SHOULDER INSTABILITY

Burton F. Elrod, M.D.,
Nashville, TN

The shoulder is probably the most complex joint in humans. It achieves a delicate balance between mobility and stability. Glenohumeral stability is achieved through both passive (static) or active (dynamic) restraints.

Passive Restraints

     The capsule, ligaments and labrum

     The geometry of the humeral and glenoid articular surfaces

     Compliance of the articular cartilage covering these surfaces

     The phenomena of limited joint volume in adhesion-cohesion

     Restraint provided by gross bony geometry

     The coracoacromial ligament

Active Restraints (Musculature)

The rotator cuff and other shoulder musculature exert forces on the glenohumeral joint. The complex integration of these restraints is such that the glenohurneral joint has a high degree of mobility yet is stable during a wide spectrum of functional situations.

Bony Geometry

     Humerus - The humeral head is spherical with thick articular surface in the middle and is thinner at the periphery. The neck shaft angle is approximately 130 degrees - l40 degrees. The head is in 30 degree retroversion relative to the trans-epicondylar line.

     Scapula - The scapula has a slight upper tilt of 3 degrees to 5 degrees, 30 degrees forward relative to the transverse plane.

     Glenoid Fossa - The glenoid fossa averaged 7 degree retroversion and 5 degree superior tilt. There is conflicting data on whether glenoid retroversion is significant in instability.

Glenoid Fossa

The glenoid is shallow, pear-shaped, relatively flat with articular cartilage thinner at the center and thicker at the periphery. It is small and approximately one-third the size of the humeral head. Only a portion of the humeral head can be articulated with the glenoid fossa in any given position.

Glenoid Labrum

The glenoid labrum is a fibrocartilaginous ridge shaped rim circumscribing the glenoid. It is attached to the periphery of the articular margin of the glenoid fossa. It is the primary attachment for the glenohumeral ligaments. Superiorly, the long head of the biceps is in continuance with the labrum. It gives off two fascicles to blend with the fibrous tissue of the labrum. The labrum has significant anatomical variability around the periphery. While it may be loosely attached superiorly, it is consistently tightly attached to the glenoid articular cartilage inferiorly. The labrum increases the depth of the glenoid by 50%. It provides a firm area of attachment for the biceps tendon and the inferior glenohumeral ligaments. The glenoid articular surface and the labrum combine to create a socket that is approximately 9mm deep in the superior inferior direction and 5mm deep in the anterior posterior direction. The labrum has been compared to a chop block which is against the wheel of a vehicle. The radius of curvature of the humeral head and glenoid are very similar.

Vacuum Effect

A layer of synovial fluid less than 1mm thick is present in a normal glenohumeral joint. The joint volume is fixed. Distraction of the joint surface increases the volume and decreases the articular pressure. A slightly negative intraarticular pressure exists in a normal shoulder and aids in centering the humeral head. It may provide strength to pathological translations. Less force is required to translate the humeral head anteriorly to posteriorly in shoulders that have been vented to the atmosphere.

Sublabral Hole Variation

It is present in approximately 10 – 19% of patients.

Coracohumeral Ligament

It is a strong band of capsule tissue. Its origin is at the base and lateral portion of the coracoid process. It blends laterally with the attachment of the rotator cuff to the greater tuberosity. Its function appears to restrict inferior subluxation. In chronic rotator cuff tears, a contracted coracohumeral ligament can make advances of the rotator cuff quite difficult. A retracted and scarred coracohumeral ligament will prevent, external rotation.

Glenohumeral Ligaments and Capsule

While the posterior aspect of the joint capsule is thin and featureless, the anterior capsule is marked by thickenings of the glenohumeral ligaments.

Superior Glenohumeral Ligament

It arises anteriorly to the origin of the biceps tendon and inserts in the fovea capitis just superiorly of the lesser tuberosity in greater than 90% of the specimens. It functions by providing minor contributions to inferior stability.

Middle Glenohumeral Ligament

The MGHL begins at the anterior humeral neck medial to the lesser tuberosity. It crosses the subscapularis tendon at a 60 degree angle and attaches to the neck of the glenoid immediately below the articular surface in the upper one half. It is absent in approximately 20 % of the shoulders. Its function is to be the primary stabilizer with the subscapularis for anterior stability at 45 degree abduction and also limits external rotation at mid-abduction with the inferior glenohumeral ligament.

