Currently there is no accepted classification system for glenohumeral joint instability, which leads to confusion in the literature. McFarland et al compared four different classification systems for patients with instability and found great variation, particularly with regard to multidirectional instability,23
leading the editors of the Journal of Bone and Joint Surgery to opine that McFarland’s article was a “…provocative call to action”, and “Until the criteria for diagnosis are clearly defined, investigators will be unable to contribute in a compelling way to understand the condition since they cannot know whether studies are comparing ‘apples and oranges’.30
This was supported by work by Chahal et al,3
where physicians had poor agreement when asked to classify patient scenarios of glenohumeral joint instability.
This difficulty may stem from the fact that much of the historic literature in orthopaedics is treatment-based
, whereas all patients who received a particular treatment are reviewed retrospectively. This is exemplified by Neer’s classic paper on multidirectional instability.25
Neer included all patients who had an inferior capsular shift, yet his patient population was diverse with a variety of instability features (17% atraumatic, 73% traumatic; 73% with anterior symptoms, 73% with posterior symptoms; 5% dislocations, and 95% subluxations). These treatment-based studies with a mixed population of patients leads to confusion defining and classifying diagnoses, and as a result leads to confusion regarding which treatments are effective. Ideally research should be condition-based
, where a clearly defined group of patients is isolated and different treatments are compared. A valid and reliable classification system for glenohumeral joint instability will allow for this kind of condition-based research.
The FEDS system for classifying instability meets this challenge. It has content validity based on published literature and a survey of experts in the field. It has been shown to be very reliable as well. It is simple to use and does not require expensive diagnostic imaging or examinations under anesthesia. Interestingly, none of the individual components of the FEDS system (frequency, etiology, direction or severity) demonstrated higher agreement among physicians than the others. Also of interest was the unexpected finding that patients and physicians agreed on the direction of instability 82% of the time, with substantial kappa strength, suggesting that patients may be able to accurately describe the direction of their instability.
It could be argued that this classification system is limited as it does not include some of the accepted instability descriptors historically used to classify patients, namely voluntary instability, subtle instability, and multidirectional instability.
Voluntary instability is a concept best explored by Rowe in 1973.34
In this landmark study, Rowe collected a group of patients who were able to demonstrate their instability to the clinician. Rowe administered psychological profile testing to these patients and determined that those who scored poorly did not do as well with surgical intervention. These data suggest there are two populations of people who can demonstrate their shoulder instability. Some are reluctant, but can show their instability to the treating physician, typically with pain or discomfort, a group we call demonstrable instability
. Others can demonstrate their instability for secondary gain or other issues, which we call volitional instability
. However, because physicians do not perform psychological testing on their patients, this concept has led to a great amount of confusion with a variety of other descriptors for this condition in the literature including “habitual instability”14
(which has erroneously included voluntary and involuntary by some authors11
) and “involuntary positional instability”.38
Because these definitions are confused in the literature and it is difficult to distinguish which patients may have psychological issues without psychological testing,19,34
we believe the term “voluntary instability” is not particularly accurate in classifying glenohumeral joint instability.17
We would suggest using the FEDS classification system to describe these patients and using the terms “demonstrable” or “volitional” as subcategories only if psychological profile testing is used to distinguish these patients.
Carter Rowe also described the “Dead Arm” Syndrome in 1987.33
Many of his patients were aware of their arm slipping, others were not. He considered all to have instability and performed instability surgery to treat them. In the FEDS classification system, only those who feel as if their arm is slipping would be considered to have instability. The problem is, as Rowe noted, pain is not specific for instability. Many of Rowe’s patients had “signs and symptoms of bursitis, biceps tendonitis, nerve impingement, cervical spine referred pain, and thoracic outlet syndrome”.33
As such, it is not clear if these patients truly had instability. We cannot include these patients in a classification of instability without severely diluting the accuracy of the diagnosis. Similarly, Frank Jobe, in 1989 created a term for an athlete with shoulder pain called “subtle instability”12
(also known as “occult instability” as described by Garth et al8
). In this condition the patient may not have symptoms of the shoulder subluxing or dislocating. Yet excessive laxity presumably leads to other pathologies and other symptoms like pain. Jobe used an instability operation to treat these patients and reported good success.12
We would argue that the term “subtle instability” is a poor choice, and that perhaps “presumptive excessive laxity” would have been better, as these patients have symptoms of pain and not a sensation of a loose, slipping, or dislocating shoulder.17
We believe that as our understanding of the pathomechanics of the thrower’s shoulder develops, a unique system for classifying different grades of pathology in the painful shoulder of the athlete will evolve.
