Background
Traumatic dislocations of the shoulder are often ongoing with injuries of the glenoidal labrum or the bony glenoid. In consequence a loss of glenoidal concavity occurs and may have a strong influence on shoulder stability.
Recent studies indicate, that Bony Shoulder Stability Ratio (BSSR) based on bony glenoid concavity, is a precise tool to determine glenoidal stability and therefore optimize surgical decision making. Although the bony glenoid already brings some concavity, the major influence on morphological concavity may be given by the cartilaginous and labral structures. Therefore it was our aim to investigate differences between the cartilaginous and bony concavity of the glenoid, including the influence on shoulder stability.
Methods
10 human cadaveric shoulders were investigated by CT and MRI. Radii and depths in anteroposterior (AP) and superoinferior (SI) direction, as well as the radius of a sphere, which was best fitting to the glenoid were measured. MRI measurement was performed twice. Once including the labrum (MRI-i) and another visually ignoring the labrum (MRI-e). By using the BSSR formula, an estimation about the stability for each group was possible. Moreover tactile measurements of glenoidal cartilage were performed to investigate precision of MRI. Linear regressions were used to analyze interrelationships between the groups. Significant differences were recognized by using paired t-tests.
Results
No relevant correlations between stability estimations and radius measurements of CT and MRI-I were found. However, a correlation of r = 0.68 concerning the depth-measurements in AP direction was observed.
MRI-i showed a lower radius and a significantly higher glenoidal depth and stability than CT in AP direction. Comparing MRI-i with MRI-e we could find significant differences in the same way, without significanct differences for the radius. Comparing MRI-e and CT no significant differences could be found for depth and stability measurements. Radius was significantly smaller in CT, regarding the AP as well as SI direction. Using sphere radius instead of an unidirectional radius the deviation between the groups were reduced with a consequently higher comparability. Between MRI-e and tactile measurements a high correlation of r = 0.9 for the radius and r = 0.77 for depth could be shown.
Conclusion
Comparison between CT and MRI provided no comparable results as expected. Cartilage and especially labrum did highly influence the estimation of glenohumeral stability. Moreover MRI seemed to be precise to present the real morphological glenoidal structure. Nevertheless because of inaccuracy in dividing labral and capsular structures and the given bias by regarding the labrum as a static structure in MRI the stability increasing effect of the labrum seemed to be overestimated in MRI-i in AP direction. Using a sphere radius resulted in more realistic information about stability.