Friday, April 10, 2015

side-view mirror strikes his right shoulder

Posted by Dr. Sophia Charlotte MD

Question 1
A young man is walking along a road when a car travelling at 30mph (48 km/h)
drives past. The side-view mirror strikes his right shoulder and he sustains an
injury to his humerus.


With regards to Image 1.1:
I. Identify the greater tubercle.
II. Identify the lesser tubercle.
III. Identify the anatomical neck of the humerus.
IV. Identify the surgical neck of the humerus.

A radiograph shows a fracture at the site marked on Image 1.1.
V. What nerve is at risk following this type of injury?
VI. What clinical features would be present if the nerve was damaged?
VII. Which nerve runs in the spiral groove of the humerus?
VIII. If a fracture was to injure this nerve, what clinical findings might be
IX. What muscles attach to the coracoid process, and what nerves innervate
X. At what site is the clavicle most commonly fractured?
XI. In which direction are the resultant fragments displaced? Explain your


Correct answer :



I. The greater tubercle is the larger of the two tubercles and lies lateral to the lesser tubercle when the humerus is in the anatomical position; it projects
lateraltotheacromion.It has threefacets, which provideattachmentfor three
of the rotator cuff tendons (supraspinatus, infraspinatus and teres minor).
II. The lesser tubercle is smaller but still prominent. It is the site of attachment of the fourth rotator cuff tendon (subscapularis).

III. The anatomical neck of the humerus follows the articular margins of the
head of the humerus. The capsule of the shoulder joint is attached to
the anatomical neck, except medially and inferiorly where it attaches to the
surgical neck of the humerus.

IV. The poorly defined surgical neck is at the upper end of the shaft of the

V. Fractures through the surgical neck of the humerus are the most
common type of proximal humeral fracture. They are extracapsular and
therefore rarely compromise the blood supply to the head of the humerus.
Fractures of the anatomical neck of the humerus are rare. The axillary
nerve (C5,6) passes immediately behind the surgical neck of the humerus,
where it lies in contact with the bone just below the capsule of the shoulder
joint. Consequently, it is susceptible to injury with fractures of the surgical
neck and with shoulder dislocation.

VI. The axillary nerve is a branch of the posterior cord of the brachial plexus.
It supplies motor innervation to the deltoid and teres minor muscles and
sensory cutaneous supply to the upper lateral arm. Injury to the axillary
nerve may result in the following signs:
• Weakness (and later wasting) of the deltoid muscle, causing a loss of
power during abduction and later ‘flattening’ of the normally rounded
contour of the shoulder.
• Sensory loss on the upper outer aspect of the arm, over the ‘regimental
patch’ area.
The main differential diagnosis of an axillary nerve lesion is a C5 nerve
root lesion. With the latter, the normal function of the suprascapular nerve
(C5,6), which supplies the supraspinatus and infraspinatus muscles, will
also be affected.

VII. The radial nerve (C5–8, T1). The radial nerve is the continuation of the
posterior cord of the brachial plexus. It descends posterior to the axillary
artery and enters the spiral (or radial) groove alongside the profunda
brachii artery and its venae comitantes. It passes between the medial and
long head of the triceps muscle and is in contact with the periosteum of
the humerus in the lower part of the spiral groove.
VIII. In a fracture involving the spiral groove, the radial nerve could be
injured, resulting in sensory and motor disturbances. Sensory impairment would be apparent in the territory supplied by the superficial radial
nerve, but due to overlap of cutaneous innervation only a small area of
anaesthesia would be evident, usually over the dorsum of the hand
between the first and second metacarpal bones. This is therefore the
18 Chapter 2: Limbs and vertebral column answers
autonomous zone and so the best area to test for sensory function of the
radial nerve. Knowledge of where the motor branches leave the radial
nerve can be used to predict the site of the lesion. The motor branches to
the long and medial heads of the triceps leave the radial nerve proximal
to the spiral groove; whereas the branch to the lateral head, and a second
branch to the medial head, are generally given off more distally. Consequently, a fracture involving the spiral groove is likely to affect the lateral
head with relative sparing of the long and medial heads. Elbow extension
is weak rather than lost. Brachioradialis function will also be impaired.
Both wrist and finger drop may be evident due to denervation of the
wrist and long finger extensors, extensor pollicis longus and abductor
pollicis longus. The interphalangeal joints of the fingers can still be
extended due to the retained action of the intrinsic muscles of the hand.

IX. Three muscles attach to the coracoid process. They are the:
• biceps brachii, short head (musculocutaneous nerve);
• coracobrachialis (musculocutaneous nerve);
• pectoralis minor (medial and lateral pectoral nerves).

X. The clavicle is the most commonly fractured bone in the body. The
junction between middle and lateral thirds is most commonly fractured
(~75% of all clavicular fractures) for two reasons: (a) the medial twothirds are circular in cross-section whilst the lateral third is flatter; the
junction between the two regions is comparatively weak and also has no
muscular attachments; (b) the powerful coracoclavicular and costoclavicular ligaments stabilise the lateral and medial third of the clavicle,

respectively, and fractures therefore tend to occur between these points.
XI. The medial fragment of bone is elevated superiorly by the unopposed
action of the sternocleidomastoid muscle. The lateral fragment is
depressed by the weight of the arm. The proximal humerus may be pulled
medially by the action of pectoralis major.


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