Introduction

Glenohumeral instability is a commonly encountered problem in active populations, especially in young athletes. Our understanding of the anatomy and pathologic entities has evolved significantly since the initial descriptions of shoulder instability. As our knowledge has grown, so too has our armamentarium of procedures and therapies aimed at resolving the condition. A thorough history, clinical exam, appropriate radiologic studies, and a thorough characterization of each patient’s instability pattern will enable proper therapeutic interventions.

Definitions

The clinician must first discern instability from laxity. Laxity is a normal finding of asymptomatic glenohumeral translation, the degree of which may be affected by age, gender, fitness, and congenital factors. There is significant variability in the amount of shoulder translation in asymptomatic, normal subjects. On the other hand, instability is a pathologic process that results in excessive translation of the humeral head on the glenoid that results in pain, weakness, or performance degradation. Clinical instability is a disruption in the static restraints to glenohumeral motion, the osseous structures, capsule, labrum, and glenohumeral ligaments. Clinical instability is readily detected on translational assessment during routine physical exam maneuvers. A second category of instability is functional instability, a dynamic instability most common in overhead athletes. It is not readily detected on translational glenohumeral exam maneuvers and is commonly due to muscular imbalances, loss of proprioceptive control, and scapulothoracic dyskinesia. A thorough history and physical exam enable the clinician to discern between laxity and clinically significant instability.

It is also important to adequately define the terms used to describe instability. A glenohumeral dislocation event is a complete instability event in which no contact remains between the articular surfaces. This usually results in the humeral head coming to rest in the dislocated position, requiring a manual reduction by a trained provider. However, a spontaneous reduction is possible, especially in the case of glenoid or combined bone loss. The presence of a manual reduction maneuver in a patient’s history has often been used to ensure that a dislocation has occurred for research purposes.¹^,² The definition of an incomplete instability event has been elusive in the literature. Most clinicians use the term “subluxation” to refer to all events that fall short of being complete events (dislocations), and that is the definition preferred by the authors. However, a subluxation event should not be confused with a microinstability event, which is a microtraumatic overuse-type injury mostly seen in throwing athletes.

Pearls

A glenohumeral dislocation event is defined by complete dissociation of articular surfaces and usually requires a manual reduction maneuver.

A subluxation event can encompass a wide spectrum of symptomatic translation, including near-dislocation events.

Classification historical background

Among the earliest means of differentiating shoulder instability was direction, with most dislocations being anterior. In 1952, McLaughlin reported on patients with posterior events and found them to comprise only 4% of all dislocations.³ This breakdown by directionality has held true over time, with a recent prospective cohort study showing 5% of dislocations in young athletes were posterior in direction.⁴ However, while complete instability events (dislocations) in the posterior direction are rare, posterior subluxation events are more common, comprising 11% of all subluxation events.⁴

The most commonly used dichotomy of instability events (both dislocation and subluxation) is related to mechanism of injury. Rowe was the first to use traumatic versus atraumatic mechanisms to classify dislocations and found a higher recurrence rate in atraumatic patients.⁵ Rockwood classified subluxations into traumatic and atraumatic injuries as well as separate voluntary instability patients,⁶ and showed good results with nonsurgical management of atraumatic and voluntary patients.⁷ Thomas and Matsen continued this theme and produced the most useful classification system to date: AMBRI and TUBS (Table 2-1).⁸ The authors separated shoulder instability into two broad categories, the first of which was made up of patients with Atraumatic, Multidirectional, Bilateral involvement, and who were treated initially with Rehabilitation or Inferior capsular shift. The second broad category included patients with a Traumatic dislocation, Unilateral direction, and who had commonly sustained a Bankart lesion requiring Surgery (TUBS). These two descriptors accurately describe both ends of the instability spectrum but do not address the gradations between.

Table 2-1 Thomas and Matsen Classification of Instability

T – Trauma	A – Atraumatic
U – Unidirectional	M – Multidirectional
B – Bankart	B – Bilateral
S – Surgery	R – Rehabilitation
	I – Inferior (capsule)

Gerber and Nyffeler parted from previous authors division of shoulder instability based on direction or mechanism and proposed a classification scheme based on static and dynamic shoulder instabilities.⁹ The authors identified three separate groups of instability: static, dynamic, and voluntary. The static group is subdivided by directionality and the dynamic group is subdivided based on the presence or absence of ligamentous laxity. Patients with voluntary instability comprised the third category.

