Posterior shoulder dislocation

Posterior dislocation of the shoulder is rare, accounting for less than 2% of shoulder dislocations.

May be bilateral in up to 15% of cases.

During posterior dislocation, an osteochondral impression fracture (also termed an encoche fracture or a reverse Hill-Sachs lesion) is produced on the anterior aspect of the humeral head as it impacts on the posterior aspect of the glenoid.

Posterior dislocations are still being missed.

The terminology dislocation is a slight misnomer in reality it is a subluxation because some of the articular surface of the humeral head is in contact with the glenoid and some behind it, often with an impression defect in the humeral head.

Treatment is determined by the size of the defect and the duration of the dislocation.

In his review article Cicak suggested 3 weeks as the cut off for chronic dislocations, Robinson suggested 6 weeks and acknowledged that it is a continuum, the longer the dislocation the larger the defect becomes the harder treatment becomes.

Posterior fracture dislocations are a separate entity from pure posterior dislocations. (see posterior fracture dislocation)

Anatomy

Stability

Although the humeral head and the glenoid are both normally retroverted with respect to their long axes, the scapula is protracted on the chest wall. Consequently, in its normal position of function, the shoulder is protected from posterior dislocation by the strong buttressing action of the posterior aspect of the glenoid.

Static stabilization is provided by the posterior capsulolabral complex and the posterior band of the inferior glenohumeral ligament,
Dynamic stability is provided by the rotator cuff and the shoulder girdle muscles.

Excessive posterior translation is also prevented by anterior constraints that contribute to capsuloligamentous stability. These include the rotator interval capsule, the superior and middle glenohumeral and coracohumeral ligaments, and the subscapularis tendon.

The relative contribution of these structures to stability varies with the position of the shoulder.

Stability also depends on coordinated glenohumeral and scapulothoracic movements.
Acute dislocation may produce an injury to the posterior stabilizers, which can present as capsulolabral tears or avulsions, glenoid rim fractures, or rotator cuff tears.

There is some evidence to suggest that posterior capsular tears will heal spontaneously following relocation of the shoulder.

Blood supply of humeral head

The humeral head has a segmental blood supply, mainly derived from the ascending branch of the anterior circumflex humeral artery. There is a risk of osteonecrosis after fracture dislocations through the anatomical neck, although if the fracture extends below the articular surface medially, the head may be perfused by intact posteromedial vessels.

History

The dislocation may be caused by:

Epileptic fit
Electric shock
Trauma

In seizures and electric shocks, spasm in the strong internal rotators (latissimus dorsi, pectoralis major, subscapularis and teres major) overpower the weak external rotators (infraspinatus and teres minor).

The main symptom is loss of movement of the involved shoulder, particularly external rotation

On Examination

Clinical findings may be subtle. The arm is held in internal rotation and adduction. Classically external rotation is limited, abduction and forward elevation is limited to between 80° and 100° (scapulothoracic movement).

Rowe and Zarins described a test in which there is inability to supinate the forearm when the arm is flexed forwards because of the internal-rotation deformity of the shoulder.

The coracoid process may be more prominent anteriorly and the humeral head palpable posteriorly.

Radiography

Standard views of the shoulder are:

Anteroposterior - Often looks normal or only has subtle abnormalities as pathology only subluxation. The anterior part of the humeral head is inside the joint and the posterior part outside. Several signs have been described on the AP view which suggest the diagnosis of posterior dislocation. These include internal rotation of the hmerus because of the fixed position of the humeral head on the posterior glenoid rim, the vacant glenoid sign since the anterior glenoid fossa looks empty, the ‘light-bulb’ appearance of the humeral head, the ‘rim sign’ in which there is more than 6 mm between the anterior glenoid rim and the humeral head, and the ‘trough line’ which is a vertical line made by the impaction fracture of the humeral head
Axillary view - The axillary view is essential for diagnosis and estimates the size of the anteromedial defect of the humeral head. It may be difficult to obtain because of pain and limitation of abduction, but may be obtained with the patient’s arm held passively in at least 20° of abduction. If pain and muscle spasm do not allow enough abduction to obtain a good axillary view, a modified axillary view should be performed.
Lateral scapular view - The lateral scapular view is particularly helpful in determining the relationship of the humeral head to the glenoid. In anterior dislocations of the shoulder, the humeral head lies anterior to the glenoid; in posterior dislocations it is posterior.

