Joints Lecture Notes

Joint Structure and Movement

Joint Classifications According to Structure and Movement (Module 8.1)

• Joints, or articulations, are locations where two or more bones meet.
• Movements of bones occur at joints, allowing mobility while preserving bone strength.
• The amount of movement allowed is determined by the anatomical structure.
• Joints are categorized functionally by the amount of motion allowed (range of motion, ROM) and structurally by anatomical organization.

Functional Classification

• Synarthrosis: No movement allowed; extremely strong.
• Amphiarthrosis: Little movement allowed (more than synarthrosis); much stronger than diarthrosis. Articulating bones are connected by collagen fibers or cartilage.
• Diarthrosis: Freely movable.

Structural Classification

• Fibrous:
  • Suture: Synarthrotic joint connected by dense fibrous connective tissue; located between bones of the skull.
  • Gomphosis: Synarthrotic joint binding teeth to bony sockets in maxillae and mandible.
  • Syndesmosis: Amphiarthrotic joint with bones connected by a ligament; example: distal joint between tibia and fibula.
• Cartilaginous: Held together by cartilage.
  • Synchondrosis: Synarthrotic joint formed by a rigid, cartilaginous bridge between two articulating bones; example: between ends of the first pair of ribs and the sternum.
  • Symphysis: Amphiarthrotic joint where articulating bones are separated by a pad of fibrocartilage; example: joint between the two pubic bones.
• Bony:
  • Synostosis: Synarthrotic, totally rigid, immovable joint formed when bones fuse; example: frontal suture and epiphyseal lines.
• Synovial:
  • Diarthrotic joints that permit a wider range of motion than any other joint type; located at the ends of long bones.

Synovial Joints (Module 8.2)

• Synovial joints are freely movable and lined with a synovial membrane.

Components of a Synovial Joint

• Articular cartilage: Covers bones at the joint. Its structure resembles hyaline cartilage but with no perichondrium. The matrix contains more water than other cartilages.
• Joint capsule (articular capsule): Sac enclosing the articular ends of the bones in a joint. It is reinforced with accessory structures (tendons, ligaments) and is continuous with the periosteum of each bone, adding strength and mobility to the joint.
• Synovial membrane: Lines the interior of the joint capsule and secretes synovial fluid into the joint cavity.

Synovial Fluid

• Synovial fluid is a clear, straw-colored, viscous fluid with the consistency of raw egg white.
• Viscosity is due to the high concentration of hyaluronic acid.
• It is produced by the synovial membrane and circulates from areolar tissue into the joint cavity and percolates through articular cartilage.
  • Provides oxygen and nutrients to chondrocytes and carries away metabolic wastes.
• The total quantity of synovial fluid is usually less than 3 \, \text{mL}.

Functions of Synovial Fluid

• Lubrication: Under compression, fluid squeezes out of the cartilage and into the space between bones, reducing friction.
• Nutrient distribution: Circulates continuously, providing nutrients and carrying away wastes. Compression and expansion of articular cartilage assist in circulation.
• Shock absorption: Viscosity increases with increasing pressure.

Accessory Structures Supporting the Knee

• Provide support and additional stability.
  • Tendon of the quadriceps muscle: Not part of the knee joint itself, but it limits range of motion and provides mechanical support.
  • Bursa: Small, thin, fluid-filled pocket filled with synovial fluid and lined by synovial membrane. Forms in connective tissue outside a joint capsule, reduces friction, and acts as a shock absorber.
  • Fat pads: Localized masses of adipose tissue covered by a layer of synovial membrane, usually superficial to the joint capsule. Protect articular cartilage and fill in spaces created as the joint moves and the joint cavity changes shape.
  • Meniscus (articular disc): Pad of fibrocartilage between opposing bones in a synovial joint, may subdivide a synovial cavity, may channel synovial fluid flow, and allows variations in the shapes of the articular surfaces.

Accessory Ligaments

• Support, strengthen, and reinforce synovial joints.
  • Capsular ligaments (intrinsic ligaments): Localized thickenings of the joint capsule.
  • Extrinsic ligaments: Separate from the joint capsule.
    • Extracapsular ligaments: Pass outside the joint capsule (e.g., patellar ligament).
    • Intracapsular ligaments: Pass inside the joint capsule (e.g., cruciate ligaments).

