Computer-Assisted Implant Surgery

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Computer-Assisted Implant Surgery

Marco Rinaldi, Alessio Esposti, Angelo Mottola, and Scott D. Ganz

IMPLANT PLANNING SOFTWARE AND STEREOLITHOGRAPHIC MODELS Alessio Esposti In recent years, after having revolutionized all fields of everyday life, computer technologies and information technology are revolutionizing the dental implantology industry with the same force that accompanied the introduction of sterility in surgical practice.

including the patient. To review the potential desired end result of treatment is reassuring for the patient and helps eliminate any false expectations that may lead to a sense of dissatisfaction. Once the plan has been established there must be a link between the plan and the surgical intervention. This is made possible by the wide range of tools available to the physician to export the simulation to a surgical guide or template that merges what is on the computer screen to the patient at time of surgery.

Presurgical Planning Software Presurgical planning software offered a new set of tools for the members of the dental implant team. First and perhaps foremost are the diagnostic tools. Until approximately 20 years ago, the main diagnostic radiographic modalities available were simple two-dimensional images. The brain, in fact, requires considerable effort to estimate the distance of an object based only on its size within the confines of the human eye. The same concept can be applied to presurgical planning tools. For an overview of the mandible with a panoramic radiograph the clinician can determine only relative height about the alveolar nerve, but no one knows exactly where the nerve is located spatially within the bone. Until recently, only clinical experience and statistics provided an indication of distance from the nerve (Fig. 3.1). From this point of view, computed tomography (CT) and magnetic resonance imaging (MRI) have filled this information gap, the first consisting of a series of several hundred images in which the physician tried to assimilate all of these images to find the answers needed. The ability to visualize three-dimensional images through interactive software provided the doctor the ability to view structures exactly as they would be seen in reality. As an example, the same story illustrated with a well-drawn two-dimensional cartoon on paper is not as impressive as a movie animation of the same scene. After the diagnostic capability of three-dimensional imaging has been realized, the interactivity is further defined by the ability to accurately simulate surgery. Depending on the software application, the sophisticated tools can be used to simulate virtually any action with regard to determining proper implant placement within the confines of the bone receptor site combined with the functional and aesthetic prosthetic result. When the bone is deficient the software can aid in simulating bone grafts to support implant placement and reconstruction of the shape of the soft tissues with the intervention of craniomaxillofacial surgery. The simulations of the surgical procedures act as the foundation for communication to all members of the reconstructive team, 96

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b Figure 3.1 Through software available on the market (SimPlant Hasselt, Belgium), the image allows you to see the difference between a traditional two-dimensional view (a) and a three-dimensional view (b) of the alveolar nerve.

CHAPTER 3  Computer-Assisted Implant Surgery

Three-Dimensional Simulation The ability to plan an intervention in three dimensions instead of two opens new frontiers for all members of the implant team. The following extreme example fully clarifies what is meant by broadening the planning possibilities. If an implant is required for a premolar site in the mandible, it may first be assessed using two-dimensional panoramic radiography (OPT). The OPT shows that in the area of interest the distance between the bone crest the nerve is only 7 mm. Therefore, in this case, because of the apparent lack of vertical height above the nerve, the implant placement could be questioned. The anatomy as revealed with the undistorted three-dimensional views provided by CT or cone beam CT (CBCT) images, shows that the distance between the nerve and the bone crest is actually 6 mm. What cannot be

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established with two-dimensional images is the spatial location of the inferior alveolar nerve. The CT image allows precise evaluation of the location of the nerve, acknowledging that it still possible to insert an implant laterally to the nerve (Fig. 3.2). If one leaves a zone of safety distance of more than 1 mm from the nerve, it is possible to insert an implant with a length of 13 mm (Fig. 3.3). The ability to diagnose in three dimensions enhances accuracy and predictability for surgical and restorative results. Most dental implant manufactures provide a clear template of their implant sizes to aid clinicians in planning. However, superimposing an implant template to two-­ dimensional images does not allow the clinician to fully appreciate the patient’s actual anatomy in assessing potential implant receptor sites and could result in potentially

Figure 3.2  Based on the panoramic images it would appear that the only possible placement that could be done was to place a 7-mm implant, given the scarcity of available space between the ridge and nerve. On three-dimensional view it can be seen that, in fact, space is available.

Figure 3.3 Using full three-dimensional images, the next installation can be planned relatively close to the nerve.

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dangerous complications. In fact, when planning to use a transparent template, it is assumed that the implant is going to be inserted exactly along the plane of the considered image. Although it can be argued that the implant direction may be corrected manually during the surgery, this implies that the implant placement may be positioned in a plane that was not appreciated in the images (Fig. 3.4). It is the author’s opinion that it is essential to visualize all the relevant structures as obtained through three-dimensional CT/CBCT imaging modalities.

