Stability of the Glenohumeral Joint with Combined Humeral Head and Glenoid Defects: A Cadaveric Study

OBJECTIVES: Shoulders with recurrent anterior shoulder instability often have combined defects of the humeral head and glenoid. The aim of this study is to define the relationship of combined humeral head and glenoid defects on anterior shoulder instability. We hypothesize that combined humeral head and glenoid defects will produce greater instability than either defect found alone, and that the size of humeral head and glenoid defects which are “critical” and need to be repaired in order to restore glenohumeral stability will be smaller when the defects are found together than when either defect is found alone. METHODS: Eighteen shoulder specimens (mean age 57 years) were tested at 60° of glenohumeral abduction and 80° of glenohumeral external rotation (equivalent to 90° of abduction and 90° of external rotation relative to the trunk). Humeral head defect sizes included 6%, 19%, 31%, and 44% of the humeral head diameter. Glenoid defect sizes included 10%, 20%, and 30% of the glenoid width. Outcome measures included Percent of Intact Stability Ratio (stability ratio for a given trial divided by the stability ratio in the intact state for that specimen) and Percent of Intact Translation (distance to dislocation for a given trial divided by the distance to dislocation in the intact state for that specimen). RESULTS: There was a progressive decrease in stability as humeral head defect size increased and as glenoid defect size increased. The decrease in Percent of Intact Stability Ratio reached statistical significance for humeral head defects of 44%, for glenoid defects of 30%, and additionally for a combined 19% humeral head defect with a 20% glenoid defect. The mean Percent of Intact Stability Ratio was 65% for a combined 19% humeral head defect with a 20% glenoid defect. The decrease in Percent of Intact Translation reached statistical significance for humeral head defects of 31% and 44%, for glenoid defects of 20% and 30%, and additionally for a combined 19% humeral head defect with a 10% glenoid defect. The mean Percent of Intact Translation was 69% for a combined 19% humeral head defect with a 10% glenoid defect. CONCLUSION: The humeral head and glenoid defect sizes required to produce instability are smaller when the defects are found together than when either defect is found alone. In patients with combined humeral head and glenoid defects bony reconstruction may be indicated for humeral head defects as small as 19% of the humeral head diameter and glenoid defects as small as 10-20% of the glenoid width.