From Volume 40, Issue 7, September 2013 | Pages 577-582
The optimal positioning of oral implants ensures good biomechanical, functional, aesthetic and phonetic results. The concept of surgical guidance in implant dentistry has been developed to bridge the gap between pre-operative treatment planning and surgical site preparation. This article discusses its evolution, from the early surgical guide systems which used only diagnostic casts as reference, to the latest in computer-assisted navigation.
When modern dental implantology was introduced in the early 1980s, implant placement was often carried out based only on available residual bone. Several studies have clearly demonstrated that implants placed in this manner often emerge in a buccal or lingual position, ending with arduous, or even impossible, aesthetic problems to solve.1 On the other hand, it has been shown that implants that are not working in their long axes were exposed to detrimental lateral forces, ending with numerous biomechanical problems and even breakages.2 Owing to too many associated problems and the functional compromises of the final prosthesis, new concepts were developed and new methods produced by first considering the prosthesis rather than the surgery. Thus the concept of ‘Prosthesis driven implantology’ was born.
In the early days, clinicians who believed in this concept mostly depended on diagnostic wax-up and/or surgical templates made on hard gypsum surfaces of master casts3,4 (Figures 1 and 2). Although these templates could provide prosthetic guidance, they could not take into consideration the anatomy of the underlying bone.5 It was also discovered that the hard surface of casts may not entirely replicate the soft tissues of the oral cavity, and therefore this method may not be as accurate as necessary for some treatment purposes.
Work on the subject finally led to ‘double-purpose templates’, which could not only be used for radiographic examination and evaluation of the patient, but also during surgery and placement of the implants.6 Conventional dental panoramic tomography and, possibly, plain film tomography are usually performed with the patient wearing a radiographic template with integrated metal spheres and guide tubes at the position of the wax-up (Figure 3). Based on the known dimensions of the metal sphere, the magnification factor could be calculated and the depth and dimensions of the implants estimated. Double-purpose templates may relieve the problem of directing the implant to a good position, but using these templates is only confined to the first drilling sequences and further drilling is not possible without difficulty. Furthermore, it was soon well established that all plain radiographs suffer the limitation of being two-dimensional projections of complex, intrinsically three-dimensional anatomy. Essentially, conventional radiography, which is widely used, has important diagnostic limitations, such as expansion and distortion, setting errors and position artifacts.
Perhaps the most important technological advancement that dramatically enhanced the clinician's ability to diagnose and treatment-plan dental implants, with an ability to view 3D anatomy, has been the computed tomography (CT) scan. Some of the promising advantages of CT are the lack of superimpositions and its high level of accuracy in comparison to other modalities. However, in order to address the requirements of the concept of prosthetic-driven implantology, some type of radio-opaque CT scan template was required, incorporating valuable information about the position, occlusion, form and contour of missing teeth (usually in the form of radio-opaque markers incorporated into a patient's existing denture, or via some type of barium coating), which would give new data that could be viewed in relationship to the underlying structures.7,8,9,10 In this way, it would be possible for a prosthodontist to visualize the location of planned implants from an aesthetic and biomechanical standpoint.
One must consider that the images obtained from CT are really 2D printing, requiring a process of mental integration of multiple sections by the observer to derive 3D information. This problem is discussed in detail in the literature. These 2D views are easier to perceive visually on the computer, but they are basically a digitized version of printed images, so are less predictable for the implant size needed and poor for anatomical complications. To overcome these obstacles, we need systems that allow simultaneous visualization of 2D reformatted images, as well as 3D-derived bone surface representations.
Since the 1990s, medical research teams have approached the problem of implant planning with the assistance of interactive computer applications.11,12 Several techniques have been developed, using software and hardware, to represent the anatomical data in 3D on a 2D screen. Using the first commercialized software program, SIMPLANT (Columbia, MD, USA) in 1993, clinicians had the ability to view and interact with the CT scan data to place the implant body virtually and visualize the prosthodontic implications at the same time (Figure 4). For the first time since the inception of osseointegrated implants and CT scans, it was possible to reformat the CT scans so as to provide an accurate 3D view of the anatomy of the bone, in which the clinician can interactively introduce implant planning into the CT images. But there was still a familiar problem – the scanning appliance became the surgical device. In fact, this was more of a visual guide, not a true surgical guide. Transferring the computer plan into actual patient treatment seemed to have a missing link, which was quickly filled by the revolutionary CAD/CAM technique.
Early published data regarding the incorporation of CAD/CAM techniques into implant dentistry were for eliminating the surgical bone impression phase of the subperiosteal implant modality.13 In this manner, and for the first time, it was possible to reproduce the area of interest, such as the mandible or maxilla, physically. Physical models like these are attractive because they offer an opportunity to hold the model in the hand and view it in a natural fashion, thus providing the clinician with a direct, intuitive understanding of complex anatomical details which otherwise cannot be obtained from imaging on screen. Unfortunately, this method has some limitations. The conventional milling machines have restricted motion capability. Practically, complex geometries are difficult to program and can result in tool/work piece collisions. Yet another limitation is that it usually requires skilful human intervention to help plan the operations and to operate the equipment. In order to circumvent these problems, another revolution was required.