Stereolithography and Powder Sintering Stereolithography is one of the most common technologies to allow a virtual computer-generated three-dimensional object as depicted on a computer to be “printed” into a solid object. The first stereolithography machine was introduced in 1987 by 3D Systems, Inc., a company founded by the inventor Charles Hull in 1986 that gave start to the industrial exploitation of this technology. Over the years, an increasing number of similar machines have appeared on market. Desktop three-dimensional printers for the office have been produced, although they may not have the excellent resolution and quality of larger, more expensive machines, but

they may soon become popular (MakerBot® Industries, LLC, Brooklyn, NY USA). The physical principle underlying the phenomenon of stereolithography is a property of certain plastic materials. Plastics are divided into two categories: thermoplastic and thermosetting. Thermoplastics behave like most plastic materials in common use: when heated they melt. Thermosetting ones, on the contrary, become harder when receiving energy. Resins used in stereolithography are engineered to do exactly this. They are in a fluid state, midway between water and honey. To provide the energy needed for transition from liquid to solid state, the resin surface is hit by a laser beam, typically HeCd or Ar + gas at ultraviolet frequencies, with a power of some tens of mW. Where the laser beam hits the surface, the molecules of resin receive energy to form bonds, called sulfur bridges, with adjacent molecules, thus preventing the molecules moving relative to each other. Macroscopically, this transformation involves the transition from the fluid to the solid state (Fig. 3.5). Exploiting this principle, a three-dimensional volume, such as a model representing the anatomy of a patient, is virtually “sliced,” that is, the object in question is transformed in a number of sections, from bottom to top (Fig. 3.6).

Curing area

Figure 3.5 Where the laser beam hits the surface of the resin, the molecules get the energy needed to form bonds between them. This phenomenon results in a macroscopic change of the state of matter from liquid to solid.

Figure 3.4 Good prosthetic planning on the panoramic images (top). Observing the three-dimensional view it can be seen that, in fact, those same implants are completely misaligned with respect to the prosthesis, hence the importance of three-dimensional planning (bottom).

Figure 3.6 The file is virtually sliced. Of course, the effect in this picture (right) has been purposely exaggerated; in fact, the slice thickness usually ranges from 0.05 to 0.15 mm.

CHAPTER 3  Computer-Assisted Implant Surgery To provide an illustration, visualize a deck of playing cards, made up of 52 individual cards. The first step would require the individual cards to be designed in a three-dimensional software program starting at the bottom card of the deck, which represents the first layer. At this point, it must be communicated to the three-dimensional printing machine that, to build this first layer, the laser must hit the surface of the resin throughout a region that has the same shape, size, and thickness of the card. Subsequently, you virtualize a second card from the bottom of the deck, continuing in the same manner until the deck is finished. The result would be a block resin, which is an exact replica of the original deck. Figure 3.7 shows the profile of the polymerization created by the passage of the laser. As can be imagined, in fact, when the laser hits the surface, it does not hit just one point because the laser beam has a known diameter and around the area of impact it creates a volume of polymerization, roughly paraboloidal in shape. Once the first section is built, on a perforated steel plate called a “truck,” it is lowered by a specific amount equal to the thickness of the desired layer. The photopolymer solidifies to a certain depth, and it is important that the thickness of the solidified volume is greater than the thickness of the new overlying layer of liquid resin: this allows the creation of an overlapping area (Fig. 3.8). The layers are connected to each other by this overlap. If this did not happen, there would be layers of resin floating and drifting on the surface of the resin itself. This may happen when there are problems with the laser calibration. At this point the phase of overlap begins (the systems to cover the section

Figure 3.7  An ultraviolet laser beam hits the resin surface creating a volume of polymerization.

Overlap

Figure 3.8 The layers are connected to each other by an overlapping area, called “overlap.”

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just built with a layer of liquid photopolymer are different). A very thin layer of the hardened section, in contact with the atmosphere, remains liquid because oxygen inhibits the chemical reaction. This facilitates the subsequent and necessary bonding between the sections (Fig. 3.9). The piece produced contains liquid trapped inside, and the walls are not completely cured (green part). Therefore the phases of washing resin excess after construction and curing in ultraviolet (UV) ovens are of fundamental importance. In these ovens the parts are irradiated for a sufficient time to complete, in depth, the polymerization initiated by the laser beam. Stereolithography allows users to reconstruct any shape that can be designed on the computer using appropriate three-dimensional design software, and therefore it has become the main technology used for rapid prototyping. This technology has been used to produce in a very short time any component of a car or any piece of design to be evaluated before mass production of these parts (Fig. 3.10). Therefore rapid prototyping has become a great design tool. A significant problem is the limited variety of materials suitable for stereolithography, so it is difficult to achieve high mechanical properties—meaning that the final components will eventually be fabricated by a different manufacturing process. Over the past few years, however, the range of materials has been increasingly expanding. Recently, stereolithographic materials suitable to be kept in contact with human body for several hours without damage to the tissues have been also been developed. These materials cannot be implanted but can be used for the time required for the surgery. Materials research is focusing on the identification of implantable materials suitable for stereolithography or other similar technologies. Powder sintering, well suited to medical applications, is a similar technology. Its principle is similar to that of stereolithography: the use of very fine dust particles, which can be plastic, such as polyamide (commercially known as nylon), or metal, for example, titanium, are warmed. Once heated, these particles can be fused by applying heat selectively; it is possible to make the nylon melt in only certain places based on a predefined design, such as stereolithography (Fig. 3.11). Once finished, the powder block cools and melted areas solidify. At this point, the powder that was not sintered (technical term that indicates precisely this construction technique based on the fusion of fine particles) is aspirated. What remains is the object that was planned to be fabricated by this process (Fig. 3.12). The significant advantage of this technology is the possibility of using materials with higher technical characteristics: they are not materials that change their molecular structure during the construction process, but finished materials, such as nylon or titanium, that are only melted and left to cool.