In 1980, Neer described the condition of multidirectional instability, which gained widespread acceptance.25
We purposefully decided to avoid the concept of “multidirectional instability”, and instead focused on the primary direction
of symptoms when describing the direction of the instability. We did this for the following reasons: 1.) the term “multidirectional instability” has been used by different authors to mean different things.23
As a result the literature is very confusing,3,13,20,40
and it is doubtful that a consensus for this term will ever be reached. 2.) Neer originally described the condition of multidirectional instability as having the sine qua non feature of an increased sulcus sign.25
His patients would not be neglected in the FEDS system, which would classify these patients in the primary direction inferior groups. In our opinion, the FEDS classification would provide better resolution, as the other important features would segregate these patients with consistently less variation. 3.) it could be argued that every form of shoulder instability could have excessive translations in multiple planes as biomechanical research and clinical studies suggest that the capsule of the glenohumeral joint behaves as a circle and that injuries are unlikely to produce damage in only one part of the capsule.26,41,45–46,48
These points argue for the elimination of the concept of multidirectional instability and argue for the concept of a primary direction of the instability. Interestingly provocative physical examinations tests looking for a reproduction of the patient’s symptoms for instability, including the anterior apprehension test, the sulcus sign, and translation tests that reproduce symptoms have been found to be sensitive, specific, and have high predictive values, with reasonable inter-examiner reliability.2,21,37,42,43,48
Therefore, these features are the best available to evaluate patients with shoulder instability. In the FEDS system, they are used in a comparative fashion to identify the primary direction of instability by finding which provocative test is most uncomfortable or most closely reproduces the patient’s symptoms.
One potential criticism is the timing of the second visit for intra-rater reliability. Patients returned for repeat evaluation between 2 and 4 weeks after their initial evaluation. This interval was chosen as it is likely narrow enough to prevent changes in the status of the instability (e.g. a second event which could be more severe), and wide enough to prevent patient or physician recall which could influence the outcome.15,16
The sub-classifications of the FEDS system were based on a consensus of a group of shoulder experts as there is little data available to provide guidance. For example the frequency of instability was divided into solitary, occasional (2–5/year), and frequent (>5/year) somewhat arbitrarily. When clinical data becomes available, if a clinically meaningful threshold exists between occasional and frequent episodes, the FEDS classification system can be modified. With regard to severity, we defined a dislocation as requiring assistance in reducing the shoulder. Any patient who has required assistance at any time would be classified as a dislocation. Anatomically a dislocation is a complete dissociation of the humeral head from the glenoid. This would require radiographs in all patients to confirm the severity of the injury. Using the requirement for assistance to reduce the shoulder is a surrogate definition, yet likely correlates well with the anatomic definition. Future studies that correlate radiographic, magnetic resonance imaging, and surgical pathology to the FEDS definitions for instability may be required to validate this concept.
Another criticism is that anatomic features are not part of the general FEDS classification system. For example, a Frequent, Traumatic, Anterior Dislocation with a large bony defect likely requires a different surgical procedure than a patient without that pathology. We would argue that many features (anatomical, generalized laxity, activity level, occupation) may influence outcome, but before we can study their influence on outcome, we must first clearly define the population under study. As such, a researcher would first identify a population of Frequent, Traumatic Anterior Dislocation patients and then study the influence of these features as subtypes and assess their impact on outcome.
Another limitation is that our survey of the American Shoulder and Elbow Surgeons had a relatively low response rate (23.8%), which may bias the result. We attempted to remedy this problem by using only those features that were rated as extremely important with a high average score (>6.0 of 7.0 points) on the Likert scale. Interestingly the results of the literature search looking at the frequency of characteristics reflect the survey results in that features that are derived from the history and physical examination are not only the most commonly used features in previous studies, but are also those considered as extremely important by the respondents suggesting there is some content validity to the survey results.
Finally another potential criticism of the FEDS classification may be that it is capable of classifying patients into too many groups. The FEDS system has 36 potential classes of shoulder instability (), and each class represents a distinctly defined diagnosis. While this seems excessive, it is important to note that 15 classes would be extremely uncommon (e.g. atraumatic dislocations). It is clear that the system does have enough breadth to include other commonly described types of instability ().
Various Forms of Shoulder Instability Described using the FEDS System