A top and bottom plate (10,11) both have guide elements (15,16) which hold the plates in their therapeutic closed position and partly in their open position in a sagittal desired position. The guide elements have catches which fit in the therapeutic closed position of the plates and in at least one open position. The one guide element (15) of the plate (10) has at least one guide pin (28) guided in a cavity (20) in the opposite plate (11). The guide pin has at least one transverse groove (30,31) into which fits a spring element (38). A second guide element (16) in the form of a spherical body (35) on the one plate fits into a trough (21) in the opposite plate.