Computed Tomography

CT is useful to evaluate the size of the defect in the humeral head and associated glenoid changes. It may also detect radiographically occult anatomical neck fractures and reveal evidence of degenerative joint disease.

Magnetic Resonance Imaging

Magnetic Resonance Imaging is not routinely necessary because soft-tissue injury is very rare in posterior dislocation of the shoulder.

Ultrasound

Ultrasound can be used to differentiate anterior from posterior dislocations but as yet has not replaced standard radiographs.

Treatment

Treatment depends on :

Size of the defect
Duration of the dislocation
Age and activity of the patient

Size of defect:

Small - defect up to 25% of the articular surface of the head, can be treated by closed or open reduction. If the shoulder is unstable, a transfer of the upper one-third of the subscapularis can be performed.
Medium - defect between 25% and 50% of the articular surface, can be treated by transfer of the lesser tuberosity
Large - defect greater than 50% of the articular surface, treat by shoulder arthroplasty.

Non-operative treatment

Despite the obvious deformity of the shoulder and loss of rotation, a chronic posterior dislocation can be surprisingly well tolerated, especially in elderly patients. There is usually little pain and enough forward elevation may be regained to allow the performance of many activities of daily living. Gerber recommends “supervised neglect” for elderly patients who have limited demands on the affected shoulder, an acceptable functional range of movement and a normal contralateral shoulder. Non-operative treatment must be considered for patients with uncontrolled fits or in any patient unable to comply with a postoperative rehabilitation programme.

Closed reduction.

Closed reduction may be attempted IF:

Its an acute dislocation (ie less than 3-6 weeks old)
The defect in the humeral head is small (less than 25 %)

Method of reduction

General anaesthesia and muscle relaxation
Gentle reduction is attempted by 90° flexion and adduction with axial traction on the arm.
Direct pressure to the humeral head from behind can facilitate reduction.
Gentle internal rotation may help to stretch out the posterior capsule and rotator cuff if the head is locked on the glenoid rim
Lateral traction on the proximal humerus may allow the humeral head to unlock from the glenoid rim.
Once it is unlocked, the humerus is gently externally rotated.

After successful reduction, stability of the shoulder is assessed. If it is stable in internal rotation, the arm is immobilised in neutral rotation for 3 weeks. If unstable, the shoulder is immobilised with the arm at the side and in external rotation of 20° for six weeks.

If closed reduction is unsuccessful, open reduction is performed under the same general anaesthetic.

Operative treatment

Approaches

Deltopectoral most common, provides limited access to posterior aspect of glenoid to allow disimpaction of humeral head. May require additional posterior approach/ incision.
The deltoid-splitting, superior subacromial approach is an alternative favoured by some authors as it provides more direct access to the dislocated humeral head. Most reconstructive procedures can be performed through this approach, although there is a risk of postoperative detachment of the deltoid from the acromion and injury to the axillary nerve if the split is continued too distally.
Irrespective of the approach used, following arthrotomy at the rotator interval and prior to relocation, the impression fracture should be disengaged from the glenoid rim under direct vision.

Small defect: (defect < 25% of humeral head)

In an irreducible dislocation with a defect of less than 25% of the humeral head an open reduction via a deltopectoral approach can be used.

Semi beach-position elbow nearly at the same level as the shoulder. The arm must be freely mobile.
Incision is made from the tip of the coracoid process along the deltopectoral groove, slightly laterally in case it needs to be extended distally to the insertion of the deltoid.
Because of the considerable internal-rotation deformity of the arm, the biceps tendon is used to find the lesser tuberosity and rotator interval. It should be found immediately beneath the upper margin of the insertion of pectoralis major into the humerus.
The rotator interval is opened and sometimes the upper margin of the subscapularis tendon is divided with the anterior capsule to allow better visualisation of the joint.
The shoulder is reduced under direct visison. Internal rotation to unlocks the defect in the humeral head followed by lateral distraction, external rotation and pressure on the humeral head from behind.
Once the reduced examine the defect and the articular surfaces. The posterior glenoid rim is usually damaged but rarely contributes to further instability. The reduction is satisfactory if the articular cartilage is good, the impression fracture small and the shoulder stable. The rotator interval is then closed.

