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Updated: Sep 9 2022

Glenohumeral Joint Anatomy, Stabilizer, and Biomechanics

Images question 3_moved.jpg Comples -  scope view_moved.jpg labrum_moved.jpg image_moved.jpg foramen illustration- edit copy_moved.jpg
  • Planes of Motion
    • Reference
      • scapular plane is 30 degrees anterior to coronal plane.
    • Abduction
      • abduction requires external rotation to clear the greater tuberosity from impinging on the acromion.
        • therefore if someone has an internal rotation contracture they can not abduct > 120
      • 180° of abduction present in shoulder
        • motion occurs at two joints
          • 120° from the glenohumeral joint (2/3)
          • 60° from the scapulothoracic joint (1/3)
  • Glenohumeral Stability
    • Static restraints
      • glenohumeral ligaments (below)
      • glenoid labrum (below)
      • articular congruity and version
      • negative intraarticular pressure
        • if release head will sublux inferiorly
    • Dynamic restraints
      • rotator cuff muscles
        • the primary biomechanical role of the rotator cuff is stabilizing the glenohumeral joint by compressing the humeral head against the glenoid
      • rotator interval
      • biceps long head
      • periscapular muscles
  • Glenohumeral Ligaments (static)
      • Ligamentous Restraints in different Arm Positions
      • Arm position
      • Anterior restraint 
      • Inferior restraint
      • Posterior restraint
      • 0° (side) and adduction
      • SGHL/CHL
      • 45° (ER) and 45° abducted
      • MGHL
      • MGHL
      • 90° (ER)
      • Anterior band IGHL
      • Anterior band IGHL
      • Posterior band IGHL
      • 90° (forward flexed, abduction, and IR)
      • Anterior band IGHL
      • SGHL/CHL
    • SGHL
      • from anteriosuperior labrum to humerus
      • restraint to inferior translation at 0° degrees of abduction (neutral rotation)
      • prevents anteroinferior translation of long head of biceps (biceps pulley)
    • MGHL
      • resist anterior and posterior translation in the midrange of abduction (~45°) in ER
    • IGHL
      • posterior band IGHL
        • most important restraint to posterior subluxation at 90° flexion and IR
        • tightness leads to internal impingement and increased shear forces on superior labrum (linked to SLAP lesions)
      • anterior band IGHL
        • stability
          • primary restraint to anterior/inferior translation 90° abduction and maximum ER (late cocking phase of throwing)
        • anatomy
          • anchors into anterior labrum
          • forms weak link that predisposes to Bankart lesions
      • superior band IGHL
        • most important static stabilizer about the joint
        • 100% increased strain on superior band of IGHL in presence of a SLAP lesion
    • Coracohumeral ligament (CHL)
      • from coracoid to rotator cable
      • limits posterior translation with shoulder in flexion,adduction, and internal rotation
      • limits inferior translation and external rotation at adducted position
  • Glenoid Labrum (static)
    • Function
      • helps create cavity-compression and creates 50% of the glenoid socket depth
    • Composition
      • composed of fibrocartilagenous tissue
    • Blood supply
      • suprascapular artery
      • anterior humeral circumflex scapular
      • posterior humeral circumflex arteries
      • labrum receives blood from capsule and periosteal vessels and not from underlying bone
      • anterior-superior labrum has poorest blood supply
    • Stability
      • anterior labrum
        • anchors IGHL (weak link that leads to Bankart lesion)
      • superior labrum
        • anchors biceps tendon (weak link that leads to SLAP lesion)
    • Anatomic variants
      • normal variant
        • the labrum attached to the glenoid rim and a flat/broad middle glenohumeral ligament is the most common “normal” variation.
        • a cord-like middle glenohumeral ligament is present in 86% of population
      • sublabral foramen
        • seen in ~12% if population
      • sublabral foramen + cordlike MGHL
      • Buford complex (absent anterosuperior labrum + cordlike MGHL)
        • seen in ~1.5% of population
        • cordlike middle glenohumeral ligament with attachment to base of biceps anchor and complete absence of the anterosuperior labrum
        • attaching a Buford complex will lead to painful and restricted external rotation and elevation.
      • meniscoid appearance (1%)
  • Soft Tissue Stabilizers
    • Posterior capsule(static)
      • thin (< 1mm) with no ligaments
    • Rotator Interval (static)
      • contracture of the rotator interval is seen with adhesive capsulitis (frozen shoulder)
      • laxity of the rotator interval results in a visible sulcus sign with inferior laxity with the shoulder in adduction
      • includes the capsule, SGHL, coracohumeral ligament and long head biceps tendon that bridge the gap between the supraspinatus and the subscapularis.
      • boundaries
        • medially by lateral coracoid base
        • superiorly by anterior edge of supraspinatus
        • inferiorly by superior border of subscapularis
        • lateral apex formed by transverse humeral ligament
    • Rotator cuff(dynamic)
      • the primary biomechanical role of the rotator cuff is stabilizing the glenohumeral joint by compressing the humeral head against the glenoid 
    • Biceps Long Head(dynamic)
      • long head of biceps acts as humeral head depressor.
      • variable origin from superior labrum
      • forms weak links that predisposes to SLAP tear
      • SGHL and subscapularis thought to play role in stabilizing long head of biceps
  • Osteology
    • Humeral head
      • greater and lesser tuberosities are attachment sites for the rotator cuff
      • spheroidal in shape in 90% of individuals
      • average diameter is 43 mm
      • approximate retroversion 20° from transepicondylar axis of the distal humerus
      • articular surface inclined upward 130° from the shaft
    • Glenoid
      • pear-shaped surface with average upward tilt of 5°
      • average version is 5° of retroversion in relation to the axis of the scapular body and varies from 7° of retroversion to 10° of anteversion
    • Coracoid
      • serves as an anatomic landmark or "lighthouse" for the deltopectoral approach
      • coracobrachialis, pectoralis minor, and short head of the biceps attach to the coracoid
    • Acromion
      • 3 ossification centers
        • meta (base), meso (mid), and pre-acromion (tip)
      • acromiohumeral interval is 7-8mm
        • AHI may be normal on Xray but decreased on MRI when pt is supine and weight of arm is removed. This usually signifies multiple tendon tear.
      • acromial morphology
        • I=flat
        • II=curved
        • III=hooked
  • Blood Supply
    • Humeral head
      • ascending branch of anterior humeral circumflex artery and arcuate artery
        • provides blood supply to humeral head 
        • vessel runs parallel to lateral aspect of tendon of long head of biceps in the bicipital groove
          • beware not to injure when plating proximal humerus fractures
        • arcuate artery is the interosseous continuation of ascending branch of anterior humeral circumflex artery and penetrates the bone of the humeral head
      • posterior humeral circumflex artery
        • most current literature supports this as providing the main blood supply to humeral head
  • Free Body Analysis of Deltoid
    • Free body diagram if the arm was at 90 degs of abduction (not pictured)
      • assuming A = 3cm and B = 30 cm
      • sum of moment M = 0
      • (A x D) - (B x 0.5W) = 0
        • 3D = 0.5W (30)
        • D = 5W
    • Arthrodesis
      • optimal position
        • 15-20° of abduction
        • 20-25° of forward flexion
        • 40-50° of internal rotation
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