Rockwood Adults CH34

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CHAPTER 34 • Glenohumeral Instability

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Figure 34-28.  A: Arthroscopic view of the shoulder shows the superior glenohumeral ligament (SGHL), middle glenohumeral ligament (MGHL) anterior labrum, and the subscapularis tendon. B: Alternative arthroscopic view showing the MGHL, anterior inferior band of the IGHL (AIGHL), and subscapularis muscle. C: Close arthroscopic view of the AIGHL and anterior labrum. D: Posterior view of the posterior labrum and posterior inferior glenohumeral ligament (PIGHL).

taut and moves beneath the humeral head to prevent anterior translation. However, with internal rotation and abduction, the IGHLC functions to limit posterior translation. 132,149

head into the glenoid cavity stabilizes it against translating forces (Fig. 34-29). Stability was greater in the hanging arm position compared with arm abduction-external rotation under the concavity-compression mechanism. 83 This indicates that the effect of concavity-compression may be an important sta- bilizer of the glenohumeral joint in the mid-ranges of motion when the capsuloligamentous structures are lax. When the arm is in the extremes of motion, the capsuloligament struc- tures are stretched to enhance their contribution to stability. Warner et al. 252 further demonstrated that rotator cuff muscle strength differs in patients with shoulder instability compared to normal. McMahon et al. 150 have also shown significantly reduced EMG activity in the supraspinatus muscle from 30 to 60 degrees of abduction in patients with anterior shoulder instability. In a dynamic shoulder model, 50% reduction in the rotator cuff forces resulted in increased anterior displacement by 46% and posterior displacement by 31%. 268

DYNAMIC STABILIZERS Rotator Cuff Musculature and Biceps Tendon

The rotator cuff musculature is comprised of the supraspi- natus, infraspinatus, teres minor, and subscapularis muscles. Contribution of the rotator cuff muscle group to glenohu- meral stability occurs through three distinct mechanisms: (1) joint compression, (2) coordinated contraction of the rota- tor cuff muscle to guide the humeral head onto the center of the glenoid, and (3) dynamization of the glenohumeral ligament with shoulder ROM through the rotator cuff attach- ments. 200,229,248,268,270 Lippitt et al. 135 first described the effect of “concavity-compression” where compression of the humeral