All You Need to Know About Joints

Joint are the area where two or more bones articulate (make contact) with each other. This allows the skeleton to function practically. While most of them allow many degrees and planes of movement, the skull sutures are immobile and only serve to form a tight cavity that protects the brain.

They are largely grouped based on their structure or function.

Structural Classification

Structural classification considers how the bones interact with each other. It is based largely on the histology and takes into account what type of tissue is present at the union of the bones.

Fibrous Joints:

They have white collagenous fibers arranged regularly and connective tissue cells between the bone surfaces. These include sutures which have fibrous connective tissue and syndesmoses which have dense connective tissue.

Example: skull sutures and tympano-stapedial connection.

Cartilaginous Joints:

They have cartilage as the connecting material. On the basis of the type of cartilage they are further divided into primary and secondary:

        • Primary: This has hyaline cartilage between the bone interfaces. Hyaline cartilage is composed of type II collagen and chondrocytes. This is where bones elongate and growth in length. Example: epiphyseal cartilage plates
        • Secondary: This has fibrocartilage which is a mixture of white fibrous tissue and cartilaginous tissue. It includes Type I and Type II collagen fibers. Example: pubic symphysis.

Synovial Joints:

This joins together two different skeletal elements but does not let them make direct contact. Instead the connecting bone surfaces are covered by an articular (hyaline) cartilage and the area where bones meet has a capsule surrounding it. This capsule has an outer articular capsule and inner synovial membrane. This synovial membrane secrets synovial fluid which fills the joint cavity.

Example: elbow joint, wrist joint, hip joint

Functional Classification

Functional classification is determined by the type and degree of movement that can occur. This classification is as follows:

Synarthroses:

These allow little to no movement. This includes sutures, gomphosis (joints between the jaw and teeth) and synchondroses (primary cartilaginous joints).

Amphiarthroses:

These allow slight movement and include symphysis (secondary cartilaginous joints) and interosseous membrane (such as between fibia and tibia).

Diarthroses:

This allows a wide range of movement and includes the synovial joint. This is further classified based on the plane of movement:

  1. Plane Joints: They consist of flat articular surfaces and allow a simple gliding movement. Example: carpometcarpal joint and intercarpal joints
  2. Hinge Joints: They allow flexion and extension. Example: elbow joint and ankle joint
  3. Pivot Joint: They consist of a bony ring within which has another bone acting as a pivot within it. They allow only rotation. Example: superior radioulnar joint and inferior radioulnar joint
  4. Condylar Joint: They consist of two convex surfaces articulating with two concave surfaces. They allow flexion and extension. Example: radiocarpal joint and tibiofemoral joint
  5. Saddle Joint: They allow flexion/extension, abduction/adduction, and circumduction. There is no axial rotation. Example: joint between femur and patella and the carpometacarpal joint of the thumb.
  6. Ball-and-socket joint: They consist of a ball like head into a cupshaped cavity. They allow flexion/extension, abduction/adduction, medial rotation/lateral rotation and circumduction. Example: shoulder joint and hip joints.

Other ways to categorize joints include biomechanically and anatomically. In biomechanical classification the number of bones involved is considered. The divisions are:

  • Simple joints: two joining bone surfaces
  • Compound joints: three or more joining bone surfaces
  • Complex joints: two involving bones surfaces and an articular disk/meniscus

In anatomical classification, they are divided based on their position in the skeletal framework and the part of the body they constitute i.e. knee joints, wrist joints, joints of the hand, sacroiliac joints.

Clinical Relevance

A disease of the joint is known as arthropathy. The common signs may include joint pain, stiffness, and difficulty in movements. Effusion, the presence of fluid in the joint space and bone erosion, loss of osseous content are also signs of arthritis. Arthropathy includes:

  • Joint dislocations: This is when there is an abnormal separation of the joint that occurs as a result of physical trauma that damages either the bone, cartilage or ligament that constitute the joint. Some conditions such as hypermobility syndrome and Ehlers-Danlos Syndrome increase the probability of joint dislocations. Shoulder joints are the most common point of joint dislocations.
  • Hemarthrosis: This is when bleeding into the joint space occurs. This can be as a result of physical injury but are most commonly associated with those that are prone to hemorrhages, such as people with hemophilia or those under treatment with warfarin.
  • Arthritis: This is when there is an inflammation of one or more joints. This is the most common cause of joint disorders in individuals over the age of 55. Arthritis can take many different forms such as:
  • Osteoarthritis: It is the most common cause. It is degenerative joint disease caused by inflammatory mediators that cause degeneration of the articular cartilage. Obesity, infection and injury can be predisposing factors.
  • Rheumatoid arthritis: It is caused as a result of an autoimmune disease where antibodies formed against the joint attack and deform cartilages and membranes.
  • Psoriatic arthritis: It occurs in those that suffer from the autoimmune disease psoriasis.
  • Gout: It occurs when uric acid crystals deposit into joint spaces. This leads to pain and inflammation along with redness, heat and swelling
  • Pseudogout: It is when calcium crystals deposit in joint spaces usually after a chronic injury.

Monoarticular arthropathies involve one joint and polyarticular arthropathies involve two or more joints. The can be diagnosed based on a collective evaluation of patient history, physical examination lab tests and medical imaging such as X-rays and MRIs.

Temporomandibular Joint Syndrome is an arthropathy involving the joint between the mandible (jaw) and temporal bone of the skull. It can be caused by mental stress that causes one to clench the jaw. The signs include facial pain as the nerve that innervates the joint also innervates the facial skin and muscles, clicking sounds and a lesser degree of movement.