Human Anatomy and Physiology - Joints

Classification of Joints

• Joints (articulations): where two or more bones meet, providing mobility and holding the skeleton together.

• Structural Classification

  • Fibrous: material binds joints; no cavity.

  • Cartilaginous: material binds joints; no cavity.

  • Synovial: joints with a cavity.

• Functional Classification:

  • Synarthroses: Immovable joints.

  • Amphiarthroses: Slightly movable joints.

  • Diarthroses: Freely movable joints.

Fibrous Joints

• Bones joined by dense fibrous connective tissue; no joint cavity.

• Mostly immovable; depends on connective tissue fiber length.

  • Sutures: Rigid, interlocking joints of the skull with short connective tissue fibers.

  • Syndesmoses: Bones connected by ligaments.

  • Short fibers: little to no movement (e.g., inferior tibiofibular joint).

  • Longer fibers: larger movement amount (e.g., interosseous membrane connecting radius and ulna).

  • Gomphoses: Peg-in-socket joints; teeth in alveolar sockets.

  • Periodontal ligament holds tooth in socket

Cartilaginous Joints

• Bones united by cartilage; no joint cavity; not highly movable.

  • Synchondroses: Hyaline cartilage unites bones; mostly synarthrotic (immovable).

  • Example: Temporary epiphyseal plate joints.

  • Symphyses: Fibrocartilage unites bone; amphiarthrotic (slightly movable).

  • Example: Intervertebral joints, pubic symphysis.

Synovial Joints

• Bones separated by a fluid-filled joint cavity; all are diarthrotic (freely movable).

• General Features:

  • Articular cartilage: Hyaline cartilage prevents crushing.

  • Joint (synovial) cavity: Fluid-filled space unique to synovial joints.

  • Articular (joint) capsule: Fibrous layer and inner synovial membrane.

  • Synovial fluid: Lubricates and nourishes; contains phagocytic cells.

  • Reinforcing ligaments: Capsular, extracapsular, intracapsular.

  • Nerves and blood vessels: Nerves detect pain and monitor position; capillaries supply filtrate for synovial fluid.

Bursae and Tendon Sheaths

• Bags of synovial fluid that act as lubricating "ball bearings".

  • Bursae: Reduce friction.

  • Tendon sheaths: Elongated bursae wrapped around tendons.

Stability of Synovial Joints

• Determined by:

  • Shape of articular surface (minor role).

  • Ligament number and location (limited role).

  • Muscle tone (most important).

Movements Allowed by Synovial Joints

• Range of Motion:

  • Nonaxial: Slipping movements.

  • Uniaxial: Movement in one plane.

  • Biaxial: Movement in two planes.

  • Multiaxial: Movement in/around all three planes.

• Types of Movements: Gliding, angular, rotation.

  • Angular Movements: Flexion, extension, hyperextension, abduction, adduction, circumduction.

  • Rotation: Medial, lateral.

  • Special Movements: Supination, pronation, dorsiflexion, plantar flexion, inversion, eversion, protraction, retraction, elevation, depression, opposition.

Types of Synovial Joints

• Based on articular surface shape and movement capability:

  • Plane

  • Hinge

  • Pivot

  • Condylar

  • Saddle

  • Ball-and-socket

Selected Synovial Joints

• Temporomandibular Joint (TMJ): Modified hinge joint; easily dislocated.

  • Movements: Hinge (depression/elevation), gliding (lateral excursion).

• Shoulder (Glenohumeral) Joint: Most freely moving; stability sacrificed.

  • Ball-and-socket joint.

  • Stabilized by muscle tendons (rotator cuff).

• Elbow Joint: Hinge joint; flexion and extension only.

  • Humerus articulates with radius and ulna.

• Hip (Coxal) Joint: Ball-and-socket joint; good range of motion.

  • Femur articulates with acetabulum.

• Knee Joint: Largest, most complex joint

  • Femoropatellar, lateral, and medial joints.

  • Intracapsular ligaments: ACL (prevents forward sliding of tibia), PCL (prevents backward sliding of tibia).

Common Joint Injuries

• Cartilage Tears: Due to compression/shear; repaired arthroscopically.

• Sprains: Ligaments stretched/torn; slow repair.

• Dislocations (Luxations): Bones out of alignment; require reduction.

Inflammatory and Degenerative Conditions

• Bursitis: Inflammation of bursa.

• Tendonitis: Inflammation of tendon sheaths.

• Arthritis: >100 types; cause joint damage.

  • Osteoarthritis (OA): Degenerative, "wear-and-tear".

