Joint Biomechanics - Elbow

Question 1

Elbow and Forearm Complex Articulations

Answer 1

• Humeroulnar joint
• Humeroradial joint
• Proximal radio-ulnar joint
• Distal radio-ulnar joint
  Radio-ulnar do not require motion of the shoulder. Pronation and supination can be performed in conjunction with, or independent from, elbow flexion and extension (it can happen in any position flexion/extension of elbow/shoulder).

Question 2

The Elbow Joint

Answer 2

Powerful modified hinge joint (because ulna experiences a slight axial rotation and side to side motion as it flexes and extends) used for pulling and pushing.
Elbow is the most important articulation regarding functionality.
Humeroulnar joint provides much of its stability through the tight fit between the trochlea and trochlear notch.
Humeroradial joint provides stability through the radial head against the capitulum, in conjunction with its many capsuloligamentous connections.

Question 3

Static Stabilizers of the Elbow

Answer 3

• Medial collateral ligament (anterior fibers): Valgus stress and extension increase tension.
• Medial collateral ligament (posterior fibers): Valgus stress and flexion increase tension.
• Radial collateral ligament: Varus stress increase tension.
• Lateral (ulnar) collateral ligament: Varus stress, external rotation of the elbow complex, flexion increase tension.
• Annular ligament: Distraction of the radius increases tension.

Question 4

Dynamic stabilizers of the Elbow

Answer 4

Main:

• Pronator teres
• Flexor carpi radialis
• Flexor carpi ulnaris* (Attaches to the capsules and ligaments and tightens them)(Medial collateral ligament)
• Extensor carpi radialis brevis
• Supinator* (Attaches to the capsules and ligaments and tightens them)(Lateral collateral ligament)

Question 5

Flexion/Extension of the Elbow

Answer 5

Sagittal plane - transversal axis
Flexion ROM variable due to muscle mass of the arm and forearm (soft endplate).
Extension ROM defined as 0°, but it is normal to have hyperextension.
Flexion: 140-150°
Hyperextension: 10-15°

Question 6

Functional Range of Motion of the Elbow

Answer 6

Range of motion we mainly use for daily life activities.

30°- 130° flexion.

Question 7

Arthrokinematics - Elbow Flexion/Extension

Answer 7

Concave-Convex rule
Humeroradial joint - Concave foeva of the radius is moving on the convex capitulum. Roll and slide in the same direction.
Humeroulnar joint - Concave trochlear notch of the ulna is moving on the convex trochlea of the humerus. Roll and slide in the same direction.

Question 8

Elbow Muscles - Pronation/Supination

Answer 8

Muscles that attach distally on the ulna flex or extend the elbow but possess no ability to pronate or supinate the forearm.
Muscles that attach distally on the radius, may flex or extend the elbow, but also have a potential to pronate or supinate the forearm.

Question 9

Synergists - Elbow Flexors

Answer 9

• Biceps Brachii: Primary flexor in supination
• Brachialis: Primary flexor in pronation. Strongest because of longest cross-sectional area.
• Brachioradialis: Flexion with thumb pointed up.
• Flexor carpi radialis (secondary)
• Flexor carpi ulnaris (secondary)
• Palmaris longus (secondary)
• Pronator teres (secondary)
• Extensor carpi radialis longus (secondary)
• Extensor carpi radialis brevis (secondary)

Question 10

Synergists - Elbow Extensors

Answer 10

Triceps brachii: Medial head is the primary

Anconeus: Contracts first (secondary)

Question 11

Where does the axis of rotation of the forearm lie?

Answer 11

Extends from near the radial head through the ulnar head/styloid process.

Question 12

The radius and ulna are bound together by what?

Answer 12

The interosseous membrane and proximal and distal radio-ulnar joints.

Question 13

Forearm Pronation and Supination

Answer 13

Happens primarily because radial head is rotating on the ulna.
In anatomical position the forearm is supinated. 0° pronation/supination is when the thumb is facing up.
A stable humeroulnar joint provides an essential rigid link on which the radius, wrist and hand can PIVOT.
Transversal Plane.

Question 14

Forearm Stability - Proximal Radioulnar Joint

Answer 14

Proximal radioulnar joint, humeroulnar joint and humeroradial joint all share one articular capsule.
The quadrate ligament stabilizes the proximal radioulnar joint, and is stretched throughout movement, most notably during supination.

Question 15

Forearm Stability - Distal Radioulnar Joint

Answer 15

• Triangular fibrocartilage complex (TFCC)(disc): connects the head of ulna to the ulnar notch of the radius. Edges of the disc are continuous with the deeper layers of the anterior and posterior radioulnar capsular ligaments. Important reinforcement!
• Pronator quadratus
• Tendon of the extensor carpi ulnaris
• Distal oblique fibers of the interosseous membrane

Question 16

Interosseous Membrane

Answer 16

Directed medial to distal from the radius to the ulna at an angle of 20°.
Attachment for extrinsic muscles of the hand.
Transmits force from the radius to ulna:
- Compressive forces of flexors and supinators against the capitulum.
- Distal pull on the radius.
During pronation, membrane wraps around ulna providing compression.

Question 17

Range of Motion of Forearm Pronation and Supination

Answer 17

85° Supination
75° Pronation
Functional range of motion for daily activities is 50° both ways.

Question 18

Arthrokinematics of Forearm Pronation and Supination

Answer 18

Distal radioulnar joint (where most of the movement is): the concave radius is moving on the convex ulna making the roll and slide in the same direction.
Proximal radioulnar joint: Radius spinning on the fixed ulna.

Question 19

Humeroradial joint

Answer 19

Spin of the foeva of the radial head against the capitulum of the humerus.
Mechanically linked to the kinematics of both elbow and forearm.

Question 20

Stability of the Humeroulnar Joint

Answer 20

Contraction of pronator teres generates significant compression, especially when the elbow is near extension.
Interosseous membrane is less taut in pronation, less able to resist proximal migration and compression of the humeroradial joint during active pronation.

Question 21

What is the relationship between shoulder external/internal rotation and forearm pronation/supination?

Answer 21

They are functionally associated with each other, allowing the hand to rotate nearly 360° in space.

Question 22

What are the two fundamental features of the pronator/supinator muscles?

Answer 22

1. The muscle must attach on both sides of the axis of rotation (from medial side to lateral or opposite).
2. The muscle must produce a force that acts with an internal moment arm around the axis of rotation.

Question 23

Synergists of Forearm Supination

Answer 23

Biceps Brachii
Supinator
Brachioradialis (secondary)

Question 24

Synergists of Forearm Pronation

Answer 24

Pronator Teres
Pronator Quadratus
Brachioradialis (Secondary)