Types of Joints

No, we aren’t talking about marijuana, people. Damn you urban dictionary! But it’s true, the term “joint” is a colloquial name for weed. A story for another day. Right now, however, our area of interest in this article is the types of joints in the human body.

Joints are essentially the point of contact between bones. They act as the gears of our bodies that make movement possible. 

We are going to attempt to provide a comprehensive summary of the different types of joints and their main characteristics.

The answers to everything under the sun is hidden, and all we need to do is ask the right questions! So let’s attempt that. 

What is a Joint?

Author: Madhero88

Joints are anatomical structures that connect two or more bones. 

We do know from the physical laws that any area of contact between two surfaces results in friction. However, if the bones come in contact with each other, for years together, the degenerative changes would lead to catastrophic results. 

Therefore, it is the articular surfaces of the bone are separated from each other by fibrous or cartilaginous tissue. There’s additionally the synovial fluid within the joint space that nourishes these cartilages as well as reduces the frictional damage. 

There are 360 joints in the human body, distinguished by their shape and degree of mobility.

Classification

Joints can be classified according to their function, but also according to which type of tissue composes them.

From a functional point of view, we can distinguish three types of joints:

Diarthroses

Author: OpenStax College

Diarthroses are essentially synovial joints that allow free mobility of the joint. Examples of diarthroses include joints of the fingers, elbows, knees, and shoulders. 

If you wish to know what kind of movements movable joints are responsible for, think of holding a pen, typing on a keyboard, but also jumping, yawning, or walking. 

These types of joints allow a wide range of movements, in one or more directions.

Amphiarthroses 

Amphiarthroses are joints with reduced flexibility. These allow the body to make smaller movements. The vertebrae, ribs, spine, and ankles are an example of semi-mobile joints.

Synarthrosis 

These are joints that tightly bind the bones together, just like a zipper system. These joints do not allow any movement at all!

Now the question arises, why have these joints at all! To understand this, let us look at a few examples of synarthroses.

The sutures of the skull is a very good example of synarthrosis. Now, they allow absolutely no movement. But, they allow the expansion of the skull while simultaneously allowing the cranial vault to protect the vital organ within.

Now, if the cranial vault was a rigid structure, the slightest increase in intracranial pressure (for example, in the case of meningitis) would cause a brainstem herniation and kill the patient. 

The sutural joints prevent this by allowing the expansion of the skull vault buying us doctors some time to save the patient. 

Author: OpenStax College

Now, from a structural point of view, joints can be classified in the following way:

Fibrous joints 

Author: OpenStax College

where bones are connected by fibrous tissue. They are characterized by very low mobility and they can be divided into three main categories:

1. Syndesmosis: These joints connect bones that are separated by a great distance, either using a membrane or a fibrous ligament. The vertebral arch is a great example of this type of joint.
2. Gomphosis: These joints are only found in the teeth and maxillary bones. Its name comes from the Greek word "gonfos" which means nail, Alluding to the way in which people believed the teeth were shaped and fitted in the mouth!
3. Sutures: These joints are only located in the skull to help increase its mobility. Suture joints are made of collagen fibers, also known as Sharpey's fibers, which connects the periosteum (the membrane which covers the outer layer of the bones) to the bone.

Cartilaginous joints

Author: OpenStax College

They are also known as symphyses. These structures are made of cartilage, And they allow joints to keep their flexibility while still being able to resist any force that is exerted on them. An example of cartilaginous joints is the tissue connecting the vertebral discs. According to their functions, these joints can also be divided into three categories:

1. Synarthrosis: joints with very little mobility which can be subclassified into two types, depending on the tissue that composes them: (I) Synchondrosis, if the tissue is made up of cartilage, or (II) Symphibrosis if the tissue is made of fiber. 
   An example of (I) Synchondrosis would be the sternoclavicular joint, while an example (II) Symphibroses are the sutures in the skull.
   
    
2. Amphiarthrosis: flexible cartilaginous structures with low mobility, which can be found in the vertebral discs, for example.
   
    
3. Diarthrosis, very flexible joints characterized by great mobility. They are made of cartilage, joint by the synovial capsule. We've already seen this type of joint, which can be found in fingers, knees, elbows, and hips.

Synovial joints

Author: OpenStax College

These constitute most of the joints in the lower extremities, and they are called synovial because they are united by the synovial capsule, full of synovial liquid, which nourishes and lubricates the cartilage of the joint. Synovial joints are classified into six groups:

1. Ball-and-socket joints or enarthrosis: They are ball-shaped and can be moved in many directions. An example would be the shoulder joint.
    
2. Arthrodia: These are flat joints that can move, but not rotate or move in different directions. An example of these joints would be the intercarpal joints of the hand.
    
3. Trochlear joints: Also known as hinge joints.They are joints that can perform motion only along 1 axis that can only extend and stretch. An example of this type of joint would be the trochlea of the humerus, the articulation of the elbow joint.
    
4. Pivot or Trochoid joints: These joints can only perform lateral rotation, such as the atlantoaxial joint in the neck.
    
5. Condyloid or Ellipsoidal joints: These joints articulate a bone with a concave shape with another bone with a convex shape, such as the temporomandibular joints on the sides of the head, which allow jaw movements. 
    
6. Saddle or Sellar joints: They are shaped like a saddle, for example, the sternoclavicular joint.

References:

1. Gray's Anatomy 41st Edition - Page no: 96 - 103
2. https://www.ncbi.nlm.nih.gov/books/NBK507893/

Author’s footnote

The types of joints were frequently tested in the competitive exams of the previous years. They were rarely tested in recent times. However, the basics are important for discerning the movements of the joints from an orthopedic perspective.

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