Tools for Transfer of Simulations to Clinical Practice: Surgical Guides and Rapid Prototyping Models For many years, tools such as rapid prototyping, stereolithography, and, more recently, powder sintering, have been used as an aid to surgery to transfer the virtual

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c Figure 3.9 This picture shows an archeology project. The statue of a kouros was scanned optically to avoid damaging it with the usual impression materials. The three-dimensional file was reconstructed using stereolithography (a, b) by Materialise, Leuven, Belgium. The statue (c) measures 1.85 m in height.

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b Figure 3.10  (a, b) Design pieces from the MGX collection (Materialise NV, Leuven, Belgium) created through stereolithography.

CHAPTER 3  Computer-Assisted Implant Surgery surgical plans to the patient (Fig. 3.13). The usefulness of three-dimensional surgical planning has already been described, but it is equally obvious that even the planning alone cannot be as effective unless there are appropriate tools to transfer the planning to the patient with a surgical guide. However, it should be noted that guides produced with rapid prototyping tools are not the only way to achieve surgical guidance for implant placement. Virtual navigation systems are the major competitors to the guidance systems through rapid prototyping techniques. Through the virtual navigation systems, the surgeon sees on the screen the position of the surgical instrument he or she is holding (drilling handpiece) with respect to the patient and therefore to the surgical plan in real time during the surgical intervention. The biggest problems of these systems (in addition to the prohibitive costs of equipment) are their limitations for applications such as dental implants. In fact, it is true that they indicate the position and direction for the osteotomy, but when drilling in a transitional zone between cortical

Figure 3.11 In powder sintering, particles of material are fused locally by the passage of the laser beam.

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and cancellous bone, the density difference can divert the drill toward the lower density area, that is, the cancellous bone. It is likened to making a mark on the wall to make a hole in it; after positioning the drill precisely on the mark and perpendicular to the wall, as soon as you start drilling, the drill moves to the right or left because of an obstacle. In such situations, navigation systems give a precise estimation of the error but cannot do anything prevent it. Surgical guides, on the contrary (in addition to relatively low costs), have the advantage of providing mechanical support to the drill to keep it in the correct position throughout the whole site preparation. The principle of stereolithographic guides is very simple and is derived from old laboratory techniques; the drilling guide is built on the plaster cast of the patient, and direction and position of the drill are simulated through small tubes or straws glued to the cast, which become incorporated in the resin to create a guide for drilling. The construction of stereolithographic guides, of course, is fully automated, as described in the section on stereolithography. The threedimensional volume construction is carried out by first identifying the type of anatomical support that is going to be used to stabilize the guide during surgery. There are three basic guide designs: (1) The guide can be supported by the bone of the patient (bone-supported), which requires the use of an open flap technique to expose the underlying bone; (2) the guide is planned to be supported by the oral mucosa, which can be performed with a flapless surgical approach (mucosal-supported); or (3) the construction can be supported by the remaining natural teeth (Fig. 3.14). In all these cases the three-dimensional volume is different as defined within the interactive treatment planning software. In the case of a bone-supported guide, the (only) reference data are those contained in CT/CBCT because it is the patient’s maxillary

b Figure 3.12  This figure shows how the pieces are extracted from the completely cooled powder block (a), and a finished product used as a lamp (b).

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Figure 3.13  Examples of surgical guides: (a) virtual simulation, (b) cutting guide for a hand used during surgery; (c) surgical guide for implants insertion with the relative bone model.

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Figure 3.14  The different types of support for a surgical guide: (a) mucous support, (b) bone support, and (c) dental support.

CHAPTER 3  Computer-Assisted Implant Surgery or mandibular bone that must be properly separated by a process called segmentation1 from the rest of the anatomical structures usually through differences in the density value of the object. In the case of mucosal support, soft tissues are difficult to fully visualize in the CT/CBCT images. It is necessary and required to make the soft tissue diagnostically visible through the use of a diagnostic prosthesis (scan or scannographic prosthesis), to be used during the scan acquisition providing accurate information of the existing soft tissue topography. The methods to fabricate these scan prostheses are mainly two: to make a radiopaque prosthesis of the desired restorative result or to create a prosthesis containing radiopaque fiducial markers. In the first case, the resin used to make the prosthesis is mixed with barium sulphate or other radiopaque substances. The patient undergoes CT/CBCT scan wearing this prosthesis so that the volume between the bone and prosthesis will become clearly visible as the soft tissue (Fig. 3.15). In the second case, a duplicate of an existing denture, or a duplicate of a diagnostic wax-up representing the proper vertical dimension, centric relation, lip support, phonetics,

1. In image processing, segmentation is the process of identification of volume portions that belong to the volume of interest and those that do not.

