DEMOGRAPHIC DIFFERENCES BETWEEN STRUCTURAL AND NON-STRUCTURAL (MUSCLE PATTERNING) INSTABILITY.

Abstract

This study identifies variations in presentation and demographics between structural and non-structural (muscle patterning) shoulder instability.

We analysed 1020 unstable shoulders (855 patients) from our institution database. Demographic details, direction and aetiology were obtained from medical records. Anterior dislocations comprised 67%, posterior 31% and inferior 2% of all directions of instability and 75 shoulders had multidirectional instability.

Structural causes were dominant in anterior instability (traumatic 39% and atraumatic 38%) and muscle patterning in posterior (81%) and inferior (90%) instability. Males accounted for 64% of all patients (73% of all structural patients and 53% of muscle patterning patients. Mean age at presentation was 25 years old (structural patients 28 years and muscle patterning patients 21 years old). There were 690 unilaterally unstable shoulders (57% right- and 43% left-sided); the dominant arm was affected in 58% overall, in 42% of all left-handers and only 33% of left-handers with muscle patterning. Bilateral shoulder instability occurred in 19% of all patients (12% of patients with structural instability and 28% of those with muscle patterning instability). For muscle patterning, the mean age at onset of symptoms was 14 years, and mean length of symptoms before presentation was 8 years. There was a trimodal distribution of age at onset of symptoms corresponding to peaks at 6, 14 and 20 years. In the group with onset of muscle patterning under 10 years old, there was a higher proportion of females (71% vs 47%), laxity (63% vs 29%) and bilaterality (54% vs 42%), and fewer presenting with pain (17% vs 50%).

Muscle patterning instability is associated with a demographic and presentation profile which may help distinguish it from structural forms of instability. As age at presentation increased, pain increased and joint laxity decreased. Bilaterality did not appear to be associated with gender, the presence of laxity or pain.