Abstract
Background
Glenoid baseplate fixation for reverse shoulder arthroplasty relies on the presence of sufficient bone stock and quality. Glenoid bone may be deficient in cases of primary erosions or due to bone loss in the setting of revision arthroplasty. In such cases, the best available bone for primary baseplate fixation usually lies within the three columns of the scapula. The purpose of this study was to characterise the relationship of the three columns of the scapula independent of glenoid anatomy and to establish the differences between male and female scapular anatomy.
Methods
Fifty cadaveric scapulae (25 male, 25 female) were analysed using CT-based imaging software. The surface geometries of the coracoid, scapular spine and inferior scapular column were delineated in the sagittal plane. A linear best-fit line was drawn to establish the long axis of each column independent of the glenoid. The width of the glenoid was measured and points marked at the midpoint of each measurement. A best-fit line starting at the supra glenoid tubercle passing through the midpoints was chosen as the superior inferior (SI) axis of the glenoid.
An orthogonal plane to the scapular plane was developed parallel to the glenoid face. The axis representing each of the three columns of the scapula and the SI axis of the glenoid, were projected onto this plane. The relationship between each column was analysed with respect to each other and with respect to the SI glenoid axis. Thus, measurements obtained gave the relationships of the three columns of the scapula (independent of the glenoid) and their relationships to the long axis of the glenoid (dependant on the glenoid). Comparisons were made between males and females using the independent t-tests.
Results
The mean angle between the scapular spine and the coracoid column was 93±13° with no significant difference between males (91±15°) and females (95±10°) (p=0.29). The angle between the inferior scapular column and the scapular spine was 6.5° greater (p=0.03) in females (134±10°) than in males (128±11°). Similarly, the angle between the inferior scapular column and the coracoid column was 11° greater (p=0.009) in males (141±15°) than in females (130±12°). No significant sex difference was found between the acromion and inferior scapular pillar with respect to the SI glenoid axis. However, the female coracoid was found to be more horizontal than the male coracoid in relation to the SI axis of the glenoid (p=0.037).
Conclusion
This study demonstrates that the relationship between the scapular spine and the coracoid column is independent of sex. However, sex has significant effects on the positions of the scapular spine and coracoid columns with respect to the inferior scapular column. The inferior scapular column is noted to be positioned more anterior (closer to the coracoid) in females than in males. Sex variations may be important when directing screws for baseplate fixation in bone deficient glenoids undergoing reverse shoulder arthroplasty.
