Joint of robot arm and surgical instrument

A joint of a robot arm, the joint coupling links of the robot arm to each other,

• the joint comprising:
• a plurality of plate-shaped segments lined up in a plate thickness direction, each of the plurality of segments having a first-direction middle portion positioned at a middle of the segment in a first direction perpendicular to the plate thickness direction;
• and
• an operating wire extending through the plurality of segments in the plate thickness direction at sides of the first-direction middle portion of each of the plurality of segments, the sides being opposed to each other in the first direction, wherein:
• a groove having a partially circular cross section and extending in a second direction perpendicular to the first direction is provided at a main surface of the first-direction middle portion of a first of the plurality of segments;
• a projection having a partially circular cross section and extending in the second direction is provided at a main surface of the first-direction middle portion of a second of the plurality of segments, the first and second segments being adjacent to each other and the main surfaces facing each other in the plate-thickness direction;
• a curvature of a cross-sectional shape of the projection is larger than a curvature of a cross-sectional shape of the groove;
• the projection is fitted in the groove, and a bottom portion of the groove and a top portion of the projection contact each other;
• and
• a dimension of the projection in the plate thickness direction is larger than a dimension of the groove in the plate thickness direction and smaller than a length of a line connecting the bottom portion of the groove and an edge portion of the groove.