Question #49

The shoulder joint is a highly mobile joint capable of abduction, adduction, flexion, extension and rotation. Considering abduction, which of the following statements is the most accurate?

Deltoid initiates abduction

The action comes entirely from the gleno-humeral joint.

Serratus anterior and trapezius are required to complete abduction

Medial rotation of the humerus is required to give full range of abduction

The nerve roots important in shoulder abduction are C6,7

The action of shoulder abduction has several different stages:

1. Initiation of abduction is by supraspinatus (deltoid on its own will tend to just lift the shoulder).
2. The first 30 degrees is due to the glenohumeral joint.
3. Deltoid is considered the principal abductor of the shoulder once the action has been initiated.
4. Infraspinatus, subscapularis and teres minor help by holding the humeral head down.
5. At 90 degrees the humeral head lies in maximum contact with the glenoid fossa. To allow further abduction the humeral head needs to be laterally rotated to increase the articular surface contact area.
6. From 30 degrees onwards scapular rotation occurs.
7. The glenohumeral joint alone can only achieve 120 degrees of abduction.
8. Serratus anterior and trapezius control scapular rotation.
9. The root value that supplies the deltoid muscle for shoulder abduction is C5.
10. As the supraspinatus tendon passes over the humeral head and inserts into the greater tuberosity it is at risk of compression under the coracoacromial arch during abduction. This compression can occur between 60 degrees and 120 degrees and will give a 'painful arc'

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