Posterior Shoulder Laxity: More Common than we Thought?

Posterior instability is a relatively uncommon condition, typically reported in athletes who participate in contact or overhead sports.  It is reported to represent as little as 10% of all shoulder instability cases^(1,2). It can occur as a result of either acute trauma or repetitive micro trauma, and is commonly seen in athletes with multidirectional instability⁽¹⁾.

Posterior instability can be hard to diagnose clinically, and it often coexists with other shoulder pathologies. However, with increasing awareness of this condition, the incidence of posterior instability - particularly in the overhead athlete - is likely to be more common than previously thought.

Anatomy

The glenohumeral joint is the most mobile joint in the body. This mobility is integral to the performance of the overhead athlete but it often comes at the expense of stability. At any given time only about one third of the humeral head is articulating with the glenoid fossa⁽¹⁾. The glenoid labrum provides some passive stability by attaching onto the periphery of the glenoid fossa, and it effectively deepens the glenoid socket⁽¹⁾. The postero-inferior capsule and inferior glenohumeral ligament are the primary restraints to posterior instability at 90 degrees of abduction⁽¹⁾. Any injury to these structures, or abnormalities of bony morphology (glenoid hypoplasia, excessive humeral or glenoid retroversion or posterior glenoid erosion), can predispose an athlete to posterior instability⁽³⁾.

As well as the passive stabilisers in the shoulder (the rotator cuff muscles), the long head of biceps and the deltoid also provide dynamic stabilization of the shoulder. Contraction of these muscles compresses the head of the humerus into the glenoid and as a consequence, they become particularly important to stabilise the shoulder if there is any disruption of the passive stabilising structures⁽¹⁾.

Pathology and classification

Classification of posterior instability remains controversial and currently, no standardised system has been agreed upon or is routinely used. The mechanism of injury, severity, direction of instability, presence of ligamentous laxity and presence of volitional instability have all been considered when classifying glenohumeral instability.

Posterior instability can be described as either ‘traumatic’, ‘repetitive overuse’ or ‘due to soft tissue disorders’⁽²⁾. Acute traumatic posterior shoulder dislocation is 15-20 times less common than anterior dislocation⁽²⁾. The most commonly described mechanisms of frank posterior dislocations are as follows:

1. Acute trauma- ie landing on hand/elbow with arm in flexed/horizontally adducted and internally rotated position and/or car accident with hand on steering wheel.
2. Electric shock
3. Seizure

(NB - Seizures and shock can cause shoulder dislocations because they produce extreme, unbalanced muscle contractions that can wrench the humerus out of place).

Athletes who experience a posterior shoulder dislocation may present with detachment of the posterior labrum from the glenoid rim (a ‘reverse Bankart lesion’ – see Figure 1). When the labral detachment is associated with a piece of the glenoid rim being avulsed, this is referred to as a ‘reverse bony Bankart lesion’⁽¹⁾.

Structural lesions associated with posterior instability may include:
Inferior glenohumeral ligament injury.

• Reverse Bankart lesion.
• Reverse Hills-Sachs lesion.
• Reverse humeral avulsion of the glenohumeral ligament (HAGL).
• Posterior labral tears.
• Kim lesion (tear between the posterior labrum and glenoid cartilage without detachment).

The clinician should also be aware that a locked dislocation can occur; in this instance, the patient would have altered contour (flattened) of the joint anteriorly⁽³⁾.

The predictors of recurrent instability in the acute setting are:

• An age of less than 40years.
• A large reverse Hills-Sachs lesion.
• Seizure as the initial cause.

More commonly in the overhead athlete, posterior instability occurs due to either generalised hypermobility or repetitive microtrauma. It is hypothesised that in positions of flexion, adduction and internal rotation (ie follow through throwing or backhand in tennis), the posterior structures of the shoulder will become stretched over time. Without adequate dynamic control in these positions, the posterior structures of the shoulder stretch, resulting in laxity and eventually instability.

Clinical examination

Subjective

The overhead athlete with posterior instability often reports vague symptoms - either anterior or posterior shoulder pain - which in most cases does not describe a dislocation or instability. They may describe weakness, a feeling of fatigue or they may have mechanical symptoms such as clicking or locking⁽¹⁾. They will typically describe pain in their shoulder during activities that involve a combination of flexion, horizontal flexion and internal rotation⁽²⁾. This includes activities such as throwing, turning a steering wheel, bench press or overhead weights.

To determine whether the injury is due to acute trauma or repetitive overuse, initial questioning should focus on whether there has been a single traumatic event, as a specific incident may be indicative of structural lesion of significance⁽²⁾. Commonly, a feeling of instability is a secondary concern - particularly in the hypermobile population - as they may not be aware that their humerus has developed increased translation because they have always ‘clicked’ or ‘clunked’⁽²⁾.

Objective

As with all shoulder injuries, a thorough assessment of both shoulders should be conducted including scapula position, shoulder ROM, strength and palpation. Cervical spine assessment should also be conducted to determine if any cervical spine involvement is present.If posterior instability is suspected than specific tests for the posterior shoulder should be conducted. These include the posterior drawer, jerk, and Kim tests.

The posterior drawer test (Figure 2)

This test can be conducted in either supine or sitting. The examiner stabilises the glenoid with one hand whilst the other hand applies a posteromedial directed onto the humerus. The examiner assesses for any excessive posterior translation. This can be graded as:

Grade 1- less than glenoid margin.
Grade 2- beyond glenoid margin/reduces.
Grade 3- beyond glenoid margin/does not spontaneously reduce.

Jerk test (Figure 3)

The patient sits with their arm in forward flexion, adduction and internal rotation. The examiner applies a posterior directed force along the line of the humerus, which causes posterior translation of the humeral head. A positive test demonstrates a clunk (+/- pain) as the patient’s arm is then horizontally extended back to neutral. A positive clunk may be indicative of a posterior labral tear⁽¹⁾.

Kim test (Figure 4)

The patient sits with the shoulder abducted and internally rotated. The examiner holds the patient’s bicep with one hand and their elbow with the other. Whilst the arm is elevated to 135 degrees, the examiner applies an axial load to the elbow causing it to posteriorly translate out of the socket. A positive test is represented by pain, clunk or a click⁽¹⁾.