Mobility in Joints

• Greater range of motion results in a weaker joint.
  • Synarthroses are the strongest joints and have no movement.
  • Diarthroses are the most mobile joints and the weakest.
• Dislocation (luxation): Movement beyond the normal range of motion, where articulating surfaces are forced out of position, damaging joint structures. Pain is from nerves monitoring the capsule and surrounding tissue; there are no pain receptors inside a joint.

Anatomical Organization and Motion at Synovial Joints (Module 8.3)

General Types of Movement

• Gliding: Linear motion that permits sliding motion in any direction on a relatively flat surface.
• Angular motion: Movement along two axes in one plane, involving a change in angle.
• Circumduction: Complex angular movement where the proximal end of the bone remains fixed while the distal end moves in a path corresponding to drawing a circle.
• Rotation: Movement around the longitudinal axis where one bone end remains fixed, and the shaft rotates.

Movement Described by Number of Axes

• Monoaxial: Around one axis.
• Biaxial: Around two axes.
• Triaxial: Around three axes.

Types of Synovial Joints

• Plane joint
  • Examples: Acromioclavicular and sternoclavicular joints, intercarpal and intertarsal joints, vertebrocostal joints, sacro-iliac joints
• Hinge Joint
  • Examples: Elbow joints, knee joints, ankle joints, interphalangeal joints
• Pivot joint
  • Examples: Atlanto-axial joint, proximal radio-ulnar joints
• Condylar joint
  • Examples: Radiocarpal joints, metacarpophalangeal joints II-V, metatarsophalangeal joints
• Saddle joint
  • First carpometacarpal joints
• Ball-and-socket joint
  • Shoulder joints, hip joints

Movements with Reference to Anatomical Position (Module 8.4)

Flexion and Extension

• Refer to movements at hinge joints of the long bones of the limbs.

  • Flexion: Decreases the angle of the joint.
  • Extension: Increases the angle of the joint.
  • Hyperextension: Extension past the anatomical position.

• Lateral flexion: Refers to bending the vertebral column to the side, most pronounced in cervical and thoracic regions.

• Movements of the foot:

  • Dorsiflexion: Upward movement of the foot or toes.
  • Plantar flexion: Movement extending the ankle, as in standing on tiptoe.

Abduction and Adduction

• Refer to movements of the appendicular skeleton.
  • Abduction: Movement away from the longitudinal axis in the frontal plane.
  • Adduction: Movement toward the longitudinal axis in the frontal plane.

Circumduction

• Moving a body part such that the distal end traces a circle while the proximal end stays in one position.

Rotation and Special Movements (Module 8.5)

Rotation

• When referring to the trunk, described as left and right rotation in reference to the anatomical position.
• When referring to the limbs:
  • Medial rotation (internal, or inward, rotation): Anterior surface of a limb turns toward the long axis of the trunk.
  • Lateral rotation (external, or outward, rotation): Anterior surface of a limb turns away from the long axis of the trunk.
  • Special Terms for Rotation of the Forearm
    • Pronation: Distal epiphysis of radius rolls across the anterior surface of the ulna, turning the wrist and hand from palm facing front to palm facing back (posteriorly).
    • Supination: Opposing movement where the palm is turned anteriorly.

Special Movements

• Opposition: Movement of the thumb toward the surface of the palm or pads of other fingers, enabling grasping objects.
• Movements of the Foot
  • Inversion: Twisting motion turning the sole inward.
  • Eversion: Opposing motion turning the sole outward.
• Protraction: Moving a part of the body anteriorly in the horizontal plane.
• Retraction: Reverse of protraction; returning the body part to the normal position.
• Depression: Moving a body part inferiorly (as in opening your jaw).
• Elevation: Moving a body part superiorly (as in closing your jaw).

Axial and Appendicular Joints (Module 8.6)

Joints of the Axial Skeleton

• Strong with very little movement permitted.
  • Examples:
    • Atlanto-occipital joint: Articulation between the occipital bone and atlas.
    • Atlanto-axial joint: Articulation between C1 and C2.

Joints of the Appendicular Skeleton

• Extensive range of motion, often weaker than axial skeleton joints.
  • Examples:
    • Sternoclavicular joint: Articulation between the axial skeleton and pectoral girdle and upper limb.
    • Sacro-iliac joint: Attaches the sacrum of the axial skeleton to the pelvic girdle.

