Shoulder instability occurs when the tissues responsible for keeping the head of the upper arm bone (humerus) securely within the glenohumeral joint—such as the joint capsule, ligaments, and labrum—become overstretched, torn, or detached. This kind of instability, resulting from tissue damage, is referred to as structural instability. In cases where abnormal muscle activity causes the joint to experience excessive or insufficient stress, the condition is known as functional instability.
Adolescents and young adults most commonly experience functional shoulder instability, with prevalence rates reaching up to 2.6% in this group. Among these individuals, the most frequent type is posterior positional functional shoulder instability (PP-FSI).
Patients with PP-FSI typically report significant shoulder pain during movement within the mid-range of motion. This discomfort is due to muscle imbalances: the external rotator cuff muscles and the posterior deltoid are underactive, while the internal rotator muscles are overactive. Additionally, the muscles surrounding the shoulder blade (periscapular muscles) display disrupted coordination. Functional MRI studies have revealed that individuals with PP-FSI may have atypical communication between the brain and the shoulder muscles during movement. These patterns are comparable to those seen in infants still developing motor control or in individuals recovering from neurological injuries such as strokes.
Conservative management of PP-FSI includes a variety of therapeutic approaches. Manual therapy is used to improve joint mobility, and targeted exercises aim to reactivate and strengthen underused muscles. Recommendations may also include cold therapy and dietary adjustments to manage inflammation, as well as the use of electronic muscle stimulation to assist with muscle retraining. Modifying activities during early recovery is important to minimize the chance of aggravating the injury. Gradually, as the patient’s symptoms improve and no sharp, stabbing pain is present, normal activities can usually be resumed.
It is also essential to evaluate and address any other related musculoskeletal injuries in areas such as the shoulder, arm, neck, or upper back that either preceded or followed the onset of PP-FSI, in order to support full recovery and a return to usual activities.
Although surgery may be considered for some patients with PP-FSI, most treatment guidelines recommend starting with conservative, non-surgical options, making approaches such as chiropractic care particularly valuable.
