Anatomy: The Shoulder Joint

• The shoulder joint is part of the anatomy unit 182.
• The shoulder joint is a synovial ball and socket joint.
• The articulating bones of the shoulder joint are the head of the humerus and the glenoid cavity of the scapula.

Capsule

• The capsule surrounds the joint
• The capsule is thin and lax, allowing a wide range of movement.
• The capsule attaches medially to the margins of the glenoid cavity.
• The capsule attaches laterally to the anatomical neck of the humerus.
• The capsule attaches inferiorly and is redundant (weak), enclosing the upper part of the shaft of the humerus for 1.5 cm (weak point).
• The tendon of the long head of biceps is intracapsular.

Ligaments of the Shoulder Joint

• The labrum glenoidale is a fibrocartilage rim attached to edges of glenoid fossa
• The glenohumeral ligament has a superior, middle, and inferior band
• The coracohumeral ligament strengthens the upper part of the capsule.
• The transverse humeral ligament acts as a retinaculum for the long head of biceps.

Stability

• The capsule is strengthened by the tendons of several muscles:
• Subscapularis m.: anterior
• Supraspinatus m.: superior
• Infraspinatus m. & teres minor: posterior
• The shoulder joint is not stable due to:
• The head of the humerus being 3 times larger than the glenoid cavity
• A redundant capsule
• A wide range of movement
• It is the most mobile joint in the human body
• The main support comes from muscles around the joint (Rotator Cuff).

Movements

• Flexion is enabled by the Pectoralis major (clavicular head), Deltoid (anterior fibers), and Coracobrachialis & biceps (assist).
• Extension is enabled by the Latissimus dorsi, Teres major, and Deltoid (posterior fibers).
• Abduction is enabled by the Supraspinatus (0-15 degrees) and Deltoid (15-90 degrees).
• Adduction is enabled by the Pectoralis major, Latissimus dorsi, and Teres major.
• Medial rotation is enabled by the Pectoralis major, Latissimus dorsi & teres major, subscapularis, and Anterior fibers of Deltoid (assist)
• Lateral rotation is enabled by the Infraspinatus, Teres minor, and post fibers of Deltoid (assist).
• Circumduction is flexion, abduction, extension, and adduction in order.

Bursae

• Bursae are closed sacs or envelopes, lined with synovium and containing fluid, usually in areas subject to friction.
• Bursae in relation to the shoulder joint:
• Reduce friction between tendons, joints, capsules, & bone.
• They are liable to be inflamed following injury of rotator cuff muscles.
• Subscapularis bursa: between subscapularis tendon & capsule.
• Subacromial bursa: between deltoid, supraspinatus and capsule.
• Infraspinatus bursa: between infraspinatus tendon & capsule.

Clinical anatomy

• Supraspinatus (Impingement) syndrome occurs when a supraspinatus tendon is thickened by calcification or degenerative changes and becomes trapped underneath the acromion, impinging upon the subacromial bursa, when the arm is abducted.
• Pain from this supraspinatus, or impingement, syndrome occurs in an arc of motion between 60° and 120° of abduction.
• Dislocation is commonly dislocated due to freedom of movement and instability.
• Most common site of dislocation is anterior-inferior
• Causes: Fall on shoulder or excessive extension and lateral rotation of shoulder.

Case Scenario

• A right-handed 21-year-old college student visits his physician because of pain in his right shoulder that developed after starting a summer job on a construction crew 2 weeks ago.
• He explains that on his job site he regularly lifts heavy construction materials over his head. During physical examination, the patient experiences sharp pain in the range of 60 to 150 degrees of abduction at the glenohumeral joint.
• Possible diagnoses include:
• Infraspinatus tendonitis
• Supraspinatus tendonitis
• Acromioclavicular (AC) joint arthritis
• Degenerative arthritis of the shoulder
• Broken clavicle