[Biomechanical evaluation of glenohumeral stability through muscle force vector analysis. Effect of a decreased glenoid inclination in shoulders with global rotator cuff tears].
OBJECTIVE
The aim of the study was to quantify the decrease in glenohumeral stability following a global rotator cuff tear and to evaluate the effect of a decreased glenoid inclination angle through analysis of muscle force vectors in a computer model.


METHODS
The lines of action of eight shoulder muscles were integrated into a standard geometric model. Muscle force magnitudes were estimated based on physiological cross-sectional area and normalized electromyographic activity. The magnitude and elevation angle of the resultant force vector was calculated at 0, 30, 60, and 90 degrees of abduction. A rotator cuff tear was simulated by reduction of the corresponding muscle force vectors.


RESULTS
At 0 and 30 degrees of glenohumeral abduction a global rotator cuff tear showed a resultant force vector pointing outside the glenoid. In the computer model, decreasing the inclination angle of the glenoid by 30 degrees increased the stability in rotator cuff-deficient shoulders.


CONCLUSIONS
The results of this study provide a biomechanical rationale for clinical complications of global rotator cuff tear such as superior humeral head translation. The decreased glenoid inclination simulated in the computer model may represent a biomechanical basis for the development of new operative techniques to treat global rotator cuff tears.
Konrad
G
G
Department Orthopädie und Traumatologie, Universitätsklinikum Freiburg, Hugstetter Strasse 55, 79106 Freiburg. gerhard.konrad@uniklinik-freiburg.de
Markmiller
M
M
Rüter
A
A
Südkamp
N
N
D001696:Biomechanical Phenomena; D003198:Computer Simulation; D004576:Electromyography; D006801:Humans; D007593:Joint Instability / Q000503:physiopathology*; D018482:Muscle, Skeletal / Q000503:physiopathology*; D016059:Range of Motion, Articular / Q000502:physiology; D017006:Rotator Cuff / Q000503:physiopathology; D000070636:Rotator Cuff Injuries; D012783:Shoulder Dislocation / Q000503:physiopathology*; D012785:Shoulder Joint / Q000503:physiopathology*