NMDA Journal Summer 2020

Limited FOV can be an advantage when eval- uating partially edentulous patients. ADA rec- ommendations for the reduction of radiation exposure state that, “In accordance with as low as reasonably achievable guidelines, limit radiation dose by optimizing image quality and using the smallest field of view available to image a specific area. Variable volume CBCT offers less cost, smaller volumes and greater resolution, reduced radiation expo- sure, time savings, less technique sensitivity, capability for a panorex and reduced medico- legal concerns. Both CT and CBCT images allow us to use interactive computed tomog- raphy (ICT) to plan for implant placement. By using ICT we can determine bone height, width, and angulation, virtually place restora- tions and implants, and construct surgical guides for implant placement. These images are indispensable in patient selection for implant placement. The evaluation of edentulous sites involves a number of anatomical considerations. Both the quantity of available bone in three dimen- sions and the quality of available bone should be considered. Misch suggested a classifica- tion system for the density of bone found in varying areas of the maxilla and mandible. D1 bone is generally found in the anterior mandi- ble. The bone in this area is usually dense cor- tical bone which has the consistency of oak wood. This area has the highest rate of implant success. D2 bone is found in the ante- rior maxilla, posterior mandible, and anterior mandible. The bone in this area is usually porous cortical and coarse trabecular bone which has the consistency of pine wood. D3 bone is found in the anterior maxilla and pos- terior maxilla. The bone in this area is usually porous cortical and fine trabecular bone which has the consistency of balsa wood. D4 bone is found in the posterior maxilla and is usually composed of fine trabecular bone which has the consistency of styrofoam. The Hounsfield unit values fromCT or gray scale values fromCBCT images may also be used to estimate bone density. D1 bone will have greater than 1,250 units; D2, 850-1250; D3 350-850; and D4, 150-350. This estimation of bone quality can be useful in case selection and treatment planning. It is advantageous to increase the length, width, and numbers of implants in less dense bone to provide more support for the prosthe- sis. Alternative techniques for implant place- ment such as osseodensification may also be advantageous when the bone is less dense. The quantity of bone limits the length and width of the implants that may be placed unless we plan for expansion or augmenta- tion. It is desirable to leave a 2mm zone of safety between the apex of the implant and the opposing anatomical landmark. In the maxilla, the cuspid and bicuspid areas gener- ally present with the best available bone for implant placement with the least possible complications. In the anterior mandible, the area frommental foramen to mental foramen presents with the best available bone with the least possible complications. So, in terms of case selection if you are beginning to place implants, these will generally be the best areas to insure success. The position of the inferior alveolar nerve in the posterior mandible limits the length of implants that can be used without nerve repo- sitioning surgery, vertical augmentation, or the use of angled implants. It is important to plan for a 2mm zone of safety superior to the nerve and also to remember that a number of implant companies do not include the tip of the drill in the calculation of the length of the drill. The presence of lingual concavities in the posterior mandible can also limit the length of the implants. In the posterior maxilla, the floor of the maxil- lary sinus limits the length of the implant unless a sinus lift is planned. This is also an area where the least dense bone is found. So, in terms of case selection, if you are not com- fortable with the sinus lift procedure, you may want to refer cases where there is less than 10mm of bone height below the sinus. Good- acre found that implants less than 10mm in length had a 16% greater failure rate in the posterior maxilla. Buccal concavities in the anterior maxilla are areas that need to be avoided and can affect the angulation of implant placement, as shown in Figure 4. Failure to account for these undercut areas can result in less than optimal implant placement as seen in Figure 4. The height of bone available for implant placement is evaluated by measuring from the crest of the ridge to the opposing anatom- ical landmark and subtracting a 2mm zone of safety. The width of bone helps determines the width of the implant, we need at least 1mm of bone buccal and lingual to the implant. In the anterior it is preferable to have 2mm of bone buccal to the implant. When there is inade- quate width of bone to place implants graft- ing or bone expansion can be considered. The angulation of the bone should be less than 25 degrees buccal-lingually for implant place- ment. If the angulation is greater than 25 degrees, the site should be grafted prior to implant placement. For conventional crown and bridge the crown, height should be 15mm or less. The minimum distance between adjacent implants determined by Tarnow using con- ventional implants was 3mm to maintain inter-implant bone. The minimum distance between an implant and an adjacent natural tooth determined by Tarnow using conven- tional implants was 2mm to maintain inter- proximal bone. His studies were repeated by Rodriguez and Vela following the introduction of platform switched implants. They found that only a 2mm distance between platform switching implants was needed to maintain inter-implant bone. They also found that only a 1mm distance between a platform switching implant and an adjacent tooth was needed to Case Selection to Optimize Success in Implant Dentistry continues on page 18  17 Figure 4— nmdental.org