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Accuracy and reliability of a new method of measuring three-dimensional scapular kinematics Bourne, Douglas Allen

Abstract

The shoulder is a very complex and mobile joint which relies on movement of the clavicle, scapula as well as the humerus. Measuring movement of the scapula is a problem that has plagued both researchers and clinicians alike. The scapula is a relatively flat bone that moves with considerable sliding under the skin, which makes it difficult to use skin markers to monitor its movement. The current gold standards for measuring scapular motion are invasive, involving the use of metal implants with or without radiation. The goal of this research was to develop and validate a non-invasive method to measure scapular movement. This new technique uses a grid of skin markers located over the scapula to monitor its movement. In addition, a patient-specific correction factor using digitization of bony landmarks was developed which takes into account skin movement. The accuracy of this new method was determined using percutaneous bone pins as a gold standard. Eight university aged healthy subjects were recruited but the data from one subject had to be discarded due to pin loosening. An optoelectonic marker grid was applied to the skin overlying the scapula. Two 1.6mm bone pins with optoelectronic marker carriers were then inserted into the spine of the left scapula by an orthopaedic surgeon. During glenohumeral abduction, glenohumeral horizontal adduction, hand behind back, and forward reaching, three bony landmarks on the scapula were digitized at six arm positions to enable calculation of a subject-specific skin correction factor. The marker positions were recorded as the subjects performed each movement 10 times dynamically. The scapular movement from the bone pin method and the skin-based system were then compared. The root mean square (rms) errors for the digitization of bony landmarks (which the skin correction factor depends on) were found to range between 3.5°-4.6°. The rms errors for the new skin-based technique were found to range between 1.4° - 3.0° when corrected with the subject-specific skin correction factor. Rms differences (reliability) between days ranged between 3.8° - 7.5°. A preliminary cadaver study showed that the pins have a small effect on skin movement. Few studies have validated non-invasive measurement techniques of scapular motion using a gold standard. The rms errors of the new skin-based technique are equal to or less than currently available or reported methods. Therefore, it may be well suited to explore the role of scapular motion in shoulder pathologies.

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