Brushing regularly is one of the most fundamental steps in maintaining good oral health. However, overbrushing (toothbrush abrasion) can lead to sensitive teeth and receding gums. Over-brushing not only refers to "how much" you brush your teeth but also "how" you brush them. Vigorous brushing can wear down the enamel, damage and push back the gums, exposing the sensitive root area, and even expose the nerve, which will cause severe toothache and require root canal treatment. It is usually caused by brushing too long, pressing too firmly or using a hard- or medium-bristled brush. Thoroughness is what is required for plaque removal, not aggressive brushing.
In this case, we can clearly see the sign of dental abrasion - a notch, wedge or V-shaped indentation of the tooth along the gumline. It was really close to the nerve. The tooth was covered up and protected by bonding a tooth-colored filling over the abraded area. Also a proper brushing technique was instructed to prevent it happens again.