Anterior instability following anterior dislocation

Instability of the shoulder encompasses a wide spectrum of conditions which can be distinguished by their aetiology, severity, constitutional factors and volition. For this reason there is no generally accepted all-inclusive system of classification.

Definitions and classifications

• Laxity - normal translation of the humeral head over the surface of the glenoid.

• Instability - occurs when the degree of translation becomes excessive and leads to symptoms. Instability may vary in:

  • Degree (subluxation, dislocation and micro-instability),

  • Direction (anterior, inferior, posterior and multidirectional),

  • Aetiology (traumatic or atraumatic)

  • Volition (voluntary or involuntary)

Incidence

Anterior instability rates range from as low as 17% to close to 100%. Many of the series which describe a high incidence have been drawn from selective populations, such as patients presenting to sports injury clinics and in military cadets and may therefore be unrepresentative of the general population.

The true incidence of recurrent instability in an unselected population approximates to:

• 75% to 80% among individuals aged between 13 and 20 years

• 50% in those aged between 20 and 30 years

Risk factors for recurrent instability:

• Age at the time of primary dislocation is the chief prognostic factor

• Family history of recurrent instability

• Radiologically- visible avulsion fracture of the rim of the glenoid

• Large Hill-Sachs lesion

• Re-dislocation is rare if the greater tuberosity is fractured at the time of the initial injury, but these patients typically have a more protracted recovery, as a result of secondary dysfunction of the rotator cuff and impingement

•  The risk of re-dislocation may also be altered by the initial treatment.

Two separate groups of individuals who develop glenohumeral instability can be described

• TUBS - traumatic, typically unilateral, with a Bankart lesion and usually requiring surgery to stabilise the shoulder)

• AMBRI - atraumatic, multidirectional, commonly bilateral, treatment by rehabilitation and inferior capsular shift in some refractory patients).

Many patients in the AMBRI group also have evidence of ligamentous laxity in other joints. These two acronyms are viewed as the ends of a spectrum of disease in which both traumatic and atraumatic elements may be present.

The final group of patients with unstable shoulders are those with voluntary instability. These individuals are normally considered to be able to dislocate their shoulders at will and should be treated non-operatively. Increasingly, this view is seen as oversimplistic since some individuals may develop an acquired instability after many repetitive voluntary dislocations, and others may have an involuntary posterior ‘positional’ element to their instability, which may respond to biofeedback or, more rarely, to surgery.

Pathology

Joint restraints are divided into:

• Static stabilisers - Glenoid fossa, the labrum, the joint capsule and the glenohumeral ligament

• Dynamic stabilisers - Rotator cuff, long head of biceps and stabilisers of the scapula.

The Bankart lesion

Avulsion of the anterior capsulolabral complex inferior to the equator of the glenoid. Invariably present in patients with traumatic instability, it is now thought that it does not produce instability in isolation. As the arm is brought into the ‘at-risk’ position of greater abduction, the resistance to external rotation is provided by the anteroinferior structures, especially the inferior glenohumeral ligaments, which act like a hammock to retain the humeral head in place. Plastic deformation of these structures occurs during the initial dislocation before avulsion of the labrum and becomes progressively more severe with subsequent episodes of instability. Repair of the Bankart lesion alone is consequently thought to be insufficient to stabilise the joint and re-tensioning of the anteroinferior capsuloligamentous complex is now considered to be an important adjunctive procedure in patients with repeated episodes of instability.

Glenoid rim

Up to half the patients with glenohumeral instability have an osseous avulsion of the glenoid rim, a bony Bankart lesion.

A cadaver study has shown that an osseous defect in which the width is at least 20% of the length of the glenoid can cause instability of the shoulder. Clinically, impression lesions and avulsion fractures have been associated with recurrent instability and failure of arthroscopic surgical stabilisation. Patients with large anteroinferior osseous defects of the glenoid, the ‘inverted pear’ sign, may therefore be better served by an open bony procedure which directly addresses the defect in the glenoid, rather than a conventional soft tissue stabilisation.

Hill-Sachs lesion

An impression fracture of the humeral head is present in most patients with anterior instability. It is usually relatively small and does not contribute to instability. However, when the defect is larger and involves more than 30% of the humeral articular surface, it may ‘engage’ with the anterior glenoid during external rotation of the shoulder causing re-dislocation.

Other lesions associated with instability

• HAGL (humeral avulsion of the glenohumeral ligaments) -probably represent a variant from the normal pattern of anterior capsular stretching or rupture

• ALPSA (anterior labroligamentous periosteal sleeve avulsions) - in which the detached sleeve heals medially to the scapular neck, allowing excessive humeral translation.

