Articulations & Synovial Joints Lecture

Terminology & Core Concepts

• Articulation (joint) = location where two or more bones meet and can potentially move; word also means to "communicate/contact."
• Functional importance: joints define where bone movement occurs; classification is based on range of motion (ROM).
• ROM spectrum: immovable → slightly movable → freely movable; the more motion a joint permits, the more clinically significant wear-and-tear & surgical interventions.

Functional Classifications of Joints

• Synarthrosis ("syn" = together)
  • No movement; bones essentially fused.
  • Examples: cranial sutures, teeth in alveoli, rib–sternum junction.
• Amphiarthrosis ("amphi" = partial)
  • Slight movement; bones linked by collagen fibers or cartilage.
  • Classic example: pubic symphysis.
  • Other examples: interosseous ligaments classed as syndesmoses.
• Diarthrosis ("dia" = through/complete)
  • Freely movable; synonymous with synovial joints.
  • Examples: hinge joints (elbow, knee), ball-and-socket (shoulder, hip).

Structural Classifications (Terminology Only)

• Suture – "sewing together" joints between skull bones.
• Gomphosis – peg-in-socket attachment of a tooth to the mandible or maxilla.
• Syndesmosis – bones connected by a ligament/band; less emphasized in exam review.
• Synchondrosis / Symphysis – cartilage-based unions; instructor advises: focus primarily on synarthrosis, amphiarthrosis, diarthrosis for testing.

Synovial Joint Anatomy

• Joint (articular) capsule – fibrous envelope continuous with each bone’s periosteum; encloses entire joint.
  • Outer fibrous layer = strength.
  • Inner synovial membrane = secretes synovial fluid.
• Articular cartilage (hyaline)
  • Smooth, slick, coats epiphyses of long bones.
  • Function: reduce friction; degenerates in osteoarthritis (“bone-on-bone” pain).
• Reinforcements: intrinsic & extrinsic ligaments + tendons crossing the capsule add strength & guide motion.

Synovial Fluid

• Secreted by synovial membrane; viscous, egg-white consistency.
• Functions
  1. Lubrication → minimizes friction.
  2. Nutrient distribution → feeds avascular cartilage.
  3. Shock absorption → hydraulically cushions impact (analogy: car shock absorbers filled with fluid; when fluid leaks, ride becomes harsh – same with aging joints).

Accessory Structures

• Bursa – thin, fluid-filled pillowlike sac; positioned where tendon/ligament rubs bone; common in shoulder, knee.
  • Reduces friction & absorbs shock.
  • Inflammation = bursitis (often from overuse in swimmers, pitchers; mimics rotator-cuff tears but usually resolves with NSAIDs + ice).
• Fat pads – localized adipose masses inside capsule; fill spaces as joint moves, add cushioning.
• Meniscus (articular disc) – fibrocartilage pad between opposing bones (e.g., medial & lateral menisci in knee).
  • Poor blood supply → tears rarely heal without surgery.
  • Experimental biologics: PRP (platelet-rich plasma) or stem-cell injections may help but results vary.
• Ligaments vs. Tendons
  • Ligament = bone-to-bone stabilizer.
  • Tendon = muscle-to-bone connector; tendon of quadriceps crosses knee as patellar tendon, adding anterior support.

Types of Synovial Joints (Structural Sub-types)

• Gliding (plane) – flat surfaces slide: clavicle ↔ manubrium.
• Hinge – angular motion in one plane, like door: elbow, knee, interphalangeal joints.
• Pivot – rotation around longitudinal axis: C1 (atlas) spinning on C2 (axis).
• Condylar (ellipsoid) – oval articular surface nests in depression: radiocarpal joint, metacarpophalangeal joints.
• Saddle – rider-in-saddle shape: trapezium ↔ first metacarpal (thumb base).
• Ball-and-socket – spherical head in cup; greatest ROM, most injury-prone: shoulder (glenohumeral), hip (acetabulofemoral).

Movements at Synovial Joints

(Applied relative to anatomical position)

• Flexion – decreases angle; Extension – increases angle.
  • Example elbow: flex = curl, extend = straighten.
• Hyperextension – extension beyond 180°.
• Abduction – move away from midline; Adduction – move toward ("adding") midline.
• Circumduction – circular conical motion (arm circles).
• Rotation
  • Pronation – forearm rotates palm posterior (dump soup).
  • Supination – palm anterior (hold soup bowl).
• Opposition – thumb pad touches other fingertips; used in neurological testing.
• Inversion / Eversion – sole turns medially (common ankle sprain) vs laterally (rare).
• Dorsiflexion / Plantar flexion – ankle pulls toes upward vs presses foot down ("plant").
• Lateral flexion – trunk/neck bends to side.
• Elevation / Depression – closing vs opening jaw.
• Protraction / Retraction – jutting jaw/chin forward vs pulling backward.

Intervertebral Discs & Spinal Pathology

• Disc structure
  • Annulus fibrosus – tough fibrocartilage outer ring.
  • Nucleus pulposus – gelatinous core.
• Bulging (protruding) disc – annulus weakens; nucleus shifts, creating lateral bulge that presses on nerves.
• Herniated disc – nucleus pulposus breaks through annulus into vertebral canal → severe nerve compression.
• Misnomer "slipped disc" discouraged; discs do not slip out entirely.
• Diagnosis: CT or MRI; treatment may require microdiscectomy.

Bone Density Disorders

• Osteopenia – localized reduction in bone mass; early-stage loss.
• Osteoporosis – systemic metabolic disease; porous bone matrix, high fracture risk. \text{osteo (bone)} + \text{poros (pore)} + \text{sis (condition)}

Degenerative Joint Disease

• Osteoarthritis (OA)
  • Progressive erosion of articular (hyaline) cartilage → bone-on-bone contact, pain, limited ROM.
  • Visualized via arthroscope or imaging.
• Imaging modalities
  • Radiograph (X-ray) – good for bone, limited soft tissue.
  • MRI / CT – excellent for soft tissue injuries (discs, menisci, ligaments).
  • Arthroscopy – fiber-optic camera inserted into joint; may lead to minimally invasive surgical repair.

Clinical & Practical Highlights / Anecdotes

• Cranial sutures are immovable; claims of manipulating sutures to cure headaches/attention issues are unfounded.
• High-mobility joints (shoulder, knee) incur more wear, repair, and replacement—similar to frequently replaced car parts.
• Instructor’s car-shock analogy: leaking hydraulic fluid = diminished damping → parallels reduced synovial fluid & worn cartilage in aged joints.
• Personal story: colleague with L4–L5 herniation required microdiscectomy; CT scan essential despite "alternative" approaches.
• Experimental biologics (PRP, stem-cell) may delay surgery for cartilage/meniscus injuries but success is variable.

Exam Emphasis (Instructor Tips)

• Prioritize remembering functional trio: synarthrosis, amphiarthrosis, diarthrosis.
• Know major movement terminology & joint type examples (hinge, ball-and-socket, etc.).
• Ligaments vs. tendons distinction; ligament = bone↔bone, tendon = muscle↔bone.
• Elevation = closing jaw; depression = opening → common test trick.
• Terms like syndesmosis, synchondrosis, detailed ligament names less likely to be tested for this class cycle.