The layer data format of CT scanners quickly prompted the realization that it may be possible to convert this data so as to make it compatible with rapid prototyping (RP) technology. In principle, the process works by taking a 3D computer file and creating a series of cross-sectional slices. Each slice is then printed one on top of the other to create the 3D objects. It was theorized that this type of mechanical prototyping is capable of quickly fabricating complex-shaped, 3D parts directly from CAD models. This is an additive process, unlike milling which is a subtractive one. The potential transformation was recognized early in RP development, and accurate anatomical RP models were fabricated. This physical realization of CT data has been termed ‘real virtuality’ or ‘virtual reality’. Clinical pre-operative planning for the placement of dental implants, and on the design of surgical aids in the form of drill guides to transfer the planning to the patient, have resulted in an interactive manipulation of computer-generated implant models which could simulate the placement of implants, and an automatic procedure to generate a surgical aid at later stages. In this manner, a new therapeutic protocol was developed that included not only case planning based on 2D and 3D scanner data, but also the transfer of implant planning into the mouth of the patient, through the use of custom-made stereolithographic drill guides14 (Figures 5 and 6).
In addition to using RP technology, a different approach has been developed for placing implants accurately and in accordance with the pre-operative treatment planning. This methodology, which has gained a place over the last 10 years in implant dentistry, is image-guided solution/navigation system (IGS) or image-guided navigational implantology (IGI). These systems were principally designed for relieving the common drawbacks of RP technology; if the proposed jaw is severely atrophied, it is difficult to handle the templates without dislodgement. Moreover, it is always a matter of concern how best to control the distance of the drill tip from critical structures, such as the inferior alveolar nerve, the floor of the nasal cavity or the maxillary sinus, during precise surgery.15 The IGS/IGI systems provide sensors as well as software programs to transfer the pre-surgical plan to the patient. They also provide automated monitoring of the surgical procedure. The process is limited by the physical navigation control of the dental practitioner placing the implants and the fact that the sensing device is sensitive to the line of sight.16 Basically, the system produced by this concept uses marker-based referencing methods (called registration points, fiducial markers, etc) to establish the transference of the tool co-ordinate system to the patient. The markers are principally devices designed to act as reliable surrogates for imaging anatomical structures of interest. By incorporating the light or sound generator markers, the clinician is able to be guided, audially and/or visually, to set the implant by simply moving the drill in the appropriate position with the support of the navigation system. The system tries to assist the surgeon during the pre-operative planning and also during the intra-operative procedure, while the optimal treatment plan is applied directly to the patient. It has been argued that the use of intra-operative navigation systems in implant dentistry allows the surgeon to transfer a detailed pre-surgical treatment plan to the patient precisely. The accuracy of the site preparation can be verified during surgery in real time17 and, if required, intra-operative modification to the plan can be made. In contrast to the ‘virtual reality’ concept, which was explained above for RP–CT technology, IGS/IGI introduced a new concept called ‘augmented reality’, ie the technique does not rely solely on artificially generated environments but expands into the real world with additional elements (information content).18 The basic principle for application of augmented reality and IGS/IGI in cranio-maxillofacial surgery is the visualization of 2D and 3D views of the surgical site superimposed on the real image of interest, which is called ‘overlay-graphics’.19
CT-based technologies available today have limitations and questions that require further investigation as to their effect on guided surgery outcomes. The resolution and accuracy of specific CBCT machines compared with the gold standard of medical-grade CT scanners has been questioned.20 NobelBiocare markets a calibration object that calibrates an individual CBCT/CT machine to an acrylic object of a known contour and density specifically for the NobelGuide protocol. Although theoretically the concept of a calibration object of this type makes sense, its efficacy is yet to be proved in the scientific literature. Materials used in rapid prototyping have inherent potential problems that can lead to light sensitivity and expansion and/or shrinkage. Leaving them exposed to light for extended periods of time, as well as sterilization in high-temperature autoclaves, distorts these materials. As a general rule, authors recommend a safety zone of 2 mm from vital structures. The literature also concludes that implant site preparation using surgical drill guides generates more heat than classic implant site preparation, regardless of the irrigation system used.21
More recently, a novel approach with a different concept was introduced. This new methodology, named the ‘tactile imaging and registration concept’,22 uses an electronically-driven intra-oral device to which tactile sensors containing needle arrays are affixed. Principally, the penetration of these needles to the soft tissue of the proposed jaw makes the analogue data which can then be retrieved and translated by a computer to a digital 3D image. This process is repeated several times until the surface-acquired data are completely recorded, transposed and matched (registered) to CT scan data. A specially designed machine is then required for making the required templates at the prescribed co-ordinates of optimal axis. Other systems, including magnetic tracking systems and a robot with a mechanical arm, have also been tried but not commercialized as yet.
Experience shows that, in most clinical situations, oral implants can be inserted without a guiding system for the transfer of pre-operative planning. The accuracy is apparently sufficient for a successful outcome.
However, when there is a relatively limited quantity of bone available, highly accurate implantation becomes mandatory and any malpositioning of the implant would result in perforation of the cortical bone, the mandibular nerve or the schneiderian membrane.
The template provides a link between diagnostic and surgical phases, as it contributes to cosmetically and functionally correct implant-supported prosthetic rehabilitation. However, template-based systems, where the planning is merely based on a wax-up, helps to optimize the positioning of the implants for restorative treatment only. Conversely, those template systems where the planning is based on volume image data as well can also help to take the hidden part of the anatomy into consideration. Nevertheless, they still lack the flexibility of image-guided implantology based on a navigation system.
