Surgical instrument for robotic surgery comprising: - a tendon-actuated articulating end, and - at least one actuating tendon, and - a backend portion comprising: - at least one elongated transmission body moveable along a longitudinal direction (X-X), which is coinciding with or parallel to a longitudinal extension axis of said at least one elongated transmission body; - at least a first resting wall, forming a first resting part for a first transversal side of the at least one elongated body; - at least a second resting part for a second transversal side, opposite to said first transversal side, of the at least one elongated transmission body; - at least one return element for the at least one actuating tendon; wherein: - said at least one actuating tendon is operatively connected to said elongated body and is operatively connected to said at least one return element; - the at least one elongated transmission body comprises on the first side thereof a first lateral surface, which slidably rests against said at least a first resting wall; - said first resting wall comprises a convex resting surface; and wherein: - the at least one elongated transmission body comprises on the second side thereof a second lateral surface, which slidably rests against said second resting part; - said backend portion further comprises at least one pivotable organ and said second resting part belongs to said pivotable organ, so that the sliding rest of the of the second lateral surface of the elongated transmission body on the second resting part determines rolling the pivotable organ; and wherein: - said at least one actuating tendon is integral with said elongated transmission body; - said at least one return element locates the traction action applied by the actuating tendon at a certain distance from the longitudinal axis (X-X) of the elongated transmission body, so that said actuation tendon applies a traction action generating a torque aiming to bring the second side of the at least one elongated transmission body towards said second resting part of the pivotable organ and to bring the first side of the elongated transmission body towards the convex surface of the first resting part. - a tendon-actuated articulating end, and - at least one actuating tendon, and - a backend portion comprising: - at least one elongated transmission body moveable along a longitudinal direction (X-X), which is coinciding with or parallel to a longitudinal extension axis of said at least one elongated transmission body; - at least a first resting wall, forming a first resting part for a first transversal side of the at least one elongated body; - at least a second resting part for a second transversal side, opposite to said first transversal side, of the at least one elongated transmission body; - at least one return element for the at least one actuating tendon; - said at least one actuating tendon is operatively connected to said elongated body and is operatively connected to said at least one return element; - the at least one elongated transmission body comprises on the first side thereof a first lateral surface, which slidably rests against said at least a first resting wall; - said first resting wall comprises a convex resting surface; - the at least one elongated transmission body comprises on the second side thereof a second lateral surface, which slidably rests against said second resting part; - said backend portion further comprises at least one pivotable organ and said second resting part belongs to said pivotable organ, so that the sliding rest of the of the second lateral surface of the elongated transmission body on the second resting part determines rolling the pivotable organ; - said at least one actuating tendon is integral with said elongated transmission body; - said at least one return element locates the traction action applied by the actuating tendon at a certain distance from the longitudinal axis (X-X) of the elongated transmission body, so that said actuation tendon applies a traction action generating a torque aiming to bring the second side of the at least one elongated transmission body towards said second resting part of the pivotable organ and to bring the first side of the elongated transmission body towards the convex surface of the first resting part.