury.

Condyloid Joint

This joint type allows for flexion, extension, abduction, adduction, and rotation. The joint between the radius and the carpal bones of the wrist is of this type. Though movement of these joints is similar to that of the saddle joint, the bone structure of these joints is closer to that of a ball and socket joint than a saddle joint.

Non-Bone Tissues

There are several types of non-bone tissue that function to hold adjacent sections of bone in place, prevent bones from rubbing together, or that attach muscles to bones. These include cartilages, ligaments, and tendons, each of which we will cover separately below.

Cartilage

Cartilage consists of cells that are high in water content, making cartilage able to absorb compression and movement quite easily. There are neither nerves nor a blood supply inside cartilage, so it is not a source of pain nor is cartilage likely to bleed if injured.

Because there is no blood flow directly into cartilage the living cells in cartilage must obtain their nutrients, water, and waste removal through a process of diffusion. This means the cells receiving nutrients simply by absorbing them from the surrounding fluids. This fluid (synovial fluid) can flow more readily if the cartilage is moved or flexed in some way, most commonly through movement of an associated joint. This is not an efficient means for obtaining nutrients and removing waste. As a result cartilages are generally very slow to heal when injured. Healing may be slightly improved by slow movement of the joint to stimulate the flow of synovial fluid (though this should only be done when recommend by a physician).

Cartilage is semi-rigid yet remains quite pliable. As a result it is found in areas of the body that need some rigidity but that also need to be generally flexible. Some obvious examples include the tip of your nose, your ears and between the bones in most joints in the body.

There are three different types of cartilage in the human body. Each has a different purpose and is therefore found in different locations of the body.

Hyaline cartilage is the most common form of cartilage in the body. It is the cartilage found in your nose. It is also found in the joints of your arms and hips, at the ends of ribs, and in the trachea. The structure of hyaline cartilage makes it quite strong and highly resistant to compression and pressure.

Fibrocartilage is the strongest of the three types of cartilage. It is found in the joints that undergo the most stress and impact in the body, including the knee joints. For this reason it also is the cartilage found in the discs between the vertebrae in the spine.

Elastic cartilage is quite flexible and is found in areas of the body that require flexibility yet need to maintain a specific shape. Elastic cartilage is found in the outer ear (making it quite flexible), Eustachian tube, larynx, and the epiglottis.

The first skeletal diagram above shows the typical locations for each type of cartilage. Each cartilage type is represented by a different color.

Ligaments

Ligaments are composed of fibrous collagen strands that are extremely strong and flexible. They are found throughout the body at joint locations where they bridge the joint, connecting to both bones in the joint. This connectivity serves to hold the bones together and to control how the joint can be moved. As a result ligaments are just as integral to a joint as the bones themselves. Ligaments tightly bind a joint together while also allowing the joint to move within its normal range of motion. Without ligaments joints would easily be dislocated and move in undesirable and potentially injurious manners.

All movable joints use ligaments to hold them together and to define or limit the normal range of motion for the joint. Ligaments are extremely strong and are relatively non-elastic. If a ligament is subjected to excessive stress then it may be stretched or even torn. Such an injury is referred to as a sprain. These injuries commonly heal over a period of a few weeks, but in some cases a torn ligament may require surgical repair. This is a relatively common surgical procedure for torn ligaments in the knee.

Tendons

Tendons are similar to ligaments, but instead of attaching from one bone to another, tendons attach a muscle to a bone. When a muscle is flexed it pulls on the tendon which in turn causes the bone to move. Not all muscles move bones as a lever. Facial muscles, for example, do not move bones. Yet many facial muscles are attached to bones via tendons so that the bones serve as anchor points for the muscles.

When a tendon or associated muscle is stretched or torn the resulting injury is referred to as a strain. Strains generally heal quickly (over a few weeks), but a severe tear in a muscle or tendon might require surgical intervention.

Repetitive movements that result in frequent small strains may cause the associated tendon to become inflamed. This condition is known as tendinitis. If you find inexplicable pain when performing a movement then you could be experiencing tendinitis. The immediate remedy is to stop the activity that is creating the repetitive motion strains. You should seek medical advice from a qualified professional if the pain continues after you have ceased the repetitive motion.

Opening and Closing the Hand

By Dr. Johannes Sobotta [Public domain], via Wikimedia Commons

The muscles that open and close your hand are located in your forearm above your wrist. From these muscles long tendons stretch through the broad area of the hand (where the Carpals and Metacarpals are located) and attach to the top or bottom of your fingers. Tendons that attached to the bottom of your finger bones are called flexor tendons. Those that attached to the top of your finger bones are call extensor tendons.

Flexor tendons pass through the wrist and the palm of your hand before attaching to the underside of each finger. If you bend a finger you can feel these tendons rise slightly in the palm of your hand (especially near any creases in your hand). There are two flexor tendons for each finger. One tendon attaches to the distal phalange of each finger. This tendon is called the flexor digitorum profundus (FDP). A separate tendon called the flexor digitorum superficialis (FDS) attaches to the intermediate phalange of each finger. Together these tendons allow your fingers to bend inward as the muscles in your forearm flex. When all fingers bend inward you can close your hand and make a fist.

Similarly, two tendons run over the back of the wrist and attached to the distal and intermediate phalanges of each finger. These tendons, which have several different anatomical names, are very near to the surface on the back of the hand. These tendons allow your fingers to bend upward and are what allow you to open your closed hand.

As a general summary, the tendons along the inside of the wrist and palm are used to close the hand. The tendons along the back of the wrist and hand are used to open the hand. Here’s a relevant martial arts question. If you are confronted by someone who is brandishing a weapon, which set of tendons might you consider disabling and why?

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