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

The role of morphologic variation in the etiology of antero-inferior 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).

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 (control group) were semi-automatically segmented into three-dimensional 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 anatomic landmarks were generated automatically. Models were aligned to the glenoid plane and center, and three-dimensional distances and angles were measured in both the original (absolute) models and the uniformly scaled models.

With both measurement techniques, the CP in GHAI was more superior to that in CTRL (mean difference [MD] = 6.5 mm, P = .001 absolute; MD = 6 mm, P = .003 for scaled) and 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 that in GHPI (MD = 3°, P = .009) and in CTRL (MD = 9°, P = .003). The lateral coracoid coverage in GHAI was lower than that in CTRL (MD = 13°, P = .008). The posterior acromial coverage of GHPI was decreased compared with that in both GHAI (MD = 5°, P = .006) and CTRL (MD = 5°, P = .018). All differences reported here remained statistically significant after Benjamini-Hochberg correction.

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 GHAI. Future studies should also determine whether, and under exactly which conditions, these morphologic findings should influence the surgical treatment decisions regarding recurrent anterior glenohumeral shoulder instability.