Biomechanics of the shoulder

Kinematics

· four articulations

· glenohumeral

· scapulothoracic

· acromioclavicular

· sternoclavicular

Range of motion

Abduction

· elevation in coronal plane

· range is 180^o

· internal rotation required to prevent impingement of tuberosities

· abduction sometimes described in plane of scapula

· 35^o anterior to coronal plane

Flexion

· elevation in sagittal plane

· range is 180^o

Rotation

· varies with elevation of arm

· internal and external range is generally 90^o each

Extension

· range is 60^o

Adduction

· range is 75^o

Glenohumeral joint

Humeral head

· nearly hemispherical

· makes angle of 135^o with humeral shaft

· retroverted 30-40^o to axis of elbow flexion

Glenoid fossa

· surface area 1/3 of humeral head

· longitudinal diameter 75% of humeral head

· tranverse diameter 60% of humeral head

· retroverted 7.5^o

· shallowness

· allows significant movement

· makes joint susceptible to instability

Stability

· primarily soft tissue

Glenoid labrum

· deepens glenoid fossa

· provides soft edge for humerus

Capsule

· reinforced anteriorly by glenohumeral ligaments

· superior, middle and inferior glenohumeral ligaments

· reinforced superiorly by coracohumeral ligament

· runs from base of coracoid to proximal bicipital groove

Coracoacromial arch

· coracoacromial ligament and acromion

· prevents excessive upward displacement

Rotator cuff

· tendons blend with capsule

· dynamic restraint to anterior, posterior and inferior displacement

Surface motion

· primarily rotational (spin)

· some translation (slide) and rolling (turn) takes place

· in 1st 30^o abduction, humeral head moves upwards 3 mm

· probably due to sag during dependency

· instant centre of rotations close to each other in centre of humeral head

Acromioclavicular joint

Anatomy

· stability provided mainly by coracoclavicular ligaments

· meniscus divides joint into two segments

Movement

· scapula moves on clavicle in three axes

· around vertical axis around conoid ligament

· around transverse axis in coronal plane hinging on trapezoid ligament

· around transverse axis in sagittal plane through acromioclavicular joint

· rotation through conoid ligament takes place between acromion and meniscus

· rotation through trapezoid ligament takes place between meniscus and clavicle

· clavicle elevates at acromioclavicular joint during shoulder elevation

· range is 20^o

· during first and last 30^o of elevation

Sternoclavicular joint

Anatomy

· principal stabiliser is costoclavicular ligament

· meniscus divides joint into two segments

Movement

· costoclavicular ligament acts as fulcrum for gliding motion at joint

· anteroposterior gliding occurs between sternum and meniscus

· superoinferior gliding occurs between clavicle and meniscus

· clavicle elevates at sternoclavicular joint during shoulder elevation

· 4^o for every 10^o shoulder elevation from 0^o to 90^o

· clavicle also rotates during shoulder elevation

· 40^o during shoulder range

Scapulothoracic articulation

Anatomy

· scapula attached to thorax only by

· sternoclavicular joint via

· clavicle via

· acromioclavicular joint

· scapulothoracic articulation is bone-muscle-bone articulation

· subscapularis and serratus anterior lie between scapula and thorax

Movement

· ratio of GH : ST movement is about 2 : 1

· 60^o of scapular movement possible because

· elevation of clavicle of 20^o at AC joint and 40^o at SC joint

· rotation of clavicle of 40^o

· ratio changes during range

· relatively more ST movement during initial elevation

· external rotation of scapula occurs during elevation

· associated with external rotation of humerus

kinetics

· complex effects of numerous muscles acting across shoulder

Muscles

· EMG studies during abduction show

· deltoid and upper trapezius responsible for moving shoulder

· supraspinatus responsible for stabilising shoulder

· deltoid contraction produces primarily upward shear

· supraspinatus contracture produces primarily compressive force and some downward shear

· during elevation

· contraction of deltoid (middle fibres) alone does not produce elevation as fibres parallel to humeral shaft

· contraction of oblique cuff muscles (infraspinatus, subscapularis, teres minor) stabilise humeral head on glenoid and provide fulcrum

· combined contraction allows elevation

Loads

· at 90^o of abduction

· deltoid force is 0.7 W

· joint reaction force is 0.9 W