Multidirectional instability (MDI) of the shoulder is an instability pattern of the glenohumeral joint characterized by excessive glenohumeral translation in the inferior direction as well as in the anterior direction, posterior direction, or both.
A detailed description of MDI was first outlined in 1980 (Neer CS II, Foster CR: Inferior capsular shift for involuntary inferior and multidirectional instability of the shoulder: A preliminary report. J Bone Joint Surg Am 1980;62:897-908) they distinguished it from the more commonly recognized forms of anterior and posterior instability and described the use of an open inferior capsular shift to address capsular redundancy and instability.
Thomas and Matsen noted that most patients with recurrent glenohumeral instability could be classified into one of two groups (TUBS & AMBRI); the first characterized by a history of definite trauma causing unidirectional instability. These patients usually were found to have a Bankart lesion and frequently required surgery to achieve stability. In contrast, patients in the second group had no history of trauma and were more prone to have multidirectional instability that was bilateral. Rehabilitation involving strengthening of the rotator cuff was the preferred treatment; however, if surgery was to be performed, it should be an inferior capsular shift. (Thomas SC, Matsen FA III: An approach to the repair of avulsion of the glenohumeral ligament in the management of traumatic anterior glenohumeral instability. J Bone Joint Surg Am 1989;71:506-513).
The aetiology of MDI can be multifactorial, involving anatomic, neuromuscular, and biomechanical abnormalities. MDI can also have a traumatic (usually repetitive microtrauma) aetiology.
Potential sites of structural pathology include the rotator cuff interval, the inferior gleno-humeral ligament complex and the glenoid labrum.
Patients with MDI may develop symptomatic instability without a clear history of trauma. These patients may have a genetic predisposition with generalised ligamentous laxity (determined by the % mix of collagen types in each individual). It is also possible for a previously asymptomatic but lax gleno-humeral joint to become symptomatically unstable after a traumatic dislocation or subluxation or after repetitive microtrauma.
Secondary rotator cuff tendonitis and sub-acromial impingement as well as long head of biceps tendonitis can occur as a result of the underlying instability and higher demand activity.
Physical examination is required to classify the instability: patients must have instability in the inferior direction, as well as in the anterior or posterior direction (or both).
Assessment of a patient who may have recurrent shoulder instability should include the sulcus sign test for inferior instability, the load and shift test for anterior and posterior instability; the apprehension & relocation tests for anterior instability; and the jerk test for posterior instability..
Patients with suspected MDI should also be evaluated for generalized ligamentous laxity with an examination for elbow, thumb, metacarpo-phalangeal & knee joint hyperextension and spine flexion range (Beiton criteria).
Evaluation of the scapula is an often overlooked aspect of the physical examination of the patient with shoulder instability. The scapula has a relatively large arc of motion on the thoracic wall, which allows the glenoid to better maintain an efficient link with the humerus during activities.
It would be virtually impossible to restore normal gleno-humeral joint stability and movement in the presence of an unstable scapula with poor muscle patterning.
Evaluation of the scapula should consist of observation of its position at rest, during active forward elevation, and abduction, and during resisted activity, such as when performing a wall press, or resisted protraction (punching action). In patients with MDI, primary scapular winging can, rarely, be the result of a long thoracic or spinal accessory nerve lesion. It is however, usually a secondary condition, the result of pain inhibition of the normal scapular stabilizers, or compensation for poor gleno-humeral movement.
The scapula may also be evaluated with the lateral slide test, which assesses scapular stabilizer function In this test, the distance from the inferomedial border of the scapula to the midline of the spine is measured in three positions: 1)at rest, 2)with the hands on hips, and 3)with the shoulder at 90° of abduction and maximal internal rotation. A difference of greater than 1.5 cm between the affected and unaffected sides in any of the positions is considered a positive test result. (Kibler WB: The role of the scapula in athletic shoulder function. Am J Sports Med 1998;26:325-337).
Because of the broad spectrum of pathology, the diagnosis of MDI requires a combination of history, physical examination, and imaging. If unresponsive to conservative measures, examination under anesthesia and arthroscopic examination may be required to confirm the diagnosis and instability pattern.
AP, axillary, and scapular lateral Y radiographs of the gleno-humeral joint should be obtained. They should also be obtained after any acute traumatic event that initiated shoulder instability, as well as a Stryker-notch view. In patients with a definite history of trauma, these views can be used to confirm reduction of the glenohumeral joint and to assess for any humeral head (Hill-Sachs) or glenoid rim fractures.
MRI is the test of choice for evaluating the soft-tissue structures around the shoulder. Although not required for the diagnosis of MDI, MRI allows imaging of other potential sources of pathology, such as the rotator cuff, long head of biceps or of concurrent injury to the glenoid labrum.
MRI may be performed with the addition of intra-articular contrast. Some studies have assessed MRI arthrography to be more accurate (Flannigan B, Kursunoglu-Brahme S, Snyder S, Karzel R, Del Pizzo W, Resnick D: MR arthrography of the shoulder: Comparison with conventional MR imaging. AJR Am J Roentgenol 1990;155:829-832); other studies have not shown any advantage of magnetic resonance arthrography over conventional MRI (Connell DA, Potter HG, Wickiewicz TL, Altchek DW, Warren RF: Noncontrast magnetic resonance imaging of superior labral lesions: 102 cases confirmed at arthroscopic surgery. Am J Sports Med 1999;27:208-213)
With MDI, bony abnormalities are uncommon and radiographs are generally normal. Abnormalities in glenoid morphology / alignment are better assessed with CT, whereas soft-tissue pathology is more likely to be identified with MRI.
Differential diagnosis of MDI
Management of patients with proven MDI should always begin with a course of nonsurgical treatment. A sufficient period of activity modification and good focused physical therapy should be attempted as initial treatment. Should a patient continue to be symptomatic after an appropriate period of rehabilitation, or should there be any diagnostic doubt, surgery, in the form of initial diagnostic arthroscopy and EUA (examination under anaesthesia) should be considered.
Indications for arthroscopic capsular plication and very rarely capsular shrinkage, in the treatment of MDI include symptomatic instability (pain and functional impairment) failed nonsurgical treatment, and an additional traumatic element of the instability.