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