Locomotor system; joints

📌 Definition & Classification
🦴 Joint (articulation): point where 2 or more bones meet.
Classified by:
1️⃣ Type of connecting tissue.
2️⃣ Degree of movement allowed.

🖇 1. Fibrous Joints (Synarthroses)
🔹 Bones connected by dense fibrous connective tissue.
🔹 Little or no movement.

Types:

• Suture: bones interlocked, later ossify.
  🧠 Example: skull sutures.
• Syndesmosis: bones connected by ligament.
  🦵 Example: inferior tibiofibular joint.
• Gomphosis: peg-in-socket joint.
  😬 Example: tooth in its socket (periodontal ligament).

🖇 2. Cartilaginous Joints (Amphiarthroses)
🔹 Bones united by cartilage.
🔹 Allow limited movement.

Types:

• Primary (Synchondrosis): hyaline cartilage, immobile.
  🦴 Example: epiphyseal plate, 1st sternocostal joint.
• Secondary (Symphysis): fibrocartilage pad, slightly mobile.
  💪 Example: pubic symphysis, intervertebral discs.

🖇 3. Synovial Joints (Diarthroses)
🔹 Most common + most movable joints.

Main Structures:

• Articular cartilage → covers bone ends.
• Joint cavity → contains synovial fluid.
• Fibrous capsule → encloses joint.
• Synovial membrane → lines capsule & secretes fluid.
• Ligaments → stabilize joint (extra-, intra-, capsular).
• Bursae → reduce friction.
• Articular discs / menisci → absorb shock.

🟠 Structures Inside Synovial Joints
A. Cartilaginous structures:

• Disc → sternoclavicular joint.
• Labrum → hip joint.
• Meniscus → knee joint.
  B. Tendons inside joint:
• Long head of biceps (shoulder).
• Popliteus tendon (knee).
  C. Ligaments:
• Cruciate ligaments (knee).

🟣 Classification of Synovial Joints

A. By Number of Bones:

• Simple → 2 surfaces (elbow).
• Compound → >2 surfaces (superior tibiofibular).
• Complex → has disc/meniscus (knee, sternoclavicular).

B. By Axes of Movement:

• Uniaxial:
• • Hinge (elbow, interphalangeal).
  • Pivot (superior radio-ulnar, atlas-axis).
• Biaxial:
• • Ellipsoid (wrist, MCP joints).
  • Bicondylar (knee).
• Polyaxial:
• • Ball & socket (hip, shoulder).
  • Plane (intertarsal, vertebral facets).
  • Saddle (thumb CMC, calcaneo-cuboid).

📌 Joints with Articular Discs

• Temporomandibular (TMJ).
• Sternoclavicular.
• Ulnocarpal.

🚨 Joint Stability Factors
1️⃣ Shape of articular surfaces.
2️⃣ Ligaments supporting joint.
3️⃣ Surrounding muscle tone & tendons.

🟠 Nerve Supply (Hilton’s Law)
🔹 The motor nerve that moves the joint also:

• Sends sensory fibers to the joint.
• Supplies skin over it.
  📍 Explains referred pain from joint to nearby skin & muscles.