DENTAL GROSS ANATOMY: Answers to Sample Questions

Week 1 -- September 5, 2007

  1. Initiating the abduction of the arm at the shoulder joint (i.e. movement from 0 to 15 degrees) would be most seriously affected by damage to which of the following nerves?
  2. Which of the following arteries is involved in collateral circulation around the shoulder joint?
  3. The most important supinator of the forearm is the:
  4. The nerve which supplies the extensor muscles of the forearm is the:
IDENTIFICATIONS
  1. Pectoralis major
  2. Sternum
  3. Serratus anterior
  4. Biceps brachii
CLINICAL CASES

CASE #1

After leaving the axilla, the radial nerve passes backwards, downwards and laterally, spiraling around the shaft of the humerus in the radial groove. It enters the region between the upper part of the medial head and the long head of the triceps brachii muscle and is then sandwiched between the medial and lateral heads. For a part of its course it is directly in contact with the bone, and here it is vulnerable in fractures of the shaft. It pierces the lateral intermuscular septum so that it comes to lie in front of the lower end of the humerus and is therefore lateral to brachialis, and medial to two muscles of the forearm (brachioradialis and extensor carpi radialis longus).

Muscle groups supplied by the radial nerve: Extensor muscles of the arm (three heads of the triceps); near the lateral epicondyle, it supplies anconeus, brachioradialis, and extensor carpi radialis longus muscles.

If the extensors are paralyzed by a radial nerve injury, a power grip becomes impossible because the wrist flexes as well as the fingers, so the fingers are pulled open by the relative shortness of the extensors (try gripping with your wrist fully flexed). Also, muscle fibers can only contract by a fixed percentage of the resting length. So you need an extended wrist to allow sufficient shortening force supplied to the fingers (not wrist).


CASE #2

The anatomical snuff box is the triangular depression observed on the dorsum of the hand when the thumb is fully extended. It is bounded by the tendons of the extensor pollicis longus on the medial side and the extensor pollicis brevis on the lateral side.

She broke the scaphoid bone, which can be palpated in the floor of the anatomical snuff box. The scaphoid is the most frequently fractured of the carpal bones.

Other injuries that can occur following a fall on outstretched hands include dislocation of the lunate bone, posterior displacement of the distal radial epiphysis (in children), fracture of the clavicle (in adolescents), and fracture of the distal radius (Colles' fracture--usually in older adults).


CASE #3

The loss of upper limb function on one side of this newborn is likely due to stretching or tearing of the ventral primary rami (C5 and C6) that form the upper trunk of the brachial plexus. Such an injury can occur during delivery if the neck is hyperextended and sharply flexed laterally. This is known as Duchenne's or Erb's paralysis. Muscles involved are the deltoid, spinati, and flexor muscles of the forearm. Hand function is spared since finger flexors and extensors are innvervated by the lower roots of the brachial plexus. With prompt physiotherapy, full function usually returns and the prognosis for the infant is very good.