A surgical instrument includes a mount body, a joint member, an arm, and a working end. The mount body has a top portion, a distal end, a proximal end and a bottom portion. The joint member is pivotally mounted at a distal end portion of the mount body, to allow positioning of a proximal portion of an arm extending distally from the joint member. The joint member is also configured to at least partially constrain movement of the proximal portion of the arm to a plane. The working end is mounted to a distal end portion of the arm. The surgical instrument can be configured as a heart stabilizer or a heart positioner. The joint member may further be configured as a slotted ball, a disk member, or a combination thereof.