Elbow - Biomechanics

Joint Stability Factors

  • Congruence: The distal end of the humerus and proximal aspect of the ulna provide joint stability by minimizing localized pressures

    • minimized pressures also minimize degenerative changes

  • Ligamentous Support: Varus and valgus movements are limited by ligaments and bone contact:

    • Valgus: Capitulum contacts the radius.

    • Varus: Ulna contacts the trochlea of the humerus.

  • Limits to Elbow Flexion:

    • Limited by muscle mass contact (e.g., biceps with forearm)

    • in severe atrophy:

      • coronoid process and coronoid fossa,

      • radial head and radial fossa.

  • Limits to Elbow Extension:

    • Contact between the olecranon process and the olecranon fossa.

Arthrokinematics of the Humeroulnar Joint

  • Demonstrates the Concave on Convex Rule: Flexing the elbow causes the ulna to slide in the same direction as flexion.

    • Interosseous Membrane: fibers are aligned to limit proximal glide of the radius relative to the ulna; facilitates force transmission from distal radius to proximal ulna.

Dislocation Prevention

  • prevent elbow dislocation from distracting forces

    • Opposed by:

      • oblique cord

      • brachioradialis

      • annular ligament

    • clinically important relative to humeroradial and radioulnar joints

Radioulnar Joint Movements

  • Pronation and Supination Mechanics:

    • Not actual forearm rotation but radius rotating around the ulna.

    • Approximate oblique axis: Imaginary line from radial head to ulnar styloid.

  • Joints Involved:

    • Proximal radioulnar joint

      • radial head

      • radial notch

    • annular ligament

      • lined with hyaline cartilage

        distal radioulnar joint

      • ulnar notch on radius

      • ulnar head on ulna

  • Triangular Fibrocartilage Complex: Stabilizes the distal radioulnar joint; prevents ulna from articulating with carpals.

    • lies between ulna and first row of carpals

    • common culprut when “wrist sprains” don’t heal

Range of Motion Considerations

  • Important for therapists to know minimal range of motion for daily activities due to potential injuries.

  • Limitations to Supination:

    • pronator teres

    • pronator quadratus
      flexor carpi radialis

    • extrinsic finger flexors

    • triangular fibrocartilage complex

      • especially in palmar capsular ligament at the distal radio-ulnar joint, interosseous membrane, and quadrate ligament

  • Limitations to Pronation:

    • biceps brachii

    • radio-styloid tensions

    • extensor pollicis longus

    • triangular fibrocartilage complex

      • especially the dorsal capsular ligament at the distal radio-ulnar joint

Kinematics in Closed Chains

  • Closed Chain Kinematics: Both ends of a kinematic chain are fixed, which allows muscles to create torques about joints they do not directly cross.

  • Example: Anterior deltoid can function as an elbow extensor during push-ups due to fixed hand position.

    • it is impossible to extend the elbow without also horizontally adducting the GH joint

      • hand is fixed to floor

      • humerus is fixed to trunk