The Effect of Variation in Definition on the Diagnosis of Multidirectional Instability of the Shoulder

Background: There currently is a wide variation in the definition of multidirectional instability of the shoulder in the literature. The purpose of this study was to determine if these variations influence the distribution of the diagnoses in a cohort of patients with shoulder instability. Methods: A cohort of 168 patients who underwent shoulder surgery for instability of any type was studied. Statistical analysis was performed in two steps. First, the instability of the shoulder in each patient was classified with the use of four existing systems, and the number of patients classified as having multidirectional instability was compared among the classification systems. Second, the definition of multidirectional instability was modified so that the result of laxity testing was the criterion for making the diagnosis, and the changes in the distribution of patients with a diagnosis of multidirectional instability were analyzed. Results: Classification with the four existing systems resulted in significant differences in the number of patients diagnosed as having multidirectional instability, with two (1.2%), seven (4.2%), thirteen (7.7%), and fourteen patients (8.3%) so diagnosed (p < 0.05). Modification of the definition of multidirectional instability so that it was based on laxity testing resulted in a wide variation in the number of patients diagnosed as having multidirectional instability; these numbers ranged from fourteen (8.3%) to 139 (82.7%) (p < 0.05). Conclusions: This study demonstrated that variations in the criteria used for the diagnosis of multidirectional instability significantly affect the distribution of patients with that diagnosis. The use of laxity testing tends to result in an overestimation of the number of patients with this condition. This observation is important because the results of studies may vary if patients with traumatic instability are considered to have multidirectional instability on the basis of laxity testing. Investigators studying patients with multidirectional instability should carefully define the inclusion criteria that they used.