IN-VIVO ESTIMATION OF THE GLENOHUMERAL JOINT ROTATION CENTER FROM SCAPULAR BONY LANDMARKS BY LINEAR-REGRESSION

In this paper, a method is described for in vivo prediction of the gle nohumeral joint rotation center (GH-r), necessary for the construction of a humerus local coordinate system in shoulder kinematic studies. T he three-dimensional positions of five scapula bony landmarks as well as a large number of data points on the surface of the glenoid and hum eral head were collected at 36 sets of cadaver scapulae and adjacent h umeri. The position of GH-r in each scapula was estimated by mathemati cally fitting spheres to the glenoid and humeral head. GH-r prediction from scapula geometry parameters by linear regression resulted in a R MSE between measured and predicted GH-r of 2.32 mm for the x-coordinat e, 2.69 mm for the y-coordinate and 3.04 mm for the z-coordinate. Appl ication in vivo revealed a random humerus orientation error due to mea surement inaccuracies of 1.35, 0.29 and 1.26 degrees standard deviatio n per rotation angle. The estimated total humerus orientation error in cluding the offset error due to the regression model inaccuracy was 2. 86, 0.84 and 2.69 degrees standard deviation. As these errors were abo ut 15 and 20% of, respectively, the intra-and inter-subject variabilit y of the humerus orientations measured, it is concluded that the metho d described in this paper allows for an adequate construction of a hum erus local coordinate system. (C) 1998 Elsevier Science Ltd. All right s reserved.