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Innervation
  • Motor
    • deltoid
      • anterior and posterior branches of axillary nerve
    • teres minor
      • posterior branch of axillary nerve
  • Sensory
    • superior lateral brachial cutaneous nerve
      • lateral shoulder sensation
Origin
  • Brachial plexus 
    • middle trunk - posterior divison - posterior cord   
  • C5 and C6 fibers
Course
  • Arises from the posterior cord of the brachial plexus
    • anterior to the subscapularis muscle and posterior to the axillary artery
    • courses along anterior subscapularis to inferior edge
    • passes beneath glenohumeral joint capsule
      • gives off articular branches that supply the capsule
  • Travels through the quadrangular space 
    • location 
      • lateral to triangular space and superomedial to triangular interval 
    • boundaries
      • superior - teres minor
      • inferior - teres major
      • medial - long head of triceps brachii
      • lateral - surgical neck of the humerus
    • contents
      • axillary nerve 
      • posterior circumflex humeral artery
    • divides into anterior and posterior branches within the quadrangular space
  • Terminal branches
    • anterior branch
      • wraps around the surgical neck of the humerus, running in the deep deltoid fascia with the posterior circumflex humeral artery
      • supplies the anterior deltoid muscle 
      • traditional "safe zone" from lateral acromion is 5 cm
        • anterior branch has been shown to run 5-7 cm distal to the lateral edge of the acromion
          • arm abduction to 90° decreases the distance between acromion and nerve by 30%
        • damage to nerve with a muscle split will denervate the anterior deltoid
      • terminates in small cutaneous branches supplying the anterior and anterolateral shoulder
    • posterior branch
      • supplies the teres minor   and posterior deltoid  muscles
        • branch to the teres minor is closest to glenoid labrum and most susceptible to injury during arthroscopy
      • pierces the deep fascia
      • terminates as the superior lateral brachial cutaneous nerve supplying the lateral shoulder
Clinical Conditions
  • Quadrangular space syndrome  
    • compression of the axillary nerve within the quadrangular space
    • overhead athletes
    • muscle hypertrophy or paralabral cysts
    • present with vague posterior or posterolateral shoulder pain
    • weakness +/- atrophy of teres minor and deltoid muscles
    • most treated successfully without surgery but can undergo nerve decompression in recalcitrant cases
  • Iatrogenic injury
    • arthroscopic surgery
      • the posterior branch has an intimate association with the inferior glenoid and capsule
      • can be damaged during arthroscopic HAGL repair or inferior capsule release
    • Hornblower's test 
      • detects teres minor weakness
      • place shoulder in 90° abduction and 90° external rotation
      • positive if patient unable to maintain external rotation, arm falls into internal rotation
 

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