(Science: dentistry) a lip bumper is used to push the molars on your lower jaw back to create more space for other teeth. The lip bumper consists of an arch wire which is attached to a moulded piece of plastic. You mount the arch wire in the buccal tubes on your lower jaw, and plastic piece rests against your lips. When you eat or talk, you push the plastic piece back which pushes on your molars. That pushes your molars back.