Anatomy Of The Teres Minor Muscle

Courtesy: Prof Nabil Ebraheim, University of Toledo, Ohio, USA

Anatomy of the teres minor muscle

• The teres minor is a narrow muscle which originates from the posterior lateral surface of the
  scapula and inserts into the greater tuberosity of the humerus .
• Teres minor is innervated by the posterior branch of axillary nerve .
• The same nerve that gives cutaneous innervation to the lateral part of the shoulder .

Important anatomical spaces

• Teres minor contribute to important anatomical spaces in the posterior part of the shoulder such as the quadrangular space
• Boundaries of quadrangular space : teres minor superiorly , teres major inferiorly , long head of triceps medially , surgical neck of humerus laterally.
• The quadrangular space contains the posterior humeral circumflex artery and axillary nerve . The
  tere minor muscle also contributes to the triangular space .
• Boundaries of triangular space : teres major , teres minor and long head of triceps . This triangular
  space also contains circumflex scapular artery . The teres minor muscle does not contribute to the
  triangular interval .

Boundaries of triangular interval : teres major , humeral shaft , long head of triceps .

• The triangular interval contains the radial nerve and deep branch of brachial artery
• The teres major muscle contributes to 3 important anatomical spaces while the teres minor only
  contributes to 2 important anatomical spaces.
• The good way to see the teres minor muscle is MRI sagittal view .The infraspinatous is the main
  external rotator of the shoulder with the arm to the side(adducted).The teres minor muscle is the
  main external rotator of the shoulder with the arm abducted.

Clinical examination

• Horn Blowers test-The test is used to determine the strength of the teres minor muscle. The patient
  and the examiner are both standing and the patients arm is elevated to 90 degrees,the patients
  elbow is then flexed to 90 degrees and the patient is asked to laterally rotate the shoulder.
  Weaknesses and /pain against resistance signals a positive test.

Why teres minor is important?

• Innervation of the teres minor muscle is unexpected because it comes from the posterior branch of
  axillary nerve .
• It is important in posterior approach to the shoulder joint, yhe approach is usually
  done in the interval between the infraspinatous and teres minor muscle.
• If you go below the teres minor muscle then you could be in the quadrangular space and could injure the neurovascular
  structure including the axillary nerve.
• Check if the teres minor muscle is atrophic on the MRI,atrophy of the muscle can be idiopathic or it may be due to quadrangular space impingement or syndrome.
• Idiopathic atrophy is associated with rotator cuff pathology.