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.