INTRODUCTION
Cone beam computerized tomography, commonly known as CBCT, is an integral part of implant dental treatment. A strong case can be made that every implant treatment plan, small or large, requires a pretreatment CBCT scan. A CBCT scan offers a low-radiation 3-D view that provides the clinician with a presurgical pathology screening, an accurate assessment of the available bone with regard to height and width, an assessment of bone density, a comprehensive view of anatomical features, increased medical legal protection, and more. The advancements of CBCT have allowed this diagnostic modality to be a low-radiation diagnostic tool, offering our patients many benefits with minimal risk. During the last 10 years, there has been an increasing number of companies focusing on CBCT technology, showing how the medical industry has come to recognize the importance of CBCT in relation to implant treatment planning and bone grafting procedures. This article will demonstrate, through case examples and concepts, how a CBCT scan can be utilized for implant treatment when addressing full-arch implant dental treatment.
Full-Arch Implant-Supported Reconstruction
Treatment of an entire arch with implants imparts a large responsibility on the treating dentist. Treatment of full-arch dentistry using implants involves a large scope of clinical and practice management concepts that must be considered from a treatment planning, surgical, and prosthetic standpoint. When treating entire arches for tooth replacement, a comprehensive treatment plan that involves many aspects of dentistry is required. A CBCT scan is the starting point for all aspects of full-arch implant-supported treatment.1
A CBCT scan can offer the information that starts the treatment planning conversation with a patient (Figure 1). The conversation to treatment plan a case based on CBCT data can relay objective clinical data, which can then be used to help explain to a patient the final results. The CBCT scan can also help direct the clinician toward the best prosthetic end result. When a radiographic template that represents the final prosthetic positon is utilized for a CBCT scan, the final prosthesis can best be translated to the bone and implant positions (Figure 2).2
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| Figure 1. Image of i-CAT FLX Cone Beam 3-D system (Imaging Sciences International). | Figure 2. Lower denture used as radiographic template with radiographic markers for a dual-scan technique. |
BENEFITS OF A PRE-EXTRACTION CBCT SCAN
When treatment planning full-arch reconstruction with dental implant support that involves removal of existing teeth, a pre-extraction CBCT scan offers many advantages (Figure 3).3 The advantages include a 3-D presurgical comprehensive view of existing pathology, a radiological estimate of bone density (with relation to immediate loading), a pre-extraction assessment of implant spacing and of the amount of alveoloplasty required, information on the estimated implants that might be needed for a case, and more. Doing both a pre- and post-extraction CBCT scan offers many benefits and inherent medical legal protection. Once teeth are removed from an arch, a post-extraction CBCT scan will offer implant placement treatment planning information related to any bone changes that might have occurred during the extraction process (Figure 4). However, capturing a pre- then post-extraction CBCT scan can present issues of accessibility to CBCT equipment. The ideal situation is for a clinician to have his or her own CBCT machine and the associated software in the dental office. Clinicians have a choice to utilize a mobile CBCT or one CBCT that is available locally.
CBCT for FP1 Cement-Retained Fixed Prosthetics
Misch4 has categorized implant fixed prosthesis (FP) as the following: FP1, FP2, or FP3. FP1 are prosthetics that most resemble natural teeth with little or no artificial pink gingiva (Figure 5). FP1 full-arch prosthetics are comparatively reduced in prosthetic height to FP2 or FP3 prosthetic options. FP2 and FP3 inherently, according to Misch,4 have an artificial pink gingival area and, therefore, increased prosthetic space. FP1 full-arch bridges, with their inherent reduced prosthetic height, are traditionally either cement-retained or screw-retained implant-supported PFM bridges.
When planning for implant placement in FP1 cases, CBCT scans are often used to gauge the height of bone in relation to the final FP1 prosthetic position.5 It is an advantage, when a CBCT scan is taken, to utilize a radiographic template that represents the desired final prosthetic position. From a CBCT scan taken with a radiographic template, the clinician can see if there is a need for bone grafting. FP1 prosthetics require exacting implant placement positions in relation to tooth positions. A CBCT scan with or without a guide can assist in not only ideal positions but also in choosing the ideal width and length of dental implants. Many CBCT software programs allow for virtual placement of accurately represented implant fixture sizes into a CBCT plan (Figure 6).