  • Rheumatoid Arthritis (RA): Autoimmune disease.

  • Gouty Arthritis: Uric acid crystal deposition.

• Lyme Disease: Bacterial; transmitted by ticks.

Classification of Joints

• Joints (articulations): points where two or more bones meet, serving crucial roles in providing skeletal mobility and structural integrity by holding the skeleton together.

• Structural Classification

  • Fibrous Joints: Characterized by bones connected through dense connective tissue with no joint cavity present. These joints provide stability and limited to no movement.

  • Cartilaginous Joints: Bones are connected via cartilage, also lacking a joint cavity, allowing for slight to moderate movement depending on the cartilage type.

  • Synovial Joints: These joints feature a unique fluid-filled cavity separating the bones, enabling a wide range of movements and are the most common type of joint in the body.

• Functional Classification

  • Synarthroses: Immovable joints that provide strong connections between bones, offering protection for internal organs such as the skull sutures.

  • Amphiarthroses: Slightly movable joints that allow limited movement, providing both stability and flexibility like the intervertebral discs.

  • Diarthroses: Freely movable joints that enable a wide range of motions and are typical of the limbs, facilitating activities such as walking and grasping.

Fibrous Joints

• Bones are joined by dense fibrous connective tissue, characterized by the absence of a joint cavity. The degree of movement largely depends on the length of the connective tissue fibers.

  • Sutures: These rigid, interlocking joints are found in the skull, connected by short connective tissue fibers, providing minimal to no movement and critical protection for the brain.

  • Syndesmoses: In this type, bones are connected by ligaments, offering varying degrees of movement based on fiber length.

    • Short fibers: Allow very little to no movement, exemplified by the inferior tibiofibular joint, which provides stability to the ankle.

    • Longer fibers: Permit a larger range of movement, such as the interosseous membrane connecting the radius and ulna, enabling forearm rotation.

  • Gomphoses: Specialized peg-in-socket joints are exclusively found as teeth in alveolar sockets.

    • The periodontal ligament firmly holds each tooth in its socket, providing stability during chewing and biting.

Cartilaginous Joints

• Bones are united by cartilage without a joint cavity. Movement capability varies but is generally not highly movable.

  • Synchondroses: Hyaline cartilage unites the bones, making these joints mostly synarthrotic (immovable).

    • Example: The temporary epiphyseal plate joints in growing bones allow for bone lengthening and eventually ossify.

  • Symphyses: Here, fibrocartilage unites the bones, classifying them as amphiarthrotic (slightly movable).

    • Examples: Intervertebral joints that provide cushioning and flexibility to the spine, and the pubic symphysis which allows slight movement during childbirth.

Synovial Joints

• These joints are characterized by a fluid-filled joint cavity separating the bones, enabling them to be diarthrotic (freely movable). They are the most common and movable type of joint in the body.

• General Features:

  • Articular cartilage: Hyaline cartilage covers the bone surfaces, preventing crushing and reducing friction during movement.

  • Joint (synovial) cavity: This unique, fluid-filled space is specific to synovial joints, providing room for movement.

  • Articular (joint) capsule: A double-layered structure composed of a fibrous layer for strength and an inner synovial membrane that produces synovial fluid.

  • Synovial fluid: It lubricates the joint, nourishes the cartilage, and contains phagocytic cells that remove microbes and debris.

  • Reinforcing ligaments: These can be capsular (thickened parts of the capsule), extracapsular (outside the capsule), or intracapsular (inside the capsule).

  • Nerves and blood vessels: Nerves detect pain and monitor joint position and stretch, while capillaries in the synovial membrane provide filtrate for synovial fluid.

Bursae and Tendon Sheaths

• These are bags of synovial fluid that act as lubricating "ball bearings" to reduce friction between bony structures and moving parts.

  • Bursae: Flattened fibrous sacs lined with synovial membrane and containing synovial fluid, commonly found where ligaments, muscles, skin, or tendons overlie and rub against bone.

  • Tendon sheaths: Elongated bursae that wrap around tendons subjected to friction, particularly in confined spaces like the wrist.

Stability of Synovial Joints

• Stability is determined by several factors:

  • Shape of articular surface: Plays a minor role. Deep sockets, like that of the hip joint, provide more stability.

  • Ligament number and location: Also has a limited role. More ligaments generally mean a more stable joint.

  • Muscle tone: This is the most important stabilizing factor. Constant, low-level muscle contraction stabilizes joints by keeping tension on tendons crossing over joints.

Movements Allowed by Synovial Joints

• Range of Motion:

  • Nonaxial: Allows slipping movements only, as there is no axis around which movement can occur.

