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Biomechanics of the shoulder

Biomechanics of the Shoulder

Osteology

The shoulder complex formed by

  • Sternum
  • Clavicle
  • Scapula
  • Humerus

Manubrium

 

 

Body

 

Xyphoid process

Superior view

Inferior view

Anterior aspect

Posterior aspect

Lateral border is thicker than medial

– Because of all the muscle attachments

Anteriorly
– Glenoid Cavity

Shallow, oval- shaped depression

– Coracoid Process

Looks like a bent finger

Posteriorly

– Separated by ribs by thin layers of muscle

– Spine

Strong, sharp ridge

Shoulder Girdle

Joints of the Shoulder

  • Glenohumeral joint
  • Sternoclavicular joint
  • Acromioclavicular joint
  • Coracoclavicular joint
  • Scapulothoracic joint

 

Glenohumeral Joint

• Ball and socket joint (note: not a real socket) in which the head of the humerus articulates with the glenoid fossa of the scapula.

• Referred to as the shoulder joint.

Sternoclavicular Joint

  • Strong enough so that dislocation of joint is less common than fracture of clavicle.
  • Provides major axis of rotation for movement of clavicle and scapula.
  • Elevation: 60°.
  • 24-30° motion anterior, posterior, and inferior.
  • Helps position GH joint.

               

Acromioclavicular Joint

  • Irregular diarthrodial joint between the acromion process of the scapula and the distal clavicle.
    – Allows limited motions in all three planes.
  • Rotation occurs during arm elevation
  • Close-packed position with humerus abducted to 90 degrees.

                

Coracoclavicular Joint

A syndesmosis with coracoid process of scapula

– Bound to the inferior clavicle by the coracoclavicular ligament.

Permits little movement.

      

         

Scapulothoracic joint

  • Not a true joint
  • The movement of the scapula on the wall of the thoracic cage is critical to shoulder motion
  • Contraction of the scapular muscles that attach the scapula to the axial skeleton, is critical in stabilizing the scapula and providing a base for joint function

              

   

Ligaments of the Sternoclavicular Joint

Interclavicular ligament
– Strengthens the capsule superiorly.

Costoclavicular ligament
– Limits elevation of the pectoral girdle.

              

Ligament of the Acromioclavicular Joint

Acromioclavicular ligament
– A fibrous band
– This ligament strengthens the joint superiorly.

Ligaments of the Coracoclavicular Joint

Coracoclavicular ligament
– A strong pair of bands that unites the coracoid process of the scapula to the clavicle

– Consists of two ligaments

  • Conoid ligament
  • Trapezoid

Ligaments of the Glenohumeral Joint

Glenohumeral ligaments
– Strengthen the anterior aspect of the articular capsule
– Three fibrous bands

Superior Glenohumeral ligament

Taut during external rotation and plays a small role in the stability of the shoulder

      Middle Glenohumeral ligament

Taut during external rotation and plays a small role in stability of the shoulder

Inferior Glenohumeral Ligament

Taut during external rotation, and plays a small role in stability of the shoulder

              

Ligaments of the Glenohumeral Joint

Coracohumeral ligament

  • Strengthens the capsule superiorly
  • Resists the pull of gravity and limits external rotation of the shoulder

Transverse humeral ligament

  • Strengthens the capsule and bridges the gap between the greater and lesser tubercles of the humerus
  • keeps the biceps tendon in its groove during movements

               

Shoulder Neural Function

Movements of the Shoulder Complex

Movements of the Scapula
– Abduction

(Protraction)

      

Serratus Anterior

      

Movements of the Shoulder Complex

• Movements of the Scapula
– Adduction (Retraction)

      

Trapezius

Rhomboid major and minor

      

Movements of
the Shoulder Complex

• Movements of the Scapula

– Elevation

Levator Scapulae

Trapezius

Rhomboid major and minor

Movements of
the Shoulder Complex

• Movements of the Scapula

– Depression

      

Trapezius

Serratus Anterior

      

Movements of
the Shoulder Complex

• Movements of the Scapula

– Upward rotation

      

Trapezius

Serratus Anterior

      

Movements of
the Shoulder Complex

• Movements of the Scapula

– Downward rotation

Levator Scapulae

      

Scapulohumeral Rhythm

  • A regular pattern of scapular rotation that accompanies and facilitates humeral abduction
  • Scapulothoracic : glenohumeral joint movements = 1:2

    

      

Composite Shoulder Girdle Movement

Shoulder Goniometry

Flexion / Extension – 180° / 50°-60°

Abduction/Adduction – 180° / 45°-50°

Internal/External Rotation

70°-80° / 80°-90°

Horizontal Abduction / Horizontal Adduction

– 45° / 120°

     

Movements of the Shoulder Complex

Movements of Glenohumeral joint

– Flexion

               

Deltoid

Coracobrachialis

Pectoralis major

Biceps brachii

   

   

Movements of
the Shoulder Complex

• Movements of Glenohumeral joint

– Extension

  

Teres major

Deltoid

Triceps brachii

Latissimus Dorsi

      

Abduction Adduction

Movements of
the Shoulder Complex

Abduction     &     Adduction

Abduction

• Movements of Glenohumeral joint

– Abduction

  

Deltoid

Supraspinatus

      

Adduction

• Movements of Glenohumeral joint

– Adduction

  

Pectoralis major, Teres major, Latissimus dorsi

Movements of the Shoulder Complex

      

External Rotation

• Movements of Glenohumeral joint

– Medial rotation

  

Infraspinatus

Teres minor

      

Internal Rotation

• Movements of Glenohumeral joint

– Medial rotation

  

Internal Rotation

The body limits internal rotation – thus placing the arm behind the body increases the amount of internal rotation

           

   

Subscapularis

Teres major

Latissimus dorsi

Pectoralis major

  

Movements of the Shoulder Complex

  

Biomechanics of GH Stability

The normal shoulder constrains the humeral head to the center of the glenoid cavity throughout most of the arc of movement

– Static restraints
– Dynamic restraints

Static restraints

  • Glenoid labrum and articular surfaces
  • Negative intra-articular pressure
  • Ligaments and capsule
  • Joint fluid adhesiveness

              

Dynamic restraints

  • Rotator cuff muscles
  • Deltoid and biceps
  • Concavity compression

      

Rotator Cuff

      

Stability Ratio

  • Maximal displacing force in a given direction(perpendicular to glenoid center line) that can be stabilized by compressive load
  • Affected by
    – Glenoid/labrum depth
    – rim lesions
    – Glenoid version
    – Dynamic stabilizer compromise

Load on the Shoulder

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