Objective: The objective of this study was to describe and validate a simple method to quantitatively calculate the missing area of the anterior part of the glenoid in anterior glenohumeral instability.
Materials and methods: The calculations were developed from three-dimensional (3D)-reconstructed computerized tomography en face images of the glenoid with "subtraction" of the humeral head in 13 consecutive cases with known anterior glenohumeral joint instability diagnosed by history and clinical examination. The inferior portion of the glenoid was approximated to a true circle whose center was determined by means of a femoral head gauge. The eroded anterior area was calculated as the ratio between the depth (a perpendicular line from the center of the circle to the eroded edge of the anterior glenoid) and the radius of the inferior glenoid circle. This data was then compared to the results obtained by two additional different methods: direct computerized measurements of the missing area and direct computerized measurement of the ratio between the radius and depth, on two dimensional computed tomography (CT) en face view reconstructions of the glenoid.
Results: We provide a function that correlates the ratio between depth and radius of the inferior glenoid circle and the area of the missing anterior glenoid. The results obtained by three different methods were comparable. Simple trigonometric calculations showed that a 5% area defect corresponds to 0.8 (12.5%) of the radius of the inferior glenoid, while a 20% area defect corresponds to 0.5 (50%) of the same radius (Table 1). Table 1 Results according to each different method Patient Sex Side CA1 MA1 PAM1 R2 D2 PAM2 R3 D3 PAM3 1 M R 738 19.1 2.58 15.1 13.4 2.45 16 14 2.6 2 M R 462.6 30.5 6.59 11.9 9.7 4.83 16 10 12.97 3 F L 359 24.5 6.82 17 11.8 9.86 11.8 17 9.86 4 M L 522 59.4 11.37 12.7 9.1 8.95 16 10 12.97 5 M L 670 93.1 13.89 13.6 7.6 16.84 16 9 16.31 6 M R 659 137.5 20.8 14.3 7.1 20.10 20 8 25.23 7 M L 520 137 26.34 11.6 5.1 23.49 16 8 19.55 AVG 12.63 12.36 14.21 SD 8.46 7.92 7.20 CA1 Area of circle directly measured by MPR software, MA1 missing area of circle measured by MPR software, PAM1 calculated (100 x MA1/CA1) percentage area missing for method 1, R2 radius of the circle measured by MPR software, D2 depth from the missing edge to the center of the circle measured by MPR software, PAM2 calculated Percentage area missing from R2 and D2 using the function "q" (Appendix), R3 radius of the circle measured with a femoral gauge, D3 depth from the missing edge to the center of the circle measured with a femoral gauge, PAM3 calculated Percentage area missing from R3 and D3 using the function "q" (Appendix), AVG average, SD standard deviation
Conclusion: Using this simple method and the function provided, the eroded area of the anterior part of the glenoid in anterior glenohumeral instability can be calculated preoperatively using a 3D CT reconstruction of the glenoid with "subtraction" of the humeral head, obviating the need for sophisticated software to obtain this critical information for preoperative decision making.