Teres Minor Anatomy

The teres minor is the smallest and most inferior of the four rotator cuff muscles, running obliquely from the lateral border of the scapula to the inferior facet of the greater tuberosity. It is easily dismissed as a minor contributor, and that is a mistake. Three features give it disproportionate surgical weight.

First, it is the only cuff muscle supplied by the axillary nerve, which makes the interval between it and the infraspinatus a true internervous plane and makes its selective sparing or selective loss a powerful localising sign. Second, it forms the roof of the quadrangular space, so every posterior and inferior exposure of the shoulder is oriented around its inferior border. Third, in the modern era of reverse arthroplasty, teres minor quality is the single most examined muscle on the pre-operative sagittal MRI, because a reverse arthroplasty restores a fulcrum but cannot create an external rotator.

Actions by plane

• External rotation: its principal action, contributing progressively more as the arm is abducted. With the arm at 90 degrees of abduction, teres minor becomes a major external rotator, whereas with the arm at the side it plays second fiddle to the infraspinatus.
• Head depression: its inferiorly and medially directed vector resists the superior shear of the deltoid, helping to maintain the acromiohumeral interval.
• Posteroinferior stabilisation: it resists posterior and inferior translation of the head, particularly in the abducted externally rotated position, and its capsular blending contributes to the posterior band of the inferior glenohumeral ligament complex.
• Horizontal extension: a minor contribution alongside the posterior deltoid.

Moment arm and length-tension

• The teres minor external rotation moment arm increases with abduction, unlike the infraspinatus, whose moment arm is greatest with the arm adducted. This complementary arrangement means the shoulder retains external rotation strength across the whole abduction arc.
• Because the muscle is short with a relatively small excursion, it works over a narrow part of its length-tension curve. Chronic retraction is poorly tolerated, and a teres minor that has undergone fatty change generates very little force even if the tendon remains in continuity.

Synergists and antagonists

• Synergists: infraspinatus (external rotation), posterior deltoid (external rotation in abduction and horizontal extension).
• Antagonists: subscapularis, pectoralis major, latissimus dorsi, teres major.

Palpation and positioning

• Teres minor lies deep to the posterior deltoid and is not reliably palpable as a discrete muscle. It is identified indirectly by following the lateral border of the scapula upward from the inferior angle with the arm across the body, which brings the strip of origin subcutaneous in a thin patient.
• The quadrangular space is localised by deep palpation of the posterior soft spot just lateral to the lateral scapular border and about 2 to 3 cm below the posterior glenohumeral joint line, with the arm abducted and externally rotated. Point tenderness here in an overhead athlete supports quadrilateral space syndrome.
• Wasting is best assessed on the sagittal MRI, not by inspection, because the posterior deltoid covers the muscle. This is an important difference from the infraspinatus, whose wasting is visible from across the room.

Named tests

Grading and pitfalls

• Support the arm during testing at 90 degrees; an unsupported arm recruits the posterior deltoid, which is also an external rotator in abduction and will mask teres minor loss.
• Passive external rotation must be full before a lag sign or the drop sign can be interpreted. A stiff shoulder produces a false positive in every lag test.
• The posterior deltoid can substitute so effectively that a patient with an isolated teres minor deficit appears normal until specifically tested in abduction.

Iatrogenic axillary nerve injury

• Mechanism: dissection or diathermy below the inferior border of teres minor, a posterior deltoid split extending more than about 5 cm below the scapular spine, or over-vigorous inferior retraction during posterior glenoid exposure.
• Consequence: simultaneous loss of the deltoid, teres minor and lateral arm sensation — an injury that converts a reconstructable shoulder into one for which no reliable salvage exists, since a reverse arthroplasty requires a functioning deltoid.
• Avoidance: treat the inferior border of teres minor as an absolute limit; obtain direct vision before coagulating in the depths of a posterior wound.

Denervation from splitting the wrong plane

• Splitting the infraspinatus raphe instead of the infraspinatus-teres minor interval denervates the lower infraspinatus, but a plane taken too low divides teres minor itself across its motor entry near the inferior border.
• Avoidance: identify both aponeurotic laminae and the circumflex scapular vessels in the triangular space.

Persistent external rotation deficit after arthroplasty

• The commonest functional disappointment after an otherwise successful reverse arthroplasty. The patient elevates well and is pleased for six weeks, then becomes frustrated at being unable to reach the back of the head, place a hand on the opposite shoulder, or bring food to the mouth without lifting the elbow.
• Avoidance: identify it before surgery with hornblower's sign and the sagittal MRI, and either plan a combined transfer or set the expectation explicitly at consent.

