A new subject-specific skin correction factor for three-dimensional kinematic analysis of the scapula

Noninvasive measurement of scapular kinematics using skin surface markers presents technical challenges due to the relative movement between the scapula and the overlying skin. The objectives of this study were to develop a noninvasive subject-specific skin correction factor that would enable a more accurate measurement of scapular kinematics and evaluate this new technique via comparison with a gold standard for scapular movement. Scapular kinematics were directly measured using bone pins instrumented with optoelectronic marker carriers in eight healthy volunteers while skin motion was measured simultaneously with optoelectronic markers attached to the skin surface overlying the scapula. The relative motion between the skin markers and the underlying scapula was estimated over a range of humeral orientations by palpating and digitizing bony landmarks on the scapula and then used to calculate correction factors that were weighted by humeral orientation. The scapular kinematics using these correction factors were compared with the kinematics measured via the bone pins during four arm movements in the volunteers: abduction, forward reaching, hand behind back, and horizontal adduction. The root-mean-square (rms) errors for the kinematics determined from skin markers without the skin correction factors ranged from 5.1 deg to 9.5 deg while the rms errors with the skin correction factors ranged from 1.4 deg to 3.0 deg. This technique appeared to perform well for different movements and could possibly be extended to other applications.