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and esthetic position of the teeth can be used as long as the adherence or stabilization during the scan acquisition is adequate. This type of prosthesis is generally not radiopaque and therefore must have radiopaque markers such as stainess-steel balls of known diametes: for example balls of 5 mm diameter attached to or incorporated within the prosthesis to become visible in the CT/CBCT images. Once the scan has been completed the images of the patient’s bony anatomy and the incorporated markers will be visible. A second scan of the prosthesis alone is then performed, allowing for the prosthesis to be identified with its markers. The software will then allow for the two different DICOM (digital imaging and communication in medicine; standard for images and digital communication in medicine) datasets, one of the patient with the prosthesis, and the second of the prosthesis alone to be superimposed or merged using the fiducial markers (dual-scan technique). In this manner there is a total understanding of the relationship of the prosthesis as it relates to the underlying bone (Fig. 3.16). Then a tooth-supported guide is required; often there is a metal artifact from adjacent crowns or restorations that can interfere with the diagnostic phase because of “scatter.” The scatter also can interfere with the fabrication of an accurate surgical guide supported by teeth—as the patient’s anatomy cannot be segmented with adequate detail necessary for template fabrication. Therefore a stone or plaster cast fabricated from an intraoral elastomeric impression material is used. The cast is then optically scanned (digitized), allowing for the

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d Figure 3.15  (a-d) Example of scan prosthesis created according to the single-scan method: a special prosthesis is built mixing resin and barium sulfate, which makes it radiopaque.

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new volume to be imported into the planning software. The cast volume is accurately superimposed on the images of the patient using specific software applications providing tooth morphology in high detail that is free from scattering artifact (Fig. 3.17). In the three types of support described, a three-dimensional volume is obtained and will be used to construct the guide. In the case of dental or bone support, the planning software is used to process these volumes and to build the guide after the placement of the implants has been established. The trajectory of implants determines the direction in which the guide will be placed in the mouth, eliminating all undercuts and creating a surface that fits the patient by removing all obstacles that may arise during the positioning

and removal of the guide. The guide will incorporate metal cylinders of a known diameter that corresponds to the drilling sequence, that is, the diameter of the drills. In the case of mucosal support, the prosthesis itself will be used as the preliminary virtual structure for the guide. Based on the position of the virtual implants, small metal tubes are inserted, in steel or titanium, of various sizes and heights, according to surgical instruments that are to be used. These tubes are located virtually in the software; the physical tubes, which are thick enough to withstand mechanical stresses, are then positioned within the resin structure and bonded. The three-dimensional volume will be built as described in the section on stereolithography and the metal tubes bonded to the built parts. The tubes work as stable support for the drill

a Figure 3.16 Diagnostic prosthesis carried out with the method of double scanning. (a) The patient undergoes CT scan while wearing the prosthesis. The prosthesis is invisible in the scan; only the radiopaque markers in gutta-percha can be seen.

Continued

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b Figure 3.16, cont’d  (b) A second scan of the prosthesis alone is carried out. In this scanning, by filtering conveniently the levels of gray, the prosthesis becomes visible, as well as the gutta-percha radiopaque markers, which are perfectly visible.

and protect the resin, to prevent particles of the guide from dropping into the osteotomy (Fig. 3.18). Regardless of the type of guide used it should reduce the difficulty of the operation provided it is used correctly. Well-fabricated surgical guides should be inserted into the patient’s mouth to ensure the position is accurate and stable before surgical intervention. In many cases, such as in totally edentulous subjects, it is essential to mechanically fixate the guide with screws or inserts, which are planned for in the design and fabrication process. A silicone bite registration also enhances stability and positioning of the guides. Even in cases of a bone-supported guide, when the bone crest is rather resorbed, fixation can be useful.

CLINICAL INDICATIONS Marco Rinaldi, Scott D. Ganz, and Angelo Mottola Indications for the use of computer-assisted surgery are manifold. The utility of stereolithographic models in the

design of bone grafts and harvests execution has been already examined. This section provides general guidelines on the use of preoperative planning software. These interactive systems increase the diagnostic accuracy, allow for a more detailed study of the case, and can be considered indicated in all clinical situations, limiting potential complications by taking the guesswork out of the process. As an example, determining implant position in an aesthetic area could in fact be more complex than deciding the position of multiple implants for an overdenture prosthetic design. Certainly the proximity of delicate vital anatomical structures adjacent to prospective implant sites could benefit enormously from the technologic advances offered by computerized or CTassisted surgery. Three-dimensional computerized planning is essential in planning angled implants, because the implant outline cannot be superimposed to dental-scan oblique sagittal images, because an angled implant would cut them obliquely, showing only a section of the planned implant (Fig. 3.19). The outline of an implant can be superimposed

