Shoulder instability is one of the most commonly diagnosed shoulder pathologies [1]. It describes a wide clinical spectrum that ranges from complete dislocation requiring mechanical reduction of the joint to shoulder subluxation in which some glenohumeral contact remains [2–6]. Anterior shoulder dislocations are the most common subtype, accounting for 98 % of cases [7–10], and are the most common large joint dislocation [11] with a reported prevalence of approximately 2 % within the general population [7, 10, 12, 13]. The incidence of anterior shoulder dislocation has a bimodal age distribution, with peaks occurring in the second and sixth decades; however, 90 % of cases occur in young individuals below the age of 30 years [14, 15]. Trauma is by far the most common cause of primary shoulder dislocation, accounting for 95 % of cases [9]. Other risk factors for shoulder dislocation include male sex and participation in contact sports [14].

Depending on the mechanism of injury, the humeral head may dislocate anteriorly, posteriorly, or inferiorly. Traumatic posterior dislocations are considerably more rare, accounting for approximately 2 % of cases [9, 15, 16], and are most frequently caused by direct trauma to the anterior shoulder or by an indirect force that is directed posteriorly through the arm to the shoulder [8]. Inferior dislocations are extremely rare, accounting for approximately 0.5 % of cases, and are due to hyperabduction of the arm that forces the neck of the humerus against the acromion, resulting in displacement of the humeral head inferiorly [10].

The glenohumeral joint is a careful balance of mobility and stability [1, 17]. Glenohumeral mobility is the result of the small surface area of the humerus that articulates with the glenoid, the shape and minimal depth of the glenoid fossa, and the relative laxity of the surrounding joint capsule [18, 19]. Glenohumeral stability is the ability to keep the humeral head centered within the glenoid fossa and is achieved through the combined action of noncontractile, static stabilizers (such as the bony articulation) as well as more dynamic stabilizers [2, 20–22]. The inherent osseous stability of the shoulder is relatively small as only about 30 % of the humeral head articulates with the glenoid fossa at any one time [8, 22].

The primary dynamic stabilizers of the glenohumeral joint are the rotator cuff tendons and muscles, the biceps tendon, and the scapular muscles [1, 10, 13, 17, 22–24]. Coordinated muscle contraction of these dynamic stabilizers produces a synergistic effect that increases the compression of the humeral head against the glenoid fossa, resulting in an increase in the load required to translate the humeral head [17, 18, 21, 22, 25].

The primary static stabilizers of the glenohumeral joint are the osseous articulation, the glenoid labrum, and the capsuloligamentous structure [21, 22, 24]. The glenoid labrum is a rim of fibrocartilaginous tissue attached at the periphery of the glenoid that increases the depth of the socket by approximately 50 %, allowing for increased articulation between the humeral head and glenoid fossa. It also provides an attachment site for the long head of the biceps tendon and glenohumeral ligaments [13, 21, 22]. The superior, middle, and inferior glenohumeral ligaments are distinct capsular thickenings that limit excessive humeral translation at the extremes of motion [2, 21]. These ligaments only contribute to mechanical stability when stretched beyond their rest length; thus, depending on the humeral location within the glenoid, there is variable contribution of these ligaments to joint stability [2, 17, 18, 22, 25].

The inferior glenohumeral ligament is a complex of three distinct parts, the anterior and posterior bands and axillary pouch, all of which originate from the anteroinferior labrum and glenoid rim, and it is the most important stabilizing factor against anteroinferior shoulder dislocation [2, 13, 17, 24, 25]. Clinically, the anterior band is the most important because it is the primary stabilizer preventing excessive anterior glenohumeral translation when the shoulder is abducted and externally rotated, the same position that most anterior humeral dislocations occur [10, 13]. The superior glenohumeral ligament arises from the anterosuperior labrum and is the primary restraint of inferior translation when the shoulder is adducted and in a neutral position [20, 21]. The middle glenohumeral ligament arises adjacent to the superior glenohumeral ligament and is the most variable of the three [18]. The middle glenohumeral ligament prevents anterior glenohumeral translation when the shoulder is held in midrange abduction [20].

Glenohumeral joint instability occurs when excessive or repetitive force applied to the joint exceeds the force of these dynamic and static stabilizers, and depending on the mechanism of injury and the direction of dislocation, typically there is associated injury to one or more of these stabilizers [10, 22]. All dislocations have the potential to damage surrounding structures and cause various complications, such as neurological injury, vascular injury, fracture, and rotator cuff tears, which may negatively impair functional outcome and increase the risk of recurrence [25, 26]. Rotator cuff tears, fractures of the greater tuberosity, and neurological injuries are more common in women and in patients over the age of 60 [27].