  • Uniaxial: Permits movement in one plane only, like the hinge joint of the elbow.

  • Biaxial: Enables movement in two planes, such as flexion/extension and abduction/adduction at the wrist.

  • Multiaxial: Allows movement in or around all three planes, offering the greatest range of motion as seen in the shoulder and hip joints.

• Types of Movements:

  • Gliding Movements: One flat bone surface glides or slips over another similar surface.

  • Angular Movements: Increase or decrease the angle between two bones.

    • Flexion: Decreases the angle of the joint.

    • Extension: Increases the angle of the joint.

    • Hyperextension: Excessive extension beyond normal range.

    • Abduction: Movement of a limb away from the midline.

    • Adduction: Movement of a limb toward the midline.

    • Circumduction: Movement of a limb in a circle.

  • Rotation: Turning of a bone around its own long axis.

    • Medial Rotation: Anterior surface moves toward the midline.

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

  • Special Movements: Movements unique to certain joints.

    • Supination: Turning the palm upward.

    • Pronation: Turning the palm downward.

    • Dorsiflexion: Lifting the foot so its superior surface approaches the shin.

    • Plantar Flexion: Depressing the foot (pointing the toes).

    • Inversion: Turning the sole of the foot medially.

    • Eversion: Turning the sole of the foot laterally.

    • Protraction: Moving a body part forward.

    • Retraction: Moving a body part backward.

    • Elevation: Lifting a body part superiorly.

    • Depression: Moving a body part inferiorly.

    • Opposition: Touching the thumb to the fingertips on the same hand.

Types of Synovial Joints

• Classified based on the shape of their articular surfaces and the movement they allow.

  • Plane Joints: Flat articular surfaces allow for gliding or translational movements.

  • Hinge Joints: Cylindrical end of one bone fits into a trough-shaped surface on another bone, allowing angular movement in one plane.

  • Pivot Joints: Rounded end of one bone fits into a sleeve or ring of bone (and possibly ligaments), allowing uniaxial rotation of one bone around its own long axis.

  • Condylar Joints: Oval articular surface of one bone fits into a complementary depression in another bone, allowing biaxial movement.

  • Saddle Joints: Each articular surface has both concave and convex areas, allowing more freedom of movement than condylar joints.

  • Ball-and-Socket Joints: Spherical or hemispherical head of one bone articulates with the cuplike socket of another bone, allowing multiaxial movements.

Selected Synovial Joints

• Temporomandibular Joint (TMJ): A modified hinge joint known for its ease of dislocation.

  • Movements: Primarily involves hinge-like depression and elevation, but also includes gliding movements for lateral excursion.

• Shoulder (Glenohumeral) Joint: The body's most freely moving joint, which consequently sacrifices stability.

  • This ball-and-socket joint is primarily stabilized by muscle tendons, particularly by the rotator cuff muscles.

• Elbow Joint: A stable hinge joint that allows only flexion and extension.

  • Formed by the articulation of the humerus with the radius and ulna.

• Hip (Coxal) Joint: Another ball-and-socket joint, which offers a good range of motion but is not as mobile as the shoulder.

  • The head of the femur articulates with the acetabulum of the hip bone.

• Knee Joint: The body's largest and most complex joint, consisting of three joints in one.

  • Includes the femoropatellar joint, as well as lateral and medial joints between the femur and tibia.

  • Critical intracapsular ligaments include the ACL (anterior cruciate ligament), which prevents forward sliding of the tibia on the femur, and the PCL (posterior cruciate ligament), which prevents backward sliding of the tibia or forward sliding of the femur.

Common Joint Injuries

• Cartilage Tears: Often occur due to compression and shear forces; frequently require arthroscopic surgery for repair.

• Sprains: Result from ligaments being stretched or torn; have a slow repair process due to poor vascularization.

• Dislocations (Luxations): Occur when bones are forced out of alignment; require manual or surgical reduction to restore proper positioning.

Inflammatory and Degenerative Conditions

• Bursitis: An inflammation of the bursa, often due to overuse or direct trauma.

• Tendonitis: Inflammation of tendon sheaths, typically caused by overuse.

• Arthritis: A broad term covering more than 100 types of inflammatory or degenerative diseases that damage joints.

  • Osteoarthritis (OA): A common degenerative condition, often described as "wear-and-tear" arthritis, involving cartilage breakdown.

  • Rheumatoid Arthritis (RA): An autoimmune disease where the body's immune system attacks the joints.

  • Gouty Arthritis: Results from the deposition of uric acid crystals in joints and soft tissues, leading to inflammation and