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c Figure 3.16, cont’d (c) The prosthesis, reconstructed in the second scan (b) is imported and reconstructed in the first one (c).

to a sagittal oblique section when the implant site is parallel to it. Therefore without the help of computers it becomes difficult to fully appreciate the reality of individual patient’s three-dimensional anatomy and the necessity of determining accurate and wide safety limits. The surgical capabilities of the operator do not count as much as a proper threedimensional diagnosis because, regardless of surgical skills, some deep structures are not detectable in any way during the surgery without a three-dimensional scan. In the case of angled implants placed anterior to the maxillary sinuses (Fig. 3.20), it is possible (if permitted by the sinus conditions) to open the sinus and probe the anterior wall, but it is unthinkable to explore directly the lateral wall of the nose (Figs. 3.21, 3.22). The same also applies to angled implants anterior to the mental foramina; foramina and mental nerves certainly can be identified during surgical dissection, but the exact location of frequent mesial bend of the nerve cannot be appreciated because often it is completely intraosseous (Figs. 3.23 to 3.25). For these reasons, computerassisted surgery can be considered indispensable in graftless surgery, when angled implants are planned and necessary to support the desired restoration. In conclusion, these systems must be used whenever you want to increase accuracy and study a case in detail. The authors leave to the clinician the freedom to decide in which cases it can be convenient to

use the enhanced capabilities of computer-assisted implant surgery, but it is important to remember the following: There is a danger when we are bound by two dimensional concepts in a three-dimensional world. Scott D. Ganz

ACQUISITION PROTOCOLS AND SCAN PROSTHESIS Marco Rinaldi, Scott D. Ganz, and Angelo Mottola Data acquisition through an appropriate CT device is a necessary requirement for computerized surgery. Both spiral CT and cone beam systems can be used. The recommended acquisition protocol establishes a set of standards more or less common to all modern equipment. Acquisition of images must be performed with CT, the transaxial plane aligned with the occlusal plane or with the base of the jaw or the bone palate in the case of edentulous patients with no radiographic templates (Fig. 3.26). The data, in DICOM format, are delivered in a CD or via the internet in a raw format and must be converted for use within differing interactive software applications. Once converted and imported into specific software the anatomical structures may be managed by “segmentation.” Segmentation is a preliminary image elaboration

CHAPTER 3  Computer-Assisted Implant Surgery

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c Figure 3.17  In the case of dental support a plaster cast is scanned (a), then the three-dimensional file obtained (b) is superimposed to the images from the CT scan (c).

Figure 3.18  SurgiGuide Æ surgical guide (Materialise NV, Leuven, Belgium): metal tubes work as a guide for drills.

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f Figure 3.19  Planning of an angled implant anterior to the maxillary sinus; determination of the inclination on the coronal image (a). Implants cannot be planned the same way as those parallel to the oblique sagittal sections, because in their inclined course they cross several images (b-f).

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Figure 3.20  Three-dimensional CT view, axially sectioned, showing the inclined course of an implant to avoid the involvement of the maxillary sinus, affected by disease. In cases such as this, it is contraindicated to open the sinus to explore the anterior wall.

Figure 3.21  An error in the inclination of an implant could involve the nasal floor, whose integrity is not visible during the surgery (compare with Figure 1.7 of Chapter 1).

Figure 3.22  The angled implant avoided the maxillary sinus but perforated the nasal cavity (compare with Figure 1.7 in Chapter 1).

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Figure 3.23  Mandibular angled implant in its tilted course as it crosses several oblique sagittal sections. It is difficult to plan accurately an angled implant without computerized surgery.

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Figure 3.24  A 27° tilted implant seen on a coronal image.

Figure 3.25  Through presurgical planning software it is possible to determine implant position in threedimensional images which makes planning angled implants easier.

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Maxillary bone properly aligned

Mandible properly aligned

Figure 3.26  (a) Schedule of the radiographic protocol recommended by Materialise NV, Leuven, Belgium. (b) CT transaxial plane should be parallel to the occlusal plane.

aimed to define the necessary volume and eliminate parts of the anatomy that are not within the region of interest and that may interfere image interpretation (Fig. 3.27).

Radiographic Templates In cases in which flapless surgery is planned it is necessary to visualize soft tissues through tomographic scanning

and the use of radiopaque scan prostheses fabricated with resin and barium sulfate. In CT/CBCT images the volume between the bone and the prosthesis corresponds to gingival tissues (Fig. 3.28). In the case of bone-supported guides, it is advisable to have a template with radiopaque teeth because they are very useful references for surgical/ restorative case planning. It is sufficient that the teeth are

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Figure 3.28 Reformatted CT image on the oblique sagittal plane. The implant axis (yellow), for a screwed prosthesis, passes correctly through the lingual surface of the scan prosthesis. The space between this and the bone is the gingival tissue.

