Rockwood Adults CH34

1088

SECTION TWO • Upper Extremity

Stabilizing Effect of Joint Compression

to the neutral position. With the arm in external rotation, the insertion of the deltoid moves more posteriorly in relation to the glenohumeral joint, and thus contraction at this position will produce a posteriorly directed compressive force and ten- sioning to reduce anterior instability. Kido et al. 117 also showed that with the capsule intact, anterior displacement is signifi- cantly reduced by application of load to the middle deltoid. However, with a simulated Bankart lesion, loading of each mus- cle portion significantly reduced anterior displacement. Thus, the stabilizing function of the deltoid becomes more essential as the shoulder becomes unstable. PROPRIOCEPTION Placement of the upper extremity and hand in space for daily function is dependent on the perception of the shoulder joint position in space and during motion. Capsule and ligaments function in joint stabilization by providing neurologic feed- back that directly mediates joint position sensibility and muscle reflex stabilization. This sensory modality is called propriocep- tion and is mediated by receptors in the muscular and cutane- ous structures of the shoulder joint. Specialized nerve endings and proprioceptive mechanoreceptors (Pacinian corpuscles, Ruffini endings, Golgi tendon endings, etc.) have been shown to exist in the capsule and ligaments. 82,237 Stimulation of these mechanoreceptors results in muscle contraction around the joint that in turn results in compressional forces which function as an adaptive control for joint stabilization to counteract sud- den movements in acceleration or deceleration. 251 It has been hypothesized that the receptors in the joint capsule respond to extremes in ROM or deep pressure that may occur because of glenohumeral translation. 46,77,78 Both Warner et al. 251 and Lephart et al. 131 have shown that the proprioception of the shoulder joint is disrupted in patients with glenohumeral instability compared to the asymp- tomatic shoulders. Zuckerman et al. 280 reported that patients after open anterior stabilization procedure had 50% improve- ment of proprioceptive ability at the 6 months postsurgery time. This improved to 100% or similar to the contralateral shoulder at the one-year mark. Overall, the literature suggests that patients with recurrent shoulder instability will have a perceivable deficit in glenohumeral proprioception, which can be restored to normal after surgical repair or reconstruc- tion. Capsuloligamentous structures may provide additional contributions to shoulder stability by providing the afferent feedback to reflexive muscle contraction of the rotator cuff, biceps, or deltoid.

Figure 34-29.  The rotator cuff muscle is responsible for the “concavity compression” in which activation of the rotator cuff results in com- pression of the humeral head into the glenoid cavity and stabilizing it against translational forces.

Many investigators have studied the contribution of the biceps tendon to glenohumeral stability. The origin of the long head of the biceps tendon arises directly from both the supraglenoid tubercle and the superior glenoid labrum. Most of the attach- ment on the labrum is posterior in orientation. 238 Itoi et al. 106 evaluated the stabilizing effect of the biceps tendon in a cadaver model and found that both the long and short heads of the biceps have similar roles in preventing anterior shoulder insta- bility with the arm in abduction and external rotation. Their role is further increased as the intrinsic shoulder stability decreases (capsule tear or Bankart lesion). Furthermore, the biceps becomes more important than the subscapularis in ante- rior stability as the stability from the capsuloligamentous struc- tures decreases. 108 DELTOID MUSCULATURE The deltoid muscle is a large triangular, bulky muscle which contributes to approximately 20% of all shoulder muscles and comprises three portions, anterior, middle, and posterior. 15 Morrey et al. 160 proposed the four essential muscle dynamic sta- bilizing effects contributing to shoulder stability. This includes passive tension from the muscle bulk, muscle contraction that results in compression of the humeral head on the articular surface, joint motion that tightens the passive ligaments of the shoulder, and the barrier effect of the contracted muscle. Using a dynamic stability index, Lee and An 129 demonstrated that the middle and posterior deltoid provided more stability by gen- erating greater compressive forces and lower shear forces than the anterior deltoid. Furthermore, the deltoid muscle produces more compressive force when the arm is elevated compared

TREATMENT OPTIONS FOR GLENOHUMERAL INSTABILITY

Treatment of shoulder instability requires a thorough under- standing of the natural history. Most of the available literature documenting the natural history of shoulder instability focuses on anterior instability. Operative treatment as well as nonop- erative treatment of anterior shoulder instability has been well studied, especially in young athletic populations. Controversy