When the crown of the tooth is widely missing due to the decay, it is called the residual crown, and when only the root part of the tooth is left, it is called the residual root. The integrity of the tooth is damaged after the occurrence of the residual root and crown, and bacteria can easily invade the root canal and reach the apical part of the root causing periapical inflammation, sometimes spreading to several interstices of the jaw face, which can cause swelling and pain. The edges of the residual roots and crowns will constantly rub against the oral mucosa, thus producing traumatic ulcers. Over time, it may induce malignant changes and cause oral malignancy. The infection of the apical part of a baby tooth with root and crown remnants can cause developmental malformation of the inherited permanent teeth, which can affect the eruption of permanent teeth and lead to malocclusion, so the root and crown remnants need to be extracted in time.