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to appear or disappear during the visualization, enhancing the diagnostic process. The scan prostheses for single acquisition are built by the dental laboratory based on a diagnostic wax-up (Fig. 3.29) or are radiopaque duplicates of the temporary prosthesis (Fig. 3.30). Scan prostheses for double acquisition, or dual-scan technique, on the other hand, are represented by the patient’s prosthesis, in which radiopaque markers were inserted before scan acquisition.

FEATURES AND PERFORMANCE

c Figure 3.27  (a, b, c) Sequence a segmentation process to “clean” the images and define the volumes of interest.

radiopaque, and the base of the prosthesis can be radiotransparent. In the case of a guide supported by bone, the base of the scan prosthesis may not be radiopaque because the flanges of the prosthesis would overlap the threedimensional views and it would not be possible to see the bone crest in three-dimensional views, depending on the software’s ability to segment or separate the volumes. In this case, during segmentation process, the radiographic template should be isolated from the bone of the patient to make it a distinct object (for the software) that can be made

Marco Rinaldi, Scott D. Ganz, and Angelo Mottola These systems offer the possibility to plan the implant sites and the clinical case by interacting with CT/CBCT dental scan images. There are a variety of software tools that can enhance the diagnosis process, helping the clinician make accurate presurgical plans. Important software functions provide the opportunity to manipulate and enhance the images to improve diagnostic assessment. It is possible to modify the grayscale values to facilitate identification of the mandibular canal. The entire path of the inferior alveolar nerve can be identified in color, to make it highlighted in all views. The software allows for specific areas of interest to be magnified with the “zoom” functionality to improve focus on any particular site. Images can be viewed in any orientation, and objects can be manipulated and moved in all positions. Three-dimensional images can be sectioned at any point on the oblique sagittal plane or on the axial plane to visualize anatomical details. To better visualize the deep structures, three-dimensional images also can be made transparent, a concept defined as “selective transparency” by Scott Ganz (Fig. 3.31). Nondis­ torted imaging inherent in CT/CBCT allows for the use

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b Figure 3.29  Construction of a scan prosthesis: (a) diagnostic wax up and (b) scan prosthesis.

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b Figure 3.30  Construction of a scan prosthesis: temporary patient’s prosthesis (a) and scan prosthesis (b).

of measuring tools to perform any linear and angular measurements with a high degree of accuracy. It is also possible when planning to harvest from a bone host site, to plan for an implant insertion to measure the distance from nearby adjacent anatomical structures. Furthermore, bone density in Hounsfield units (with medical CT) can be calculated, and certain software can help measure the volume of a graft (Fig. 3.32). Dental implants planned within the software applications are represented by their actual computer-aided drafting (CAD) shape and can be chosen by a software library of available manufacturer’s components. Once the plan has been formulated and simulated on the computer, patients can view the treatment plan either directly on the computer screen or through color printouts. This type of documentation facilitates the understanding of the treatment plan and doctor-patient communication and can be expanded to educate the entire dental implant team.

Views There are four main views that can be visualized from the dataset of a CT/CBCT scan: coronal, oblique sagittal (transverse), axial, and three-dimensional (Fig. 3.33). Views are

related to each other through colored indicators that help to easily navigate through images.

Implant Planning: Software Features Using interactive treatment planning software allows implant planning on any of the provided images. Implants can be chosen from the manufacturers’ library and then drawn directly on the images. Implants can be manipulated in many ways to provide the most accurate placement. Length and diameter can be changed to fit within the bony receptor site. The implant can then be tilted or rotated to comply with the desired restorative position. Prosthetic abutments that serve as the link to the restoration also can be provided from a library of components. The peri-implant bone quality and the volumes of a possible graft can be assessed. The level of risk for potential complication can be customized by setting the security distance from the alveolar nerve or between implants. Exceeding the set distance activates an alarm. Implants can be placed parallel to each other through an automated process, or their angle of inclination can be calculated to help with the restorative plan. Once all the implants have been placed and their positions verified in all views, the treatment plan is saved. Images can be captured

CHAPTER 3  Computer-Assisted Implant Surgery

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c Figure 3.31 Some of the possibilities offered by a presurgical planning software (SimPlant Hasselt, Belgium). (a) Three-dimensional view of implant plan. It shows the implant axis and the virtual teeth. (b) Modulation of transparencies to highlight the deep structures. Alveolar nerve has been colored. (c) Relations between implants and alveolar nerve. “Objects” can be made to appear and disappear from images.

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b Figure 3.32  Other allowed functions are (a) calculation of bone density in Hounsfield units and (b) detection of implant fenestrations.

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Figure 3.33  The four views of the SimPlant Æ software (Materialise NV, Leuven, Belgium): clockwise, starting from the top left, oblique sagittal, axial, three-dimensional, and coronal images.

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Figure 3.34  Types of surgical guides: (a) supported by mucosa, (b) by teeth and (c) by bone.

from the computer screen and printed as a form of medicallegal documentation for the patient record. The plan can then be shared with all members of the implant team and shown to the patient for case acceptance. Once accepted by the patient, the next step is to order the surgical guide.

