Functional shoulder instability (FSI) is defined as glenohumeral instability that is not caused by structural defects but rather by abnormal muscle activation patterns. Patients with FSI are able to dislocate their shoulder at will, either by motion (positional FSI) or even without moving the arm (non-positional FSI). In contrast to structural shoulder instability, little is known about the epidemiology of FSI. The aim of the following study was to further analyse this rare pathology and approximate the prevalence of FSI. A self-evaluated and anonymous online survey among 5866 medical students was conducted using the students email list of two german-speaking medical universities (Study Center 1 and Study Center 2). Possibly affected siblings were used as a supplementary group (Siblings Cohort). General sociodemographic data, dislocation mechanism, potential causes, age at the tie of developing first symptoms, general hyperlaxity, previous interventions, and sporting activity were evaluated and analyzed. The total number of email recipients and responses was used to estimate the minimal and maximal prevalence of FSI. Five hundred thirteen questionnaires were completed by the students and subsequently analyzed. In total, there were 32 participants with FSI. The minimal prevalence of FSI was found to be 0,5% and the maximal prevalence 2,6%. In most cases (67%) a positional FSI was reported. The majority of the patients reported that first symptoms developed under the age of 16?years (69%) and without any traumatic event (72%). Most of the affected participants had no therapeutic intervention for their FSI (69%) and performed non-overhead (59%) or overhead sports (28%). Functional shoulder instability (FSI) is more common than expected amongst young adults and seems to develop during childhood mostly without specific reason.
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