Joints are articulations or connections between bones.
They bind the skeleton together and enable bone growth.
Joints facilitate movement in conjunction with muscles.
Classification of Joints
Joints are classified by structure and function.
Structural Classification
Fibrous Joints: Held together by dense connective tissue.
Examples: Suture, Syndesmosis, and Gomphosis.
Cartilaginous Joints: Held together by cartilage.
Examples: Synchondrosis and Symphysis.
Synovial Joints: Have a complex structure.
Examples: Gliding, Hinge, Ball and Socket, Pivot, Ellipsoidal, and Saddle joints.
Functional Classification
Synarthrosis: Immovable joints.
Amphiarthrosis: Slightly movable joints.
Diarthrosis: Freely movable joints; includes uniaxial, biaxial, and multiaxial joints.
Fibrous Joints
Held together by dense connective tissue with abundant collagen fibers.
Types: Syndesmosis, Suture, and Gomphosis.
Syndesmosis
Bones are bound by a sheet of dense connective tissue (interosseous membrane) or a bundle (interosseous ligament).
Amphiarthrotic, flexible, and allows twisting.
Example: Between the tibia and fibula.
Suture
Thin layer of connective tissue (suture ligament) connects bones.
Synarthrotic or immovable.
Example: Flat bones of the skull.
Gomphosis
Cone-shaped bony process in a socket.
Synarthrotic or immovable.
Example: Root of a tooth in the maxilla or mandible, held by the periodontal ligament.
Cartilaginous Joints
Connected by hyaline cartilage or fibrocartilage.
Types: Synchondrosis and Symphysis.
Synchondrosis
Bands of hyaline cartilage unite bones.
Synarthrotic.
Temporary (epiphyseal plate, which ossifies into synostosis) or permanent (between manubrium and first rib via costal cartilages).
Symphysis
Pads of fibrocartilage between bones, with articular surfaces covered by hyaline cartilage.
Amphiarthrotic with limited movement.
Examples: Pubic symphysis and intervertebral discs between adjacent vertebrae.
Synovial Joints
Most common type of joint; all are diarthrotic.
Complex structure.
Structure
Articular Cartilage: Covers articular ends of bones.
Joint Capsule: Two layers:
Outer fibrous layer composed of ligaments.
Inner synovial membrane that secretes synovial fluid.
Types of Synovial Joints
Classified by shape and movements.
Ball and Socket Joints (Spheroidal)
Round head in cup-shaped cavity.
Widest range of motion: multi-axial plus rotation.
Examples: Hip and shoulder joints.
Condylar Joints (Ellipsoidal)
Oval condyle fits into elliptical cavity.
Back-and-forth and side-to-side movement; biaxial with no rotation.
Examples: Joints between metacarpals and phalanges.
Plain Joints (Gliding)
Almost flat or slightly curved.
Back-and-forth and twisting movements; non-axial.
Examples: Wrist and ankle joints.
Hinge Joints
Convex surface fits into concave surface.
Uniaxial movement in one plane.
Examples: Elbow and joints between phalanges.
Pivot Joints (Trochoid)
Cylindrical surface rotates within a ring.
Uniaxial movement: rotation only.
Example: Atlas (C1) on the dens of axis (C2).
Saddle Joints (Sellar)
Both bones have concave and convex surfaces.
Biaxial movement in two planes.
Example: Carpal and metacarpal of the thumb.
Synovial Joint Components and Features
Every synovial joint has a joint capsule surrounded by articular cartilage.
Ligaments connect bone to bone.
Synovial membrane secretes synovial fluid into the synovial cavity.
Synovial fluid allows for greater movement and nourishment of the joint.
Some synovial joints have menisci and bursae for cushioning.
Synovial Joint Movements
Flexion: Decreasing the angle of a joint (bending).
Extension: Increasing the angle of a joint (straightening).
Abduction: Moving an appendage away from the body.
Adduction: Moving an appendage toward the body.
Lateral Flexion: Bending the upper body laterally away from the midline.
Hyperextension: Taking the joint beyond its normal anatomical position, often from injury.
Dorsiflexion: Bringing the toes up (superior).
Plantar Flexion: Bringing the toes down (inferior).
Circumduction: Rotational movement possible with ball and socket joints.
Rotation: Moving a body part along an axis (lateral and medial rotation).
Supination: Palm of the hand up or superior.
Pronation: Palm of the hand down or inferior.
Inversion: Sole of the foot turning medially.
Eversion: Sole of the foot turning laterally.
Protraction: Moving a body part forward (e.g., scapula).
Retraction: Moving a body part backward (e.g., scapula).
Elevation: Raising of the shoulders superiorly.
Depression: Lowering of the shoulders inferiorly.
Specific Synovial Joints
Temporomandibular Joint (TMJ)
Modified hinge joint for chewing and grinding food.
Shoulder Joint
Ball and socket joint with a shallow glenoid cavity, allowing a wide range of motion but increased risk of displacement.
Ligaments: coracohumeral, glenohumeral, and transverse humeral ligaments to stabilize and protect the joint.
Elbow Joint
Primarily a gliding or plain joint for flexion and extension.
Aids in pronation and supination.
Ligaments: radial collateral, ulnar collateral, and annular ligaments.
Hip Joint
Ball and socket joint with less movement but more stability than the shoulder joint due to a deeper acetabulum.
Reinforcing ligaments: iliofemoral (strongest in the body), pubofemoral, and ischiofemoral ligaments.
Knee Joint
Largest and most complex joint.
Modified hinge joint between femur and tibia, and a plain joint between femur and patella.
Cushioned by bursae and fat pads, with additional support from ligaments and tendons.
Menisci separate the femur and tibia.
Important ligaments: medial collateral ligament, lateral collateral ligament, and posterior cruciate ligament.
Injuries, such as those to the lateral meniscus or tears of the ACL and LCL, can occur during athletic events. For example, a hit to the lateral knee during a football tackle.