Anterior Shoulder Instability Is Associated With an Underlying Deficiency of the Bony Glenoid Concavity

Purpose To determine whether anterior shoulder instability is associated with an inherent deficiency of the bony glenoid concavity, which results in a reduced bony shoulder stability ratio (BSSR). Methods In this case‐control study, we searched the institutional database for patients treated for unilateral recurrent anterior shoulder instability. We included 30 consecutive patients with atraumatic instability, 30 consecutive patients with traumatic instability, and 36 matched healthy controls, for a total of 96 shoulders. Computed tomography images of the unaffected shoulders of the instability patients were compared with images of the ipsilateral shoulders of age‐ and sex‐matched healthy controls for differences in glenoid morphology. By use of a mathematical formula based on Pythagorean trigonometric identities, the mean BSSRs of the different groups were calculated and compared. Validation of the formula was accomplished by finite element analysis. Results The mean BSSR of atraumatic instability patients was 17.9% ± 8.5% and therefore significantly lower than the mean BSSR of 31.1% ± 7.5% of the control group (13.2%; 95% confidence interval [CI], 9.1% to 17.4%; P < .001). The mean BSSR of the traumatic instability group was higher, at 23.9% ± 8.5% ( P  = .007), but still showed a deficit of 7.2% (95% CI, 2.8% to 11.7%; P  = .002) compared with controls. The atraumatic instability group showed a mean reduction of 0.9 mm (95% CI, 0.6 to 1.1 mm; P < .001) in concavity depth and a decrease of 2.9° (95% CI, 0.4° to 5.3°; P  = .021) in concavity retroversion, whereas the traumatic instability patients had a reduction of 0.4 mm (95% CI, 0.1 to 0.8 mm; P  = .006) in concavity depth. Neither of the instability groups differed significantly from their respective controls in terms of glenoid concavity diameter, head radius, or glenoid vault morphology. Conclusions Anterior shoulder instability is associated with an inherent flattening of the bony glenoid concavity, which significantly decreases the BSSR. The deficiency appears to be more pronounced in patients with atraumatic instability than in patients with traumatic instability. Level of Evidence Level III, case‐control study.