Shoulder Subluxation

Etiology

The etiology can be classified as traumatic, non-traumatic, and neuromuscular causes. For the traumatic cause, it is more frequent in active young individuals[1]. Shoulder subluxation is prevalent in boxers.[3] However, a non-contact sport with repetitive shoulder movements and a hand in the outstretched position can also cause subluxation. For the non-traumatic cause, the etiology could be multifactorial. The patients may have suboptimal shoulder muscle control or tendon/ligament injury in the rotator cuff interval.[4][5] These 2 patterns of shoulder instability can be defined as TUBS syndrome (traumatic, unilateral dislocations with a Bankart lesion, often requiring surgery) and AMBRI syndrome (atraumatic, multidirectional, bilateral shoulder subluxation/dislocations, often treated with rehabilitation, and sometimes requiring inferior capsular shift), respectively.[6] The neuromuscular causes, such as stroke, cerebral palsy, and brachial plexus injury, can also lead to shoulder subluxation.[7][8]

Epidemiology

A limited number of studies investigate the epidemiology of shoulder subluxation because most studies focus more on shoulder dislocations.

For traumatic causes, a study investigating U.S. military academy athletes showed that 84.6% of sport-related shoulder instabilities were shoulder subluxation, and 15.4% were shoulder dislocation. Among the cases with shoulder subluxations, 45.5% experienced the first subluxation event, while the remaining 54.5% had recurrent shoulder subluxation.[3] In non-traumatic shoulder subluxation, a study conducted in Japan reported that among the population with shoulder disabilities, 3.9% had non-traumatic shoulder subluxation. Younger patients have a higher risk of shoulder dislocation, no matter what causes it.[5] Furthermore, 17 to 81% of stroke patients had shoulder subluxation.[9]

Pathophysiology

The anterior labral detachment is the most common traumatic cause leading to anterior shoulder instability.[1] In the non-traumatic population, the loose and redundant inferior capsule, ligamentous laxity, and injury around the rotator cuff interval affect shoulder movement and lead to subsequent glenohumeral instability.[5] For neuromuscular cases, weakness of the rotator cuff and shoulder girdle muscles results in shoulder inferior subluxation. Spasticity of internal rotation muscles (latissimus dorsi, pectoralis major, and subscapularis muscles) results in a backward tendency of the humeral head and causes posterior shoulder subluxation.[10]

Histopathology

Adolescents frequently experience recurrent subluxations. There is a higher amount of collagen type III (more elastic than type I collagen) in ligaments and tendons. With the increase in age, collagen-producing cells produce more collagen type I, which is less insoluble and more stable.[11]

For the atraumatic cause, the cysteine contents, which are abundant in collagen type III, are higher in the shoulder joint capsule when compared with the normal population. In addition, the adaptation and remodeling of collagen in the joint capsules of unstable shoulders show higher stable and reducible collagen cross-links, collagen fibril diameters, and elastin content to provide strength for the shoulder capsule. The collagen study in the skin tissue shows smaller collagen fibril diameters correlate with increases in glenohumeral joint laxity.[12]

History and Physical

The patients present with stiffness, mild pain, and instability of the shoulder girdle. A history of previous dislocation, mechanism of injury (direction of shoulder movements during the traumatic event), and family history of shoulder instability are also important.[13]

For stroke patients with hemiplegia, although there is no clear association between shoulder subluxation and pain, both medical conditions can exist simultaneously. Furthermore, some patients have a limited range of motions on the hemiplegic shoulder after shoulder subluxation.

During physical examinations, subluxation is detected by palpation of the glenohumeral joint. The extent of shoulder subluxation can be quantified by the distance from the acromion to the humeral head, using fingerbreadth, caliper, or tape.[14] However, the physical examination should be performed bilaterally, and a comparison between both sides is important for establishing a correct diagnosis. Inspection of shoulder contours, the examination of the active and passive range of motion, and motor and sensory testing are suggested to determine the causes of shoulder subluxation.

The tests to examine shoulder laxity include the load and shift, drawer, sulcus, hyperabduction, and push-and-pull tests.[15][16] The apprehension and relocation test is specific for examining anterior shoulder instability.[16]

For patients with a history of stroke, if they present with concomitant subluxation and shoulder pain, the physical examination for the rotator cuff tendinopathy, acromioclavicular arthropathy, and subacromial impingement helps identify the cause of shoulder pain.[17]

Evaluation

Patients with traumatic shoulder subluxations should have plain radiographs of anteroposterior, scapular-Y, and axilla views done to evaluate the severity of shoulder subluxation and collateral injuries to the bones and joints. Diagnostic ultrasound and magnetic resonance imaging (MRI) can provide the details of soft tissue injuries such as the labral tear. Computerized tomography (CT) scan is mainly indicated for traumatic shoulder dislocation to rule out subtle fractures not visible on radiographs or to assess the severity of associated fractures already noticed on the radiographs.[18][19]

