Open Repair of Posterior Shoulder Instability

• This chapter describes the background, etiology, diagnosis, treatment, rehabilitation, and outcomes for posterior instability of the glenohumeral joint treated with open surgical repair.

Important Points

• Distinguish type: repetitive microtrauma, traumatic, atraumatic

• Voluntary: positional or muscular

• Symptoms can be similar to those of impingent or biceps pathology

• Jerk and Kim tests on examination

• Open procedure: poor tissue, revision, bony abnormality, 3+ instability

Clinical and Surgical Pearls

• Lateral or beach chair position

• Saber incision

• Deltoid and infraspinatus split

• Determine if augmentation needed

Clinical and Surgical Pitfalls

• Stay less than 15 mm medial to glenoid to avoid suprascapular nerve

• Avoid axillary nerve and posterior humeral circumflex artery inferior to teres minor

Posterior instability accounts for only 2% to 10% of all cases of shoulder instability.1,2 Posterior glenohumeral instability most commonly arises from acute traumatic instability, atraumatic instability, or repetitive microtrauma.^2,3 Repetitive microtrauma is the most common type and includes things such as bench-press, overhead weightlifting, rowing, swimming, and blocking in football linemen. The repetitive loading causes stretch and injury to the posterior capsule.³ The second most frequent type is acute trauma and includes injuries sustained by football linemen, bench-press injuries, seizures, and electrocution.² The rarest form is atraumatic instability and is most commonly associated with generalized ligamentous laxity. Accurate diagnosis relies on a detailed history, comprehensive physical examination including special tests for posterior instability, appropriate imaging, examination under anesthesia, and intraoperative confirmation of pathology. The posterior capsule is thin in comparison with the anterior capsule, but it does contain the posterior inferior glenohumeral ligament (PIGHL). The PIGHL is the primary restraint to posteroinferior instability; it is under tension with shoulder flexion and internal rotation.² This is consistent with the clinical observation that the position of posteroinferior glenohumeral instability is shoulder flexion, internal rotation, and adduction. Other anatomic factors that can contribute to posterior glenohumeral instability include a dysfunctional or torn subscapularis,⁴ glenoid or humeral retroversion,^5,6 glenoid hypoplasia,⁵ loss of glenoid concavity as a result of labral injury,⁶ compromise of the middle and superior glenohumeral ligaments,⁷ and possibly a nonfunctional rotator interval.

Preoperative Considerations

History

Although patients may report discrete episodes of posterior dislocation, the most common presenting complaint is shoulder pain, which may be localized to the deep posterior shoulder. Patients also often describe an inability to perform certain activities, or subjective feeling of weakness.²

Signs and Symptoms

Symptoms can appear similar to those of subacromial impingement or biceps tendonitis; therefore, posterior instability should be on the differential diagnosis for such a presentation. Some patients will be able to voluntarily sublux the shoulder. The two types of voluntary subluxation are positional and muscular.⁸ The particular type should be carefully distinguished because the surgical outcomes in these two groups are very different. Positional subluxators can reproduce the instability with flexion and internal rotation,^2,9 in the same way that a patient with anterior instability can with flexion, abduction, and external rotation. Voluntary muscular subluxators reproduce instability with the arm in adduction, and this is associated with global laxity or muscular dysfunction.² Patients with positional instability do well with surgery, whereas patients with muscular instability usually do poorly.^8,9

Physical Examination

Physical examination of the shoulder should always begin with examination of the cervical spine. This should include a motor and sensory examination for all nerve roots and a provocative Spurling test for reproducible nerve impingement. Examination should then proceed to the affected shoulder, with examination of the contralateral shoulder for comparison. Anatomic symmetry should be checked first, followed by a comparative range-of-motion examination of both shoulders. Any positions of apprehension should be noted. Scapulothoracic motion should then be checked for any dyskinesis. Some common findings include posterior glenohumeral tenderness, increased external rotation, and mild loss of internal rotation. The presence of a sulcus sign can indicate posterior instability or multidirectional instability. This should be assessed with the arm at the side and in neutral rotation. Loss of the sulcus with external rotation of the arm indicates that it is probably not clinically significant. Posterior instability can be assessed with several provocative tests, which include the jerk test, the Kim test, the posterior stress test, and the load-shift test.²

