Video: Main joints

Welcome back to today's weather report. Our meteorologists forecast a bright but slightly windy day ahead of us. Oh! But wait. Never mind what meteorologists say. I'm getting a live weather update right here. My arthritic knees are telling me that we're in for a terrible day of rain and thunder. And look! Here it comes.

Knee joints telling the weather? Who knew? But actually, there is some sign suggesting that both atmospheric pressure and temperature may influence joint pain and therefore give us an idea of what the weather might be like. Want to find out what else our joints can do? Stay tuned as we explore the main joints of the body.

In today's tutorial, we are going to take a look at the three different types of joints within our body – synovial, fibrous and cartilaginous joints. We'll explore the basic anatomy of synovial joints and their functional classification before describing some of the main synovial joints of the human body. We will then finish off with some clinical notes to help consolidate our knowledge.

Before we dive right in, I want to quickly go over what a joint actually is. A joint is an area where two or more bones meet as we can see here. Joints link the bones of the body together to form our skeletal system. There are approximately 360 joints in the human body which, as we have seen, can be organized into three different types. Let's take a look at these now.

In fibrous joints, the bones are connected by dense fibrous connective tissue. Fibrous joints can be further subclassified into sutures, gomphosis, and syndesmosis. Sutures are immovable joints and can be found between the bones of the skull. These joints become completely immovable at around 20 years of age. They are particularly important during the birthing process as they allow for deformation of the skull during labor. Gomphoses are another type of immovable joint. These joints can only be found where the teeth articulate with their sockets – in the maxilla and the mandible. Syndesmoses are slightly movable joints that involve an interosseous membrane between articulating bones. An example of a syndesmotic joint is the middle radioulnar joint which is also known as the interosseous membrane of the forearm.

Cartilaginous joints, as their name suggests, are joints where the bones are connected by cartilage. Cartilaginous joints can be further subdivided into synchondroses or primary cartilaginous joints and symphyses or secondary cartilaginous joints. Synchondroses are almost completely immovable and are characterized by hyaline cartilage between two articulating bones. These highly immobile joints can be observed at the costochondral joints of the anterior thoracic cavity and at the epiphyseal plates of long bones. Symphyseal joints are the second group of cartilaginous joints. The joint features include adjacent bone surfaces with hyaline cartilage and connected by fibrous tissue with some degree of mobility. The intervertebral joints, pubic symphysis, and the manubriosternal joint are all examples of symphyses.

Synovial joints are the most commonly identified joint type in the human body and therefore will be the main focus of today's tutorial. Synovial joints are a little different as the bones within the joint are not actually directly in contact with each other. The bones of a synovial joint are separated by a narrow synovial or articular cavity and the articulating bone surfaces are covered by hyaline cartilage. Synovial joints are encompassed by a fibrous capsule that strengthens and stabilizes the joint. The interior of the capsule is covered with a synovial membrane that secretes synovial fluid to the interior of the joint cavity which is responsible for lubricating the joint.

Synovial joints are the most mobile joints in our body. These joints can be further classified into six different types according to their shape and movements. Let's take a quick look at these types now.

The first type is the synovial plane joint which allows for sliding or gliding movements. The bones of a synovial plane joint lie flat against each other. In synovial ellipsoid joints, the rounded end of one bone articulates with the shallow depression in the other bone. Ellipsoid joints allow for flexion and extension, abduction and adduction as well as circumduction movements.

Synovial hinge joints are next. In the same way that a hinge on a door can only move in one direction, these joints can only move along one axis creating movements of flexion and extension. Another synovial joint type is the pivot joint which is named after the movement it elicits. Pivot joints will usually have a rounded portion of bone acting as the axis for another bone to pivot around. Saddle joints are named after their appearance as the bones of the joint look like a saddle sitting into each other as a rider does on a horse's saddle. Much like an ellipsoid joint, these joints are biaxial, allowing for similar movements of flexion and extension, abduction and adduction as well as circumduction.

The final synovial joint variation is the ball-and-socket joint. As its name suggests, a ball-shaped articular surface articulates with a concave socket creating a highly dynamic joint. Movements of a ball-and-socket joint include abduction and adduction, flexion and extension, internal and external rotation as well as circumduction.

Now that we have a basic understanding of the joint types, let's take a closer look at some of the main joints within our bodies beginning with the main joints of the upper body.

