Humeral head defects have been associated with failed anterior shoulder instability repairs. Quantitative data are required to determine (1) the critical defect size for consideration of surgical repair and (2) the ability of proposed repair techniques to restore normal joint function.
Increasing defect size will decrease stability and anterior translation before dislocation. Stability will decrease in shoulder positions where the defect is oriented in line with the anterior glenoid. Osteoarticular repair will restore joint stability to intact shoulder level.
Controlled laboratory study.
A robotic/universal force-moment sensor testing system was used to apply joint compression (22 N) and an anterior load (40 N) to cadaveric shoulders (n = 9) with all soft tissues removed (intact) at joint orientations with 60° of glenohumeral abduction and 0° and 60° of external rotation. Four posterolateral osteoarticular defects were created (12.5%, 25.0%, 37.5%, and 50.0% defect) followed by an osteoarticular allograft transplantation (repair). The loading protocol was repeated in each shoulder state for both joint orientations. The anterior translation and stability ratio (anterior load/compressive load) were recorded before dislocation.
All shoulders dislocated at 60° of external rotation with all sizes of defects. At 0° of external rotation, shoulders with the 12.5% to 37.5% defects did not dislocate, and only 2 shoulders with the 50.0% defect dislocated. At 60° of external rotation, the 25.0% defect and 37.5% defect had significantly less anterior translation before dislocation, as compared with the intact (P < .05), both of which became similar to the intact after repair (P > .05). The stability ratio at 60° of external rotation significantly decreased in the 25.0% and 37.5% defects, as compared with the intact (P < .05), representing a 25% and 40% decrease in stability ratio. The stability ratio became similar to intact after repair (P > .05).
The size and orientation of the defect has important contributions to glenohumeral joint function. Increasing defect size required less anterior translation before dislocation and decreased the stability ratio, thereby increasing the risk of recurrent instability.
Defects as small as 12.5% of the humeral head have biomechanical consequences that may affect joint stability. In addition, shoulders with large osteoarticular defects (37.5% or 50.0%) may benefit from osteoarticular allograft transplantation to restore shoulder stability.