Accuracy and Reliability of a Simple Calculation for Measuring Glenoid Bone Loss on 3-Dimensional Computed Tomography Scans.

PURPOSE: To establish the accuracy and reliability of the circle-line method (CLM) of measuring glenoid bone loss; to compare the CLM calculation with a traditionally used method of calculating a ratio; and to evaluate surgeons' ability to estimate the amount of glenoid bone loss before performing any calculations.

METHODS: Three-dimensional reconstructions of computed tomography scans of consecutive patients with anterior instability and glenoid bone loss were reviewed by 13 surgeons blinded to the diagnosis. The reviewers made estimations of bone loss before creating any measurements by viewing the available computed tomography scan as well as the 3-dimensional reconstructions. They selected an en face view of the glenoid to create a best-fit circle. Bone loss calculation with a traditional linear method as well as a CLM calculated by algebraic geometry was completed. The CLM requires calculation of the diameter of a best-fit circle on the glenoid, as well as the length of a single line along the circle representing the line of bone loss. All methods were compared with a computerized method of tracing the area of the glenoid within a best-fit circle. Interobserver and intraobserver calculations were performed. Analysis-of-variance testing was used to compare the estimates of bone loss versus the CLM-calculated bone loss. Tukey post hoc analysis was used to define the accuracy of the CLM calculation compared with a more traditional method of calculating bone loss.

RESULTS: Bone loss estimations were significantly different from CLM-calculated bone loss in all cases except those with greater than 25% bone loss. The CLM was more accurate in all types of bone loss except cases of greater than 25% bone loss. Interobserver reliability was very good for the glenoid diameter measurement and moderate for the CLM. Intraobserver reliability ranged from moderate to good for all methods of measurement.

CONCLUSIONS: Surgeon estimations of glenoid bone loss, as well as traditional line-measurement calculations, are inconsistent and unreliable for accurate determination of the optimal surgical treatment for anterior shoulder instability. The CLM is a simple, reproducible, and accurate method for determining glenoid bone loss and does not require specialized software or imaging protocols.

LEVEL OF EVIDENCE: Level II, diagnostic study.