If the shoulder is unstable with the arm in internal rotation, transfer the upper one-third of the tendon of subscapularis to the defect using transosseous non-absorbable sutures. The suture knot should be behind the bicipital groove. After this procedure the shoulder is usually stable and the arm is immobilised at the side in slight external rotation for 3-4 weeks.

Medium defect: (defect 25-50% of humeral head)

If the impression fracture of the humeral head is between 25% and 50%, an open reduction and transfer of the lesser tuberosity is recommended.

McLaughlin described the transfer of subscapularis for a defect of between 20% and 40%. The tendon of subscapularis is secured into the defect through drill holes in the bone.

Hughes and Neer modified this method by osteotomising the lesser tuberosity with the attached subscapularis.

The advantages of transfer of the lesser tuberosity are better bony filling of the humeral head and more secure reinsertion of the subscapularis.

Via a deltopectoral approach the biceps tendon is identified as a landmark for the lesser tuberosity.
The rotator interval and the lower edge of the tendon of subscapularis are identified.
The circumflex vessels are ligated.
Under direct visualisation of the joint through this interval, osteotomise the lesser tuberosity starting at the bicipital sulcus and extending to the defect of the humeral head.
The lesser tuberosity with the attached subscapularis is elevated to expose the head and the glenoid.
The lesser tuberosity with the attached tendon of subscapularis is ?xed into the defect with two cancellous lag screws. If the shoulder is stable, the arm is immobilised in neutral rotation for 4 weeks.

Other options for the treatment of a defect between 25% and 50% is rotational osteotomy of the humerus and reconstruction using autograft or allograft.

Rotational osteotomy of the humerus.

After open reduction, a transverse osteotomy of the surgical neck of the humerus is performed.
The humeral shaft is rotated internally and the osteotomy is fixed.
The increased internal rotation ensures that the defect remains anterior to the glenoid throughout the entire range of movement.

Allograft reconstruction.

The defect is filled with allograft from the femoral head which is contoured to fit the segmental defect and to restore the sphericity of the head.

The graft is fixed with cancellous screws.

This procedure has given similar results to those of transfer of subscapularis without altering the normal anatomy of the proximal humerus.

This procedure should be used in patients with good bone quality of the residual head and with no osteoarthritis.

Autograft reconstruction.

Osteochondral autograft of the humeral head may be used in patients with a medium or large anteromedial articular impression defect as may occur with bilateral acute posterior dislocation. After removing the humeral head from the contralateral shoulder during hemiarthroplasty, the articular segment of the head is fashioned into a well-fitting osteochondral autograft and fixed into the impression defect of the head with Herbert screws.

Large defect: (defect >50% of humeral head)

In patients with an impression fracture involving more than 50% of the articular surface or when the humeral head is very soft and not viable hemiarthroplasty is indicated.

It is important to decide before surgery whether to transfer the lesser tuberosity or to perform an arthroplasty. If the latter is the case it is important not to perform an osteotomy of the lesser tuberosity. If an osteotomy is performed reconstruction of the tuberosity must be carried out with the potential problems of malunion or nonunion.

Reduction can be very difficult if the defect is large and the duration of the dislocation is more than 6 months.

It is important to release the soft tissues around the shoulder and to reduce the shoulder slowly.

Retroversion of the humeral component should be decreased from approximately 35° to 20°. Excessive anteversion is not required, nor is plication of the posterior capsule. If there is a concern regarding the stability of the humeral component, the arm is immobilised in external rotation of 10° to 20°.

