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► This is an anatomy question based on USMLE Step 1 standards. It also includes extra information that will help you to find the correct answer.
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A 60-year old female presents to the emergency room with a history of falling on her right elbow. She cannot raise her right arm. Physical exam reveals loss of sensation over the lateral side of the right upper arm and shoulder. Radiology reveals fracture of surgical neck of the humerus. Which of the following muscles is supplied by the nerve that is most likely damaged?
A. Teres minor
B. Teres major
C. Subscapularis
D. Infraspinatus
E. Supraspinatus
Technical Review:
First, as usual, let’s read the last sentence which is “Which of the following muscles is supplied by the nerve that is most likely damaged?”. In this case, the question alone is insufficient to determine the answer and we need the whole stem. Next, let’s determine what subject this is; we are discussing nerves and muscles, so this is clearly anatomy.
Guidelines to solve the question:
This is a 2 step-question about the anatomy of the humerus. In the first step you need to determine which nerve was damaged as a consequence of the fracture of the surgical neck. There are hints in the stem. First, recall that the musculocutaneous, axillary, radial, median, and ulnar nerves all originate in the cervical spine from C5 – C8 and also T1 and all of these nerves pass over the glenohumeral joint and down the humerus. Due to the anatomical configuration of the arm, a fracture of the surgical neck of the humerus results in damage mainly to the axillary nerve. So, our answers likely involves the axillary nerve. Also realize that the axillary nerve provides sensation to the right upper arm and shoulder and this information strengthens our argument that the axillary nerve is involved.
Now the second step, to recall which tissues are innervated by the axillary nerve that results in the pathology discussed in the stem. Again we look to the stem for clues and hints. Recall that the axillary nerve provides sensation for the lateral portion of the upper arm and shoulder and innervates only two muscles: the deltoid and teres minor muscles. Another way to think about this is that the patient can not raise her arm, implying damage to the deltoid muscle which is innervated by the axillary nerve and the only other muscle innervated by the axillary nerve is the teres minor muscle. So the answer to the question is either deltoid or teres minor muscle. A quick review of the answers shows that only teres minor is a choice. Thus the correct answer is A. Teres minor
High Yield Facts:
The humerus is a long bone of the arm that attaches proximally to the shoulder at the glenohumeral joint and distally to two of the elbow joints, the humeralulnar joint and the humeralradial joint.
The humerus is more frequently fractured at the surgical neck, inferior to the anatomical neck.
The axillary nerve is frequently damaged with humeral fractures at the surgical neck. It innervates the deltoid and teres minor muscles and carries sensation from the lateral portion of the upper arm and shoulder, the “regimental badge” area. The posterior circumflex humeral artery may also be damaged.
The teres major muscle is innervated by the lower subscapular nerve while the subscapularis muscles is innervated by the lower and upper subscapular nerves. These nerves all stem from the posterior cord of the brachial plexus and both muscles adduct and medially rotate the arm.
The Supraspinatus and infraspinatus muscles are both innervated by the subscapular nerve which comes directly from the C5 and C6 roots of the brachial plexus. These muscles abduct the arm and laterally rotate the arm, respectively, while stabilizing the humerus.
Related Lecturio Lectures:
[ Ссылка ] starting at time 19:00 “brachial plexus”
[ Ссылка ] starting at time 19:00, when the actually start talking about specific nerve innervation.