The first joint we're going to talk about is the temporomandibular joint which forms the main connection between the skull and the lower jaw. It is a combined hinge and ellipsoid synovial joint between the squamous part of the temporal bone and the head of the mandible. This joint has an articular disc which divides the joint cavity into two parts – a superior and inferior compartment. The superior compartment permits protraction and retraction as well as side-by-side movement of the mandible while the inferior compartment allows depression and elevation of the mandible. This joint is especially important in eating and speaking.

Moving inferiorly, we meet the zygapophyseal joints, also known as facet joints. These are plane synovial joints allowing for gliding movements between the vertebrae. They also function to limit lateral movement of the vertebrae to prevent displacement. These joints are formed by the articulation of the superior and inferior articular processes of adjacent vertebrae or zygapophysis and can be found at our vertebral arches all the way down our vertebral column.

Moving anteriorly, we come across a joint of the shoulder girdle – the sternoclavicular joint. This is an important joint because it is the only skeletal articulation between the appendicular skeleton of the upper limb with the axial skeleton of the trunk. The articular surfaces are the sternal end of the clavicle and the clavicular notch of the manubrium of the sternum together with the adjacent superior surface of the first costal cartilage. The sternoclavicular joint is a synovial saddle joint allowing movements of elevation and depression, protraction and retraction and axial rotation of the clavicle. Similar to the temporomandibular joint, the sternoclavicular joint also contains an articular disc which divides the joint space into two compartments. This articular disc aids in enhancing joint concurrency by increasing the contact area of each joint surface.

Another important joint of the shoulder girdle is the acromioclavicular joint which is formed by the articulation of the acromion of the scapula and the lateral end of the clavicle. This joint, also known as the AC joint, is a plane synovial joint allowing gliding movement while also assisting in abductory motions of the upper limb at the shoulder joints. The AC joint is easily palpated at the tip of our shoulder.

Next, we came to the glenohumeral joint, which is also simply known as the shoulder joint. It is classified as a ball-and-socket synovial joint formed by the rounded head of the humerus and the concave glenoid fossa of the scapula. This joint permits a wide range of movements which include flexion and extension, abduction and adduction, internal and external rotation and circumduction of the arm at the shoulder. As a result of its extensive mobility, it is a relatively unstable joint heavily relying on surrounding ligaments, cartilaginous support, and its joint capsule to stabilize and protect the joint.

Moving distally down the upper limb, we come to the elbow joint. The elbow joint is a compound hinge synovial joint and is formed by the articulation of the distal end of the humerus with the radius and the ulna. This hinge joint permits flexion and extension movements of the arm at the elbow. The elbow joint is classified as a compound synovial joint because it is comprised of smaller separate articulations namely the humeroulnar joint formed between the humerus and the ulna, the humeroradial joint formed between the humerus and the radius, and the proximal radioulnar joint formed between the proximal part of the radius and the proximal part of the ulna. These three joints together constitute the elbow joint.

At the distal end of the forearm, we meet the radiocarpal joint which is known as the wrist joint. As its name suggests, the radiocarpal joint is made up of the distal end of the radius and the proximal row of carpal bones. The distal end of the radius specifically articulates with three carpal bones – the scaphoid, lunate, and triquetrum. The radiocarpal joint is the ellipsoid-type synovial joint allowing for biaxial movement of the hand at the wrist. This joint permits flexion and extension, abduction and adduction as well as circumduction of the hand.

Onto the joints of the hand, we meet the carpometacarpal joints. These joints are formed by the articulation of the distal row of carpal bones and the bases of the five metacarpal bones of the hand. Carpometacarpal joints of digits two to five are synovial plane joints allowing for slight gliding movements while the carpometacarpal joint of the thumb is a synovial saddle joint allowing for movements of flexion and extension, abduction and adduction as well as circumduction. This joint is a modified saddle joint because it also allows for some degree of internal and external rotation. Thanks to this joint, the thumb is much more mobile than the other digits permitting the movement of opposition of the thumb and giving us the ability to hold and manipulate objects.

The final joints of the upper limb that we're going to look at today are the metacarpophalangeal joints. These five joints are formed by the articulations between the head of the metacarpals and the base of the proximal phalanges. These joints are what we refer to as the knuckles of the hand. They are ellipsoid synovial joints, allowing for flexion and extension as well as adduction and abduction of the fingers. Why don't you try flexing your fingers now and watch your knuckles move?

Now that we've looked at the main joints of the upper body, let's take a look at the main joints of the lower body.