Complications

Acute Redislocation

This may occur following closed relocation or be due to failure of an adjunctive surgical stabilization procedure.
Recurrent episodes of posterior instability may occur following an initial dislocation and are best considered within the spectrum of recurrent posterior instability.

Osteonecrosis

Osteonecrosis of the humeral head has been reported following simple dislocation, but it is more frequently encountered following internal fixation of an anatomic neck fracture-dislocation.

The risk of osteonecrosis increases with the degree of fracture displacement and the extent of involvement of the tuberosities.

Osteonecrosis may be associated with satisfactory function if an anatomic reconstruction has previously been achieved.

Symptomatic patients are usually treated with an arthroplasty.

Posttraumatic Degenerative joint disease

Posttraumatic degeneration of the glenohumeral joint is relatively uncommon after posterior dislocation, but when it occurs the severity of the arthrosis is usually worse than that following anterior dislocation. If symptoms are severe enough to warrant treatment, a shoulder arthroplasty is usually performed.

Joint Stiffness and Functional Incapacity

Persistent shoulder stiffness and functional incapacity after a simple dislocation are associated with:

A delay in the diagnosis
Deformity
Osteoarthrosis
Osteonecrosis of the humeral head
Rotator cuff tear
A complication of an ancillary stabilization procedure or treatment with an arthroplasty

Summary

The key physical sign is fixed internal rotation of the arm.

The axillary view is essential for diagnosis and to estimate the size of the anteromedial defect of the humeral head.

‘Supervised neglect’ can be considered in a patient with limited disability and low functional expectations.

Closed reduction should be attempted if the defect is less than 25% of the articular surface and the duration of the dislocation is less than 3 weeks.

Open reduction should be carried out for an irreducible dislocation with a defect of less than 25%.

If the shoulder is unstable after open reduction transfer of the upper one-third of the tendon of subscapularis to the defect using transosseous non-absorbable sutures should be performed.

Transfer of the lesser tuberosity remains the operation of choice in patients with a defect of between 25% and 50% of the articular surface.

Hemiarthroplasty of the shoulder should be performed in patients with an impression fracture involving more than 50% of the articular surface or when the humeral head is very soft and not viable.

Total shoulder arthroplasty should be performed in patients with considerable erosion of the glenoid.

see (Posterior fracture dislocation)

see (look and learn - shoulder /proximal humerus)

References

C. Michael Robinson and Joseph Aderinto; Posterior Shoulder Dislocations and Fracture-Dislocations J. Bone Joint Surg. Am., Mar 2005; 87: 639 - 650.

Cicak, N. Posterior dislocation of the shoulder. Journal of Bone & Joint Surgery - British Volume. 86-B(3):324-332, April 2004

Clough, T.M.; Bale, R.S. Bilateral posterior shoulder dislocation: the importance of the axillary radiographic view. European Journal of Emergency Medicine. 8(2):161-163, June 2001.

Ogawa, Kiyohisa MD; Yoshida, Atsushi MD; Inokuchi, Wataru MD; Posterior Shoulder Dislocation Associated with Fracture of the Humeral Anatomic Neck: Treatment Guidelines and Long-Term Outcome. Journal of Trauma-Injury Infection & Critical Care. 46(2):318-323, February 1999

Goodrich, J. Allan; Crosland, Edward; Pye, Jacque. Acromion Fracture Associated With Posterior Shoulder Dislocation. Journal of Orthopaedic Trauma. 12(7):521-523, September/October 1998

Posterior dislocation of the shoulder with ipsilateral humeral shaft fracture: a very rare injury; Injury, Volume 28, Issue 2, March 1997, Pages 150-152; S. Naresh, J. A. Chapman and T. Muralidharan

Goodrich, J. Allan; Crosland, Edward ; Pye, Jacque; Acromion Fracture Associated With Posterior Shoulder Dislocation. Journal of Orthopaedic Trauma. 12(7):521-523, September/October 1998.

Munting, T.; de Beer, M. A.; Vrettos, B. C. MISSED POSTERIOR DISLOCATIONS OF THE SHOULDER. Journal of Bone & Joint Surgery - British Volume. 85-B SUPPLEMENT II:142, 2003.