To perform the jerk test, the examiner grasps the elbow in one hand and the clavicle and scapular spine with the other. The patient can be seated or lateral. The arm is flexed and internally rotated, the humerus is axially loaded posteriorly and inferiorly at approximately the 7-o’clock position, and the scapula is translated anteriorly. The test result is positive if a sudden jerk associated with pain occurs as the humeral head relocates.⁴

The Kim test is performed with a patient seated and the arm in 90 degrees of abduction. The elbow is grasped with one hand and the lateral arm is grasped with the other. The humerus is then axially loaded, and while the arm is elevated to 45 degrees, a posterior force is applied to the arm. Production of pain indicates a positive test result.¹⁰ Positive jerk and Kim test results together indicate 97% sensitivity for posterior glenohumeral instability.

The posterior stress test has been described in the seated position; however, it is sometimes easier to stabilize the scapula with the patient supine and lateralized off the edge of the examining table. The arm is flexed 90 degrees, adducted and internally rotated, and then translated posteriorly. A positive test result is present if dislocation, subluxation, pain, or apprehension occurs.¹¹

The load-shift test can be performed with the patient supine or standing. The arm is forward flexed and abducted 20 degrees, and the humeral head is axially loaded and then translated anteriorly and posteriorly while the scapula is stabilized. The test is graded as anterior or posterior and as 1+, 2+, or 3+. A grade of 1+ indicates that the head translates to the glenoid rim but not over it, a grade of 2+ indicates that the head translates over the rim but spontaneously reduces, and a grade of 3+ indicates that the head translates over the rim and stays dislocated.¹²

Imaging

A complete evaluation for posterior instability with diagnostic imaging includes radiographs, magnetic resonance imaging (MRI), and in some cases computed tomography (CT).

Plain radiographs should include a standard anteroposterior view and an axillary view. These are used to assess the glenohumeral articulation. The axillary view is helpful for determining if there is any humeral or glenoid bone loss or if there is any relative retroversion of the glenoid. Findings suggestive of bone loss, abnormal version, or glenoid hypoplasia should lead to obtaining a CT scan for a complete evaluation of the bony architecture.

MRI is used to evaluate the articular cartilage, labrum, capsule, glenohumeral ligaments, rotator cuff, biceps, and bony structures. Partial avulsion injuries to the posterior labrum (Kim lesion) have been described in posterior instability and can appear as a loss of posterior labral height or a marginal labral crack.⁶ Magnetic resonance arthrograms can be used; however, if the MRI is performed with a 3-tesla magnet, intra-articular contrast can often be avoided.

Indications and Contraindications

Physical therapy is the first line of treatment for posterior glenohumeral instability. It has been shown to lead to favorable improvement in two thirds of patients.3,13 It is most successful in patients with multidirectional instability and repetitive microtrauma.² Success rates for nonoperative treatment range from 89% for atraumatic subluxors to 16% for traumatic subluxors.¹³ Patients with MRI evidence of a posterior labral tear are also less likely to respond to therapy.¹⁴ A major component of therapy should focus on subscapularis strengthening because the subscapularis is an important dynamic posterior stabilizer.

For patients in whom physical therapy fails, traumatic subluxors, patients with significant posterior labral injury, patients with glenoid or humeral bone loss, and patients with abnormal humeral or glenoid version, surgery is indicated. Voluntary muscular subluxation is a contraindication for surgery because of the poor outcomes.

Arthroscopic stabilization for posterior glenohumeral instability has been described with success rates ranging from 55% to 97%.¹⁵ Arthroscopic stabilizations are indicated as primary procedures for patients with good tissue quality and no bony abnormalities. Open posterior stabilizations should be used for revisions, poor tissue quality, glenoid retroversion, or significant glenoid bone loss. Some authors suggest that laxity graded 3+ is also an indication for open stabilization.¹⁶

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