Rockwood Adults CH34

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CHAPTER 34 • Glenohumeral Instability

sia. 135 The pathoanatomy of MDI differs from both anterior and posterior instability and typically involves a large, patulous infe- rior capsule in both the anterior and posterior directions. This significantly increases the volume of the capsule. The extent of involvement of the rotator interval is a topic of debate in patients with MDI. Biomechanical studies on cadavers have demon- strated that the inferior capsule and the rotator interval are the primary restraints to inferior glenohumeral translation. 84,249 The inferior capsule is responsible for resisting inferior translation primarily with arm abduction to 90 degrees, while the rotator interval resists inferior translation with the arm at the side. The glenohumeral joint is composed of a large spherical humeral head that articulates with the smaller glenoid surface. The artic- ular geometry contributes minimally to the overall stability of the glenohumeral joint due to the small area of the glenoid sur- face relative to the large humeral head and the relative mismatch of the bony curvature of the glenoid to the humeral head. 27,200 The shape of the glenoid is smaller superiorly and larger infe- riorly, much like a “pear” configuration and produces a signif- icant surface area and radius of curvature mismatch between the joint surfaces of the glenoid and the humeral head. Further- more, unlike the hip joint, the glenoid does not constrain the humeral head as only up to 25% to 30% of the humeral head is in contact with the glenoid at various shoulder ROM. 220,221 Although the subchondral bone on the glenoid side is flatter than the humeral head, recent studies have demonstrated that the articular surface of the glenoid is highly congruent to the articular surface of the humeral head. Kelkar et al. 115 reported the average radii of curvature of the humeral head and glenoid articular surfaces were 25.5 ± 1.5 mm and 27.2 ± 1.6 mm, respectively. Thus, the mismatch in the articular cartilage in the glenoid and humeral head increases the conformity of the overall glenohumeral joint to within 3 mm. Furthermore, the glenoid concavity is deepened by the labrum that is attached circumferentially around the glenoid on the outer rim. 97 Biome- chanical studies have demonstrated that joint conformity con- tributes more in controlling translation during active motions, whereas capsular constraints become more important during passive motions. 112 In terms of humeral version, there is mini- mal evidence that abnormal version contributes significantly to glenohumeral instability. 242 Glenoid Labrum The labrum is a fibrocartilaginous bumper that forms a cir- cumferential ring around the glenoid and serves as an anchor- ing point for the capsuloligamentous structures (Fig. 34-27). Attachment to the articular cartilage occurs via a narrow fibro- cartilaginous transition zone, but it is otherwise fibrous through- out the entire structure. 97 It is loosely attached superiorly above the equator and significant individual anatomic variabil- ity exists in this particular region. 51 In contrast, the anterior- inferior labrum is intimately attached to the glenoid rim and any detachment indicates an abnormality. 132 The essential STATIC STABILIZERS Articular Geometry and Concavity

Anterior view

Posterior view

Long head of biceps tendon SGHL

Supraspinatus tendon

Rotator Interval

Teres minor tendon Infraspinatus tendon Capsule Posterior band of IGHL Posterior axillary pouch of IGHL

MGHL Subscapularis

Anterior band of IGHL

Anterior axillary pouch of IGHL

Long head of triceps tendon

Figure 34-27.  The shoulder joint relies on both static and dynamic structures to maintain stability. The anatomy and structures of the gle- nohumeral joint is illustrated in this image.

and static factors, proprioception also plays a significant role in the pathoetiology of shoulder instability. 251 Proprioception is the perception of motion of the joint, and it is an important mechanism by which the muscles receive a message to contract and guard against instability. A failure of proprioceptive feed- back may contribute to instability. Biomechanically, Warner et al. 249 showed that the primary restraint to inferior translation of the adducted shoulder is the superior glenohumeral ligament (SGHL). 88 With progressive abduction of the arm, the anterior and posterior-inferior GHL became the main static stabilizers in resisting inferior translation. Furthermore, the anterior portion of the inferior GHL was the pri- mary restraint with the arm in 45 degrees of abduction and the posterior portion was the primary restraint with the arm in 90 degrees of abduction. Additionally, Warner et al. 249 also showed that venting of the shoulder capsule resulted in significant inferior translation of the humeral head via the loss of the inferior restraint and the vacuum effect. Thus, the so-called “sulcus sign” is believed to be the result of intra-articular vacuum effect and capsular laxity. Patients with MDI have demonstrated a loss of proprio- ception compared with normal controls, 131 further confirming that MDI may play a role in stability. The rotator cuff muscu- lature specifically provides compression of the humeral head against the glenoid. 135 As the glenoid socket is a lateral fossa within the scapula, the ability of the scapulothoracic muscula- ture to position the scapula can either optimize or impair gle- nohumeral stability. For this reason, scapulothoracic dyskinesia should always be evaluated in patients who present with MDI as these patients can frequently exhibit scapulothoracic dyskine-