The Effect of Scapula Tilt and Best-Fit Circle Placement When Measuring Glenoid Bone Loss in Shoulder Instability Patients.

Purpose: To analyze the effect of lack of standardization on the reliability of current measurement techniques for glenoid bone loss in clinical practice.

Methods: Ten consecutive patients with anterior glenoid bone loss due to recurrent anterior shoulder instability and available computed tomographic (CT) scans of the affected shoulder were included in this study. One hundred seventy 3-dimensional en-face view images of the 10 glenoids with up to 20° degrees of tilt in the anterior, posterior, superior, and inferior direction were rendered. Three independent observers first identified the en-face view images and subsequently performed measurements of the defect surface and diameter as well as the glenoid surface and diameter on all 170 images. Measurements were completed based on the conventional best-fit circle technique using the edge of the visible glenoid bone as reference and additionally based on the so-called spoon technique, which places the best-fit circle on the edge of the visible glenoid concavity.

Results: The overall agreement regarding en-face view image selection between the observers was 30% (K-alpha = 0.10, 95% confidence interval 0.02-0.22). Tilt of the en-face view in any direction resulted in significant alterations of all 4 measurement parameters as well as the relative defect area and diameter (P < .05). The conventional and the spoon techniques rendered significantly different results regarding all 4 measurement parameters as well as the relative defect area (P < .05).

Conclusion: Impreciseness of scapula positioning for creation of an en-face view of the glenoid as well as varying best-fit circle placement significantly alter glenoid defect size measurement results.

Clinical Relevance: Because the glenoid defect size plays an important role in the choice of treatment for anterior shoulder instability, measurement techniques need to be as precise as possible.