Anatomy and Physiology of Joints
Chapter 09: Skeletal System: Articulations Anatomy & Physiology
9.1 Classification of Joints
Joints (articulations)
Definition: Places of union between bones, or between bones and cartilage, or between bones and teeth.
Role: Bones articulate at a joint.
Study of Joints: Arthrology
Classification: Joints can be classified by their structure and by their function.
Structural Classes of Joints
Fibrous Joint
Bones held together by dense connective tissue.
Cartilaginous Joint
Bones joined by cartilage.
Synovial Joint
Bones joined by ligaments with a fluid-filled joint cavity separating the bone surfaces.
Functional Classes of Joints
Synarthroses
Definition: Immobile joints, which can be either fibrous or cartilaginous.
Amphiarthroses
Definition: Slightly mobile joints, which can also be either fibrous or cartilaginous.
Diarthroses
Definition: Freely mobile joints, all of which are synovial joints.
Range of Motion at Joints
Movement can range from no movement (synarthroses) to extensive movement (diarthroses).
Inverse Relationship: There is a tradeoff between mobility and stability.
Example: Skull sutures are immobile but very stable.
Joint Classification by Structure
No Joint Cavity
Synarthroses (immobile) or Amphiarthroses (slightly mobile).
Joint Cavity
Diarthroses (freely mobile).
Types of Joints
Fibrous Joints: Gomphosis, Suture, Syndesmosis.
Cartilaginous Joints: Synchondrosis, Symphysis.
Synovial Joints: Plane, Hinge, Pivot, Condylar, Saddle, Ball-and-socket.
The Relationship Between Mobility and Stability in Joints
Trade-off: In every joint, higher stability means less mobility, and higher mobility suggests less stability.
Fibrous Joints:
Function: Primarily to hold two bones together.
Types:
Sutures: Immobile, fibrous joints found between skull bones.
Syndesmosis: Slightly mobile, connected by interosseous membranes (e.g. between radius and ulna).
Cartilaginous Joints:
Function: Resist compression, tension stress, and act as shock absorbers, can be immobile or slightly mobile.
Synovial Joints:
Function: Movement, hence are freely mobile.
Examples: Glenohumeral joint, knee joint.
9.2 Fibrous Joints
Characteristics
Connected by dense regular connective tissue and lack a joint cavity.
Generally immobile or slightly mobile.
Types of Fibrous Joints:
Gomphoses: Peg-like structures in a socket (e.g., teeth in the mandible and maxillae).
Sutures: Short fibers found between skull bones, interlocking edges increase stability and decrease fracture risk.
Syndesmoses: Bound by interosseous membrane, found between radius and ulna or tibia and fibula, functioning as amphiarthroses.
9.3 Cartilaginous Joints
Properties
Joined by either hyaline cartilage or fibrocartilage, lack a joint cavity, and can be immobile or slightly mobile.
Types:
Synchondroses: Joined by hyaline cartilage, generally immobile (synarthroses) e.g., epiphyseal plates.
Symphyses: Fibrocartilage pads between articulating bones providing slight mobility (amphiarthroses).
9.4 Synovial Joints
General Features
Definition: Bones are separated by a joint cavity.
Type: Diarthroses (freely mobile).
Common features:
Articular capsule, synovial fluid, articular cartilage, ligaments, sensory receptors, and blood vessels.
Articular Cartilage
Hyaline cartilage on bone surfaces at joints.
Functions: Reduce friction, cushion for joint compression, prevent damage to articulating ends of bones.
Articular Capsule
Double-layered capsule:
Outer fibrous layer (dense connective tissue) strengthens joints.
Inner synovial membrane (areolar connective tissue) covers internal surfaces not covered by cartilage and produces synovial fluid.
Joint Cavity
Space between articulating bones lined by synovial membrane secreting synovial fluid.
Functions of synovial fluid include lubricating articulating surfaces and nourishing chondrocytes.
Ligaments
Composed of dense regular connective tissues, stabilizing and reinforcing synovial joints.
Types:
Extrinsic ligaments: Physically separate from capsule.
Intrinsic ligaments: Thickening of the capsule itself, including those outside and inside the capsule.
