Joints: Structure, Classification, and Function

Introduction to Joints

Joints, or articulations, are sites where two or more bones meet. They play a crucial role in providing mobility to the skeleton while maintaining its structural integrity. Joints are classified both structurally and functionally, which helps in understanding their anatomy and physiological roles.

Classification of Joints

Structural and Functional Classification

Joints are classified based on their structural characteristics and the degree of movement they allow. The three main structural classes are fibrous, cartilaginous, and synovial joints. Functionally, joints are classified as synarthroses (immovable), amphiarthroses (slightly movable), and diarthroses (freely movable).

Fibrous Joints

Overview of Fibrous Joints

Fibrous joints are connected by dense fibrous connective tissue and lack a joint cavity. Most are immovable, but the degree of movement depends on the length of the connective tissue fibers. There are three types: sutures, syndesmoses, and gomphoses.

Sutures

Sutures are rigid, interlocking joints found only in the skull. They allow for growth during youth and become immovable (synostoses) as the connective tissue ossifies in adulthood.

Syndesmoses

Syndesmoses are joints where bones are connected by ligaments. The amount of movement depends on the length of the connecting fibers. Short fibers (e.g., inferior tibiofibular joint) allow little movement, while longer fibers (e.g., interosseous membrane between radius and ulna) permit more movement.

Gomphoses

Gomphoses are peg-in-socket fibrous joints, exemplified by the articulation of teeth with their sockets in the maxilla or mandible. The periodontal ligament secures the tooth in place.

Cartilaginous Joints

Overview of Cartilaginous Joints

Cartilaginous joints unite bones with cartilage and lack a joint cavity. They are not highly movable and are classified as either synchondroses or symphyses.

Synchondroses

Synchondroses are joints where bones are united by a bar or plate of hyaline cartilage. Most are immovable. Examples include the epiphyseal plates in growing children and the joint between the first rib and the manubrium of the sternum.

Symphyses

Symphyses are joints where fibrocartilage unites the bones, providing strength and flexibility. Hyaline cartilage is also present as articular cartilage on the bony surfaces. Symphyses are amphiarthrotic (slightly movable). Examples include intervertebral joints and the pubic symphysis.

Synovial Joints

General Features of Synovial Joints

Synovial joints are the most common and movable type of joint in the body. They are characterized by the presence of a fluid-filled joint cavity. All synovial joints are diarthrotic (freely movable) and are found in nearly all limb joints.

• Articular cartilage: Hyaline cartilage covering bone ends, preventing crushing of bone ends.

• Joint (synovial) cavity: Small, fluid-filled space unique to synovial joints.

• Articular (joint) capsule: Two layers—an external fibrous layer and an inner synovial membrane that produces synovial fluid.

• Synovial fluid: Viscous, slippery fluid that lubricates and nourishes articular cartilage and contains phagocytic cells.

• Reinforcing ligaments: Capsular, extracapsular, and intracapsular ligaments stabilize the joint.

• Nerves and blood vessels: Provide sensory input and nutrition.

Additional Structures in Synovial Joints

• Fatty pads: Cushioning between the fibrous layer and synovial membrane or bone.

• Articular discs (menisci): Fibrocartilage pads that improve the fit of bone ends, stabilize the joint, and reduce wear and tear.

• Bursae: Fluid-filled sacs that reduce friction where ligaments, muscles, skin, tendons, or bones rub together.

• Tendon sheaths: Elongated bursae that wrap around tendons subjected to friction.

Types of Synovial Joints

Synovial joints are classified by the shape of their articular surfaces and the movements they allow:

• Plane joints: Allow gliding or sliding movements (e.g., intercarpal joints).

• Hinge joints: Permit flexion and extension (e.g., elbow joint).

• Pivot joints: Allow rotation around a single axis (e.g., proximal radioulnar joint).

• Condylar joints: Permit movement in two planes (e.g., metacarpophalangeal joints).

• Saddle joints: Allow movement in two planes, with articular surfaces both concave and convex (e.g., thumb joint).

• Ball-and-socket joints: Allow movement in multiple axes and planes (e.g., shoulder and hip joints).

Movements at Synovial Joints

Synovial joints allow a variety of movements, which are classified as gliding, angular, and rotational movements.

Selected Synovial Joints

Temporomandibular Joint (TMJ)

The TMJ is a modified hinge joint where the mandibular condyle articulates with the temporal bone. It allows both hinge (depression and elevation) and gliding (side-to-side) movements. The TMJ is the most easily dislocated joint in the body.

Shoulder (Glenohumeral) Joint

The shoulder joint is a ball-and-socket joint, providing the greatest range of motion of any joint in the body. The large head of the humerus fits into the shallow glenoid cavity of the scapula, allowing for extensive movement but sacrificing stability.

Knee Joint

The knee is the largest and most complex joint in the body, consisting of three joints within a single cavity: the femoropatellar joint and the lateral and medial tibiofemoral joints. It allows flexion, extension, and some rotation. The knee is stabilized by multiple ligaments and menisci but is vulnerable to injury, especially from lateral blows.

Clinical and Developmental Aspects of Joints

Inflammatory and Degenerative Conditions

• Bursitis: Inflammation of a bursa, usually due to trauma or friction.

• Tendonitis: Inflammation of tendon sheaths, often from overuse.

• Arthritis: A group of over 100 diseases causing joint pain, stiffness, and swelling. Includes osteoarthritis, rheumatoid arthritis, and gouty arthritis.

Developmental Aspects

By embryonic week 8, synovial joints resemble adult joints. Joint structure and flexibility are influenced by use, and aging leads to decreased joint function and increased risk of degenerative conditions. Regular exercise helps maintain joint health.

Key Clinical Terms

• Arthroplasty: Surgical repair or replacement of a joint.

• Chondromalacia patellae: Softening and damage of the cartilage on the underside of the kneecap.

• Synovitis: Inflammation of the synovial membrane.