Inferior Glenohumeral Ligament Complex

It consists of an anterior band, the axillary pouch and a posterior band. It functions like a hammock to support the humeral head and its the main static stabilizer of the abducted shoulder. In internal rotation, the posterior band of the IGHLC tightens and with the axillary pouch supports the humeral head. In external rotation, the opposite occurs and the anterior band tightens and supports the humeral head anteriorly. It takes its origin from the labrum and neck of the glenoid adjacent to the labrum. It inserts the anatomical neck of the humerus.

Posterior Capsule

The posterior capsule is thin and featureless as compared to the posterior capsule but appears to be one of the primary restraints to posterior translation.

Normal Variations in the Glenohumeral Ligament

Sixty-six percent make up group I (classic arrangement with well defined SGHL, MGHL and IGHL). Group II includes seven percent which have confluent MGHL and IGHL with no separation between the two. Group III includes 19% cord like MGHL with a high running attachment and a foramen below. Group IV includes 8% with no discernable MGHL, IGHL but were confluent with an anterior capsular sheath.

Coracoacromial Ligament

It consists of two bands arising from the coracoid process to a certain single band in the anterolateral portion of the acromion. It is important in impingement but also is felt to prevent superior migration of the humerus and rotator cuff tears.

Dynamic or active (stabilizers) of the rotator cuff

It is a functional musculotendinous unit arising from both anterior and posterior aspects of the scapula to insert in a circumferential fashion at the lateral aspect of the proximal humerus. The major muscles include the subscapularis. supraspinatus. infraspinatus and teres minor.

Subscapularis

The subscapularis arises from the subscapularis fossa at the anterior aspect of the scapula and a; the broad tendon Insertion of the lesser tuberosity of the humerus. It is intimately related to the anterior shoulder joint capsule on its deep surface. The primary primary the subscapularis to provide internal rotation Secondly, it restrains the anterior humeral displacement of the glenoid In addition to its ability to center the head the glenoid, it is also intimately associated with anterior stabilization of the shoulder because of its connection of the anterior capsule.

Supraspinatus

The supraspinatus arises from the wall of the supraspinatus fossa and passes laterally and forward. It passes underneath the coracoacromial arch to insert its broad tendon in to the greater tuberosity of the humerus Location makes it vulnerable to impingement between the humerus and the coracoacromial arch as a common side of impingement in calcific tendinitis. The supraspinatus is active during the entire arc of scapular abduction.

Infraspinatus

Infraspinatus arises from the Infraspinatus fossa. It inserts into the tendon of the greater tuberosity. The infraspinatus blends into the posterior capsule.

Teres Minor

It rests on the mid-portion of the lateral border of the scapula and inserts into the most inferior facet of the greater tuberosity. It is primarily the external rotator of the humerus and is also active during abduction.

Function

The rotator cuff muscles actually pull the humeral head toward the center of the glenoid. Because the glenoid acts as a shallow cup, the humeral head will be centered with force perpendicular to the cup. The rotator cuff muscles actually pull to the face of the glenoid adding to the stabilization on the glenoid. It is felt that the posterior musculature also decreases the strain on the anterior capsule.

Seventeen muscle units control the composite thorocoscapular humeral articulation. Muscles function both concentrically and eccentrically with accelerators, decelerators, stabilizers in the composite articulation. Active motion about the shoulder is not simply a matter of muscle activity or a problem of percentages. Active motion involves a system of forced couples as the basis for motor control.

Dynamic stabilization of the humeral head on the glenoid, of the scapula and on the thorax is a fundamental prerequisite for normal active range of motion and function. Loss of this coordinated muscular action with a concomitant imbalance between agonists and antagonist muscles may render the joint dysfunctional and possibly unstable.

1. Anterior deltoid

2. Middle deltoid

3. Posterior deltoid

4. Pectoralis major

5. Latissimus dorsi

6. Long head biceps

7. Teres major

8. Supraspinatus

9. Infraspinatus

10. Subscapularis

11. Teres minor

12. Upper trapezius

13. Middle trapezius

14. Lower trapezius

15. Serratus anterior

16. Rhomboid group

17. Levator scapula

Trapezius

The upper and lower components participate in a scapular rotary couple to assist upper rotation. The entire muscle must be effective to enable to maximum scapular rotation. The serratus anterior and the upper and lower halves of this muscle are both included in this upper lower component of the scapular couple.

Serratus Anterior

During throwing, the serratus is the primary controller for the scapula. It provides a stable glenoid which serves as a pure platform of the humeral head