The size of the supraspinatus outlet during elevation of the arm in the frontal and sagittal plane: a 3-D model study.
OBJECTIVE
To quantify the size of the supraspinatus outlet as it is dictated by both the three-dimensional geometry of the shoulder and the relative orientation of the humerus with respect to the scapula during motions of the arm.


METHODS
Previously obtained data of shoulder kinematics were brought into a geometrical model of the shoulder, derived from a cadaver study.


BACKGROUND
Knowledge of the parameters dictating the size of the supraspinatus outlet is essential for a better understanding of the impingement syndrome of the shoulder.


METHODS
A geometrical model, based on fitting spheres to various anatomical items of the shoulder was derived from three-dimensional position data of the gleno-humeral joint and coraco-acromial arch of 32 cadaver shoulders. Kinematical data were collected from 10 healthy volunteers. The geometrical and kinematical data were combined to study the supraspinatus outlet during elevation of the humerus in the frontal and sagittal plane.


RESULTS
No single geometry parameter correlated significantly with the initial size of the outlet. During arm elevation, the greater tuberosity was moved away from the coraco-acromial arch quite effectively resulting in narrowing of the outlet during elevation in the frontal plane from 60 degrees to 120 degrees only. Deviations from the average were quite substantial. This was caused by kinematical and especially geometrical variability.


CONCLUSIONS
The size of the outlet is dictated by both the geometry and kinematics of the gleno-humeral joint. Assessment of the individual susceptibility to impingement requires three-dimensional viewing techniques including three-dimensional movements of both the scapula and humerus.


CONCLUSIONS
Little is known about etiology and pathogenesis of various shoulder disorders such as the impingement syndrome. The supraspinatus outlet plays probably a key role. More knowledge on the architecture of the outlet is required for a better understanding.
Meskers
Carel G M
CG
Orthopaedic Laboratory BO-57, Departments of Orthopaedic Surgery and Rehabilitation Medicine, Leiden University Medical Center, P.O. Box 9600, 2300 RC Leiden, The Netherlands. cmeskers@compuserve.com
van der Helm
Frans C T
FC
Rozing
Piet M
PM
D000173:Acromioclavicular Joint / Q000502:physiology; D001132:Arm / Q000502:physiology; D001696:Biomechanical Phenomena; D002102:Cadaver; D006801:Humans; D018482:Muscle, Skeletal / Q000502:physiology*; D011187:Posture; D016059:Range of Motion, Articular / Q000502:physiology*; D012680:Sensitivity and Specificity; D012785:Shoulder Joint / Q000502:physiology*