Joints

Joints are the structures in the body that not only work to connect the bones to provide stability, but also to allow specific movements with the help of muscles. These structures come in three different categories known as the locked/immovable joints, passive movement/slightly movable joints, and freely movable joints (also known as synovial).

Immovable Joints

Suture- This is a type of fibrous joint that does allow movement (as they are only found on the skull, and the skull does require a minimal amount of flexibility), however so little that it is still categorized as immovable. The sutures are made up of Sharpey’s fibers which hod the bones of the skull together. When born, these suture sites are unfused, however in old age these cranial sutures may ossify completely.

Cartilage- These joints are made up of exactly what you’d expect… Cartilage! (the substance from which bone ossifies/Hyaline cartilage) While having more movement than fibrous joints like the one above, they still are quite limited (meaning that it can be found categorized in immovable or slightly movable). This joint is dispersed all throughout the body and is also responsible for laying out the growth regions of immature bones in children. In more specific terms, this cartilaginous joint is synchondrosis (meaning that it is solely joined by hyaline cartilage).

Slightly Movable Joints

Fibro-Cartilage- This type of joint seems to be exactly the same as the previous cartilage joint, however it is structurally different. These joints are associated with fibro-cartilage, a more spongy material. Because of this small difference, the joint is allowed more movement and is therefore categorized under “slightly movable.” In more specific terms, this cartilaginous joint is symphyses (meaning that although the end of the bones are lined with hyaline cartilage, they are joined by fibro-cartilage). These joints are found in between the vertebrae and pubic bones.

Freely Movable Joints

Plane- When trying to understand the plane joint, it helps to reference back to the planes associated with the directional terms. These planes are flat, and therefore this joint meets bones that are flat or nearly flat in relation to each other. This means that they can glide freely upon the plane that the joint connects them at. A better way to explain it is to imagine you are at a diner ordering coffee. Someone sets the coffee mug on the table and you slide it towards yourself. Now imagine that table being one bone and that mug being another bone. The joint is what creates the direction of the plane (aka the angle of the table surface) and allows the bones to glide (although movement is limited to the surrounding ligaments).

Hinge- The hinge joint (a much simpler joint) functions like a door hinge as it only allows the bones to move along one axis. Opening and closing the door is about the only thing you can do, and the same goes for the joint. The purpose of this is so that we can extend and flex our muscles. Examples of this joint would be the elbows and knees.

Saddle- Like the hinge joint, the saddle joint has a clear relation to real life objects. In this case, it is a bit more complex in comparison because it not only deals with a saddle, but also the person on it. Imagine two gears that fit together perfectly but allow enough space for movement along the sagittal and frontal planes. In relation to the saddle, just picture someone riding a horse. The saddle is the end of one bone and the crotch/legs are the end of the other bone. The best example of this joint is the base of the thumb.

Condyloid- This joint is a bit more complicated as it allows for more movement. The structure of it is is similar to a bowl. One bone has what seems to be a bowl on the end of it, and the other bone just happens to be able to fit it perfectly. It sounds a bit dumb with that explanation, but i mean, its accurate. Anyways, The best example of this joint would be the wrist as we can extend, flex, adduct, abduct, and allow circumduction. While it sounds a lot like the ball and socket joint, it is different as it does not allow axial rotation due to the muscles and ligaments surrounding (and it isn’t a perfect circle, it just deals with a spherical surface). In fact it has a closer resemblance to the saddle joint.

Pivot- These joints are responsible for allowing rotation throughout the body. By rotation, I mean how we can turn our head back and forth to communicate “no” to others, or how we can twist our elbows to allow supination and pronation. This also presents a good example of how we can have many joints at one part of the body (as the elbow has 3 of them alone).

Ball-And-Socket- This joint is one of the more versatile joints in the body as the range of motion and movement are wide. It is composed of one bone with a rounded end, and the other bone with a depression deep enough to fit it. The best examples of this joint are the shoulders and the hips (as they are the only examples available :-). And of course with this joint having the greatest freedom of them all, it can flex, extend, abduct, adduct, internally rotate as well as externally rotate.

Ellipsoid- Although there is already a misleading connection between the condyloid and ball-and-socket joints, this one adds to the mix anyways. Most of the time condyloid and ellipsoid are categorized under the same joint, however there are subtle differences. First of all, the ellipsoid deals with ellipsoidal surfaces while the condyloid deals with spherical. Carrying it over to the other side, the ball-and-socket joint is more like the ellipsoid than the condyloid. When it comes to the condyloid joint, it is actually an extension of the saddle joint because it adds more movement however contains a similar structure. When it comes to the ellipsoid joint, it is an alteration of the ball-and-socket joint as it has the same overall structure, just different shapes.

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