Determining the accurate placement of a posterior portal in shoulder arthroscopy with the use of computerized tomography images

BACKGROUND: Portal placement is an important factor in performing a successful shoulder arthroscopy. Recent cadaveric studies have found variance in the anatomy of the glenohumeral joint. Our aim was to determine if computerized tomography (CT) images could be used to map the trajectory of the posterior portal objectively and then measure the distance between this trajectory and palpable landmarks to apply this knowledge to clinical practice. METHODS: Two-dimensional multiplanar reformatted CT images were generated using OsiriX (Pixmeo, Switzerland) from CT images performed in a tertiary hospital over a 1-month period. The center of the glenoid was identified and a trajectory through it radiologically mapped. Horizontal and lateral measurements were taken from this trajectory to both the posterolateral edge of the acromion and tip of the coracoid. RESULTS: Following application of inclusion and exclusion criteria, 226 shoulders were analyzed. Fifty scans were selected at random and re-reviewed by the primary examiner to assess intra-rater reliability which showed strong correlation and no significant differences between first and second measurements (P < .01, r > 0.6). The mean distance from acromion to portal trajectory was 1.39 cm inferiorly (95% confidence interval [CI] 1.31-1.48, standard deviation [SD] 0.65 cm) and 1.44 cm medially (95% CI 1.35-1.53, SD 0.71 cm). The mean distance from the coracoid to the trajectory was 1.71 cm inferiorly (95% CI 1.64-1.78, SD 0.55 cm) and 1.26 cm medially (95% CI 1-2-1.31, SD 0.45 cm). Paired t-test analysis between right and left shoulders within the same subject, where these data were available (n = 81), showed no significant difference (P > .05) between sides. Subset analysis was also performed between males and females, but only showed a significant difference between the vertical distance from the coracoid process to the center of the glenohumeral joint. This distance was shorter in females compared to males (1.56 cm in females compared to 1.84 cm in males, P < .001). CONCLUSIONS: Knowledge of shoulder anatomy is vital to the placement of arthroscopic portals, yet research on this topic has been based primarily on surface anatomy, small sample sized cadaveric studies or expert opinion alone. Our study shows that posterior portal placement in shoulder arthroscopy can be measured objectively using CT scanning.