Anterior shoulder dislocations are most often the result of forceful external rotation and abduction of the humerus [7, 9, 10, 13]. In this position, the excessive anterior glenohumeral translation commonly causes tear of the anterior-inferior labrum, referred to clinically as a Bankart lesion [10, 20, 22], which is seen in over 90 % of cases of traumatic humeral dislocations [10, 13]. When the humeral head dislocates anteriorly, impaction of the humeral head against the anteroinferior glenoid may cause a posterolateral humeral head compression fracture, referred to as a Hill-Sachs lesion [10, 13, 20, 25]. Furthermore, in adults over 40 years of age who sustain a shoulder dislocation, rotator cuff injury is a common sequela [26].

Patients presenting with acute shoulder instability will typically have a clear history of trauma resulting in obvious dislocation of the joint requiring mechanical reduction or the subjective feeling of instability, indicating joint subluxation [13, 28, 29]. In contrast, patients who are not able to recall a definitive onset to their symptoms may have generalized laxity [10, 28]. When obtaining the patient history, it is critical to determine the mechanism of injury as well as the position of the arm during the injury, as this may help distinguish between an anterior or posterior dislocation [28]. Patients presenting with acute traumatic anterior dislocations typically describe the arm as being extended, abducted, and externally rotated at the time of injury. Other important components of the history include the number of previous episodes of subluxation or dislocation and the age at which these occurred, previous physical therapy or surgery to the shoulder, and the presence of pain or any other associated symptoms [13, 20, 30].

Physical examination should begin with general inspection for any gross abnormalities, especially if there is a history of trauma, followed by evaluation of active and passive range of movement [12]. In the setting of a dislocated shoulder, a visible deformity of the shoulder is often present. On examination, both active and passive movements are restricted due to pain, and the arm is typically held fixed in slight internal rotation and abduction, with resultant flattening of the shoulder contour [20]. A full neurovascular examination should be performed to assess the motor and sensory functions of the axillary, musculocutaneous, median, radial, and ulnar nerves and to palpate the radial and ulnar pulses [10, 13].

Patients presenting with acute traumatic posterior dislocations typically describe the arm as being forward flexed, adducted, and internally rotated at the time of injury and may have pain or a subjective feeling of instability when the arm is adducted and internally rotated [21, 28]. Athletes involved in blocking, such as football lineman and rugby players, are prone to this type of injury [16]. Additional features of the history that may suggest posterior dislocation include a history of epilepsy, electroconvulsive shock therapy, and alcohol withdrawal seizures [10, 21].

Following physical exam, conventional radiography is often the next step in evaluating most shoulder pathology. Plain films can be tailored according to the suspected condition [31, 32]. When evaluating shoulder instability, it is important to obtain orthogonal views, typically consisting of at least an anteroposterior (AP) and axillary lateral view [31]. Some clinicians may prefer a complete shoulder series including anterior-posterior views in both internal and external rotation, axillary, outlet, and 30 ° caudal tilt views.

In most patients, acute shoulder dislocation causes significant pain and muscle spasm, particularly if it is a first-time dislocation, so it is important to offer the patient analgesia to reduce pain and promote muscle relaxation before attempting reduction [5, 11, 14, 20]. Intra-articular lidocaine (IAL) injection and intravenous analgesia with or without sedation provide effective analgesia and patient satisfaction during the reduction maneuver with equal rates of successful reduction; however, IAL is associated with less side effects and a shorter recovery time [10, 11, 13, 14]. Reduction with minimal or no analgesia may be attempted in patients presenting very soon after dislocation and/or in patients with recurrent dislocation only associated with moderate pain, as the most effective treatment is rapid reduction of the joint [11].

Once pain control is adequate and radiographs performed exclude the presence of an associated fracture, prompt joint reduction is necessary to minimize the risk of neurovascular compromise and soft tissue stretch [10, 14, 33]. Numerous reduction techniques can be used for closed reduction of anterior dislocations, and choice depends on physician preference. The most utilized techniques for anterior shoulder dislocations include the traction-countertraction, Milch, Stimson, and scapular manipulation techniques [33]. In general, these techniques move the humeral head, typically in an anterior-inferior position, into a more favorable position relative to the glenoid to facilitate reduction. Sometimes, a combination of techniques is utilized to achieve success. Closed reduction of posterior dislocations is considerably more difficult, and due to the risk of fracture, some authors recommend that closed reduction should only be attempted in patients with an associated humeral head defect comprising less than 20 % who are within a short time frame of the dislocation [10, 33].

Following reduction of the joint, a complete neurovascular exam should be conducted to ensure that the patient did not sustain any nerve or vessel injuries [14, 26]. Repeat radiographs should be obtained immediately after the procedure to confirm joint reduction and assess for any osseous lesions not present on initial radiographs [14]. Due to the significantly increased incidence of rotator cuff tears in patients over the age of 40, it is imperative that these patients undergo careful evaluation for possible rotator cuff lesions in the follow-up time frame [14].