Scapular inclination and glenohumeral joint stability: A cadaveric study

Background. In shoulders with multidirectional instability, translation of the humeral head on the glenoid is increased in the midrange because of the following three reasons: the increased retroversion, a hypoplastic posteroinferior rim, and decreased scapular abduction during arm elevation. This study aimed to clarify the relationship between glenoid inclination and glenohumeral joint stability. Methods. Nine fresh-frozen cadaveric shoulders were tested. With a 50-N compressive load, the translation force was measured in the 3-o'clock, 6-o'clock, 9-o'clock, and 12-o'clock directions by using a tilt of 0°, 5°, 10°, 15°, and 20°. When the glenoid was tilted in one direction, the translation force was measured in the direction of inclination and in the opposite direction. The stability ratio was then calculated. Results. The stability ratio in the 3-o'clock direction significantly decreased with a tilt of more than 5° in the 3-o'clock direction. The stability ratio in the 9-o'clock direction significantly decreased with a tilt of more than 15° in the 9-o'clock direction and significantly increased with a tilt of more than 5° in the 3-o'clock direction. The stability ratio in the 6-o'clock direction significantly increased with a tilt of more than 10° in the 6-o'clock direction. Conclusions. The posterior and inferior stability increased with an anterior tilt of more than 5° and with a superior tilt of 10°, respectively. The anterior and posterior stability decreased with an anterior tilt of 5° and with a posterior tilt of 15°, respectively.