The mouthpiece includes an arcuate base assembly (12), comprising two side pieces (14, 16) and an intermediate center piece (18), the two side pieces (14, 16) being connected rotatably to opposing ends of the center piece (18), such that the rear ends of the side pieces move outwardly and inwardly. A flexible screw thread spindle (30) having two side portions and a front portion is connected at the free ends thereof (33, 35) to the rear ends of the side pieces of the base assembly, such that the flexible spindle member follows the arcuate shape of the base assembly. Two side carriages (40, 42) are mounted on opposing side portions of the spindle (30), the carriages having bristles (43) mounted thereon for cleaning teeth. A motor and gear box assembly (36) are mounted at the front of the appliance for moving the two carriages along the respective side portions of the spindle (30), which have opposing screw threads. A front carriage (48) is mounted on and moves along a front spindle by a front motor and gear box (36).
