Introduction to the Shoulder Complex (Part I) - What structures are involved in providing shoulder stability?

Introduction

The shoulder is the most mobile joint in the human body. A normal shoulder precisely constrains the humeral head to the centre of the glenoid cavity throughout most of the arc of movement. 

Anatomy

Glenohumeral joint (GHJ):

• Multiaxial ball and socket synovial joint
• Large ROM (range of motion) up to 180 deg with 3 degrees of freedom (flexion/extension, abduction/adduction, internal rotation/external rotation)
• Glenoid fossa is relatively shallow and provides little bony support for stability of the humeral head (humeral head = 3 x glenoid fossa; at most, only ~25% of humeral head is in contact with glenoid fossa in any given position)

Shoulder Stability

Shoulder stability is the result of a complex interaction between static and dynamic restraints.

• Static stabilisers (capsule, ligaments, labrum) act as concave structures to deepen the glenoid fossa.
• Dynamic stabilisers include rotator cuff and surrounding larger muscles that provide scapular stability (serratus anterior, lat dorsi, rhomboids, trapezius, pectoralis)

Static Stabilisers

1. Capsule 

The capsule is relatively large and loose, providing little contribution to joint stability. It allows for ~2-3mm of distraction and is strengthened by glenohumeral ligaments and rotator cuff tendons. The anterior capsule is thicker than posterior capsule.

- Medial attachment = margin of glenoid fossa

- Lateral attachment = circumference of anatomical neck of humerus

2a. Ligaments - Superior Glenohumeral Ligament (SGHL)

- Origin = tubercle on glenoid just posterior to long head of biceps

- Insertion = humeral head near upper end of lesser tuberosity

- Role = To limit posterior and inferior translation of the adducted humerus

2b. Ligaments - Middle Glenohumeral Ligament (MGHL)

- Origin = superior glenoid and labrum

- Insertion = proximal anterior humerus adjacent to lesser tuberosity

- Role = to limit external rotation at 45 deg abduction, limit inferior translation with arm by side

2c. Ligaments - Inferior Glenohumeral Ligament Complex (IGHLC)

The IGHLC acts like a sling and is the most important single ligamentous stabiliser in the shoulder (longest and strongest). There are 3 bands: anterior, axillary and posterior.

- Origin = anterior glenoid rim and labrum

- Insertion = inferior aspect of humeral articular surface and anatomic neck

- Role = primary static restraint against ant/post/inf translations when humerus is abducted past 45 deg

3. Glenoid Labrum

The labrum is a fibrocartilagenous rim which increases GHJ stability by ~20% and deepens the glenoid by ~50% (4.5-9mm superior/inferior; 2.5-5mm ant/post). Its role is to: increase surface contact area, provide support/reinforcement, and act as an attachment site for GHL (anterior) and biceps tendon (superior).

Dynamic Stabilisers

1. Rotator Cuff (RC)

Produces a synergistic action to create a compressive force at the GHJ during shoulder movements and maintains the humeral head in the glenoid throughout the entire ROM. RC tendons are tightly adherent to the underlying joint capsule. Contraction of the RC muscles may tighten the underlying capsule and also acts as a soft tissue barrier to excessive humeral head translation.

Kinematics:

- Normal = centre of humeral head deviates from centre of glenoid fossa by ≤0.3mm throughout Abd in scapular plane

- Abnormal = ~2.5mm superior migration of humeral head 

2. Scapula

Normal shoulder function requires a smooth integrated movement of glenohumeral, scapulothoracic, acromioclavicular, sternoclavicular joints (ie. scapulohumeral rhythm = SHR). Synchronous scapular rotation and humeral elevation are essential for maintaining optimal alignment of humeral head in glenoid fossa. Muscles controlling scapular rotation are traps (U/M/L), serratus ant (U/L), rhomboids, levator scap, pec minor.

Abnormal SHR is a result of:

- Weakness of RC ± scapular stabilisers

- Tightness/shortening of RC muscles 

- Involuntary adaptation to avoid a painful arc

Proprioception                                                                                     

Joint position and awareness plays an important role in coordinating muscular tone and control. Proprioceptive reflex mechanisms protect against excessive translations and rotations of the GHJ. Mechanoreceptors are present within static restraints (capsule, ligaments). Joint instability secondary to trauma leads to a decrease in proprioceptive reflexes and therefore providing a predisposition to recurrence.

In Part II of my overview on the shoulder complex, I will explain what happens with Shoulder Instability and implications for Management & Rehabilitation.