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Author [[PageAuthor::William J Gibson (Reviewed by Yazan Daaboul, M.D.)]]
Exam Type ExamType::USMLE Step 1
Main Category MainCategory::Anatomy
Sub Category SubCategory::Musculoskeletal/Rheumatology
Prompt [[Prompt::An 18-year-old female softball player presents to her primary care physician with right shoulder pain and weakness. The pain becomes much worse when the patient performs over-the-head activities with her right shoulder and is often relieved by maintaining arm adduction and with NSAID administration. The patient informs the physician that she has been preparing for an upcoming championship and has been practicing several hours a day for the past few weeks. On physical examination, the patient reports pain when as soon as she begins to elevate her arm against the physician's resistance while abducting and forward flexing the shoulder with her thumb pointing downward. Which of the following muscles is most likely injured in this patient?]]
Answer A AnswerA::Deltoid
Answer A Explanation [[AnswerAExp::While the deltoid is responsible for shoulder abduction, the deltoid is not responsible for initiating shoulder abduction. Instead, the deltoid is responsible for abduction beyond 15 degrees.]]
Answer B AnswerB::Infraspinatus
Answer B Explanation [[AnswerBExp::The infraspinatus is a muscle of the rotator cuff that is responsible for lateral rotation of the arm. The infraspinatus muscle and tendon injuries are common among baseball pitchers.]]
Answer C AnswerC::Supraspinatus
Answer C Explanation [[AnswerCExp::The supraspinatus is a muscle of the rotator cuff that is responsible for the initial 15 degrees of shoulder abduction. The supraspinatus sits atop the scapula in the supraspinous fossa. Its tendon proceeds under the acromion to attach to the lateral head of the humerus. Contraction of the supraspinatus abducts the arm in the manner seen in the animation below.]]
Answer D AnswerD::Teres major
Answer D Explanation [[AnswerDExp::The teres major is a large, flat muscle that medially rotates and adducts the humerus. It is innervated by the subscapular nerve (C5-C6). The teres major is not part of the rotator cuff muscles.]]
Answer E AnswerE::Teres minor
Answer E Explanation [[AnswerEExp::The teres minor is a rotator cuff muscle that is responsible for adduction and lateral rotation of the arm. It is innervated by the posterior branch of the axillary nerve (C5-C6).]]
Right Answer RightAnswer::C
Explanation [[Explanation::The hallmark of rotator cuff injuries is the combination of pain that accompanies shoulder weakness. The patient in this vignette has injured her supraspinatus muscle, the most commonly injured muscle in rotator cuff injuries. The supraspinatus originates on the medial edge of the supraspinous fossa, where it proceeds to pass under the cover of the acromion to attach to the lateral aspect of the head of the humerus. The supraspinatus is innervated by the suprascapular nerve, which originates from the superior trunk of the brachial plexus (C5-C6). It is responsible for initiating arm abduction (initial 15 degrees of abduction). Beyond 15 degrees, the deltoid muscle, which is innervated by the axillary nerve (C5-C6), becomes more effective in arm abduction.

The rotator cuff is composed of 4 muscles: Subscapularis, Infraspinatus, Teres minor, and Supraspinatus (SITS). When patients are suspected to have rotator cuff injuries, provocative testing may be performed to further localize the injury. Although rotator cuff injuries often require MRI to identify the location of the injury, clinical tests may be helpful to evaluate for specific rotator cuff injuries:

  • Empty can test: Patient pushes against physician resistance while arm in 90 degree abduction with thumb pointing down. Pain suggests tear to the supraspinatus muscle or tendon.
  • Full can test: Patient pushes against physician resistance while arm abducted 45-90 degrees and shoulder externally rotated. Pain suggests tear to the supraspinatus muscle or tendon.
  • Neer's test: Patient places the arm in forced flexion with arm in full pronation. Pain suggests subacromial impingement due to pinching under the coracoacromial arch.
  • Hawkin's test: Patient elevates the arm forward to 90 degrees while internally rotating the shoulder. Pain suggests subacromial impingement or rotator cuff tendonitis.
  • Drop-arm test: Patient passively abducts involved shoulder, then slowly lowers the arm to the waist. Sudden arm drop suggests supraspinatus injury.
  • Cross-arm test: Patient raises arm to 90 degrees and adducts forcibly the acromion into distal end of clavicle. Pain suggests acromioclavicular joint involvement.

The following demonstrates how contraction of the supraspinatus results in 15 degrees of arm abduction:

Educational Objective: The supraspinatus muscle is responsible for initiating the first 15 degrees of arm abduction. Injury to the supraspinatus muscle or its tendon is the most common rotator cuff injury. Empty can test is a provocative test that may be clinically helpful in the diagnosis of supraspinatus injury.
References: Woodward TW, Best TM. The painful shoulder: part I. clinical evaluation. Am Fam Physician. 2000;61(10):3079-88.
First Aid 2014 page 411]]

Approved Approved::Yes
Keyword WBRKeyword::Brachial, WBRKeyword::Brachial plexus, WBRKeyword::Rotator cuff, WBRKeyword::Rotator, WBRKeyword::Cuff, WBRKeyword::Shoulder, WBRKeyword::Arm, WBRKeyword::Upper Limb, WBRKeyword::Rotator cuff injury, WBRKeyword::Supraspinatus, WBRKeyword::Supraspinatus muscle, WBRKeyword::Supraspinatus injury, WBRKeyword::Empty can test, WBRKeyword::Empty can, WBRKeyword::Provocative test, WBRKeyword::Provocative testing, WBRKeyword::Shoulder pain, WBRKeyword::Shoulder weakness, WBRKeyword::Shoulder
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