This bony growth inside the lower arch is called Torus Mandibularis which is present in 5-7% of cases. Their exact cause is not known but normally it does not pose any problem but if they are too big then denture fabrication and denture wearing becomes a problem as the dentures causes irritation and ulceration on these lumps which is quite painful. You can discuss with your dentist. If he thinks that it can be reduced by doing surgery then you can go for that. This surgery will reduce the size of these lumps.
Free Dental Consultation