Chapter 9: Joints – Structure, Classification, and Movement

Joints and Movement

Definition of a Joint

Joints, also known as articulations, are the locations where two or more bones, or bone and cartilage, come together to form a connection. These structures are essential for providing movement and stability to the skeleton.

• Articulation: The process by which bones connect at a joint.

• Function: Joints allow for varying degrees of movement, depending on their structure.

Factors Affecting Joint Type and Movement

Determinants of Joint Movement

The type and range of movement at a joint are primarily determined by:

• Structure of the joint: The anatomical arrangement of bones, cartilage, and connective tissues.

• Function of the joint: The intended purpose, such as stability or mobility.

Structural Classification of Joints

Overview of Structural Classification

Joints are classified structurally based on how the adjacent bones are connected:

• Whether bone surfaces are directly connected by connective tissue or cartilage.

• Whether bone surfaces contact each other within a fluid-filled cavity.

Main Structural Types:

• Fibrous joint

• Cartilaginous joint

• Synovial joint

Fibrous Joints

Fibrous joints are where adjacent bones are directly connected by fibrous connective tissue. These joints do not have a joint cavity and are generally immobile or allow very limited movement.

• Suture joint: Found between skull bones; provides stability and protection.

• Gomphosis joint: The connection of a tooth within its socket in the maxilla or mandible.

Cartilaginous Joints

Cartilaginous joints are where bones are joined by cartilage, either hyaline or fibrocartilage. These joints allow more movement than fibrous joints but less than synovial joints.

• Synchondrosis: Bones are joined together by hyaline cartilage. Examples include the epiphyseal plate (temporary), sacral bone, and the costochondral joints of the ribs (permanent).

• Symphysis: Bones are joined together by fibrocartilage, which is tougher and more flexible than hyaline cartilage. Examples include the pubic symphysis and intervertebral symphysis.

Synovial Joints

Synovial joints are the most common and most movable type of joint in the body. The articulating surfaces of the bones are not directly connected but come into contact within a fluid-filled cavity, allowing for free movement.

• Presence of a joint cavity filled with synovial fluid.

• Articulating surfaces covered by hyaline cartilage to reduce friction.

• Enclosed by a joint capsule lined with a synovial membrane.

Functional Classification of Joints

Overview of Functional Classification

Joints are also classified based on the degree of movement they allow:

• Synarthrosis: Immobile joint

• Amphiarthrosis: Slightly mobile joint

• Diarthrosis: Freely movable joint

Examples of Functional Types

• Synarthrosis: Suture joints between skull bones; manubriosternal joint.

• Amphiarthrosis: Cartilaginous joints such as the pubic symphysis and intervertebral symphysis.

• Diarthrosis: All synovial joints, which allow a wide range of movements and are mostly found in the appendicular skeleton.

Types of Synovial Joints

Classification by Axes of Motion

Synovial joints are further classified based on the number of axes of motion:

• Uniaxial joint: Movement in a single plane (e.g., elbow).

• Biaxial joint: Movement in two planes (e.g., metacarpophalangeal joint).

• Multiaxial joint: Movement along three axes (e.g., hip and shoulder joints).

Types and Examples of Synovial Joints

• Pivot joint (Uniaxial): Allows rotation (e.g., atlantoaxial joint between C1 and C2 vertebrae).

• Hinge joint (Uniaxial): Allows bending and straightening (e.g., elbow, knee).

• Condyloid joint (Biaxial): Allows movement in two planes (e.g., metacarpophalangeal joints).

• Saddle joint (Biaxial): Articulating surfaces fit together like a rider and saddle (e.g., thumb carpometacarpal joint).

• Plane joint (Multiaxial): Flat surfaces allow sliding movements (e.g., intercarpal joints).

• Ball-and-socket joint (Multiaxial): Rounded head fits into a socket, allowing the greatest range of motion (e.g., shoulder and hip joints).

Movements at Joints

Types of Movements

• Flexion: Decreases the joint angle; anterior or posterior movement (e.g., bending the knee or elbow).

• Extension: Increases the joint angle; returns to anatomical position (e.g., straightening the knee or elbow).

• Abduction: Movement away from the midline (e.g., raising the arm laterally).

• Adduction: Movement toward the midline.

• Circumduction: Circular movement of a limb.

• Rotation: Twisting movement about the long axis (e.g., head, vertebral column).

• Medial rotation: Anterior surface moves toward the midline.

• Lateral rotation: Anterior surface moves away from the midline.

• Dorsiflexion: Lifting the top of the foot upward at the ankle.

• Plantar flexion: Pointing the toes downward.

• Supination: Rotation of the forearm so the palm faces up.

• Pronation: Rotation of the forearm so the palm faces down.

• Inversion: Turning the sole of the foot toward the midline.

• Eversion: Turning the sole of the foot away from the midline.

• Opposition: Movement of the thumb to touch the tip of a finger.

Homeostatic Imbalances and Joint Injuries

Bursitis

Bursitis is the inflammation of a bursa near a joint, causing pain, swelling, and tenderness. It can be acute or chronic and is often due to muscle overuse, trauma, arthritis, gout, or infection.

• Treatment: Rest, ice, NSAIDs, corticosteroids, and sometimes removal of the bursa.

Osteoarthritis

Osteoarthritis is a common disorder of synovial joints, characterized by inflammation and "wear and tear" of the articular cartilage. It is associated with aging and repetitive use.

• Treatment: Mild exercise, NSAIDs, and in severe cases, joint replacement.

Gout

Gout is a form of arthritis caused by the deposition of uric acid crystals in the joint, resulting from overproduction or underexcretion of uric acid.

• Treatment: Pain relief and management of underlying conditions to improve renal excretion of uric acid.

Rheumatoid Arthritis

Rheumatoid arthritis (RA) is an autoimmune disorder where the immune system attacks the joints, leading to inflammation, cartilage destruction, and joint deformation.

• Treatment: Focuses on reducing inflammation and pain; no known cure.

Sprains and Strains

• Sprain: Stretching or tearing of the supporting ligaments of a joint.

• Strain: Stretching or tearing of muscle or tendon fibers.

• RICE technique: Rest, Ice, Compression, Elevation; used for initial management of joint or muscle injuries.

Special Joints and Clinical Notes

Temporomandibular Joint (TMJ)

The TMJ is the joint between the temporal bone and mandible, allowing for hinge and gliding movements necessary for opening and closing the mouth. Common symptoms of TMJ disorders include clicking, uneven opening, and headaches.

Shoulder Joint

The shoulder joint allows a wide range of motion due to less bony support, but is more prone to injury. Muscles provide most of the joint's stability.

Hip Joint

The hip joint supports the weight of the body and is prone to osteoarthritis. It has greater depth and stability than the shoulder joint.

Knee Joint

The knee joint is the largest joint in the body and is susceptible to hyperextension, twisting, and side trauma.

Ankle Joint

The ankle joint, located between the talus and tibia, is commonly injured and allows dorsiflexion and plantar flexion.

Summary Table: Structural and Functional Classification of Joints

Structural Type	Functional Type	Examples	Movement Allowed
Fibrous (Suture, Gomphosis)	Synarthrosis	Skull sutures, tooth in socket	Immobile
Cartilaginous (Synchondrosis, Symphysis)	Amphiarthrosis	Epiphyseal plate, pubic symphysis, intervertebral discs	Slightly mobile
Synovial (Pivot, Hinge, Condyloid, Saddle, Plane, Ball-and-socket)	Diarthrosis	Shoulder, hip, elbow, knee, thumb	Freely movable

*Additional info: Academic context and examples have been expanded for clarity and completeness.*