P-21 Estimation of the instantaneous center of rotation in the macaque shoulder joint: 1.Definition of the scapular orientation in the three-dimensional coordinate analyzer

The three-dimensional orientation of the scapula as a base of the Numeral kinematic analysis has thus far not been clearly defined. Further, the kinematic studies as to the glenohumeral joint were almost limited to those of human and hence the investigators lacked comparative morphological viewpoints. Since the scapular blade, particularly its half locating close to the glenoid cavity can be well regarded as a plate, it seems mechanically rational to set the part in the analyzer as a kinematic criterion. However, because the morphology of the blade is diverse among species, it is difficult to give common metrical points for defining the plate throughout species. Now the present study proposes the following definition: 1. A segment (V) can be found that touches steadily the surface of the subscapular fossa and is on the perpendicular plane (P) at midpoint of segment SGc(S: intersection point made by the vertebral border and the base line of spine; Gc: midpoint of the maximum diameter of the glenoid cavity). 2. Then a rectangle on P, one side of which corresponds to V, one angle of which is on the lateral ridge of the axillary border (LA), and one side of which contacts the cranial border and is perpendicular to V, can be determined. Now, the name M is given to the midpoint of the rectangular side that contacts LA and is perpendicular to V. 3. Finally, the scapula is fixed in the analyzer so that the segment SGc is horizontal, V, vertical (= parallel to Z axis), and the segment MGc, parallel to ZX plane. It was confirmed that the definition is easily applicable to the scapula of dog or Virginia opossum whose shape greatly differs from that of macaque, and that, after the setting, the glenoid cavity in either animal becomes oriented almost towards the positive direction of X axis.