**2. Anatomy and biomechanics of the glenohumeral joint**

Both static structures and dynamic stabilizers of the glenohumeral joint interact to produce stability. Static stabilizers include the bony anatomy, labrum, capsule, glenohumeral ligaments and rotator interval. Dynamic stabilizers include the rotator cuff, long head of the biceps, deltoid and scapular muscles. Negative pressure within the joint also contributes to stability by producing the "suction cup effect", which helps center the humeral head independently of muscular forces and is primarily important in the midrange, where the capsule and ligaments are not under tension.

The bony anatomy of the glenohumeral joint also plays a significant role in stability. The glenoid is more concave in the superoinferior than the anteroposterior direction. In addition, the articular cartilage is thicker towards the periphery of the glenoid, thus increasing the

Arthroscopic Treatment of Recurrent Anterior Glenohumeral Instability 31

underwent a muscle-strengthening exercise regimen (Burkhead & Rockwood, 1992). Sixteen percent of the shoulders with traumatic etiology had excellent or good results in contrast to 80% of those with atraumatic etiology. The authors highlighted the importance of identifying the etiology of instability to ascertain a successful result out of conservative treatment. In a prospective randomized clinical trial, active patients aged less than 30 years who were treated with supervised physical therapy showed recurrence rates of 17 to 96% whereas arthroscopic instability repair had failure rates between 4% and 22% (Bottoni et al, 2002). These findings indicate that young, highly active patients would benefit from early, arthroscopic repair after first-time traumatic anterior shoulder dislocation compared with

A **Bankart lesion** is the commonest sequel of an anterior dislocation and the main cause of instability. It is defined as a labral complex avulsion from the scapular periosteum. It usually includes some degree of capsular stretch and injury. When the lesion involves a fracture of the antero-inferior glenoid rim in addition to the soft tissue avulsion it is referred

Fig. 1. A three-dimensional reconstruction CT-image demonstrating a bony-Bankart lesion. **Humeral avulsion of glenohumeral ligaments (HAGL)** occurs when the capsuloligamentous structures are avulsed and torn off the humeral head and not the glenoid. An external rotation force in addition to hyperabduction commonly results in this lesion in contrast to a hyperabduction and impaction force that may produce a Bankart lesion (Matsen et al, 2006). The incidence of HAGL lesions after a traumatic dislocation has been reported at 39% (Bokor et al, 1999). A bony HAGL lesion occurs when the glenohumeral ligament is avulsed along with a bone fragment of the humeral head

conventional nonoperative treatment.

**4. Type of associated pathology** 

to as **bony Bankart** (Fig 1).

(Oberlander et al, 1996).

depth of the concavity. Because the size of the glenoid is limited compared with the humeral head, even a relatively small bone loss may reduce considerably the surface area for articulation and consequently compromise stability. A bone loss that exceeds 20% of the glenoid surface is considered critical for the recurrence of instability (Burkhart & De Beer, 2000; Tauber et al, 2004).

The labrum is a fibrocartilaginous structure attached to the glenoid rim. It functions to increase the anteroposterior and superoinferior depth of the glenoid and the surface contact area for the humeral head. Specifically, it increases the concavity of the glenoid up to 9mm in the superior-inferior direction and the anteroposterior depth to 5mm (Howell et al, 1988). Labral resection reduces resistance to translation by 20% (Lippitt & Matsen, 1993). The labrum also provides an attachment site for the glenohumeral ligaments. Two types of labral attachments to the glenoid have been described. The first, around the periphery through a fibrocartilaginous transition zone, which creates mobility along the central border similar to the knee meniscus. The second is securely attached both peripherally and centrally. The anteroinferior attachment of the labrum to the glenoid is normally tight. On the contrary, the superior attachment inserts directly into the biceps tendon distal to the insertion on the supraglenoid tubercle, it is loose and anatomically variant. Isolated lesions of the superior labrum do not result in instability. However, if the biceps insertion is also destabilized, significant translation occurs (Pagnani et al, 1995).

The glenohumeral capsuloligamentous system provides a restraint to excessive translation in varying positions of the joint. In particular, the anterior band of the inferior glenohumeral ligament (AIGHL) attaches to the anteroinferior labrum and primarily resists anteroinferior translation in the abducted externally rotated shoulder position.

The rotator cuff compresses the humeral head into the glenoid throughout the range of motion. An association between undersurface rotator cuff tears and instability has been described (Jobe & Bradley, 1989). The rotator interval (RI), between the leading edge of the supraspinatus and the superior edge of the subscapularis, has also been implicated in glenohumeral instability. Closure of a large defect in the RI has been shown to decrease inferior instability. There may be an inverse relationship between the size of the RI and the superior glenohumeral ligament (SGHL) contributing to the instability (Nobuhara & Ikeda, 1987).
