Posterior Shoulder Instability
Unidirectional posterior shoulder instability accounts for less than 5% of all shoulder instability. Posterior instability can be classified as either type I (traumatic) or type II (atraumatic). Atraumatic subluxations are further subdivided into involuntary and voluntary, including those who have learned through muscular control and arm position to dislocate or sublimate their shoulder. Most patients presenting with posterior instability are categorized as type II. This instability occurs as a result of repetitive microtrauma. Type I instability is much less common and occurs after a posterior shoulder dislocation. Initial management consists of extensive physical therapy. When nonoperative treatment fails to alleviate symptoms, successful surgical treatment is dependent on an anatomic diagnosis of the pathologic lesion.
Anatomic Lesions Associated with Posterior Instability
1. Redundant posterior capsule (most common)
2. Reverse Bankart lesion
3. Posterior glenoid erosion or excessive retroversion/hypoplasia
4. Reverse Hill-Sachs lesion
Mechanism of Injury
1. Repetitive microtrauma (most common)
2. Macro trauma (less common)
Uncommon Contributors to Posterior Instability
1. Suprascapular nerve palsy with associated external rotator atrophy
2. Hypoplastic glenoid or abnormal glenoid retroversion
3. Fracture malunion (glenoid or humerus)
1. Posterior stress test (Clancy or “jerk” test): Shoulder subluxes with forward elevation, adduction, and internal rotation while placing a force on the palm directed toward the body (reduces with removal of load and external rotation).
2. Load and shift test: Patient supine with posterior stress applied to humeral head at different degrees of rotation.
3. Many patients can voluntarily sublux the shoulder. This occurs when the patient initiates forward elevation with the arm in internal rotation and slight adduction. The subluxation occurs between 90 and 120 degrees of forward elevation. The humeral head then spontaneously reduces between 120 and 180 degrees of forward elevation as abduction is added. We have named this maneuver the “pivot shift of the shoulder.”
1. Standard shoulder radiographs should be normal, but glenoid erosion, excessive retroversion/hypoplasia, and reverse Hill-Sachs deformity should be obvious.
2. Axillary radiograph may demonstrate subluxation.
3. Nerve conduction/EMG is helpful if suprascapular nerve palsy suspected.
4. Magnetic resonance imaging is not necessary, but may be helpful to evaluate labral pathology.
5. Computed tomography is helpful if excessive glenoid erosion or retroversion is suspected.
Once the diagnosis of unilateral posterior shoulder instability is made, patients are placed in an aggressive therapy regimen aimed at activity modification and rotator cuff muscular strengthening. This therapy is performed for at least 6 months. The goal is full range of motion and return to prior level of activity.
Recurrent symptomatic unidirectional posterior shoulder instability refractory to aggressive physical therapy of 6 months duration.
Voluntary willful subluxators/dislocators or a more global multidirectional instability.
Multiple methods of reconstruction have been described. For those patients with unidirectional instability, we perform a posterior capsulorrhaphy. Posteroinferior capsular shift is used in patients with a more global multidirectional instability in which the principle direction is posteroinferior. Since the posterior capsule is often thin and of poor quality, we perform the posterior capsulorrhaphy procedure because it utilizes both quality infraspinatus tendon and capsule in the reconstruction. Also, if the surgeon wishes, the posterior capsule may be shifted superiorly prior to capsulorrhaphy to eliminate capsular redundancy. We have also employed glenoid osteotomy when the instability was secondary to excessive glenoid erosion or retroversion. Because posterior capsulorrhaphy is the most common procedure we use to address posterior instability, it is described below.
1. Shoulder retractor tray (Darrachs, Fakudas, etc.)
2. Suture anchors
3. #1 nonabsorbable sutures