Bone can be classified according to structure, composition, density and volume. Lekholm, Zarb et al. have classified bone quality and volume in to four types, expressed as type I, II, III and VI. This classification closely resembles a more recent classification by Misch. Misch separates bone quality and volume in to two distinct classifications that can be combined for patient specific diagnostic purposes and drill protocol procedures. Bone quality is classified in to four groups D1, D2, D3, and D4 whereby the Misch classification suggests a location, composition and a measurable density reading (Hounsfield units) for each type of bone. The interplay of bone quality and volume has a direct influence on the success rate and drill protocol for Endosseous (Endosteal) implants as well as the esthetics and function of the final prosthesis.
D1 bone is composed of almost all cortical bone mass located primarily in the anterior mandible. A Hounsfield unit reading of 1250 and above indicates D1 bone. This type of bone results in the greatest bone to implant contact (BIC) and exceptional initial implant stability. Due to its density this type of bone has fewer intrinsic blood vessels and depends for a significant portion of nutrient and blood supply on the periosteum. Conservative flap reflection is advised to lessen the impact of the disrupted blood supply during and after surgery. To minimize detrimental heat generation during the osteotomy it is advantageous to use copious amounts of pre cooled sterile saline solution for cooling. Smaller increments in drill diameter combined with drill speeds around 2000 rpm and a force of around 2 kg will ensure efficient bone cutting while heat generation is kept at acceptable levels. Each implant manufacturer recomends specific drill speed ranges for task specific drills. Using a slow pumping motion will further decrease heat generation.
D2 bone is composed of a thick crestal layer of cortical bone and coarse trabecular bone underneath the cortical bone. This type of bone can mostly be found in the anterior and posterior mandible. A Hounsfied reading between 850 to 1250 units is indicative of D2 bone. This type of bone offers a excellent BIC ratio and presents with abundant intrinsic vascularization due to its coarse structure. The drill protocol is slightly less demanding in terms of drill diameter sequence, fewer drills may be necessary to achieve final osteotomy dimensions. Drill speed, exerted force, pumping motion and pre cooled, sterile saline solution irrigation are the same as D1 bone.
D3 bone is composed of a porous crestal layer of cortical bone and fine trabecular bone underneath the cortical bone. This type of bone can mostly be found in the anterior and posterior maxilla but also in the posterior mandible. A Hounsfield reading between 350 and 850 units is indicative of type D3 bone. Due to the porous architecture of D3 bone the BIC is significantly reduced and warrants a modified drill protocol and procedure. A reduced number of drills and a osteotome may be indicated to achieve final osteotomy dimensions in combination with a reduced drill speed (1500 rpm) and force. Each implant manufacturer recomends specific drill speed ranges for task specific drills. Great care should be taken not to create a oval osteotomy or perforate the buccal plate in the anterior maxilla. The anterior maxilla presents with a challenging environment because the palatal aspect of the anterior maxilla is often denser and thicker than the labial portion, forcing the drill during the osteotomy and the implant during insertion more labially resulting in a possible elliptical osteotomy and a perforated labial plate. To further enhance osseointegration and counteract the limited bone density it may be favorable to utilize implants with a titanium plasma spray (TPS) or hydroxylapatite (HA) coating. The coarse structure of D3 bone is well vascularised and therefore aides in osseointegration.
D4 bone is composed of primarily fine trabecular bone and often the absence of cortical bone. This type of bone can mostly be found in the posterior maxilla and poses the greatest challenge in implant placement. A Hounsfield reading between 150 and 350 units is indicative of D4 bone. Due to the fine trabecular architecture and often absence of a cortical bone structure D4 bone results in the least amount of BIC. As with D3 bone a modified surgical procedure and drill protocol are necessary. A undersized osteotomy and the use of osteotomes to condense rather than remove bone will enhance the predictability and success rate in combination with TPS and HA coated implants.