Most patients present with atraumatic onset. This typically occurs in individuals with excessive joint laxity or hypermobility, which can be either congenital or acquired from overuse, such as in repetitive manual labor or participation in sports. Patients are less likely to have structural damage to the joint and more likely to exhibit signs of poor motor control and scapular dyskinesis. However, MDI can sometimes exist in the presence of a structural lesion such as a labral tear or an osseous defect, indicating a traumatic etiology.
Classic history and presentation: Symptoms can range in severity from vague shoulder pain without noticeable instability to symptomatic subluxation or dislocation during daily activities or sleep. Discomfort, aching, looseness, apprehension, and shifting of the shoulder with overhead movement are common. Transient neurologic deficits such as episodes of numbness or paresthesia may occur.
MDI is shoulder instability in at least 2 directions or planes of motion:
- Anterior shoulder instability presents as pain and difficulty with overhead activities, exacerbated with the shoulder in an abducted, extended, and externally rotated position (eg, in the early acceleration phases of the overhand throwing motion).
- Posterior shoulder instability presents as pain occurring with the shoulder flexed forward, adducted, and internally rotated (eg, pushing open a heavy door, performing push-ups, or in the progressive phase of the overhand throwing motion).
- Inferior shoulder instability presents as pain during activities that exert traction on the arm (eg, carrying heavy bags).
- Age – MDI typically affects individuals aged 12-35 years.
- Sex / gender – Among sedentary individuals, MDI may more commonly affect young women with poor muscular development, but a higher incidence has been found among young athletic males. Overall prevalence is reported to be equal between men and women.
Pathophysiology: Instability results from a combination of defects of the static and dynamic stabilizers of the shoulder. This is often due to loosening of the ligaments attached to the labrum of the shoulder capsule that surrounds and stabilizes the head of the humerus. Increased capsule length and repeated stretching of the glenohumeral ligaments due to hyperlaxity allows increased humeral head translation and predisposes to instability. Most importantly, the characteristic pathology of MDI is increased capsular redundancy, which may be congenital and associated with a systemic disease (eg, Ehlers-Danlos syndrome, osteogenesis imperfecta, Marfan syndrome) or acquired from overuse. Increased motion of the glenohumeral joint can lead to dislocation, subluxation, or repetitive microtrauma that produces tears of the labrum and/or rotator cuff.
Grade / classification system: Shoulder instability is typically classified by pathology, with MDI defined using the acronym AMBRI (atraumatic, multidirectional, frequently bilateral, responds to rehabilitation, and may require inferior capsular shift for surgical treatment). This distinguishes MDI from unilateral instability, which is defined using the acronym TUBS (traumatic, unidirectional, presence of Bankart lesion, and requires surgery).
