📝 STANDARD DESCRIPTION:
Components of a typical synovial joint:
1. Articular cartilage — hyaline; smooth gliding surface; avascular.
2. Joint capsule — fibrous outer + synovial inner.
3. Synovial membrane — secretes synovial fluid.
4. Synovial fluid — lubricant + nutrient.
5. Ligaments — collateral, intracapsular, extracapsular.
6. Menisci / Discs (where present).
Movements at synovial joints:
• Flexion / Extension — decrease/increase angle.
• Adduction / Abduction — toward/away from midline.
• Rotation — around long axis.
• Circumduction — combined motion describing a cone.
• Pronation / Supination — rotate forearm/leg.
• Gliding — sliding of flat surfaces.
📚 DETAILED DESCRIPTION:
ANATOMY OF A SYNOVIAL JOINT
1. Articular Cartilage (Hyaline):
• 1-3 mm thick; covers bone ends.
• Avascular & without nerve supply — nutrition from synovial fluid by diffusion.
• Once damaged, repair is poor (forms fibrocartilage scar, not hyaline).
2. Joint Capsule (Capsula articularis):
• Outer fibrous layer — dense connective tissue, continuous with periosteum.
• Inner synovial layer (synovial membrane) — thin, vascular, secretes synovial fluid.
3. Synovial Fluid (Synovia):
• Clear, pale yellow, viscous (egg-white consistency).
• Composition: dialysate of plasma + hyaluronic acid + lubricin + glycosaminoglycans.
• Functions: lubrication, shock absorption, nutrition of cartilage.
• Volume: small (drop in carpal joint to several ml in stifle).
4. Ligaments:
• Collateral ligaments — on either side of joint, prevent side-to-side movement.
• Cruciate ligaments — inside the joint, cross each other (cranial + caudal cruciate of stifle).
• Capsular thickenings — annular ligaments at fetlock.
5. Menisci: Crescent-shaped fibrocartilage pads (stifle has medial + lateral). Distribute load and stabilize.
6. Bursae & Tendon Sheaths — accessory structures (see below in Myology section).
MOVEMENTS — FULL LIST:
• Flexion — decreases joint angle (folding the limb).
• Extension — increases joint angle (straightening).
• Hyperextension — past 180° (e.g., fetlock when fully loaded).
• Adduction — moving toward median plane.
• Abduction — moving away from median plane.
• Rotation — around long axis (medial / lateral rotation).
• Circumduction — combination → describes a cone.
• Pronation — rotating limb so palm faces ventrally.
• Supination — rotating limb so palm faces dorsally.
• Gliding — sliding of flat surfaces.
• Protraction / Retraction — limb moved forward / backward.
• Elevation / Depression — raising / lowering (mandible).
// COMPARATIVE ANALYSIS
| HORSE |
Pronation/supination ABSENT (radius-ulna fused) — limb is locked in pronated position to save energy. |
| DOG |
Full pronation/supination retained — used to hold prey, dig, climb. |
| CAT |
Cat shoulder = nearly ball-and-socket → enormous range of motion (e.g., grooming the back). |
CLINICAL RELEVANCE
Sprain = stretched/torn ligament. Strain = stretched/torn muscle/tendon. Luxation = dislocation (bone out of joint). Subluxation = partial luxation. Effusion = excess synovial fluid (visible swelling).
