There are four divisions of available bone based on width, height, length, angulation and crown height in the edentulous site.
1. Division A edentulous ridge offer abundant bone in all dimensions.
2. Division B bone may provide adequate width for narrower, small diameter root form endosteal implants.
3. Division C edentulous ridge exhibits moderate resorption and presents more limiting factors for predictable endosteal implants.
4. Division D edentulous ridge basal bone loss and severe atrophy, resulting in dehiscent mandibular canals or a completely flat maxilla.
Misch CE: Dental Implant Prosthetics, Mosby Inc 2005