We'll begin with the sacroiliac joint of the pelvic region. This joint is formed by the articulation between the sacrum and the ilium of the pelvic bone. It is an atypical synovial joint, meaning, that it has a very limited range of motion only allowing for slight gliding and rotational movements. The main function of this joint is to bear the weight of the axial skeleton and transfer it to the hip bones and lower limbs.

Connecting the pelvic region to the lower limb is the hip joint. The hip joint formed by the articulation of the rounded head of the femur and the concave acetabulum of the hip bone is the synovial ball-and-socket joint. It is a highly mobile joint allowing flexion and extension, abduction and adduction, external and internal rotation as well as circumduction movements of the lower limb at the hip joint. It fully relies on surrounding ligaments, muscles and its own joint capsule for support and stability.

At the junction between the thigh and the leg is the knee joint. This joint is a rather unstable joint that requires support from surrounding tendons and muscles. It is formed by the articulations between the femur, the tibia and the patella. The knee joint is a modified synovial hinge joint which not only flexes and extends the leg at the knee but also allows for slight rotational movements during flexion and extension.

At the distal aspect of the leg, we meet the ankle joint. The ankle joint is also known as the talocrural joint and it is formed by the distal end of the tibia and fibula of the leg and by the talus of the foot. The tibia and fibula form a socket at the ankle known as the ankle mortise into which the body of the talus fits. The ankle joint is a hinge-type synovial joint and it allows for dorsiflexion and plantarflexion of the foot.

Sitting just beneath the ankle joint is the subtalar joint. The subtalar joint is formed by the two large tarsal bones of the foot – the calcaneus and the talus. The subtalar joint is a synovial plane joint allowing for sliding or gliding movements which are involved in inversion and eversion of the foot.

The weightbearing ankle joint complex made up of the talocrural and subtalar joints is one of the most commonly injured joints in the human body. This is due to their location, level of force travelling through the joints, and associated movements. The ligaments within this area are commonly stretched and even strained as we go over or sprain our ankles.

The final joints that we'll look at today are the interphalangeal joints of the foot. The first digit – the big toe – forms only one interphalangeal joint while the second to fifth digits form two interphalangeal joints on each digit. This is due to the fact that the big toe only possesses a proximal and distal phalanx whereas the rest of the toes possess a proximal, middle and distal phalanx. The interphalangeal joints are synovial hinge joints allowing for flexion and extension of the toes.

Now that we've had a look at the main joints of the body, let's explore some clinical notes on this subject.

One of the most common forms of joint-related pathology is osteoarthritis. Osteoarthritis is a degenerative pathology characterized by the degeneration of the articular cartilage that lines the articulating ends of the bones within a joint. Highly mobile joints such as the knees, hips and shoulders are most susceptible to injury. Since osteoarthritis affects the joints exposed to the highest stress, this condition is considered less of a disease and is rather regarded as a result of wear and tear or degeneration. This process results in a bone-against-bone grinding which leads to a reduced range of movement of the joint, loss of proprioception, swelling and pain. This degenerative joint disease is most commonly seen in older individuals, however, repeated trauma to the joint or birth defects can also predispose younger individuals to osteoarthritis. Management involves non-pharmacologic interventions such as weight management and exercise, pharmacologic therapy such as anti-inflammatory drugs, and surgery.

That brings us to the end of this tutorial. But before we finish up, let's take a look at a quick summary to highlight what we learnt today.

We began today's tutorial by taking a brief look at the three different types of joints – fibrous joints which can be divided into sutures, gomphosis, and syndesmosis; cartilaginous joints which are subclassified into synchondroses and symphyses joints; and finally, synovial joints. We focused on synovial joints today as they are the most commonly identified joint type in the human body. Synovial joints can be further divided into several varieties which include plane, ellipsoid, hinge, pivot, saddle, and ball-and-socket joint types.

We then explored the main joints of the human body beginning with the main joints of the upper body. Here we met the temporomandibular joint, the zygapophyseal joints of the vertebral column, the sternoclavicular joint, the acromioclavicular joint, the glenohumeral joint, the elbow joint, the wrist joint, the carpometacarpal joints, and finally, the metacarpophalangeal joints. We then took a look at some of the main joints of the lower body. Here we identified the sacroiliac joint of the pelvic region, the hip joint, the knee joint, the ankle joint, the subtalar joint, and finally, the interphalangeal joints of the foot. We finish this tutorial off by taking a look at a clinical note on osteoarthritis, a degenerative pathology of the joints characterized by degeneration of the articular cartilage.

Thanks for watching. I hope you enjoyed learning about the main joints of the human body, and happy studying!