Back and Scapula: Muscles, Attachments, Abduction, and Arm Spaces

SERRATUS ANTERIOR

  • Origin: Costal/anterolateral surfaces of upper ribs (commonly described as ribs 1–9).
  • Insertion: Anterior surface of the medial (vertebral) border of the scapula.
  • Function: Protracts the scapula, holds it against the thoracic wall, and assists in upward rotation of the scapula during arm elevation.
  • Innervation: Long thoracic nerve.
  • Clinical relevance: Weakness or paralysis (e.g., after injury to the long thoracic nerve) leads to winged scapula and impaired scapular stabilization during arm movements.

TRAPEZIUS

  • Origin: External occipital protuberance and superior nuchal line; spinous processes of C7–T12.
  • Insertion: Lateral third of the clavicle, acromion, and spine of the scapula.
  • Innervation: Spinal accessory nerve (CN XI) with proprioceptive feedback from C3–C4.
  • Fibers and actions:
    • Upper fibers: Elevate the scapula (shrug).
    • Middle fibers: Retract (adduct) the scapula.
    • Lower fibers: Depress the scapula and assist in upward rotation.
  • Functional note: Coordinates with serratus anterior for upward rotation of the scapula during arm elevation.
  • Practical signs: Trapezius dysfunction can affect shoulder girdle mechanics and posture.

DELTOID

  • Structure: Multipennate muscle with three FIBERS groups—anterior (clavicular), middle, posterior (spinal).
  • Origins:
    • Anterior: Lateral third of the clavicle (clavicular part).
    • Middle: Acromion (acromial part).
    • Posterior: Spine of the scapula (spinal part).
  • Insertion: Deltoid tuberosity on the lateral aspect of the middle third of the humeral shaft.
  • Nerve supply: Axillary nerve.
  • Cutaneous innervation: Skin over the lateral deltoid region (regimental badge area).
  • Actions: Primary abductor of the arm in the 15°–90° range when activated together with supraspinatus; also assists in flexion (anterior fibers), extension and lateral rotation (posterior fibers).
  • Abduction range and sequence:
    • Initiation: Supraspinatus initiates first ~0°–15°.
    • Sustained abduction: Deltoid (especially middle fibers) contributes from ~15°–90°.
    • Above ~90°: Trapezius and serratus anterior permit further elevation of the arm by rotating the scapula upward.
  • Clinical note: Axillary nerve injury leads to loss of abduction and flattening of the shoulder contour; sensory loss over the lateral shoulder (regimental badge area).

AXILLARY NERVE INJURY (Surgical neck of humerus fracture case)

  • Features expected with axillary nerve damage:
    • Loss of sensation over the upper lateral aspect of the arm.
    • Loss of rounded (normal) contour of the shoulder.
    • Atrophy of the deltoid muscle.
    • Impaired overhead abduction of the arm.
  • Classic clinical sign: Patch of sensory loss over the lateral shoulder (regimental badge area).

SCAPULA & ATTACHMENTS (muscles attaching to the scapula)

  • Coracoid process region:
    • Short head of biceps brachii and coracobrachialis originate from the coracoid process.
    • Pectoralis minor attaches to the coracoid process and is involved in stabilizing the scapula against the thoracic wall.
  • Supraspinatus and Infraspinatus (attachment sites):
    • Supraspinatus originates from the supraspinous fossa and inserts on the greater tubercle of the humerus.
    • Infraspinatus originates from the infraspinous fossa and inserts on the greater tubercle.
  • Teres minor and Teres major:
    • Teres minor originates from the lateral border of the scapula and inserts on the greater tubercle.
    • Teres major originates from the inferior angle of the scapula and inserts on the medial lip of the intertubercular groove of the humerus.
  • Levator scapulae and Rhomboids:
    • Levator scapulae: from transverse processes of C1–C4 to the superior angle of the scapula; elevates the scapula.
    • Rhomboid major and minor: origin from spinous processes (lar to T1–T5) to the medial border of the scapula; retracts and elevates the scapula.
  • Latissimus dorsi and other trunk-muscle attachments:
    • Latissimus dorsi attaches to the floor of the intertubercular groove of the humerus; originate from the thoracolumbar fascia and iliac crest, among other sites.
  • Pectoralis minor:
    • Origin: ribs 3–5.
    • Insertion: Coracoid process of the scapula.
    • Action: stabilizes and protracts the scapula; pulls the shoulder girdle anteriorly.
  • Supraspinatus, Infraspinatus, Subscapularis (not all listed in slides but part of scapular attachments):
    • Supraspinatus and infraspinatus insert on the greater tubercle of the humerus (superiorly and posteriorly, respectively).
    • Subscapularis attaches to the lesser tubercle (not detailed in slides but part of scapular attachments).

