Shoulder Anatomy

shoulder-anatomyJoints

The shoulder girdle consists of:
• Glenohumeral joint
• Acromioclavicular joint
• Scapulothoracic articulation

 

Glenohumeral Joint (GHJ)

The joint is formed by the articulation between the head of the humerus and the glenoid fossa of the scapula. It is a synovial ball and socket type joint. Both articular surfaces are covered by hyaline cartilage. The glenoid fossa is little more than 1/3 of the humeral head so the socket is deepened to by the glenoid labrum. The outer margin of the labrum provides attachment to the long head of Biceps superiorly, with the tendon of the long head of Triceps partly arising from the inferior labrum. The GHJ capsule is surrounded by a loose fibrous capsule. It is thick and strong in some parts, particularly anteriorly, where it is strengthened by the 3 glenohumeral ligaments. However due to the laxity of the capsule it provides little stability to the GHJ.

The GHJ has both capsular and accessory ligaments.

Capsular ligaments:
• Superior glenohumeral ligament
• Middle glenohumeral ligament
• Inferior glenohumeral ligament
• Transverse humeral ligament

Accessory ligaments:
• Coracohumeral ligament
• Coracoacromial ligament

The blood supply to the GHJ is by branches from the suprscapular branch of the Subclavian artery, the acromial branch of the thoracoacromial artery and branches from the anterior and posterior circumflex humeral arteries.

The GHJ is almost completely surrounded by muscles, which protect the joint by helping to suspend the upper limb from the pectoral girdle. These muscles are Supraspinatus, Infraspinatus, Teres Minor and Subscapularis – also known as the Rotator Cuff. Tendons of these four muscles blend closely with each other and the shoulder joint capsule. Their primary function is to centralise the humeral head, limiting superior translation during abduction, and counteracting the strong upward pull of the deltoid muscle.

Rotator Cuff Muscles

Supraspinatusanatomy_rotator cuff muscles 2

Proximal Attachment
Arises from supraspinous fossa of the scapula and from fascia overlying the muscle. The muscle converges on a tendon which passes under the acromion and above the shoulder
joint.

Distal Attachment
The tendon inserts on the upper facet of greater tuberosity of the humerus, and is separated from acromion, coracoacromial ligament, and deltoid by the subacromial/subdeltoid bursa.

Nerve Supply
Suprascapular nerve (C4,5,6).

Actions
Supraspinatus was once described as being active in the early phase of abduction only however it is clear that it has a role in external rotation, abduction and flexion of the shoulder. As part of the rotator cuff, it helps to compress the humeral head in the glenoid fossa throughout its range of movement.

Infraspinatus
Proximal Attachment
The infraspinous fossa of the scapula, and from the thick fascia overlying the muscle.

Distal Attachment
The tendon inserts on the middle facet of the greater tuberosity of the humerus. There is a bursa between infraspinatus and the neck of scapula.

Nerve Supply
Suprascapular nerve (C4,5,6).

Actions
It is a lateral rotator of the humerus, but it’s more important action is as a member of the rotator cuff, where it contributes to stability of the joint by compressing the humeral head into the glenoid fossa.

Teres Minor

Proximal Attachment
Teres minor arises from the upper two thirds of the lateral border of the scapula.

Distal Attachment
The lowermost of the three facets of greater tuberosity of the humerus.

Nerve Supply
Axillary nerve (C4,5,6).

Actions
Teres minor is an adductor and lateral rotator of the humerus, and helps to oppose upward subluxation of the humerus due to the powerful actions of deltoid, biceps brachii, and triceps. Its main action is as a member of the rotator cuff.

 

Subscapularis
Proximal Attachment
Subscapularis arises from the medial two thirds of the costal surface of the scapula, separated from the neck of scapula by subscapularis bursa.

Distal Attachment
It passes laterally, forming a broad tendon which inserts on the lesser tuberosity, shoulder joint capsule, and the front of the upper shaft of the humerus. Some of the superficial fibers blend with the transverse humeral ligament.

Nerve Supply
Subscapularis is innervated by both subscapular nerves (C5,6,7).

Actions
Subscapularis is an adductor and medial rotator of the humerus. It also resists anterior translation of the humeral head, and acts with the other cuff muscles to stablise the humeral head within the glenoid fossa.

Acromioclavicular joint

The Acromioclavicular joint connects the clavicle with the scapula. The articulation is between the lateral end of the clavicle and the anteromedial border of the acromion process. Both joints surfaces are covered with fibrocartilage. A wedge-shaped fibrocartilaginous articular disc is present and compensates for some of the incongruity between the articular surfaces. It is a synovial joint and has a relatively loose fibrous capsule.The capsule is thickest and strongest above, where it is reinforced by the fibres of Trapezius.
The capsule is reinforced by 2 ligaments. These are:

Capsular ligaments
• Superior acromioclavicular ligament
• Inferior acromioclavicular ligament

Accessory ligament
• Coracoclavicular ligament

Scapulothoracic articulation

scapula on skeletonThe scapulothoracic articulation is formed by the convex surface of the posterior thoracic cage and the concave surface of the anterior scapula. The scapula is a flat bone, with the gliding surfaces formed by the subscapularis and the serratus anterior. The scapulothoracic articulation allows for increased shoulder elevation. For every 2° of glenohumeral elevation, there is 1° of scapulothoracic elevation. The structure of the scapulothoracic articulation can be divided into three layers: superficial, intermediate, and deep. The trapezius and latissimus dorsi muscles and an inconsistent bursa between the latissimus and inferior angle of the scapula comprise the superficial layer. The intermediate layer is composed of the rhomboid major and the rhomboid minor, the levator scapulae muscles, and the spinal accessory nerve and bursa between the superomedial scapula and trapezius muscle. The spinal accessory nerve travels closely along this bursa at an average of 2.7 cm lateral to the superomedial scapular angle. Finally, the deep layer consists of the serratus anterior and subscapularis muscles and the corresponding scapulothoracic and subscapularis bursa.