Sensory Receptors and Blood Vessels
Numerous receptors detect painful stimuli and report on movement.
Tendons attach muscle to bone, not part of the joint but help stabilize it.
Bursae
Sac-like structures filled with synovial fluid, found in synovial joints.
Functions: Alleviate friction where bones, ligaments, muscles, skin, or tendons rub together.
Accessory Structures
Include tendon sheaths and fat pads which act as protective and cushioning structures in joints.
9.4b Classification of Synovial Joints
Movement Classification
Uniaxial: Movement in one plane or axis.
Biaxial: Movement in two planes or axes.
Multiaxial or Triaxial: Movement in multiple planes or axes.
Types of Synovial Joints
Ball-and-socket:
Globular or egg-shaped head articulates with a cup-shaped cavity providing a wide range of motion (e.g. shoulder and hip joints).
Condylar:
Ovoid condyle fitting into an elliptical cavity, allows back and forth and side-to-side movements within 2 planes (e.g. joints between metacarpals and phalanges).
Additional Joint Types
Plane (gliding): Allows sliding or twisting movement (e.g. joints of wrist and ankle).
Hinge: Movement in one plane, similar to a door hinge (e.g. elbow and phalange joints).
Pivot: Rotation around central axis (e.g. joint between atlas and dens of axis).
Saddle: Variety of movements in two planes (e.g. joint between trapezium and metacarpal of thumb).
Types of Joint Movements
Flexion: Bending parts at a joint, decreasing the angle between them.
Extension: Straightening parts at a joint, increasing the angle between them.
Lateral Flexion: Bending the head, neck, or trunk sideways.
Hyperextension: Straightening beyond normal anatomical positions.
Abduction: Moving a body part away from the midline.
Adduction: Moving a body part toward the midline.
Dorsiflexion: Movement that brings the foot closer to the shin.
Plantar Flexion: Movement that moves the foot farther from the shin, pointing the toes.
Rotation: Movement of a part around an axis.
Circumduction: Movement that traces a circular path.
Pronation: Forearm rotation that faces the palm downward.
Supination: Forearm rotation that faces the palm upward.
Inversion: Turning the sole of the foot medially.
Eversion: Turning the sole of the foot laterally.
Protraction: Moving a part forward.
Retraction: Moving a part backward.
Elevation: Raising a part of the body.
Depression: Lowering a part of the body.
Synovial Joints and Levers
Movement at synovial joints can be compared to mechanics of a lever system.
Terminology of Levers
Lever: An elongated, rigid object that rotates around a fixed point called a fulcrum.
Effort: Applied at one point.
Resistance: Located at another point.
Effort Arm: Distance from the fulcrum to the point of effort.
Resistance Arm: Distance from the fulcrum to the point of resistance.
Types of Levers
First-Class Levers: Fulcrum is located between effort and resistance (e.g., atlanto-occipital joint of the neck).
Second-Class Levers: Resistance is between fulcrum and effort (e.g., standing on tiptoe).
Third-Class Levers: Effort is applied between resistance and fulcrum (e.g., elbow joint).
Major Joints in Human Body
Joints are reinforced by multiple strong ligaments that stabilize them.
Key Joints
Temporomandibular Joint: Articulates between the head of the mandible and the temporal bone; only mobile joint in the skull.
Shoulder Joint:
Sternoclavicular Joint: Saddle joint allowing for elevation, depression, and circumduction of the clavicle.
Acromioclavicular Joint: Plane joint between acromion and clavicle.
Glenohumeral Joint (Shoulder Joint): Ball-and-socket joint allowing for greatest range of motion but also unstable.
Elbow Joint: Composed of two articulations, humeroulnar and humeroradial, both within a common articular capsule.
Hip Joint:
Articulation between head of the femur and acetabulum of hip bone; more stable than shoulder joint.
Knee Joint:
Largest and most complex diarthrosis, primarily a hinge joint allowing slight rotation and lateral gliding.
Talocrural (Ankle) Joint: Highly modified hinge joint allowing dorsiflexion and plantar flexion, includes two articulations within one capsule, secured by ligaments preventing excessive movements, prone to sprains and tears.