Classification of Glenohumeral Instability: A Proposed Modification of the FEDS System

Type of shoulder instability

Causative lesion

Traumatic

Capsule avulsion (Bankart lesion)

Atraumatic

Capsule excessive laxity

Transient (“dead arm syndrome”)

Bone Hill-Sachs lesion

Voluntary

Bone-fractured glenoid rim

Involuntary chronic laxity, (multidirectional)

Glenoid tilt

Muscle rupture (rotator cuff)

He reasoned that there was no one “essential lesion” of shoulder instability; the treatment of shoulder instability addresses the anatomic lesion.

Neer [7] classified anterior shoulder instability into three categories, based on the mechanism of the initial instability event: (1) atraumatic, including patients with generalized joint laxity; (2) traumatic, one major injury resulting in a dislocation; and (3) acquired, patients with repeated microtrauma including throwers, swimmers, etc. He advised exercise for groups 1 and 2, immobilization and surgery if recurrences continue, and inferior capsular shift for group 3. His classification did not specifically address specific anatomic lesions.

Gerber and Nyffeler [8] classified shoulder instability as static or dynamic. Superior static instability was attributed to massive rotator cuff tears. Static posterior instability was due to progressive static subluxation, usually associated with glenoid dysplasia or degenerative joint disease. Inferior subluxation was usually neurologic or luxatio erecta.

Dynamic instabilities were classified according to direction including multidirectional, presence or absence of hyperlaxity, and involuntary versus three types of voluntary dislocations. They included locked anterior and posterior dislocations in the dynamic group. The treatment of the instability was directed at the type of pathology.

Despite the common occurrence of recurrent shoulder instability, no single classification system has been adopted by the orthopedic community.

2.3 The FEDS System

Kuhn [10, 11] introduced the FEDS classification of shoulder instability to address the inconsistencies in defining shoulder instability. He recognized that most systems were procedure based and not condition based. Additionally, the many terms used to describe instability, e.g., voluntary, multidirectional, traumatic, bidirectional, etc., were poorly defined and thus made it difficult to directly compare the results of many published studies. As an example, in one of the first reports on multidirectional instability, Neer and Foster included several different patterns of shoulder instability under the term “multidirectional”: the common theme was that he treated every type with an anterior-inferior shift.

From the outset, he reasoned that the term “instability” needed to be defined. After a review of the literature, he concluded that instability required both discomfort and a feeling of looseness, slipping, or the shoulder “going out” of joint. He then performed a systematic review of the orthopedic literature to identify criteria used by previous authors to define types of shoulder instability. Etiology, direction, severity, and frequency (all define below) were the four most commonly used features. He then used this information to develop the FEDS system.

Frequency was felt to be an indirect measure of the severity of the pathology and was divided into three categories: solitary, one episode of instability; occasional, two to five episodes; and frequent, more than five episodes. The difference between occasional and frequent is somewhat arbitrary.

Etiology was divided between those with a traumatic etiology and those who did not have a specific event that led to their instability (atraumatic). Athletes with pain with overhead activities, but no feeling of slipping, were not included as the author felt that they did not have true instability by their definition.

Direction was defined as the direction of the patient’s most severe symptoms (anterior, inferior, posterior). If the patient could not tell the physician which direction was most symptomatic, the physician was to use common provocative examination tests, i.e., apprehension test, sulcus sign, and jerk test, to determine the most symptomatic direction of instability. They purposely did not include the term “multidirectional” in the classification because of the confusing and often contradictory descriptions of this entity in the literature.

Severity was divided into subluxations and dislocations depending on whether the shoulder auto-reduced or required a maneuver to reduce it.

The advantages of the system are obvious. The FEDS system required data only from a history and physical examination to classify the patient’s instability type. It has been shown to have content validity and is highly reliable for classifying glenohumeral instability [10, 11]. The classification of instability is immediately identified without additional imaging. For the most part, the classifications follow the outline as set forth in ICD-10, making the system also very useful for coding and billing purposes as well.

2.4 Deficiencies of the FEDS System

While extremely useful and easy to utilize, classifying a patient’s shoulder instability in FEDS system does not classify instability sufficiently to direct appropriate treatment or to allow several researchers at multiple institutions assurance that their patients all have the same type of instability. Consider the three following cases of first-time shoulder dislocations that illustrate this point.

Case #2.1

A 17-year-old male suffered a first-time anterior shoulder dislocation when his dominant right arm was hit while shooting on goal in a water polo game. The shoulder did not relocate spontaneously but did relocate with gentle internal rotation once he got out of the pool. The MRI is seen in Fig. 2.1a.