SYNOVIAL JOINTS

[SURIYA.KARTHIKEYHAN]

Synovial Joint

Most of the joints of the body are synovial which permits free movements.

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§ Synovial Joint- characteristics

• Articular surfaces of bone are covered by articular cartilage.
• Cavity present filled with fluid- synovial fluid.
• Joint cavity is enveloped completely by articular capsule- outer fibrous and inner Synovial membrane.
• Articulating bones are connected by ligaments additional to capsule.
• In Some joints, Joint cavity is divided by articular disc or menisci .

 

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Continued…

1.Articular Cartilage: Hyaline cartilage.

• Avascular, non nervous and elastic.
• Once damaged, it cannot be replaced by hyaline tissue.
• Functions - Regulates growth of epiphysis, Reduces force of compression during weight bearing.
• 3 layers - Superficial, intermediate and deep layer.

2.Synovial fluid:

• Rich in hyaluronic acid that maintains viscosity of the fluid.   
• Functions: Maintains nutrition of the articular cartilage and provides lubrication.

3. Joint (Articular)

• Capsule encloses joint cavity-(Outer) Fibrous capsule: continuous with periosteum-(Inner) synovial membrane; secretes synovial fluids Synovial joints.

4. Articular discs or menisci:

• Made of fibrocartilage, divides joints completely or incompletely into 2 compartments.
• Eg: Temperomandibular,  sternoclavicular and knee joints.
• absorbs shock, guides bone movements and distributes forces.
• Tendon: attaches muscle to bone.
• Ligament: attaches bone to bone.

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Tendon Sheaths and Bursae 

• Bursa =  a sac filled with synovial fluid.
• Located  between muscles or between a tendon & a bone etc.
• Example – Shoulder, knee joints.
• Tendon sheaths = elongated bursae.
• Located where there is a considerable friction; such as in hand, wrist, fingers, the ankle, foot, toes etc.
• Functions of bursa and tendon sheaths:
  • Reduce friction in joints (such as the shoulder).
  • Cushion  the movement of  one body part over another.

Tendon Sheaths and Bursae

• Bursa =  a sac filled with synovial fluid.
• Tendon sheaths = elongated bursae lined with synovial membrane and wrapped around a tendon.

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SHOULDER( Humeroscapular joint)

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Types of synovial joints

Based on number of articulating bones

• Simple joints: Only 2 bones. eg: Interphalangeal joints.
• Compound joints: More than 2 bones sharing a common space. eg: Ankle joint.
• Complex joint: More than 2 bones but joint space is divided into 2 compartments by a articular disc/menisci. eg: Knee joint.

Right Knee Joint – Superior View

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According to axis of movements and shape of articular surfaces

• Uniaxial joints:
  • 1)Hinge 2)Pivot 3)Condylar joints.
• Biaxial joints:
  • 1)Ellipsoid 2)Saddle joints.
• Mutiaxial or Polyaxial joints:
  • 1)Ball and Socket joint 2)Plane joints..

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1. Hinge (Ginglymus) Joints

• Features: One bone with convex surface that fits into a concave depression on other bone.
• Examples:
  • Elbow joint (trochlea of the humerus fits into the trochlear notch of the ulna).
  • Ankle joint(Tibia and Talus).
  • Interphalangeal (finger and toe joints).
• Type:  Monaxial (uniaxial) joints.
• Axis: Transverse axis.

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2. Pivot(Trochoid) Joints

• Features: One bone rotates on its longitudinal axis relative to the other.
• Examples:
  • proximal radioulnar joint( the radius pivots during pronation and supination).
  • atlantoaxial joint (dens of axis is fixed and ring formed by anterior arch of atlas).
• Type: Monaxial joints.
• Axis:Vertical axis.

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3. Condylar(Modified hinge) Joints

• Features: Each bone has 2 distinct condyles and these articular condyles may be enveloped in same articular capsule or separate capsule.
• Examples:
  • Temporo-mandibular joint.
  • Knee joint.
• Type: uniaxial joints.
• Axis: Transverse axis mainly but partly on vertical axis.

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4.Ellipsoid joint

• Features: One articular surface is convex and other is concave, reciprocally curved.
• Examples:
  • Metacarpophalangeal joint.
  • Metatarsophalangeal joint.
  • Atlantooccipital joint.
• Type: Biaxial joints.
• Axis: Transverse and anteroposterior axis.

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5. Saddle Joints

• Features: Each articular surface is shaped like a saddle, concave in one direction and convex in the other bone (like a sitting rider).
• Examples:     
  • Carpometacarpal joint at the base of the thumb.   
  • Sternoclavicular joints.
• Type: Biaxial joint.
• Axis: Transverse and anteroposterior axis.
• (opposition)primates’ anatomical hallmark.

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6. Ball-and-Socket  (Spheroidal) joints

• Features: Smooth hemispherical head fits within a cuplike depression.
• Examples:
• Shoulder joint(head of humerus into glenoid cavity of scapula).
• Hip joint(head of femur into acetabulum of hip bone).
• Talocalcaneonavicular joint.
• Type: multiaxial joints.
• Axis: Transverse,vertical and anteroposterior axis.

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7. Plane(Gliding) Joints

• Features: Flat articular surfaces in which bones slide over each other.
• Examples:
  • Intercarpal joints.
  • Intertarsal joints.
  • Facet joints (B/w articular processes of the vertebrae).
• Type: Multiaxial joints.
• Axis: Transverse, vertical and anteroposterior axis.

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