Antero-inferior glenohumeral instability is associated with an abnormal position of the coracoid process

Background The role of morphological variation in the etiology of anteroinferior glenohumeral instability (GHAI) remains debated. Clinically, the coracoid process (CP) varies in size and position, and simple coracoid lowering ("Trillat procedure") can prevent recurrent anterior dislocation. Additionally in some cases, fracture of the coracoid is necessary for an anterior dislocation to occur. Therefore, the CP position relative to the glenoid center was compared in stable and unstable shoulders, hypothesizing that the CP tip is more superior, more medial, and more anterior in patients with recurrent GHAI than in those with stable shoulders or posterior glenohumeral instability (GHPI). Methods Computed tomographic scans of shoulders of 60 patients with GHAI, age- and sex- matched to 30 shoulders of patients with GHPI and 13 unmatched individuals with healthy shoulders (CTRL) were semiautomatically segmented into three-dimensional (3D) models. All shoulders were scaled to uniform size and left-sided cases were mirrored to the right side. A statistical shape model was created for each cohort, and anatomical landmarks were generated automatically. Models were aligned to the glenoid plane and center, and 3D distances and angles were measured in both the original (absolute) models and the uniformly scaled models. Results With both measurement techniques, the CP in GHAI was more superior than in CTRL (mean difference (MD) 6.5 mm, P = .001 absolute; MD 6 mm, P = .003 for scaled) and more superior than in GHPI (MD 2.4 mm, P = .015 absolute; MD 2.2 mm, P = .021 scaled). The anterior coracoid coverage in GHAI was significantly lower than in GHPI (MD 3°, P=.009) and in CTRL (MD 9°, P = .003). The lateral coracoid coverage in GHAI was lower compared to CTRL (MD 13°, P = .008). The posterior acromial coverage of GHPI was decreased compared with both GHAI (MD 5°, P=.006) and CTRL (MD 5°, P=.018). All differences reported here remained statistically significant after Benjamini-Hochberg correction. Conclusions The position of the CP is more superior in patients with GHAI compared to GHPI and CTRL cohorts. Further research is needed to determine the biomechanical and clinical context in which a more superior CP position is associated with anteroinferior glenohumeral instability. Future studies should also determine whether, and under exactly which conditions, these morphological findings should influence the surgical treatment decisions of recurrent anterior glenohumeral shoulder instability.