Functional Identification of Shoulder Joint Centers
Date
2009-05
Authors
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Journal ISSN
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Publisher
University of Delaware
Abstract
Analysis of upper extremity kinematics is difficult due to the lack of bony
landmarks on the upper arm and shoulder. The medial and lateral epicondyles provide
two potential bony landmarks for the upper arm, and the shoulder joint center provides
a potential third landmark. Two methods commonly used to estimate the location of
the shoulder joint center are the constant offset method (Rab, Petuskey, and Bagley,
2002) and the spherical fit method (Hicks and Richards, 2005). The constant offset
method is susceptible to error due to variability of human geometry. The spherical fit
method has been shown to be more accurate than constant offset methods for the hip;
however, the methods have not been compared for the shoulder joint center. The
objective was to compare the accuracy of the shoulder joint centers found using the
functional spherical fit method as well as the constant offset method proposed by Rab,
Petuskey, and Bagley (2002) relative to a physical measure of the shoulder joint center
found using ultrasound. The physical measure was compared to the centers found
with the constant offset method and functional identification for four positions:
adduction-ER, 90º abduction-ER, 180º abduction-ER, and 90º flexion, IR. The centers
were found for each method using the elevation of the humerus as a constraint.
Specifically, motion trial data was analyzed at maximum elevation limits ranging from
-30º below horizontal to 80º above horizontal at 10º intervals. Results indicated that
the angle of elevation when using the spherical fit should be limited to zero degrees
relative to the horizontal in order to minimize error. Using this minimized difference,
the functional identification is most accurate in the adduction-ER position. Since both estimation methods are relative to the acromion marker, both are susceptible to
errors in the other positions. In positions that require abduction, the scapula displays
posterior tilt, resulting in the acromion rotating behind the glenoid cavity and causing
the estimated centers to become posterior to the actual shoulder joint center.
Conversely, in the 90º flexion-IR position, the scapula tilts anteriorly, causing the
acromion to rotate in front of the glenoid cavity and produce estimated centers anterior
to the actual shoulder joint center. Therefore, although the functional identification is
more accurate in the adduction-ER position, both methods are characterized by
substantial errors when implemented through a full range of motion.