A Concise Look at Shoulder Instability
Patients with a history of shoulder dislocations are frequently seen in sports medicine clinics. The most common sport in which the initial dislocation episode occurs is football (Matthew Kraeutler, 2018). However, wrestling and hockey also report high numbers of glenohumeral instability events (Brett Owens J. D., 2012). The term ‘glenohumeral instability’ encompasses a large spectrum of disease from joint subluxation to a complete dislocation (Brett Owens J. D., 2012).
When considering shoulder instability, knowledge of the anatomy of the shoulder complex is necessary. The glenohumeral joint is the articulation between the humeral head the shallow glenoid of the scapula (Mologne, 2010). There are both static and dynamic stabilizers of the shoulder. The dynamic stabilizers of the shoulder include the deltoid muscle, the long head of the biceps, and the rotator cuff (Elizabeth Cody, 2014). The static stabilizers are the glenoid labrum and glenohumeral ligaments (Mologne, 2010). The glenoid labrum should be thought of as the bumpers on the the edge of the glenoid. They are small triangularly shaped structures that act to prevent translation of the humeral head (Mologne, 2010). They also serve to deepen the glenoid cavity (Peter Millett, 2006). The joint capsule is a 1-4.5mm thick capsule that is made up of linked collagen fibers (Mologne, 2010). There are then an inferior, middle, and superior glenohumeral ligaments which also serve as stabilizers of the joint and are found within the joint capsule (Mologne, 2010, Peter Millett, 2006). Each glenohumeral ligament acts to stabilize the humeral head in different arm positions (Peter Millett, 2006).
Ultimately, the term shoulder instability describes a humeral head which is not remaining in the glenoid fossa due to an injury or defect in one of the dynamic or static stabilizers (Leslie Fink Barnes, 2017). There have been multiple changes in classification of shoulder instability. In the past they were classified based on if the event was a dislocation or subluxation or if the instability was traumatic, atraumatic, or due to repetitive microtrauma. For the purpose of this review we will be using the direction of instability for classification. This includes anterior, posterior, inferior, and multidirectional (Edward McFarland T. K., 2003).
Prior to evaluating each direction for instability, an evaluation for general ligamentous laxity should be performed. An article published in 1973 in the Annals of the Rheumatic Diseases created the Beighton Score. Patients are taken through a variety of maneuvers to assess ligamentous laxity. These include dorsiflexion of the fifth digit past 90 degrees, apposition of the thumb to the forearm, hyperextension of the elbow greater than 10 degrees, hyperextension of the knees greater than 10 degrees, and flexion of the trunk so that hands can lay flat on the floor. Then the patient is given a score between zero and nine. Historically scores greater than 4 or 6 are considered positive for joint laxity (P. Beighton, 1973). The sensitivity of this scoring system has been called into question. The MOON Cohort found that only 13% of their study population were found to have a Beighton score >4 (Matthew Kraeutler, 2018).
Image 1. AP view of the right shoulder demonstrating anteroinferior shoulder dislocation.
Image 2. AP view of the right shoulder demonstrating a posterior shoulder dislocation. Note this is a subtle finding and the so-called “lightbulb sign”
After an initial dislocation event, three view radiographic images (AP, scapular Y, and axillary) are typically obtained (Matthew Milewski, 2013). According the American College of Radiology, a noncontrast MRI is as good as a CT scan in evaluating for glenoid and humeral head bone loss if a Bankart or Hill Sachs lesion is seen on the initial xray (Behrang Amini, 2018). In an acute shoulder, a joint effusion can act as intra-articular contrast to better evaluate the soft tissue structures (Behrang Amini, 2018). However, according to the ACR for subacute and chronic dislocations an MRI arthrography is preferred (Behrang Amini, 2018).
Inferior Instability
The primary restraints of the glenohumeral joint inferiorly is the superior glenohumeral ligament and the coracohumeral ligament when the shoulder is adducted and the inferior glenohumeral ligament when the shoulder is abducted (Edward McFarland J. B., 2017).
Image 3. Demonstration of sulcus sign for inferior instability (courtesy of researchgate.net)
Evaluating for a sulcus sign is the test of choice when evaluating for inferior instability. The provider will apply a downward traction to the upper arm and assess for the amount of depression between the acromion and humeral head. This can be classified as Type I through III. Type I is less than 1cm, type II is between 1 and 2 cm, and type III is more than 2cm (KJ Hippensteel, 2018). The provider should also be evaluating for pain and provoking the feeling of instability with maneuver (Edward McFarland J. B., 2017). In order to stress the inferior glenohumeral ligament there is also the hyperabduction test described by Gagey (Edward McFarland J. B., 2017).