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| Figure 3. Pre-extraction CBCT software image (SimPlant [Dentsply Sirona Implants]) showing cross-sectional, panoramic, axial, and 3-D view (Materialise). |
Figure 4. Post-extraction CBCT software image (SimPlant) showing cross-sectional, panoramic, axial, and 3-D view. |
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| Figure 5. Example of a Misch classification fixed prosthetics 3 (FP3) showing prosthetically reproduced pink gingiva. | Figure 6. Virtual anatomically correct implants on a maxillary and mandibular CBCT software plan. |
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| Figure 7. CBCT software plan showing an FP1 implant positioned superior to the inferior alveolar nerve. | Figure 8. Before (above) and After (below) images of a grafted site for an FP1 prosthetic plan. |
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| Figure 9. An example of a full-arch hard-tissue-supported surgical guide. | Figure 10. The 3-D view on a CBCT showing planned implant positions in the Misch A to E positions for a mandibular bar overdenture. |
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| Figure 11. Example of ideal screw hole positions in maxillary and mandibular FP3 full-arch zirconia screw-retained bridges. |
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| Figure 12. A 45° angle correcting multiunit abutment (BioHorizons) used to redirect a screw hole. |
Often with FP1, sinus grafting is needed in maxillary cases. CBCT scans are indispensable to gauge sinus anatomy and diagnose presurgical pathology. In the mandible with FP1 cases, implants are often placed distal to the mental foramen. CBCT scans can offer an accurate assessment of the inferior alveolar nerve position (Figure 7).
When substantial bone grafting is needed prior to implant placement for an FP1 prosthesis, a second CBCT taken post-graft healing is recommended prior to implant placement. This second CBCT will show the post-grafted morphology and also give accurate information for implant placement. This second CBCT will also show if there is a need for additional bone grafting, either before or during implant placement (Figure 8).
Due to the exacting positioning of implant placement for an FP1 prosthesis, a surgical guide can be an advantage, especially for the less experienced clinician.6 The CBCT information can accurately be used for the formation of a surgical guide that is either soft-tissue- or hard-tissue-supported (Figure 9).
As can be seen in these examples, there are many advantages to utilizing a CBCT scan when treatment planning an FP1 prosthesis as a full-arch tooth replacement option.
CBCT for Overdentures
When an overdenture is treatment planned, implants are usually placed between the sinuses on the maxilla and the foramen on the mandible (Figure 10). A CBCT scan can help identify the mesial locations of both the anterior sinus wall and the location of the foramen and the location of an anterior loop. Through placing implants as distal as possible at a safe distance from the sinuses or foramen, the anterior posterior (A-P) spread of the implants can be maximized. Maximizing the A-P spread reduces stress on the implants and creates a more stable prosthesis.7 A CBCT scan for overdentures will also help gauge the anatomy in the anterior maxilla and mandible and offer information on alveoloplasty needs, incisal canal anatomy, presence (or not) of pathology, and more. A CBCT scan will also offer information on bone anatomy so that the clinician can gain ideal angulations for implant placement. Nonangled implants are preferred when overdentures are supported by implants that are not connected by a bar. An important aspect of overdenture treatment planning is determining if there is enough prosthetic space. A bar overdenture can require up to 15.0 to 20.0 mm of prosthetic space.8 A CBCT scan can help gauge and treatment plan this required prosthetic height, especially when a radiographic marker of the planned teeth is used for a CBCT scan.
CBCT for Screw-Retained FP3 Prosthetics
Screw-retained FP3 prosthetics, either an acrylic hybrid or zirconia, require certain surgical objectives for prosthetic success. While the exact positions of the implants are not as demanding as in an FP1 prosthesis, the amount of alveoloplasty and angulations are crucial. As with an overdenture, alveoloplasty is an important surgical step for prosthetic success.9 In general, 12.0 mm of prosthetic space is needed of a zirconia bridge, and, for an acrylic hybrid bridge, the minimal prosthetic space is 15.0 mm, according to the literature.10 Once again, a radiographic marking of the planned teeth in the correct prosthetic position with a radiographic guide will accurately show how much prosthetic space is available. That information can then direct the clinician toward the amount of alveoloplasty that is needed for a case.
