The "angle" method of calculating glenoid bone loss: a validation study and review of current methods.

Hypothesis/Background: This study compares our "angle" method to the "best-fit circle" method and examines existing literature on glenoid bone loss measurement. We hypothesized that the "angle" method of calculating glenoid bone loss would perform comparably to the "best-fit circle" method. Methods: The "angle" method calculates bone loss by subtracting the area of the triangle placed along the length of the defect from the sector subtended by the arc of the bone defect and is an adaptation of the Lederman ratio to estimate glenoid bone loss. To validate our method, we measured glenoid bone loss on 26 shoulder computed tomography scans using both the "angle" and "best-fit circle" methods. All patients had the diagnosis of anterior shoulder instability and had a computed tomography arthrogram performed due to suspected bone loss; those with glenohumeral arthritis were excluded. Results: Twenty-six patients were included. The glenoid bone loss measured using the "angle" method was 17.5% (standard deviation [SD], 7) on average, while the "best-fit circle" method yielded 23.2% (SD, 7). The mean difference between the 2 methods was 5.4% (P ≤ .0001; SD, 2.1), with the "best-fit circle" method finding higher average bone loss. Intra-rater reliability was excellent for the "angle" method, with a correlation coefficient of 0.965-0.995 compared to 0.953 and 0.992 for the "best-fit circle" method. Conclusion: The "angle" method of calculating glenoid bone loss predicts a 5.4% decreased absolute bone loss than the commonly used "best-fit circle" method and shows high intra-rater reliability, with a correlation coefficient of 0.965-0.995.