Algorithmic approach

Because of the multitude of etiologies, mechanisms, directions, and contributing factors to glenohumeral instability, no encompassing classification scheme accurately accounted for patients’ unique instability. As a result, several authors have gone to a logarithmic or descriptive approach of shoulder instability.¹⁰^,¹¹ The variables in the algorithmic tree are acuity, direction, presence of trauma, degree of instability, and volitional control of the instability episodes. The algorithmic approach assists the surgeon in assessing the important variables in the patient’s condition, which identifies the presumed pathologic lesion and therapy.

The four variables from the patient history that are commonly linked to the shoulder pathology and subsequent treatment are direction of instability, etiology, frequency, and degree. These four elements of shoulder instability are not independent variables, but describing each characteristic of a patient’s condition allows for the most accurate and precise description. Many algorithmic approaches fail in rare circumstances, such as a patient with a traumatic Bankart and unilateral findings of an inferior sulcus sign associated with multidirectional instability.¹⁰

While many algorithmic approaches encompass the entire spectrum of glenohumeral instability, we offer a simplified algorithm that focuses on traumatic anterior instability that represents 80% of instability that occurs.⁴ We also have attempted to provide general treatment recommendations (surgery vs. rehabilitation) based on the characteristics of the instability (Fig. 2-2).

FIGURE 2-2 This figure is the authors’ proposed treatment algorithm for traumatic anterior instability, which comprises 80% of instability events. MDI, multidirectional instability; MRI, magnetic resonance imaging.

Physical exam

Although a patient’s history may guide the examiner to focus on specific aspects of the physical exam, a generalized upper examination is important to avoid missing contributing pathology that was misrepresented or omitted from the history. The physical exam of shoulder instability follows the same routine of thorough musculoskeletal exams with inspection, palpation, range of motion assessment, testing of motor strength, neurovascular examination, and specialized tests for shoulder instability. Examination of the shoulder should be preceded by an examination of the neck and cervical spine. The patient should have both shoulders completely exposed, and the examination should begin at each phase with the asymptomatic shoulder.

Inspection

During inspection of the shoulder girdle, muscular contour should be evaluated for asymmetry indicative of muscular atrophy of the deltoid, supraspinatus, or infraspinatus muscles. Squaring of the lateral shoulder border with a prominent acromion may represent deltoid atrophy from an axillary nerve injury (Fig. 2-3). Atrophy of the supraspinatus above the scapular spine may represent a large rotator cuff tear or suprascapular nerve entrapment at the suprascapular notch. Atrophy of the infraspinatus below the scapular spine may likewise represent a massive rotator cuff tear in an older patient or could indicate suprascapular nerve entrapment at the spinoglenoid notch in a throwing athlete, commonly from a ganglion cyst.

FIGURE 2-3 This clinical photograph shows a 23-year-old patient 3 months after sustaining traumatic glenohumeral dislocation from a motorcycle accident. This photograph shows deltoid atrophy from an axillary nerve palsy. The patient also sustained a bony Bankart lesion and displaced lesser tuberosity fracture repaired through an open anterior approach.

The position of the glenohumeral joint can be assessed by the position of the humeral head relative to the acromion. In a posterior dislocation, the humeral head may be prominent posteriorly, and the lateral acromion may be prominent in an inferior subluxation. The scapular body position also should be assessed for scapular winging. Finally, the skin should be inspected around the shoulder for evidence of previous surgical incisions, thin atrophic skin, or excessive scar widening that may indicate a collagen disorder.

The rotator cuff muscles also should be assessed specifically for strength. Jobe’s empty can test isolates the supraspinatus muscle (Fig. 2-4). The test is performed with the arm flexed to 90 degrees in the scapular plane, thumb pointed down, while the examiner applies a downward force. Pain or weakness resisting the downward pressure indicates pathology in the supraspinatus muscle. The “full can” test also can isolate the supraspinatus with the thumb pointed up, which may be less painful in patients with impingement.