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Biomechanics of shoulder and
elbow
Mr C A Busch FRCS
Rowley Bristow Unit
St Peter’s Hospital
Chertsey
Biomechanics of shoulder and elbow
Overview
• What is biomechanics ?
• Stability
• Kinematics
• Kinetics
• Elbow
Biomechanics of shoulder and elbow
• Definition of biomechanics:
Science that deals with the effect of
energy and forces on biological systems.
Biomechanics of shoulder and elbow
Stability
• Active constraints
• Passive constraints
• Static constraints
Biomechanics of shoulder and elbow
Active constraints
Normal joint laxity
• Anterior 8 mm
• Posterior 9 mm
• Inferior 11 mm
• 2 cm with anaesthetised patient
Biomechanics of shoulder and elbow
Passive constraints
• Adhesion /cohesion
• Glenoid labrum
• Intracapsular pressure
Biomechanics of shoulder and elbow
Kinematics of “ Shoulder joint ”
• Glenohumeral
• Scapulothoracic
• Acromioclavicular
• Sternoclavicular
Biomechanics of shoulder and elbow
Normal ROM gleno-humeral joint
Forward flexion Male 171-175 degrees
Female 167-168 degrees
Rotation 180 degrees (^ Ext rot)
Rotation in 90 degrees abduction 120 degrees (^ Int rot)
Biomechanics of shoulder and elbow
Acromio-clavicular joint
• Diarthrodial joint (non-congruent)
• Fibro-cartilagenous meniscus
• AC capsule and ligaments
• Coraco-clavicular ligaments
Biomechanics of shoulder and elbow
Sterno-clavicular joint
• Only articulation between upper extremity and the
rest of the body
• Diarthrodial joint , Fibro-cartilagenous meniscus
• Ligamentous restraints
Biomechanics of shoulder and elbow
Clavicle
• Strut to keep glenoid and scapula lateral to thorax
• Allows pectoralis major and latissimus dorsi to
function
Function of scapula
• To maintain optimum length of deltoid
• To maintain shoulder stability during overhead activities
• To avoid impingement
Biomechanics of shoulder and elbow
Elevation Lifting Force
30 degrees 50 %
90 degrees 43 %
150 degrees 18 %
Biomechanics of shoulder and elbow
Flexion
Prime mover
• Anterior and middle deltoid
Stabiliser
• Infraspinatous , supraspinatous , and latissimus
dorsi
Biomechanics of shoulder and elbow
External rotation
Prime mover
• Infraspinatus , Teres minor , posterior deltoid
Stabiliser
• Subscapularis
Biomechanics of shoulder and elbow
Internal rotation
Prime mover
• Pectoralis major , latissimus dorsi, subscapularis
• All three muscle group’s activity decreases with
increased abduction
Biomechanics of shoulder and elbow
Extension
Prime mover
• Posterior and middle deltoid , subscapularis
• Increased supraspinatus activity with extension
Stabiliser
• Subscapularis , supraspinatous
Biomechanics of shoulder and elbow
Elbow
• Component link in lever arm system which places the
hands
• Acts as fulcrum for lifting activity
• In power and fine work stabilises upper extremity linkage
• In crutch walking it becomes weight bearing
Biomechanics of shoulder and elbow
Stability (Valgus)
• MCL anterior oblique ligament
posterior oblique ligament
Biomechanics of shoulder and elbow
Stability (Valgus)
• In 90 degrees of flexion MCL provides 54% of valgus
stability.
• The radial head provides 30 % of valgus stability
(0-30 degrees)
• In extension olecranon locks in fossa
Biomechanics of shoulder and elbow
Stability (Varus)
90 degrees of flexion LCL 9 %
Joint 78 %
Capsule 13 %
extension LCL 14 %
Joint 54 %
Capsule 32 %
Biomechanics of shoulder and elbow
Kinematics
• 2 degrees of freedom
• “ functional arc “ = 100 degrees
Biomechanics of shoulder and elbow
Kinetics
• short moment arms
• efficient kinematically but not kinetically
• joint compressive forces largest in 0-30 degrees
extension
Biomechanics of shoulder and elbow
Kinetics
• Flexion max power 90-130 degrees
• Extension max power 60-140 degrees
• Most flexion strength in supination
• Pronation only 86 % of supination
Biomechanics of shoulder and elbow
Kinetics
• Ulna-humeral joint 40 %
Radio-humeral joint 60%
• Direction of force of forearm muscles is constant
in relation to the trochlear notch