• SLAP (superior labral anterior and posterior detachment) - may occur in continuity with the inferior labral avulsion, and defects of the rotator interval.

The exact importance of many of these lesions in the pathogenesis of shoulder instability is not fully understood at present.

Treatment of the patient with recurrent instability

Patients with predominantly traumatic instability, benefit more from surgical treatment, than those patients with predominantly atraumatic or voluntary instability, who are best treated non-operatively in the first instance

Look for:

• Evidence of generalised ligamentous laxity

• Provocative tests to define the direction and extent of instability.

• Plain radiographs - delineate any bony abnormality, including defects of the rim of the glenoid and head of the humerus. 

• Examination under anaesthesia or diagnostic arthroscopy may be used for patients in whom the precise diagnosis is in doubt.

• MRI of the shoulder is superior to CT in the assessment of instability of the shoulder because of the better soft-tissue definition provided.

Surgical treatment for recurrent instability.

Open surgical stabilisation options

• The Bankart repair - most commonly used open technique in the treatment of recurrent anterior instability. Involves repair of the Bankart lesion, usually combined with an adjuvant capsular shift. Many variations of the Bankart repair have been used, but most surgeons now tend to favour suture anchors to secure the glenoid labrum to the decorticated anterior rim of the glenoid. A plication is performed to re-tension the anteroinferior capsuloligamentous complex. Care must be taken not to overtighten the repair in order to avoid the restriction of external rotation post-operatively.

• Bristow-Helfet procedure - Coracoid osteotomy and fixation to the anteroinferior rim of the glenoid, technically demanding procedure, preferable for patients with a large defect in the glenoid rim. This applies particularly to epileptics with recurrent traumatic instability who often have a large anteroinferior deficiency.

• In patients with large, engaging Hill- Sachs defects, allograft bone grafting or rotational osteotomy may also be considered.

Arthroscopic stabilisation.

In acute dislocation repair of the Bankart lesion  is undertaken in patients with recurrent instability, the anteroinferior capsule becomes progressively more attenuated with each episode of subluxation or dislocation. In these patients tightening of the anterior capsule can be achieved by:

• Superior advancement of the labrum at the time of Bankart repair

• Thermal or laser-assisted shrinkage of the anterior capsule

• Suture plication of the capsule or closure of the rotator interval are increasingly advocated.

Judging the extent to which this is required may be difficult and incurs the risk of producing excessive capsular tightness, limiting external rotation.

Results of stabilisation procedures for recurrent traumatic instability.

Open Bankart repair is slightly better than  other open procedures. However, combination of all the reported series using open techniques during the last ten years still reveals a rate of recurrence after operation of less than 10%.

The overall average rate of recurrence after arthroscopic stabilisation in case series without control groups is 15.2%, compared with 6.3% for open Bankart repair and 8.8% for all open procedures. However, the rates of recurrence vary considerably between the four categories of arthroscopic techniques and there is often considerable heterogenicity in the results of studies reporting the same method. Although the rates of recurrence are greater in studies with longer follow-up, there appears to have been a gradual improvement with the use of the more modern arthroscopic techniques and implants, particularly suture anchors. The few available studies suggest that the results of arthroscopic Bankart repair with a separate adjuvant capsular tightening, either by closure of the rotator interval, capsular shrinkage or plication, may be better than a Bankart repair alone. Most of the comparative studies have shown rates of recurrent instability to be higher after arthroscopic stabilisation than after open repair, and a meta-analysis of these series shows the risk of recurrence to be statistically significant, and more than twice that of an open procedure. As with primary stabilisation, no consistent benefits of the arthroscopic procedure over an open stabilisation have been demonstrated in terms of long-term residual shoulder pain, functional scores or restriction of movement. However, many studies report a shorter hospital stay, decreased post-operative pain and analgesic requirements, together with an improved cosmetic result when using the arthroscopic method.

Although revision arthroscopic stabilisation has been described, most failed open and arthroscopic stabilisations caused by technical failure are treated by a further open procedure.

Complications of surgical stabilisation

• complications of surgery

• recurrent instability

• loss of movement

• infection

• failure of the implant, loosening, breakage and impingement after the use of staples, pull-out after repair with suture anchors

• neurovascular injury

• late degenerative joint disease

• adhesive capsulitis

• synovial fistula

References

Robinson CM, Dobson RJ.
Anterior instability of the shoulder after trauma.
JBJS Br. 2004 May;86(4):469-79

Last updated 21/08/2004