Anterior shoulder instability
Based on the MOON shoulder instability cohort, anterior shoulder instability is the most common direction of instability (Matthew Kraeutler, 2018). Men account for ¾ of the anterior shoulder instability population (Matthew Kraeutler, 2018). A study done at the US Military Academy found that in 97% of the athletes who had an initial shoulder dislocation experienced an anteroinferior glenoid labrum avulsion (Bankart lesion) (Brett Owens J. D., 2012). Damage to the glenoid rim will decrease the depth of the glenoid cavity and can put the shoulder at higher risk of re-dislocation (Dana Piasecki, 2009). Several studies have tried to look at how much glenoid bone loss needs to occur before a shoulder is unstable. Different thresholds ranging from 21 to 30% have been proposed (Dana Piasecki, 2009).
Image 4. Illustration of apprehension test for anterior shoulder instability (courtesy of shoulderdoc.co.uk)
Multiple studies have demonstrated that in young athletes <35 years old, there is a high rate of recurrence after initial episode of anterior dislocation (Brett Owens J. D., 2012). However, in our older population it is important to rule out concomitant rotator cuff tears, as the incidence of rotator cuff tears with a dislocation increases as we age (Christopher Dodson, 2008).
Typical nonsurgical management includes immobilization in a sling for the first three to ten days (Brett Owens J. D., 2012). There are currently no evidence based guidelines on when to initiate physical therapy. The overall trend is for early range of motion exercises (Brett Owens J. D., 2012). The athlete will then be taken through a progressive strengthening program that focuses on the glenohumeral stabilizers and scapula stabilizers (Brett Owens J. D., 2012). If a patient regains full motion and strength and no longer has instability symptoms it can be argued that they can return back to play within the same season (Brett Owens J. D., 2012). This typically occurs at 3 weeks (Brett Owens J. D., 2012).
The two types of procedures in repairing recurrent anterior instability are capsulolabral repairs or bony augmentation (Guillaume Dumont, 2014). Typically for an anterior shoulder dislocation the labrum will be repaired and a capsular plication will be performed to tighten the capsule (Brett Owens J. D., 2012). One of the most common procedures currently is the Latarjet procedure. This involves transposition of the coracoid to the anteroinferior portion of the glenoid rim and repair of the anterior capsule (Mithun Joshi, 2015).
An antero-inferior glenohumeral dislocation can also lead to the development of a Hill Sachs lesion. A posterosuperior defect on the humeral head can occur when the humeral head hits the glenoid rim (Matthew Provencher, 2012). Small lesions can be managed nonoperatively but large lesions that encompass >20% of the humeral head surface should be evaluated for surgical repair (Matthew Provencher, 2012).
Posterior shoulder instability
Episodes of posterior shoulder instability can be seen in blocking lineman and during bench presses (Brett Owens S. C., 2013). There are far less episodes of posterior instability compared to anterior instability with posterior instability accounting for only 10% of all instability events (Brett Owens S. C., 2013). One risk factor identified is glenoid retroversion where each degree of retroversion leads to an increase in the risk of posterior shoulder instability (Brett Owens S. C., 2013). Compared to the other muscles of the rotator cuff the subscapularis provides the most posterior support (Peter Millett, 2006).
Image 4. Demonstration of load and shift test (courtesy of researchgate.net)
The load and shift test can assess for both anterior and posterior instability. The physician will grasp the proximal humeral head and use the other hand to stabilize the shoulder and scapula. Then the humeral head is translated anteriorly and posteriorly and the physician is assessing the amount of translation (KJ Hippensteel, 2018). The test should be done at 0, 45, and 90 degrees in abduction. Injury of the inferior glenohumeral ligament will allow more laxity with the higher degrees of abduction. Alternatively the jerk test can be used to assess for posterior shoulder instability which involves translating a posterior force through an abducted and internally rotated shoulder and assessing for subluxation or pain (KJ Hippensteel, 2018).
Most patients with posterior shoulder instability can be treated nonoperatively (Peter Millett, 2006). Physical therapy will focus on developing the rotator cuff and muscular stabilizers to try and compensate for a damaged labrum or glenohumeral ligament (Peter Millett, 2006). Surgery can be considered in patients who fail physical therapy. The same surgical principles remain for posterior instability surgical repair including labrum repair and capsular tightening (Peter Millett, 2006). One of the most common risks following surgical repair is loss of internal rotation (Peter Millett, 2006).
Multidirectional shoulder instability
The treatment of choice is typically nonoperative management. The goal is to repair the static or dynamic structure that is deficient (Trevor Gaskill, 2011). This may include rotator cuff strengthening, scapular stabilization, and proprioceptive exercises working on glenoid positioning (Trevor Gaskill, 2011). Those patients that continue to have instability symptoms despite physical therapy may benefit from operative repair (Trevor Gaskill, 2011).
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