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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 comes from motion in two joints (2:1 ratio)
      • 120° from the glenohumeral joint
      • 60° from the scapulothoracic joint
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 Res.
Inferior Res.
Posterior Res.
0° (side) and adduction
x
SGHL/CHL
xxx
45° (ER) and 45° abducted
MGHL
x
MGHL
Adduction   SGHL/CHL  
90° (ER)
Anterior band IGHL
Anterior band IGHL
Posterior band IGHL
90° (forward flexed, abduction, and IR)
Anterior band IGHL
 
Posterior 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 often
      • 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
    • retroverted 30° 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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