TYPES OF SURGICAL GUIDES Marco Rinaldi, Scott D. Ganz, and Angelo Mottola As already noted earlier, surgical guides can be supported by mucosa (Fig. 3.34a), teeth (Fig. 3.34b), and bone (Fig. 3.34c).

CHAPTER 3  Computer-Assisted Implant Surgery Each of these types is used to perform a specific technique of implant surgery. The bone-supported guides provide a more traditional surgery, with the opening of a flap to expose the bone, whereas tooth-supported or mucosal-supported guides can provide for a flapless approach to surgery. The term graftless surgery, instead, indicates those cases in which, by tilting the implants distally to gain posterior restorative support, grafts can be avoided. Each approach presents surgical indications and limitations, advantages and disadvantages, that are analyzed in the following section.

Indications for Guide Types Mucosal-Supported Guides

Mucosal-supported guides are used for completely edentulous arches and allow for implant insertion to be carried out without flap elevation (flapless) using a circular mucotomy at the level of the implant site. Flapless surgery has the advantage of being minimally invasive compared to the open flap technique because it avoids the trauma of flap elevation and reduces the amount of anesthetic needed and the surgical time required to complete the procedure; it is also significantly faster. For these reasons this method is termed “minimally invasive surgery.” It also allows the insertion of implants in several short sessions, even when the treatment plan involves many implants. Flapless surgery provides the potential of satisfactorily treating even elderly or medically compromised patients. The guide should be positioned intraorally using a silicone bite index with the patient closing into a predefined bite. To stabilize mucosalsupported guides it is recommended to fixate the template with osteosynthesis screws or other suitable fixation systems. It is very important to be able to rely on the position of the surgical guide because with flapless surgery it is difficult to verify.

Tooth-Supported Guides

This type of guide has the same advantages of flapless surgery, or it can be accomplished with minimal flap designs if desired. Furthermore, the presence of teeth ensures stability of the guide and its correct position. Of course, it cannot be used in fully edentulous patients. Dental (tooth) support reduces the main problem of mucosal-supported guides, that is, the correct position and stabilization of the guide.

Bone-Supported Guides

These guides do not change the approach to the patient in terms of surgical trauma, but allow more control of the intervention because the operator can stop at any time and easily check the situation. It is the author’s opinion that either tooth-supported or bone-supported templates are excellent teaching tools for novices who are at the beginning of their learning curve for guided surgical implant applications. Tooth-supported surgical guides are generally fabricated by stereolithography on the basis of tomographic images. DICOM data from CT/CBCT represent a “still image” on which to build the entire surgical plan; therefore it is recommended to respect the timing of radiographic examination,

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surgical planning, and intervention. Indications for bonesupported guides are listed below and, by exclusion, those for flapless surgery can be found. According to the opinions of the authors, indications for bone-supported guides are the following: • The need or the desire to visually check the preparation of implant sites. • Narrow safety margins (implant planned near dangerous anatomical structures). • Instability of mucosal support for a scan prosthesis and the surgical guide, such as in cases of advanced atrophy. • Reliability of the other team members in carrying out the CT study and positioning the radiographic guides. • Need to make corrections to the implant site (bone grafting, osteoplasty, guided bone regeneration techniques, bone chips, membranes, etc.). • Need to remove grafts means of fixation used in pre-prosthetic surgery (plates, grids, osteosynthesis screws). • Type of anesthesia (general or local) and the operating environment (operating room or dental surgery, invasive or noninvasive surgical techniques, stereolithographic). • Classification of the patient according to the American Society of Anesthesiologists (ASA) system and treatment plan. These general guidelines can have several exceptions (Fig. 3.35). Bone-supported surgical guides must be positioned without interference by soft tissue; therefore they require a large flap to obtain an adequate exposure of the bone. The crucial moment of the intervention is the localization on the patient’s bone at the same position that the surgical guide has on the stereolithographic model (Fig. 3.36). The guide adaptation check must be made with great care because an error in positioning could have disastrous consequences and cause serious damage to the patient. Often the guide will “snap” onto the stereolithographic resin model. It is therefore advisable to try to find the same “snap” from the model with the bone of the patient. On the model, in fact, the guide should have practically no freedom of movement and, in general, is placed with a particular movement, which should be found when the guide is tried on the bone. The correct position is the position of stability, which is the one that does not allow the guide to slide to other positions. If there is any doubt about stability, the bone-supported guide can also be fixated with screws. It is convenient to check visually the correspondence between the position of the guide on the model and the bone, even if it takes some time to remove and replace the guide. It is possible to check the distance of the guide border from the mental foramen, the nasal spine, or other vital anatomical landmarks or structures. “Windows” on the resin guides usually serve to aid in irrigation during drilling, but can also be used to verify seating. They also allow the assessment of the distance of the guide from the bone surface. If the guide position creates any doubt, or more positions of stability should be found, it is advisable to leave the guided procedure and proceed with the conventional free-hand surgery.

