BIOMECHANICS AND FUNCTIONAL ANATOMY OF THE SHOULDER

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					BIOMECHANICS AND FUNCTIONAL ANATOMY OF
THE SHOULDER JOINT
CHAPMAN chapter 76


Passive elements of function
Four joints

Glenohumeral joint
Bony anatomy
      Average radius 26.9mm in men
      Bony contact 35% of humeral head
      Total contact (labrum included) 75% of humeral head
      Socket depth: horizontal=5mm(+60% by labrum); vertical=9mm(+50% by
      labrum)
      Humerus retroverted 20 to 35o; upward 45o
      Scapular axis anteverted 30o, but glenoid to scapular axis angle varies fron 7o
      retroversion to 10o anteversion
Labrum
      Average socket depth doubled by labrum
      Dense fibrous tissue, no chondrocytes
      Labrum attached directly to biceps
      Vertical passive stability = 2x horizontal (effect of socket depth)
      Labrum contributes 20% of passive resistance to translation
Capsule
      On scapula 1cm beyond labrum
      On humerus to anatomic neck; inferiorly doun to surgical neck
      Intraarticular vacuum important to stability
Glenohumeral ligaments – thickenings of capsule; constant in presence, variable in size
      Superior glenohumeral ligament
              From labrum anterior to biceps at base of coracoid
              To superior and medial part of lesser tuberosity
      Middle glenohumeral ligament
              Most variable in size
              From inferior to sup glenohum lig
              To anatomic neck on medial part of lesser tuberosity
      Inferior glenohumeral ligament
              The thickest
              Broad: anterior, inferior and posterior parts
              Anterior margin thick, called Superior band; to anatomic neck
              Rest thin, called axillary pouch; posteriorly to surgical neck
      Ligamentous resistance to external rotation
              In neutral all ligaments slack, but there is some tension in SGHL&MGHL
              In neutral abduction + ER the MGHL and superior band of IGHL tighten
              In 45o abduction superior band of IGHL tight
              In 90o abduction major strain in superior band of IGHL
Coracohumeral ligament
      The most consistent capsular ligament
      From lateral base of coracoid process
      Over the rotator interval
      To lesser and greater tuberosities (forms part of the roof of the bicipital sheath
Coracoacromial ligament
      Passive restraint to humeral superior translation in rotator cuff deficient shoulders

Scapulothoracic articulation
Loose areolar tissue

Sternoclaicular joint
Clavicle – if excised, stability preserved by posterior sheath and muscular attachments
Large clavicular facet + small sternal facet + small 1st rib facet
Articular disc attached to clavicle superiorly and 1st rib inferiorly
Costo-clavicular ligaments + strong anterior/weak posterior sterno-clavicular ligaments

Acromioclavicular joint
Unique ligamentous arrangement: other than superior acromio-clavicular capsular
      ligament, the other ligamentous stabilizers are 2cm away (coraco-clavicular
      ligaments)
Conoid ligament: stability against anterior and superior displacement of clavicle
Trapezoid ligament: less important


Motions of the shoulder joint
Scaption: elevation in the plane of the scapula (30 to 40o anterior to coronal plane)
Elevation: 168o men/175o women (1/3 scapulo-thoracic & 2/3 gleno-humeral)
Glenohumeral rhythm: during first and last 30o of elevation humerus moves bur scapula
       does not

Glenohumeral joint mobility
Greatest balance of muscle action with scaption (true neutral plane of elevation)
Rotation in neutral abduction: ER=108o, IR=71o, total ER-IR = almost 180o
Rotation in 90o abduction: ER=90o, IR=30o, total ER-IR decreased to 120o

Scapulothoracic mobility
Largest arc: upward rotation = 60o
Elevation-depression; retraction-protraction

Clavicular joints mobility
Sternoclavicular: 30o rise during first 90o arm abduction
Acromioclavicular joint: 15o rotation during first and last 40o elevation (total 30o)
Muscular control
14 muscles
3 heads of deltoid
4 rotator cuff muscles + biceps
2 axiohumeral muscles (pec & lat dorsi)
Scapular muscles (serratus ant, trapezius, rhomboid, levator scapulae)

Biomechanics of arm elevation
Force required is greater at 90o than at lesser levels of elevation
Deltoids: primary elevators
Rotator cuff muscles: synergistic to deltoids; compress glenohumeral articulation
Long head of biceps: secondary shoulder stabilizer; primary elbow motor
All forces can be decomposed in compressive vector (horizontal, aids stability) and shear
        vector (parallel to joint line, results in motion)
Deltoids: greater component of vertical shear at lesser degrees of elevation
Supraspinatus provides compression 2.73 times more than shear; very important synergist
        to deltoid contraction
Other rotator cuff muscles provide downward shear to balance deltoid
Action of rotator cuff lessens force necessary for abduction by 41%
Complete paralysis of deltoid or the rotator cuff (but not both) does not prevent elevation
Synergistic action of serratus anterior and trapezius rotate scapula to increase elevation;
        paralysis of either muscle prevents elevation above 90o

Biomechanics of arm rotation
Rotation changes coordination between deltoid and rotator cuff from synergy to sequence
Therefore shoulder muscles provide less stability during rotation (throwing, swimming)
       than during pure elevation; propensity to injury

Biomechanics of arm suspension (carrying heavy load)
Downward pull on arm resisted mostly by supraspinatus (compressive force) and not by
deltoid (direct upward lifting)

Biomechanics of horizontal movements

				
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posted:11/18/2011
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