A 16-year-old high school quarterback comes to the office due to left shoulder pain. One week ago, he suffered an injury during a football game. The patient was running with the football when another player tackled him to the ground, causing him to land directly on the posterior aspect of his left shoulder. He developed severe pain immediately and did not return to the game. The patient has had persistent shoulder pain since the injury despite using ice and over-the-counter ibuprofen. He has been unable to resume playing football due to pain and weakness in his shoulder. On examination, there is diffuse bruising over the posterior aspect of the left shoulder. Abduction of the left arm is normal, but external rotation against resistance is weak and painful. Which of the following muscles is most likely injured?
Show Explanatory Sources
This patient's shoulder pain and weak, painful external rotation against resistance with preserved abduction are characteristic of an infraspinatus injury. The rotator cuff muscles all attach to the humeral head and stabilize the shoulder joint, in addition to moving the arm at the shoulder. A fall onto the shoulder is a common mechanism for rotator cuff injury, resulting in acute shoulder pain and characteristic weakness depending on which muscle is involved:
The supraspinatus inserts onto the superior aspect of the greater tubercle and abducts the arm (Choice D).
The infraspinatus inserts onto the posterolateral aspect of the greater tubercle and externally rotates the arm.
The teres minor inserts onto the posterolateral aspect of the greater tubercle and helps with adduction and external rotation of the arm.
The subscapularis inserts onto the lesser tubercle on the anterior aspect of the humerus and helps with adduction and internal rotation of the arm (Choice C).
(Choice A) The biceps brachii is a 2-headed muscle that originates from the scapula and converges onto a single insertion point on the upper forearm. It is responsible for flexion at the elbow and forearm supination. Biceps injury can cause shoulder pain but would not affect external rotation of the arm at the shoulder.
(Choice E) Unlike the teres minor, the teres major is not a rotator cuff muscle. It arises from the inferior scapula and inserts below the humeral head onto the anterior aspect of the humerus. It assists with humeral extension (when the arm is flexed) and internal rotation of the arm at the shoulder.
(Choice F) The trapezius originates from the occipital bone and spinous processes of the cervical and thoracic vertebrae and inserts onto the scapula and clavicle. It helps move and rotate the scapula. Trapezius injury would cause difficulty with arm abduction and adduction, but would not affect external rotation of the humerus.
Educational objective:
The rotator cuff muscles (supraspinatus, infraspinatus, teres minor, and subscapularis) all insert onto the humeral head, allowing them to stabilize the shoulder joint and move the arm at the shoulder. An infraspinatus injury would result in shoulder pain and weak, painful external rotation of the arm against resistance.