Joints Types, Protection & Location

Within the skeleton there are three types of joints which are categorised according to the degree of movement that each allows.  These categories are:

1. Fibrous joints – the bones of fibrous joints are joined by fibrous tissue, such as the sutures in the skull or the pelvis.  Fibrous joints allow no movement at all.

 

 
2. Cartilaginous joints – the bones of cartilaginous joints are joined by cartilage, such as the sternocostal joint between the sternum and first rib. These joints allow a very small amount of movement.
 

3. Synovial joints – the bones of synovial joints meet in a joint capsule, such as the knee joint where the femur and tibia meet. These joints are the most common and most moveable joints in the human body.

  

Synovial joints

Synovial joints are the most commonly occurring type of joint, which also produce the greatest range of movements.  The movements created at synovial joints allow us to do everyday activities like walking, running, writing and typing.

Before we look at specific synovial joints in more detail it is important to understand their general characteristics and factors that limit their range of movement.

 

Characteristics of synovial joints

The six key characteristics of synovial joints are listed below.

1. Articular cartilage:  This is a smooth, white, shiny mass that covers the articular (joining) surfaces of bone.  It protects bone tissue and reduces friction (rubbing) between bones when they move.

2. Articular capsule:  The articular capsule attaches to the bones near the edge of their articulating surfaces.  The capsule is made of strong fibrous tissue and surrounds the joint, adding stability and stopping unwanted material from entering and irritating the joint.  The capsule also provides a cavity for synovial fluid to work within.

3. Synovial fluid:  This is a yellowish oily fluid that lubricates the articulating surfaces, forms a fluid cushion between surfaces, provides nutrient for cartilage and absorbs debris that is produced by friction between articulating surfaces.
 
4. Ligaments:  These are strong fibrous bands that join articular surfaces, controlling movement and providing stability.  In the adjacent diagram we see the collateral ligaments on each side of the knee which limit side to side movement of the knee and protect it from impact from the side. (Please note that the articular capsule has been removed in this diagram to show the ‘inside’ of the knee joint).
 
5. Articular discs (meniscus):  Some synovial joints such as the knee have menisci.  These are made of tough fibrous tissue and their function is to absorb shock and maintain joint stability.  They lie between the articulating surfaces and as such protect the surfaces of articulating bone, as seen in the previous image.
 
6. Bursae:  These are found in some synovial joints.  They are closed sacs filled with synovial fluid.  Their role is to reduce friction that can occur with movement, such as when tendons rub over bones.  An example of bursa is shown in the adjacent diagram of the shoulder (Note the articular capsule has also been removed in this diagram).
 
  
 

Factors limiting synovial joint movement

Some joints allow virtually unrestricted movement while others are very restricted.  The range or degree of movements available at each synovial joint is determined by the following three factors:

1. Joint articulation (how the bones of the joint fit together):  If we compare the shoulder and hip joint we can see some key differences in articulation.  The shoulder has a very shallow socket which allows the humerus a greater range of movement before it contacts the unmovable object of the scapula.  In comparison the deep socket in the hip joint allows far less range for the femur before its movement is stopped by contact with the pelvic bones. 

                                     

                             Hip joint                               Shoulder joint

 

2. Joint ligaments:  Ligaments are tough, dense fibrous bands that join bone to bone and as you may remember they are a characteristic of synovial joints.  Ligaments provide extra stability to joints and help to prevent ‘undesirable movement’.  An example of this is the collateral ligaments of the knee.  The medial and lateral collateral ligaments work to help prevent side to side movement, while allowing the knee to flex and extend (bend and straighten) as normal.  This can be seen in the below images of the knee joint.

                   

 

3. Condition of muscles and tendons around a joint:  Muscles and tendons also serve to protect joints from excessive movement.  Examples can be seen at one of the most mobile joints in the body, the shoulder.  Here the extra mobility of the shoulder, due to the shallow socket and lack of bony congruence, requires additional support from the muscles and tendons around it.  The shoulder joint gets this additional support from the rotator cuff muscles and their tendons.  This group of muscles provides stability and protection to make up for the lack of ‘bony congruence’, as can be seen in the below image.

                    

 

It’s important to note here that in order to stabilise and protect joints, muscles, tendons and ligaments need to be kept strong and healthy.  When they are weak and or have been stretched beyond their normal limit, such as from poor weight lifting technique, their ability to perform these tasks is impeded and injury becomes increasingly likely.

 

Types of synovial joints

There are six types of synovial joints which allow varying types and ranges of movement to occur.  The variation in the movements at these joints is because of the differences in their characteristics and limiting factors, as previously discussed.  The six synovial joints are:

1. Gliding joints:  The joint surfaces are flat and of approximately similar length.  Movement occurs in a gliding or sliding of one bone against another.  Side to side and back and forth movement is allowed with these joints.  Examples of gliding joints are:  between the carpals of the wrist and between the tarsals of the ankle.

  

 

2. Saddle joints:  These joint surfaces resemble a saddle and allow side to side and back and forth movements.  An example is the thumb joint shown adjacent between a carpal bone known as the trapezium and the first metacarpal.

 

3. Hinge joints:  The joint surfaces are arranged to allow only back and forth movement such as bending and straightening.  Examples of these joints are the elbow where the humerus and ulna join and the knee.

 

 4. Pivot joints:  These joints allow only one type of movement, the rotation of one bone on or around another.  An example of a pivot joint is the joint between the atlas and axis (C1 & C2) vertebrae, the rotation around each other allows our heads to ‘pivot’ left and right.

 

5. Ball and socket joints:  This type of joint allows side to side, back and forth, and rotational movement.  Examples of these joints are the hip or shoulder joints, where the head (ball) of one bone fits into the cavity (socket) of another.

 

6. Ellipsoid joints:  This joint is also known as a ‘condyloid joint’.  Ellipsoid joints allow back and forth and side to side movement.  Such joints occur between the metacarpals and phalanges (between the bones of the hand and the bones of the finger) as seen in the adjacent image.

 

 

Key synovial joints of the body

Now the six types of synovial joints each allow specific joint actions and varying ranges of movement.  The following two tables detail some of the key joints of the body, their locations, common names, bones of the joints and the actions that they allow.

  

Location of joint	Common Joint Name	Bones of the Joint	Actions of the Joint
	Neck	C1, C2 and skull	Rotation
	Intervertabral	Vertabrae	Flexion, extension, lateral flexion and rotation

 

Location of joint	Common Joint Name	Bones of the Joint	Actions of the Joint
	Shoulder	Humerus, scapula and clavicle	Flexion, extension, horizontal flexion, horizontal extension, abduction, adduction, rotation, circumduction, elevation, depression, protraction and retraction
	Elbow	Humerus, radius and ulna	Flexion and extension
	Wrist	Radius, ulna and carpals	Flexion, extension, abduction, adduction and circumduction
	Sacroiliac	Sacrum and ilium	Very limited range due to strong ligaments
	Hip	Femur and pelvis	Flexion, extension, horizontal flexion, horizontal extension, abduction, adduction, rotation and circumduction
	Knee	Femur, patella, tibia and fibula	Flexion and Extension
	Ankle	Tibia, fibula and tarsals	Plantar flexion, dorsiflexion, inversion, eversion, supination and pronation