For aesthetic reasons, the screw hole positions of the implants in an FP3 screw-retained prosthesis should ideally be located in the cingulum area of the planned tooth positions in the anterior region (Figure 11). If implants are placed so that the screw hole is coming through the facial aspects of the planned teeth, then the screw holes can be redirected by using multiunit abutments (Figure 12). Multiunit abutments add another implant component, with associated costs and the additional vulnerability of another screw connection. A multiunit abutment creates an additional microgap and possible issue associated with that.11 If a clinician wants to avoid the use of multiunit abutments and attach the FP3 prosthesis direct to fixture, then implants have to be placed so the screw hole is directed toward the lingual. A CBCT scan can assist in directing the implant positions toward the lingual, either through a CAD/CAM produced surgical guide, or through information from a CBCT being used in a nonguided surgery. The cross-sectional view on a CBCT can offer invaluable information toward showing the buccal/lingual relationship of the maxillary or mandibular ridge in relation to dental implant positions.
When performing dental implant surgery in a nonguided manner, the extraction sites can be a visual aid in combination with CBCT information. The extraction sites can be used to visualize and extrapolate where the cingulum of the extracted teeth would have been, or for the location of the planned teeth positions.
IN SUMMARY
A CBCT scan is imperative for success when restoring an entire arch, or arches, with dental implants. This article has discussed the pertinence of CBCT related to Misch classification FP1, overdentures, and FP3 prosthetics on both the maxilla and mandible. The importance of pre- and post-extraction CBCT scans has been discussed as an integral part of the treatment planning process for full-arch implant cases. The relationship between surgical planning and the final prosthetic result has also been emphasized in relationship to Misch’s 3 prosthetic options.
References
- Yilmaz B. Incorporating digital scans of diagnostic casts into computed tomography for virtual implant treatment planning. J Prosthet Dent. 2015;114:178-181.
- De Kok IJ, Thalji G, Bryington M, et al. Radiographic stents: integrating treatment planning and implant placement. Dent Clin North Am. 2014;58:181-192.
- Tischler M, Ganz SD, Patch C. Success with screw-retained zirconia bridges, part one. Dent Today. 2014;33:98-103.
- Misch CE. Contemporary Implant Dentistry. 2nd ed. St Louis, MO: Mosby; 1999:68-70.
- Worthington P, Rubenstein J, Hatcher DC. The role of cone-beam computed tomography in the planning and placement of implants. J Am Dent Assoc. 2010;141(suppl 3):19S-24S.
- Tischler M, Patch C, Mirelez A. Full-arch zirconia screw-retained bridges: the advantages of a guided surgical approach. Dent Today. 2015;34:64-67.
- McAlarney ME, Stavropoulos DN. Determination of cantilever length-anterior-posterior spread ratio assuming failure criteria to be the compromise of the prosthesis retaining screw-prosthesis joint. Int J Oral Maxillofac Implants. 1996;11:331-339.
- Ahuja S, Cagna DR. Classification and management of restorative space in edentulous implant overdenture patients. J Prosthet Dent. 2011;105:332-337.
- Bidra AS. Technique for systematic bone reduction for fixed implant-supported prosthesis in the edentulous maxilla. J Prosthet Dent. 2015;113:520-523.
- Doherty MJ, Purcell BA. Treatment options for the edentulous arch: a logical approach for the restorative doctor and surgeon. Selected Readings in Oral and Maxillofacial Surgery. 2015;23. selectedreadingsoms.com/wp-content/uploads/2016/03/Doherty23.4E.pdf. Accessed July 13, 2016.
- Koutouzis T, Wallet S, Calderon N, et al. Bacterial colonization of the implant-abutment interface using an in vitro dynamic loading model. J Periodontol. 2011;82:613-618.
Dr. Tischler, the implant editor for Dentistry Today, maintains a private practice in Woodstock, NY and is the implant director and owner of Tischler Dental Laboratory as well as the founder of the Teeth Tomorrow Franchise Network. He is a Diplomate of the American Board of Oral Implantology/Implant Dentistry and the International Congress of Oral Implantologists, a Fellow and Honored Fellow of the American Academy of Implant Dentistry (AAID), and a Fellow of the AGD and the Misch International Implant Institute. He has published more than 40 articles on implant dentistry and bone grafting and has lectured in the United States and Canada, including lectures discussing his in-office live surgical course on the Prettau Zirconia Implant Bridge. He is on faculty for the AAID Maxi Course at Rutgers University and can be reached at mt@tischlerdental.com.
Disclosure: Dr. Tischler reports no disclosures.
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