In Vivo Glenoid Track Width Can Be Better Predicted With the Use of Shoulder Horizontal Extension Angle

Abstract

Background: The glenoid track concept has been widely used to assess the risk of instability caused by a bipolar lesion. The mean glenoid track width is reported to be 83% of the glenoid width. However, this width seems to be affected by the range of motion of the shoulder. By clarifying the relationship between the range of shoulder motion and the glenoid track width, a more precise determination of the glenoid track width for each individual could be possible. Purpose: To determine the relationship between the glenoid track width and the range of motion of healthy volunteers. Study Design: Descriptive laboratory study. Methods: Magnetic resonance imaging was taken in 41 shoulders of 21 healthy volunteers (mean age, 32 years) with the arm in maximum horizontal extension, with the arm kept in 90° of abduction and 90° of external rotation. Three-dimensional surface bone models of the glenoid and the humerus were created with image analysis software. The distance from the anterior rim of the glenoid to the medial margin of the footprint of the rotator cuff tendon was defined as the glenoid track width. Active and passive ranges of shoulder motion were measured in the supine and sitting positions. The correlations between the glenoid track width and the ranges of shoulder motion were investigated with Pearson correlation coefficients. Intra- and interobserver reliabilities based on the intraclass correlation coefficient were also analyzed to assess the reliability of the glenoid track measurement. Results: The intra- and interobserver reliabilities for the glenoid track measurement were excellent (0.988 and 0.988, respectively). Among all the measurements, the glenoid track width and the active range of motion in horizontal extension in the sitting position showed the greatest correlation coefficient (r = −0.623, P < .0001). A correlation between the glenoid track width and this angle was expressed as Y = −0.49X + 90, where X is the horizontal extension angle (degrees) and Y is the glenoid track width (percentage of glenoid width). Conclusion: The present data demonstrate that the greater the horizontal extension angle in abduction and external rotation, the smaller the glenoid track width. An individualized glenoid track width can be obtained by measuring the active horizontal extension angle with the arm in abduction and external rotation in the sitting position. Clinical Relevance: An individualized glenoid track width enables selection of a more precise surgical option by the on-track/off-track concept.