Elbow Anatomy and Function

Elbow Complex Function

• The elbow complex is made up of 3 bones and 4 joints.
• The elbow complex provides mobility for the hand in space.
• It allows the hand to be brought close to the face for eating and grooming or to be placed at a distance from the body equal to the length of the entire upper extremity.
• The elbow complex also provides stability for skilled or forceful movements of the hand.
• The elbow and wrist are linked together to enhance hand function.

Elbow Complex Structure

• The elbow joint is a compound joint that functions as a modified or loose hinge (ginglimus) joint.
• The elbow joint allows for flexion and extension.
• The proximal & distal radioulnar joints function as one joint to provide forearm rotation.
• The radioulnar joints are diarthrodial uniaxial pivot joints (trochoid) and permit rotation (supination and pronation).

Distal Humerus

• The shaft of the humerus ends medially as the trochlea and medial epicondyle.
• The shaft of the humerus ends laterally as the capitulum and lateral epicondyle.

Flexor Muscles

• The primary elbow flexors are:
  • Brachialis: The workhorse of the flexors, it has the largest cross-sectional area to generate the greatest force.
  • Biceps Brachii: Produces maximal activity when performing flexion and supination simultaneously.
  • Brachioradialis: Regardless of forearm position, contraction causes full flexion and rotates the forearm towards the neutral, thumb-up position.
• The secondary assisting synergists for the flexor muscles are Pronator Teres and ECRL.

Extensor Muscles

• The primary elbow extensors are:
  • Triceps Brachii: Works as a first-class lever and provides maximal efficiency between 20°-30°.
  • Anconeus: Useful for producing “background” joint stability and provides forces that stabilize the ulna during active pronation and supination.

Flexion-Extension Considerations

• Biceps and triceps are polyarticular muscles that produce force across multiple joints.
• When speed is increased, the flexors show moderate activity with a loaded forearm.
• Pronator Teres, as well as the palmaris longus, flexor digitorum superficialis, flexor carpi radialis, and flexor carpi ulnaris, is a weak elbow flexor with primary actions at the radioulnar and wrist joints.

Flexion-Extension Considerations for Extensors

• The activity of the long head of the triceps is affected by changing shoulder joint position.
• The medial and lateral heads of the triceps are not affected by the position of the shoulder.
• Maximum isometric torque generation for the triceps brachii is at a position of 90° of elbow flexion.
• The triceps is active eccentrically to control elbow flexion and concentrically to extend the elbow in a closed kinematic chain, such as in a push-up.
• The triceps act as a stabilizing synergist to prevent flexion of the elbow.

Proximal Radio-Ulnar Joint

• The main movement of the proximal radio-ulnar joint is rotation of the radial head about its axis within the fibro-osseous ring.
• The tension of the quadrate ligament limits movement during supination and pronation.
• The radial head is slightly oval, which explains why the annular cuff of the radial head cannot be bony and rigid.

Foream Mobility

• The cup-shaped surface of the radial head spins with respect to the capitulum humeri.
• The beveled ridge of the radial head glides in contact with the capitulo-trochlear groove of the humerus.
• During pronation, the radial head comes to lie transversely with a lateral shift allowing the radius to move out of the way of the ulna.

Distal Radio-Ulnar Joint

• The axis of pronation-supination extends into the hand along the medial edge of the ulna and the fifth finger while maintaining contact with the table.
• The radius glides and rolls on the ulna.
• The axis of pronation-supination is intermediate in location and passes through the lower end of the radius near the ulnar notch, which is collinear with the axis of the hand (the 3rd finger).
• In this case, not only the position of the radius but also the ulna is changing.

Function of the Elbow Complex

• Most activities of daily living require a combination of motion at both the elbow and radioulnar joints.
• A total arc of ~100° of elbow flexion (between 30° and 130°) and ~100° of forearm rotation (~50° supination and ~50° pronation) is sufficient to accomplish simple tasks.
• Mobility of the elbow complex is necessary for normal functioning.
• Compensations for limited range of motion of the elbow complex happen at the shoulder.
• The mobility afforded the hand by pronation and supination of the forearm is achieved at the expense of stability because the movable forearm is unable to provide a stable base for attachment of the wrist and hand muscles.
• Many of the muscles that act on the wrist and hand are attached on the distal end of the humerus.
• The fact that the wrist and most the extrinsic hand muscles cross the elbow create close structural and functional relationships between the elbow and wrist/hand complexes.
• Anatomically, the hand and wrist muscles help reinforce the elbow joint capsule and contribute to stability of the elbow.

Muscle Innervation and Dermatomes

• Musculocutaneous Nerve (C5-7):
  • Coracobrachialis Muscle
  • Biceps Brachii
  • Brachialis Muscle
• Axillary Nerve (C5-6):
  • Deltoid Muscle
  • Teres Minor muscle