Which of the following joints is considered to have a spheroidal type of movement

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