Evaluating Glenoid Bone Loss Location Using Best Fit and Outer Circles: Are We Pointing in the Same Direction?

Background: Clinicians commonly use a clockface model to describe glenoid lesions, but variations in the usage of best fit and outer circles are often observed. Determining the compatibility of these methods is crucial, as discrepancies may affect clinical practice and the interpretation of previous research outcomes. Hypothesis: The best fit circle and outer circle assess the location of glenoid bone loss differently. Study Design: Cohort study (diagnosis); Level of evidence, 3. Methods: This study included 231 patients with recurrent anterior shoulder instability who underwent 3-dimensional computed tomography. The authors assessed the location, orientation, and extent of glenoid defects based on clock measurements, using templates of both the outer and best fit circles. They investigated discrepancies between the 2 methods and explored factors that influenced these discrepancies. Results: The orientation of the linear defect showed subtle but statistically significant differences between the circles (3:02 ± 0:21 for the outer circle and 3:03 ± 0:22 for the best fit circle; P < .001). The outer circle frequently overestimated the location of the defect's center and the superior and inferior margins (differences of 0:42 ± 0:16, 0:30 ± 0:10, and 0:17 ± 0:13, respectively; all P < .001). Additionally, the anterior or posterior placement of the best fit circle's center relative to the glenoid axis was associated with all measurement discrepancies. While discrepancies in the measurements of the center, superior, and inferior margins correlated with glenoid defect size (P < .001 for all), the orientation of the defect line did not correlate with defect size (P = .456), indicating that the defect may expand while maintaining its original angle. Conclusion: The defect locations of the outer circle and best fit circle differed statistically. Although the mean difference was <1 hour, caution is needed when interpreting findings across studies or in surgical communication, as reference points or clockface systems may differ between them. Among these locations, the orientation of the defect line appears stable with respect to defect size in each circle, suggesting that the defect may serve as a reliable reference for describing anchor or bone block placement during surgery.