ABDUCTION AT THE SHOULDER JOINT

  • Supraspinatus:
    • Initiates abduction from 0° to ~15°.
  • Deltoid (middle fibers):
    • Abducts from approximately 15° to 90°.
  • Beyond 90°:
    • Upward rotation of the scapula is achieved by the coordinated action of the serratus anterior and trapezius (particularly the lower fibers of trapezius) to allow continued overhead abduction.
  • Combined muscle actions:
    • Supraspinatus initiates first 15°, then deltoid takes over for abduction up to ~90°, with trapezius and serratus anterior enabling further elevation via scapular rotation.

SPACES AROUND THE ARM

  • Quadrangular space (borders and contents):
    • Borders: superior border is the teres minor; inferior border is the teres major; medial border is the surgical neck of the humerus (or in some descriptions the long head of triceps forms the medial boundary); lateral border is the surgical neck of the humerus.
    • Contents: axillary nerve and posterior circumflex humeral artery.
  • Triangular space (borders and contents):
    • Borders: teres minor (superior), teres major (inferior), and long head of the triceps (lateral).
    • Contents: circumflex scapular artery.
  • Triangular interval (often called the lower triangular space) (borders and contents):
    • Borders: teres major (inferior), long head of triceps (superior), and lateral head of triceps (medial).
    • Contents: profunda brachii (deep brachial) artery and radial nerve.
  • Note on nomenclature in slides: there are references to an 'upper triangular space' and a 'lower space'; standard anatomy textbooks define the three spaces above and describe their contents as listed here (quadrangular, triangular, and triangular interval).

TRIANGLE OF AUSCULTATION

  • Boundaries (as per slides):
    • Trapezius (superior boundary)
    • Medial border of the scapula (inferior or lateral boundary, depending on description)
    • Latissimus dorsi (inferior boundary)
  • Floor: rhomboid major (and minor in some descriptions)
  • Clinical relevance: optimal area for listening to the lungs with a stethoscope due to relatively thin musculature here.

PRACTICE QUESTIONS AND CASES (FROM TRANSCRIPT)

  • Question: Deltoid is an example of a multipennate muscle with three types of fibers. The anterior fibers cause which movement at the shoulder joint?
    • Correct answer: Flexion and medial (internal) rotation.
  • Case: A 19-year-old male with fracture of the surgical neck of the humerus and suspected axillary nerve injury will show:
    • Loss of sensation on the upper lateral aspect of the arm.
    • Loss of the rounded contour of the shoulder.
    • Atrophy of the deltoid.
    • Loss or impairment of overhead abduction.
  • Clinical signs to remember:
    • Regimental badge area sensory loss (over the lateral deltoid region).
    • Deltoid muscle atrophy leads to a flattened shoulder contour and weakness in abduction beyond ~15°–90° depending on compensatory scapular rotation.

KEY TERMS TO REMEMBER

  • Deltoid tuberosity: location of insertion on the humerus.
  • Regimental badge area: skin overlying the deltoid region supplied by the axillary nerve.
  • Upward rotation of the scapula: crucial for full arm elevation; achieved by serratus anterior and trapezius.
  • Quadrangular space: transmits axillary nerve and posterior circumflex humeral artery.
  • Triangular space: transmits circumflex scapular artery.
  • Triangular interval: transmits profunda brachii artery and radial nerve.
  • Long thoracic nerve: innervates serratus anterior; injury leads to winged scapula.
  • Spinal accessory nerve (CN XI): innervates trapezius and contributes to shoulder girdle movement.

REAL-WORLD RELEVANCE

  • Understanding the scapulohumeral rhythm is essential for diagnosing shoulder girdle pathologies and planning rehab after injuries or surgeries (e.g., rotator cuff tears, axillary nerve injury).
  • Knowledge of the spaces around the arm is critical to avoid neurovascular injury during surgical approaches to the shoulder region.
  • Recognizing the signs of axillary nerve injury helps in early diagnosis and targeted rehabilitation to restore deltoid function and shoulder contour.

FORMULAS AND ANGLES (FOR CLARITY)

  • Abduction angles for shoulder movement:
    • 0^\u00b0 ext{ to } 15^ 0: initiation by supraspinatus.
    • 15^ 0 ext{ to } 90^ 0: abduction by middle deltoid.
    • 90^ 0 ext{ and above}: continued abduction with upward rotation of the scapula by trapezius and serratus anterior.