Rockwood Adults CH34

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CHAPTER 34 • Glenohumeral Instability

MECHANISMS OF INJURY FOR GLENOHUMERAL INSTABILITY

direct work-up modalities and strategies for management. It is also important to document patient age at the time of the first instability event, number of dislocation and/or sublux- ation events, requirement for manual reduction and/or seda- tion in an emergency room setting, position of the arm during the instability event, and any prior nonoperative or surgical intervention. 116 Instability events that occur while at rest or while in positions not typically associated with risk of dislo- cation (i.e., with the shoulder in an adducted position) are particularly worrisome and can serve as a harbinger of more complex instability. Physical examination should consist of inspection, pal- pation, and range of motion (ROM) assessment (passive and active) with comparison to the contralateral shoulder. 139 Increased external rotation may imply anterior hyperlaxity, and asymmetric hyperabduction greater than 15 degrees differ- ence from the contralateral shoulder (Gagey test) with scapular stabilization may indicate incompetency of the inferior gleno- humeral ligament complex (IGHLC). 67 Neurovascular exam- ination is also necessary to exclude the presence of associated injuries, in particular the axillary nerve due to its tethered posi- tion and close proximity to the axilla. Resting scapular position and dynamic scapular motion throughout an overhead arc of shoulder motion should also be documented, as the presence of scapular dyskinesia or winging may contribute to the feeling of instability and may affect the timing of any operative treat- ment. Undiagnosed scapular winging may also lead to symp- toms of glenohumeral instability. 253 There are a multitude of provocative special tests for glenohumeral instability which are usually considered to be the most critical portion of the physi- cal examination and are discussed in the Signs and Symptoms section (see below).

Glenohumeral instability is typically related to a traumatic event that can occur at any age as a result of injury during athletic competitions and falls. While externally applied forces are the most common mechanism, noncontact or muscular imbalance events such as missed punches or seizures can also result in dis- location events. Individuals with generalized laxity or genetic collagen disorders may experience instability as the result of attritional injury to the joint capsule or via a low energy mecha- nism or muscular imbalance. In general, traumatic dislocations are classified by the direction, which can be anterior, posterior, or inferior. Depending on the patient factors (age, collagen laxity, and muscle strength) and degree of force imparted to the injured shoulder, dislocations will result in varying degrees of damage during a primary or repeat dislocation. Contact sport participa- tion, and in particular tackling or collision sports, represents the most common mechanism of injury for dislocation. 144,176 Anterior shoulder dislocations can result from either falls onto a forward flexed arm in external rotation (Fig. 34-1A) or tackling in collision sports, where the arm is extended and experiences a posteriorly directed force (Fig. 34-1B). Pos- terior shoulder dislocations can result from athletic injuries and falls, but seizures and electrocution also represent com- mon mechanisms. Seizures and electrocution may also result in a locked posterior dislocations due to the relative increased combined muscular mass of anterior internal rotator muscles (subscapularis, anterior deltoid, and pectoralis major) which overcome the posterior external rotator muscles (infraspina- tus, teres minor, posterior deltoid, and latissimus) acting on an internally rotated and adducted limb. Similarly, a fall onto

A

B C Figure 34-1.  A: Fall onto a forward flexed and externally rotated arm will result in anterior shoulder sub- luxation or dislocation. B: Tackling an opponent with the arm straight and extended may result in anterior shoulder instability, especially if a posteriorly directed force occurs. C: A fall onto a forward flexed and internally rotated arm can also result in a posteriorly directed force which creates a posterior force vector of the humeral head relative to the glenoid resulting in posterior shoulder instability.