Vertebral Column Joints (Module 8.7)

Types of Joints in the Vertebral Column

• Syndesmoses: Fibrous joints including vertebral ligaments.
• Synchondroses: Intervertebral joints forming intervertebral discs.
  • Account for ~1/4 length of the vertebral column.
  • With increasing age, water content decreases, and so does cushioning ability resulting in increased risk of vertebral injury and a shorter vertebral column.
• Vertebral Synovial Joints: Joints between bony processes.

Intervertebral Disc Components

• Anulus fibrosus: Tough outer ring of fibrocartilage with collagen fibers attached to adjacent vertebrae.
• Nucleus pulposus: Soft, elastic, gelatinous core that gives the disc resilience and shock absorption ability.

Primary Vertebral Ligaments

• Ligamentum flavum: Connects laminae of adjacent vertebrae.
• Posterior longitudinal ligament: Connects posterior surfaces of adjacent vertebral bodies.
• Interspinous ligament: Connects spinous processes of adjacent vertebrae.
• Supraspinous ligament: Connects tips of spinous processes from the sacrum to C7. The ligamentum nuchae extends from C7 to the base of the skull.
• Anterior longitudinal ligament: Connects anterior surfaces of adjacent vertebral bodies.

Clinical Module: Intervertebral Disc Disease and Osteoporosis (Module 8.8)

Intervertebral Disc Disease (IVDD)

• Bulging disc: Caused by weakened posterior longitudinal ligaments, allowing compression of the nucleus pulposus and distortion of the anulus fibrosus. The tough outer layer of cartilage bulges laterally.
• Herniated disc: Nucleus pulposus breaks through the anulus fibrosus and protrudes into the vertebral canal, compressing spinal nerves.

Bone Changes with Aging

• Osteopenia: Inadequate ossification leading to loss of bone mass, often occurring with age, beginning between ages 30 and 40. More severe in women than men.
• Osteoporosis: Bone loss sufficient to affect normal function.

Loss of bone mass, along with reduced cushioning of intervertebral discs, leads to an increasing incidence of vertebral fractures in the elderly.

Shoulder and Hip Joints (Module 8.9)

Shoulder Joint (Glenohumoral Joint)

• Greatest range of motion of any joint and the most frequently dislocated joint. Demonstrates how stability is sacrificed for mobility.
• Ball-and-socket diarthrosis: Articulation between the head of the humerus and the glenoid cavity of the scapula. Stabilized by five major ligaments, surrounding muscles, and associated tendons.
  • Bursae help reduce friction with the tendon of the biceps brachii surrounded by a tubular bursa as it passes through the articular capsule.

Glenohumeral Joint Stabilizing Ligaments

• Coracoclavicular ligaments.
• Acromioclavicular ligament.
• Coraco-acromial ligament.
• Coracohumeral ligament.
• Glenohumeral ligaments.

Glenohumeral Joint Features

• Articular capsule: Permits an extensive range of motion.
• Small articular cartilage
• Glenoid labrum: Fibrocartilage rim that increases the area of the glenoid cavity.

Hip Joint

• Sturdy ball-and-socket diarthrosis joint permitting flexion, extension, adduction, abduction, circumduction, and rotation.
  • Articulation between the head of the femur and the acetabulum (deep fossa) of the hip bone.
  • The acetabular labrum is a rim of fibrocartilage that increases the depth of the joint cavity and helps to seal in synovial fluid.
• The articular capsule of the hip extends from the lateral and inferior surfaces of the pelvic girdle to the intertrochanteric line and intertrochanteric crest of the femur. It encloses both the head and neck of the femur and is reinforced by five ligaments.

Hip Joint Reinforcing Ligaments

1. Transverse acetabular ligament: Crosses the acetabular notch, filling the gap in the inferior border of the acetabulum.
2. Ligamentum teres (ligament of the femoral head): Originates along the transverse acetabular ligament and attaches to the fovea capitis.
3. Pubofemoral ligament
4. Iliofemoral ligament
5. Ischiofemoral ligament

Elbow and Knee Joints (Module 8.10)

Elbow Joint

• Complex hinge joint involving humerus, radius, and ulna.
• Extremely stable because:
  1. The bony surfaces of the humerus and ulna interlock.
  2. A single, thick articular capsule surrounds both the humero-ulnar and proximal radio-ulnar joints.
  3. Strong ligaments reinforce the articular capsule.

