Joints Anatomy and Physiology
Joints and Their Classification
Definition of Joints: Points where two bones meet (articulations), movable or not.
Categories: Four major categories of joints based on how bones are bound:
Bony Joints (synostosis)
Fibrous Joints (synarthrosis)
Three types: Sutures, Gomphoses, Syndesmoses
Cartilaginous Joints (amphiarthrosis)
Two types: Synchondroses, Symphyses
Synovial Joints (diarthrosis)
Most mobile and complex.
Fibrous Joints
Sutures: Immobile or slightly mobile, found in skull bones; types include:
Serrate (interlocking edges)
Lap (overlapping edges)
Plane (straight edges)
Gomphosis: Joint between tooth and socket, allows minor movement.
Syndesmosis: Bones connected by long collagen fibers; varies in mobility.
Cartilaginous Joints
Synchondroses: Bones connected by hyaline cartilage, e.g., rib-sternum connection.
Symphyses: Bones joined by fibrocartilage, e.g., pubic symphysis and intervertebral discs.
Synovial Joints
Structure: Separated by joint cavity containing synovial fluid; components include:
Articular cartilage
Joint capsule
Accessories (bursa, ligaments)
Classes of Synovial Joints: Six types based on movement:
Ball-and-socket
Hinge
Pivot
Condylar
Saddle
Plane
Movement Types:
Flexion: Movement that decreases the angle between two bones or parts of the body (e.g., bending the elbow).
Extension: Movement that increases the angle between two bones or parts of the body (e.g., straightening the elbow).
Abduction: Movement of a limb or body part away from the midline of the body (e.g., raising the arm out to the side).
Adduction: Movement of a limb or body part towards the midline of the body (e.g., bringing the arm back down to the side).
Circumduction: A circular movement of a limb at the far end, combining flexion, extension, abduction, and adduction (e.g., moving the arm in a cone shape).
Rotation: Pivoting or rotary movement around a longitudinal axis (e.g., turning the head from side to side).
Supination: Rotation of the forearm and hand laterally so the palm faces anteriorly or superiorly (e.g., holding a bowl of soup).
Pronation: Rotation of the forearm and hand medially so the palm faces posteriorly or inferiorly (e.g., dribbling a basketball).
Mechanical Advantage in Joints
Levers: Long bones as levers for motion; classified by arrangement of fulcrum, effort, and resistance.
Components of a Lever:
Fulcrum (F): Fixed point around which a lever pivots (joint).
Effort (E): Force applied to move the lever (muscle contraction).
Resistance (R): Load or weight being moved (body part or external object).
Classes of Levers:
First-Class Lever: Fulcrum is between the effort and resistance (R-F-E).
Example: Head resting on the vertebral column. (Effort: posterior neck muscles, Fulcrum: atlanto-occipital joint, Resistance: weight of the head anterior to the joint).
Can produce mechanical advantage (MA > 1), disadvantage (MA < 1), or balance ().
Second-Class Lever: Resistance is between the fulcrum and the effort (F-R-E).
Example: Standing on tiptoes. (Fulcrum: metatarsophalangeal joints, Resistance: body weight acting through the ankle, Effort: calf muscles).
Always produces mechanical advantage (MA > 1), favoring force over distance/speed.
Third-Class Lever: Effort is between the fulcrum and the resistance (F-E-R).
Example: Flexing the elbow. (Fulcrum: elbow joint, Effort: biceps brachii muscle, Resistance: weight of the forearm/hand).
Always produces mechanical disadvantage (MA < 1), favoring speed and distance of movement at the expense of force.
Joints and Their Classification
Definition of Joints: Points where two bones meet (articulations), movable or not.
Categories: Four major categories of joints based on how bones are bound:
Bony Joints (synostosis)
Fibrous Joints (synarthrosis)
Three types: Sutures, Gomphoses, Syndesmoses
Cartilaginous Joints (amphiarthrosis)
Two types: Synchondroses, Symphyses
Synovial Joints (diarthrosis)
Most mobile and complex.
Fibrous Joints
Sutures: Immobile or slightly mobile, found in skull bones; types include:
Serrate (interlocking edges)
Lap (overlapping edges)
Plane (straight edges)
Gomphosis: Joint between tooth and socket, allows minor movement.
Syndesmosis: Bones connected by long collagen fibers; varies in mobility.
Cartilaginous Joints
Synchondroses: Bones connected by hyaline cartilage, e.g., rib-sternum connection.
Symphyses: Bones joined by fibrocartilage, e.g., pubic symphysis and intervertebral discs.
Synovial Joints
Structure: Separated by joint cavity containing synovial fluid; components include:
Articular cartilage
Joint capsule
Accessories (bursa, ligaments)
Classes of Synovial Joints: Six types based on movement:
Ball-and-socket: A rounded head fits into a cuplike depression; allows multiaxial movement (e.g., shoulder and hip joints).
Hinge: Allows movement primarily in one plane, like a door hinge; monaxial (e.g., elbow, knee, and interphalangeal joints).
Pivot: A bone rotates on its longitudinal axis relative to another; monaxial (e.g., atlantoaxial joint of the neck, proximal radioulnar joint).
Condylar (or Ellipsoid): An oval convex surface fits into an oval concave surface; allows biaxial movement (flexion/extension, abduction/adduction) but not rotation (e.g., radiocarpal joint, metacarpophalangeal joints).
Saddle: Both bones have a saddle-shaped articular surface, concave in one direction and convex in the other; allows biaxial movement (e.g., carpometacarpal joint of the thumb).
Plane (or Gliding): Articular surfaces are flat or slightly curved, allowing limited gliding or sliding movements; nonaxial (e.g., intercarpal, intertarsal, and sacroiliac joints).
Movement Types:
Flexion: Movement that decreases the angle between two bones or parts of the body (e.g., bending the elbow).
Extension: Movement that increases the angle between two bones or parts of the body (e.g., straightening the elbow).
Abduction: Movement of a limb or body part away from the midline of the body (e.g., raising the arm out to the side).
Adduction: Movement of a limb or body part towards the midline of the body (e.g., bringing the arm back down to the side).
Circumduction: A circular movement of a limb at the far end, combining flexion, extension, abduction, and adduction (e.g., moving the arm in a cone shape).
Rotation: Pivoting or rotary movement around a longitudinal axis (e.g., turning the head from side to side).
Supination: Rotation of the forearm and hand laterally so the palm faces anteriorly or superiorly (e.g., holding a bowl of soup).
Pronation: Rotation of the forearm and hand medially so the palm faces posteriorly or inferiorly (e.g., dribbling a basketball).
Mechanical Advantage in Joints
Levers: Long bones as levers for motion; classified by arrangement of fulcrum, effort, and resistance.
Components of a Lever:
Fulcrum (F): Fixed point around which a lever pivots (joint).
Effort (E): Force applied to move the lever (muscle contraction).
Resistance (R): Load or weight being moved (body part or external object).
Classes of Levers:
First-Class Lever: Fulcrum is between the effort and resistance (R-F-E).
Example: Head resting on the vertebral column. (Effort: posterior neck muscles, Fulcrum: atlanto-occipital joint, Resistance: weight of the head anterior to the joint).
Can produce mechanical advantage (MA > 1), disadvantage (MA < 1), or no advantage () depending on the relative distances of effort and resistance from the fulcrum.