Missed quadrilateral space pathology

• Isolated teres minor atrophy is reported on MRI more often than the clinical syndrome occurs, and is rarely an isolated abnormality. Operating on the image rather than the patient is the error.
• Conversely, a genuinely symptomatic vascular quadrilateral space syndrome in a throwing athlete can present with digital ischaemia and be misattributed to a vasospastic disorder; the shoulder is the source.

Failure of an external rotation transfer

• A latissimus dorsi and teres major transfer requires an intact, functioning deltoid and adequate passive external rotation. It fails when the deltoid is compromised, when the transfer is tensioned in internal rotation, or when the patient cannot be taught to fire the transferred muscle.
• Donor morbidity is generally well tolerated but matters in wheelchair users and crutch users, who rely on latissimus dorsi for transfers and for propulsion.

Differential of isolated teres minor atrophy

Staging systems applied to teres minor

Imaging

• The working slice is the sagittal oblique MRI at the level of the scapular spine, medial to the coracoid, which shows all four cuff bellies. Teres minor sits in the inferior posterior quadrant.
• Denervation of teres minor appears first as diffuse T2 hyperintensity of the whole belly, then as uniform fatty replacement — a different pattern from the thinning with preserved fascicular architecture seen in disuse.
• Look at the deltoid on the same images. Teres minor atrophy with a normal deltoid points to the posterior branch alone; teres minor atrophy with deltoid atrophy points to the main axillary nerve.
• On ultrasound, teres minor is accessible in the posterior window between the infraspinatus and the posterior deltoid but is difficult to grade for fatty change; MRI remains the standard.

Intervals and approaches

Teres minor as a transfer target and as a preserved structure

• Teres minor is not a donor: it is small, its excursion is short, and its pedicle is shared with the axillary nerve.
• It is a target: the tendon of a latissimus dorsi transfer for an irreparable posterosuperior cuff tear is routinely sutured to the residual infraspinatus and teres minor stumps as well as to the greater tuberosity.
• It is a structure to preserve: in reverse arthroplasty, in posterior approaches and in humeral head fracture surgery, an intact teres minor is the difference between a patient who can externally rotate afterward and one who cannot.
• Modified L'Episcopo transfer (Boileau): the latissimus dorsi and teres major are detached from the medial lip of the bicipital groove and transferred posterolaterally to the humerus, converting them from internal to external rotators. It is combined with a reverse arthroplasty when the teres minor is degenerate and hornblower's sign is positive.

Anatomical variation across populations

• Cadaveric series describe variable fusion of teres minor with the infraspinatus, in which the two share a common tendon at the greater tuberosity and no discrete plane exists. Where present, this variant complicates the posterior internervous approach and increases the risk of denervating either muscle.
• The teres minor branch of the axillary nerve may arise before the nerve enters the quadrangular space in a minority of specimens, which alters the pattern of denervation after a proximal injury.
• Fibrous bands within the quadrangular space are described with varying prevalence across dissection series and are the presumed substrate for compressive quadrilateral space syndrome.
• Reported prevalence of isolated teres minor atrophy on shoulder MRI differs substantially between imaging cohorts, reflecting the fact that most of it is incidental.

Side-by-side guidance

Registry and outcome signals

• National joint registries record reverse total shoulder arthroplasty survivorship but do not capture range of motion by plane, so the functional impact of a degenerate teres minor is visible only in institutional series. Those series consistently identify teres minor quality as an independent determinant of external rotation.
• Lateralised reverse designs have been introduced partly to improve external rotation and to reduce scapular notching. Registry data on survivorship of lateralised versus medialised designs continue to accumulate; the functional claim regarding external rotation still rests on cohort studies rather than registry endpoints.
• Combined reverse arthroplasty and external rotation transfer is performed in low volumes, and the evidence base is a series of institutional cohorts rather than randomised trials.

High-resource versus limited-resource practice

• Well-resourced settings: routine sagittal MRI for Goutallier grading of all four cuff muscles, lateralised reverse implants, and combined transfer where indicated.
• Limited-resource settings: hornblower's sign performs the same triage function as the MRI at no cost, reliably identifying the patient in whom a reverse arthroplasty alone will disappoint. Where reverse arthroplasty is unavailable, an isolated latissimus dorsi and teres major transfer in a younger patient with a preserved fulcrum remains a valuable operation.
• The essential clinical skill transfers everywhere: examine hand-to-mouth before promising external rotation.