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

Anatomical factors

• Superior angulation (Cam effect)

• Scapular angle

• Retroversion

Biomechanics of shoulder and elbow

Passive constraints

• Adhesion /cohesion

• Glenoid labrum

• Intracapsular pressure

Static contraints

• Capsule

• Coracohumeral ligament

• Glenohumeral ligaments

SGHL , MGHL , IGHL

Inferior gleno-

humeral ligament

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

Scapulo-thoracic ratio 2 : 1 (Range 1.1 : 1 - 4.3 : 1)

Biomechanics of shoulder and elbow

• Kinetics

Biomechanics of shoulder and elbow

Elevation Lifting Force

30 degrees 50 %

90 degrees 43 %

150 degrees 18 %

Subscapularis 45 degrees

Infraspinatous 45 degrees

Teres minor 55 degrees

Supraspinatous 0 degrees

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)

• LCL , anconeus , joint capsule

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

Rotational axis

Axis of rotation

Carrying angle

Biomechanics of shoulder and elbow

Biomechanics of shoulder and elbow

Kinetics

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

Biomechanics of shoulder and elbow

In conclusion

• Biomechanics of shoulder and

elbow joint adapted to function.

• They combine to place hand

in the desired position of

function.