Specific Joints of the Elbow

• Humeroradial joint: Capitulum of the humerus articulates with the head of the radius.
• Humero-ulnar joint: Largest and strongest articulation. The trochlea of the humerus articulates with the trochlear notch of the ulna.
  • The shape of the trochlear notch determines the plane of movement.
  • The shapes of the olecranon fossa and olecranon limit the degree of extension.
• The proximal radio-ulnar joint is not part of the elbow joint, but the capsule and ligaments help hold the humerus, ulna, and radius in position.

Muscle Attachments of the Elbow Joint

• Muscles that extend the elbow attach on the olecranon on the posterior surface and are controlled by the radial nerve.
• The tendon of the biceps brachii attaches at the radial tuberosity, and contraction produces supination of the forearm and flexion at the elbow.

Severe stresses can still produce dislocations or other injuries, especially when epiphyseal growth is not complete (e.g., nursemaid’s elbow—partial dislocation of the radial head from the annular ligament).

Elbow Joint Reinforcing Ligaments

• Radial collateral ligament: Stabilizes the lateral surface of the elbow joint.
• Annular ligament: Binds the head of the radius to the ulna.
• Ulnar collateral ligament: Stabilizes the medial surface of the elbow joint.

Knee Joint

• Contains three separate articulations:

  1. Medial condyle of the tibia to the medial condyle of the femur.
  2. Lateral condyle of the tibia to the lateral condyle of the femur.
  3. Between the patella and patellar surface of the femur.

• These articulations permit flexion, extension, and very limited rotation.

• The fibula is not part of the knee joint.

Knee Joint Supporting Structures

• Quadriceps tendon: Continues as the patellar ligament to the anterior tibial surface.
• Fibular collateral ligament (lateral collateral ligament, LCL): Provides lateral support.
• Tibial collateral ligament (medial collateral ligament, MCL): Provides medial support.
• Popliteal ligaments: Run between the femur and heads of the tibia and fibula.
• Medial and lateral menisci: Pair of fibrocartilage pads located between femoral and tibial surfaces. Act as cushions and provide lateral stability.
• Anterior cruciate ligament (ACL)
  • At full extension, slight lateral rotation of the tibia tightens the ACL and forces the lateral meniscus between the tibia and femur.
  • This “locks” the knee in the extended position; the opposite motion is required to “unlock” it.
• Posterior cruciate ligament (PCL)

Clinical Module: Arthritis (Module 8.11)

Terminology

• Rheumatism: General term indicating pain and stiffness in the bones and/or muscles.
• Arthritis: All rheumatic diseases that affect synovial joints, always involving damage to the articular cartilage. Causes vary.
  • Three types: osteoarthritis, rheumatoid arthritis, and gouty arthritis.

Osteoarthritis

• Also known as degenerative arthritis or degenerative joint disease (DJD).
• Most common form of arthritis, generally affecting individuals age 60 or older.
  • In the United States, it affects 25 \,\% of women and 15 \,\% of men over age 60.
• Caused by:
  • Cumulative effects of wear and tear on joints.
  • Genetic factors affecting collagen formation.

Joint Changes with Arthritis

• Normal articular cartilage has a smooth, slick surface and thick cartilage with a homogeneous matrix.
• Articular cartilage damaged by osteoarthritis has rough, bristly collagen fibers on the surface, which increases friction at the joint and promotes further degeneration.

Visualization of Problematic Joints

• Arthroscope: Narrow, flexible fiberoptic tube with a tiny camera that allows exploration of a joint without major surgery. May be used in combination with other flexible instruments inserted through additional incisions to conduct surgery (arthroscopic surgery).
• Magnetic resonance imaging (MRI): Allows visualization of soft tissue outside the joint cavity, not visible with arthroscope. Cost-effective and noninvasive.

Artificial Joints

• May be the method of last resort for arthritis treatment if other methods fail to slow disease progression (e.g., regular exercise, physical therapy, anti-inflammatory drugs).
• Can restore mobility and relieve pain, but high-impact activities are restricted after replacement.
• New joints (hips/knees) can last more than 15 years.