In atraumatic shoulder subluxations, shoulder radiographs can be arranged to evaluate the translation of the humeral head with respect to the glenoid. It may be helpful to detect some bony factors leading to shoulder instability, such as a hypoplastic/dysplastic glenoid with a flat glenoid fossa. Stress radiographs, arthrography, MRI, or arthroscopy, are not mandatory for diagnosis unless some underlying pathology is suspected.[19]

For hemiplegic shoulder subluxation, the shoulder radiographic can be arranged to follow up on the treatment effectiveness or development of glenohumeral subluxation over time. However, palpation of the subacromial gap for grading the extent of shoulder dislocation seems to be reliable and unnecessary radiographs should be avoided. Also, diagnostic ultrasound can help to investigate the causes of shoulder pain with subluxation, such as bicipital or supraspinatus tendinosis and impingement.[9]

Treatment / Management

Non-Surgical Treatment

In traumatic shoulder subluxation, the immediate treatments include ice packing to reduce soft tissue swelling, avoiding postures leading to recurrent subluxation, and wearing a protective arm sling. Narcotics or non-steroid anti-inflammatory drugs can be used for pain control. The passive or active assistive range of motion exercise of the upper limb and scapular stabilization exercise can start as early as possible, followed by strengthening of shoulder girdle muscles and glenohumeral joint proprioceptive training to improve dynamic shoulder stability.[20][21]

In the atraumatic group, the goal of treatment is to restore shoulder function. The rehabilitation should emphasize the progressive strengthening of the rotator cuff, deltoid, and scapular stabilization muscles. Exercises to improve shoulder coordination with lifestyle modification are also recommended.[4]

Hemiplegic shoulder subluxation and functional electrical stimulation (FES) are effective in reducing subluxation in the acute stage. Shoulder support or orthoses such as Bobath, Rolyan humeral cuff, or standard hemi sling may reduce the subluxation. To prevent further subluxation in stroke patients, supporting the hemiplegic limb in the proper position is crucial.[9][7](A1)

The modalities for pain control include ice in an acute phase, ultrasound therapy, transcutaneous electrical nerve stimulation (TENS), electrical stimulation (ES), and moist heat.[20][21]

Surgical Treatment

Surgical management is considered in younger patients or active individuals with chronic shoulder subluxations. The indications for the operation include failed non-operative treatment, recurrent dislocation in younger age, irreducible dislocation, open dislocation, unstable reduction, the first time in young adults with traumatic unilateral dislocations, high work demand, and bony Bankart lesion.[22]

The surgical procedures are classified into arthroscopic and open surgeries. Open shoulder stabilization surgery is needed if there is a contraindication for arthroscopic management like Hill-Sachs lesions, glenoid defects of more than 30%, humeral avulsion glenohumeral ligament (HAGL) injury, shoulder instability with the bony fragment, shoulder deformity and previous surgery of shoulder stabilization. The operations commonly performed are the capsular-shift procedure to tighten the shoulder capsule, Bankart repair for labral tears, the Remplissage procedure for the Hill-Sach lesion, and the Latarjet procedure for the glenoid bone loss of more than 30%. The capsular shift, Bankart repair, and Rempissage procedure can be done with open or arthroscopic techniques. The Latarjet procedure should be performed openly.[23]

When comparing the results of open and arthroscopic surgeries, arthroscopic management is as effective as open repair in improving shoulder stability. The advantages of arthroscopic surgery include shorter operative time and hospital stay, decreased morbidities, and complications, lower cost, and less surgical scar.[24] However, this arthroscopic technique highly depends on surgeons’ experiences and is not able to correct the bony defect.

Differential Diagnosis

In patients with shoulder subluxation, other differential diagnoses include bicipital tendinopathy, acromioclavicular joint, and rotator cuff injuries. However, in traumatic shoulder subluxation, other non-traumatic factors that lead the affected shoulder more vulnerable to subluxation should be scrutinized.

The differential diagnoses causing pain in hemiplegic shoulder subluxation consist of complex regional pain syndrome[17], rotator cuff tendinopathy[2][25], and shoulder girdle muscle spasticity.[10]

• Glenoid labrum tear
• Acromioclavicular joint injury
• Clavicle fractures
• Shoulder dislocation
• Swimmer's shoulder
• Bicipital tendonitis
• Rotator cuff injury

Prognosis

In traumatic shoulder subluxation, the recurrent rate is higher, especially in younger patients. Patients who have a greater awareness of their condition and modify their daily activities accordingly have a high chance of recovery. Meanwhile, immobilization for 3 to 4 weeks does not change the outcome, and immediate gentle mobilization is then allowed.[26] Fractures of the humeral head and intensity levels of activities are not associated with an increased risk of recurrent shoulder subluxation. Moreover, the range between 12% to 62% of shoulder osteoarthritis can develop following the operative management of shoulder instability.[27]

In atraumatic shoulder subluxation without concurrent fractures or labral injuries, around 20% of the patients have spontaneous recovery.[28]

In hemiplegic stroke patients, motor weakness and Brunnstrom’s arm motor stages are the predictors of the presence of glenohumeral subluxations, and there is the spontaneous reduction of shoulder subluxation in patients with significant motor recovery.[7]