My dentist was puzzled as to why I needed fillings, despite brushing twice a day with my sonic toothbrush. I recently figured out the problem for myself. Turns out every time I brushed, some of the toothpaste was dripping down into the join between the brush head and the base, then drying out to form a powdery 'cushion' that muffled the sonic vibrations reaching the head. Easily remedied - now after brushing, I just detach the head and give both parts a quick rinse before re-assembling. Went for my check-up the other day, and my dentist said, 'Well, I'm seeing a lot of improvement here. What are you doing differently?' I replied, 'Well, I've changed my morning routine. Now whenever I'm done brushing my teeth, I pull the head off it.'