Measurement of glenoid bone loss: a comparison of measurement error between 45 degrees and 0 degrees bone loss models and with different posterior arthroscopy portal locations.

BACKGROUND: Osteotomies at an angle of 45 degrees to the long axis of the glenoid were originally used in a cadaveric model to simulate the bone loss that can occur clinically in anterior instability of the shoulder. However, this type of glenoid defect is not consistent with the usual clinical scenario, in which bone loss occurs nearly parallel (at 0 degrees) to the long axis of the glenoid.
PURPOSE: Our objectives were to compare the amount of glenoid bone loss measured after a 45 degrees glenoid osteotomy with that after a 0 degrees osteotomy and to determine differences in bone loss measurement from 2 different posterior shoulder portals. STUDY
DESIGN: Controlled laboratory study.
METHODS: Glenoids of 14 embalmed cadaveric shoulders (mean age, 81 years; range, 56-90) were mounted in a custom shoulder holder, and 2 posterior portals (2 and 3 o'clock) were fixed into place. The area of a best-fit circle of the inferior portion of the glenoid was digitally calculated, and 2 sequential osteotomies of 12.5% and 25% of anteroinferior glenoid bone loss area were created. Two different types of osteotomies were created: group 1, "inverted-pear" bone loss (45 degrees to the long axis of the glenoid); and group 2, "clinical" bone loss osteotomy (0 degrees to the long axis of the glenoid). Measurements of bone loss were performed based on the bare spot method from 2 simulated posterior portals at 2 and 3 o'clock using a calibrated probe and digital calipers. The osteotomy was measured in 3 different locations (upper, middle, and lower thirds).
RESULTS: In the 12.5% bone loss model, bone loss measurements for both groups were significantly higher than expected (22.2%-23.1% in group 1, 17.4%-17.9% in group 2; P = .031-.049). In the 25% bone loss model, the mean measured bone loss was 27.8% in group 1 and 27.5% in group 2; however, bone loss measurements varied significantly in group 1 based on measurement location along the osteotomy (upper third, 12.3%; middle third, 31.5%; lower third, 39.8% loss) (P = .01-.0001). In group 2, the bone loss measurements were less varied (23.5%-30.3%). There were no differences between the location of the posterior portal (2 vs 3 o'clock) in determination of glenoid bone loss for both the 12.5% and 25% osteotomies.
CONCLUSION: Glenoid bone loss determination in a 45 degrees osteotomy model significantly overestimates the amount of true glenoid bone loss. However, in a 0 degrees clinical bone loss simulation model, the arthroscopic bare spot method of bone loss determination was sufficiently accurate at all 3 areas (upper, middle, and lower third) of bone loss. Both the 2-o'clock and 3-o'clock posterior portals were accurate to determine the amount of glenoid bone loss as referenced from the bare spot. CLINICAL RELEVANCE: Arthroscopic determination of glenoid bone loss is more accurate than what has been previously described with the 45 degrees simulation model. Measurement of glenoid bone loss from either the 2-o'clock or 3-o'clock posterior portal is accurate in a clinical bone loss model.