Bone Quality
The bone in the maxilla and mandible may be classified according to its quality as D1, D2, D3 and D4 types, taking into account three different characteristics of the bone tissue: location, composition and density. The impact of bone quality in implant dentistry is reflected throughout the implant placement process, with factors such as its strength, modulus of elasticity, percentage of bone-to-implant contact, and tension around a functioning implant directly influencing success rates.
The internal and external structures of the maxillary and mandibular bones are known to differ depending on their location, which is simultaneously due to their different biomechanics. While the maxilla has a thin cortical bone and a large trabecular structure, the mandible has a thicker cortical and less trabecular bone tissue. The anterior and posterior regions also differ from each other. The densest bone is usually found in the anterior mandible, followed by the anterior maxilla and the posterior mandible, with the posterior maxilla having the least dense bone. It has been found in the literature that location (and therefore bone composition) also affects implant survival. Bone density can be measured with a quantitative bone density scale using computed tomography (Hounsfield scale).
This classification of bone quality was proposed in 1988 by Carl E. Misch, an American dentist who is recognized worldwide for his efforts to improve implant dentistry as a specialty. The four groups were classified according to their macroscopic cortical and trabecular bone characteristics. While cortical bone tends to always be found in the same locations, trabecular bone is not as consistent and can be located with a higher degree of variation. Each of these types has a different recommended surgical approach, including treatment plan, implant characteristics and design, surgical protocol, healing, and progressive loading times. These changes in the understanding of differences between types of bone have greatly improved implants predictability.
D1 bone type
This bone consists almost entirely of a dense cortical. It is found in the anterior mandible and can be compared to oak or maple. On a Hounsfield scale, it measures 1250 units or more. It has a large bone-implant contact (BIC) area and great initial implant stability. Its nutrition depends on the periosteum due to a lower number of blood vessels. Surgery must be conservative and special care to avoid bone heat must be taken.
D2 bone type
It has a porous cortical and coarse trabecular bone. It may have multiple localizations, such as the anterior or posterior mandible and also the anterior maxilla. This type of bone can be compared to white pine or spruce. Its density can be measured from 850 to 125 on the Hounsfied scale. It provides a good bone-to-implant contact and good vascularization, therefore fewer drills are needed during the surgery.
D3 bone type
It consists of a porous crestal layer of cortical bone and a fine trabecular bone. Tactile analogy can be made to compressed balsa wood. It is commonly found in the anterior and posterior maxilla, but may also be found in the posterior mandible. On the Hounsfield scale, 350 and 850 units indicate D3 bone. Its porous structure means less BIC, and requires a different drilling protocol to achieve osseointegration. It does have a great vascularization that promotes new bone formation. Lower BIC can be enhanced by implant surface treatments such as titanium plasma spray (TPS) or hydroxyapatite (HA) coating.
D4 bone type
This bone is trabecular without any cortical. It is located in the posterior maxilla and it can be compared to compressed styrofoam or lightweight balsa wood. It can be read from 150 to 350 units on a Hounsfield scale. As a result of drilling on low density bone, BIC is seriously compromised and the surgical technique must be delicate and precise so as not to remove bone and instead compress it around the implant.
