Crowded teeth covered with plaque and tartar. Close-up of a dental patient's crowded lower teeth covered with plaque and tartar. Plaque is a bacterial biofilm that can form on teeth and can cause tooth decay. Tartar is a mineralised and hardened form of dental plaque. If not treated, the condition can lead to gingivitis, peridontitis and tooth loss. For the teeth after cleaning with an ultrasonic scaler see image C034/2623.