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a

b

c

d Figure 3.35  (a-d) In some instances it is possible to make a correction to the implant site even if a flapless technique has been used. In this case, after the implants insertion (Certain NanoTite, Inc., Biomet 3i, Palm Beach, Fla, USA) a small linear incision was carried out by the vestibule and a biomaterial was inserted through a tunnel to correct a small bone dehiscence. (Endobon xenograft Æ, Inc., Biomet 3i, Palm Beach, Fla, USA).

a

b Figure 3.36  Checking of the surgical guide stability (fit-check): check of the stability on the STL model (a) and on the bone (b). The position must match.

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a

b

c Figure 3.37  Surgical instruments specifically designed for computer-assisted surgery: (a) Navigator™ System (Biomet 3i Inc., Palm Beach, Fla, USA). (b) SAFE Æ System (Materialise NV, Leuven, Belgium). (c) ExpertEase.

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Preparation of the Implant Sites Surgical guides are built to transfer the computer threedimensional planning to the surgical intervention phase. Surgical guides always allow us to define implant positions and inclinations, but may provide only a few indications about the depth of preparation. To prepare a full-length implant site through the guide, the implant length must be added to the thickness of soft tissue, to the distance of the guide tube from it, and to the tube length. Often the resulting length is difficult to obtain because of lack of appropriate equipment or drills of sufficient length to navigate the vertical height requirements. Therefore guided surgery requires specific hardware components and support from the implant manufacturers to develop the required parts. In these cases, although not ideal, it may be recommended to prepare with common graduated drills the last millimeters of the implant site without the guide. Some surgical kits recently developed specifically for assisted surgery contain dedicated instruments consisting of special drills and implant mounts of different lengths that enable the

complete preparation of the implant sites and insertion of the implants through the guide (Fig. 3.37). These systems enable precise control of preparation depth and implant placement. By using a kit for the laboratory it is possible to create a model with the implant position and build the temporary prosthesis even before performing the surgery. In the case it would be necessary, it is advisable to make even small corrections to the bone after implant placement, so as to not modify the bone support of the guide (Fig. 3.38).

APPLICATIONS OF 3D IMAGING IN ORAL SURGERY The interactive analysis of CT images, allowed by a surgical planning software, gives many advantages also in fields other than implantology. In fact, the three-dimensional views and the software functions are very useful in the diagnosis and surgical planning of many diseases in oral surgery (Figs. 3.39 to 3.56).

Figure 3.38 Any bone adjustments should be performed after implant insertion, because bone supported guide is built based on the presurgical CT and any bone change might compromise its stability.

Figure 3.39 Odontoma: the OPG shows a lesion in the sites 1.2 and 1.1.

Figure 3.40  Odontoma: 3D CT view in which the teeth have been isolated from the maxillary bone.

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Figure 3.41  Odontoma: 3D view made transparent to show the relations between the teeth and the neoformation.

Figure 3.42  Odontoma: making the teeth to disappear it shows the neoformation that can be easily located on the palatal side.

Figure 3.43  Odontoma: palatal flap for the removal of the neoformation. It shows the naso-palatine nerve.

Figure 3.44  Odontoma: Removal of the neoformation. Histological examination proved it to be a compound odontoma.

Figure 3.46  Odontogenic sinusitis: 3D CT view of an apical lesion on the 2.4.

Figure 3.45 Odontoma: The precise radiographic location of the odontoma allowed its excision with minimal bone removal.

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Figure 3.47  Odontogenic sinusitis: the sagittal oblique section shows as the lesion also extends inside the maxillary sinus.

Figure 3.49  Odontogenic sinusitis: panorex image showing the full obstruction of the left maxillary sinus. Additional diagnostic examination confirmed the presence of purulent sinusitis in maxillary, ethmoid and frontal sinuses. The patient will be operated per oral via, by FESS.

Figure 3.51  Impacted canine: Axial image at the level of the caine. The canine apex penetrates the nasal cavity.

Figure 3.48  Odontogenic sinusitis: the axial image shows the cystic lesion in the left maxillary sinus and the presence of sinus inflammation.

Figure 3.50 Impacted canine: maxillary 3D CT view showing the presence of an included canine, in a very deep position. The diagnosis of the position allows to perform an adequate surgical-orthodontic treatment.

Figure 3.52  Impacted wisdom tooth: oblique sagittal image shows the close relationship between the 3.8 and the mandibular canal.

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Figure 3.53 Impacted wisdom tooth: 3D view, with the alveolar nerve highlighted, sectioned in the oblique sagittal plane.

Figure 3.55  Oro-antral communication: wide oro-antral communication following a Le Fort I type orthognathic surgery.

Figure 3.54 Impacted wisdom tooth: 3D view, with the alveolar nerve highlighted, sectioned in the axial plane. Figure 3.56  Oro-antral communication: the surgery was performed by nasal (FESS) and oral approach. The intervention was planned using the tools of computer diagnosis.

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