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CBCT Images Processing Systems in modern implant-prosthetic therapy

Forna Norina

Digital techniques based on processing of CBCT images are a new trend in the modern implant-prosthetic therapy. These techniques are recommended for the analysis and planning of the surgical pro-implant and implant stage. Using these techniques, implantology specialists and oral surgeons can optimize their therapeutic decisions regarding the pro-implant and implant stages, with implementation of more safe and accurate execution of the surgical and implant procedures. Digital systems for the analysis of CBCT images (Planmeca Romexis 3D, CS9300) are used for evaluation of the quality and volume of implant sites as well as for measuring distances to anatomical elements that must be avoided during implant procedures (sinus cavity, mandibular canal). Expert applications also use CBCT images for the assessment of mucosal and bone support, planning of bone addition procedures and positioning of dental implants (Implant 3D, Universe; NobleGuide, Nobel Biocare; Digital Smile Design, DSD; SimPlant, Dental Materialize; Virtual Implant Placement, BioHorizons; ImplantMaster, iDent; Implant 3D, Media Lab; EasyGuide, Keystone Dental). 3D Navigation systems (Robodent; X-Guide) that use CBCT images has been introduced to assist the implantology specialists both in planning and execution of the implant surgical procedures. The benefits of these systems are as follows: real-time visualization of the depth and angle of the burr; fully automatic recording of the patient and of the procedures applied; one software for planning and navigation; extensive collection of generic implants and implants; measurement and analysis of bone density. Published studies demonstrate the increase in the long-term success rate in digital-assisted implant-prosthetic therapy and sustain the expansion of the digital applications in the contemporary implant-prosthetic therapy.

Radiography is considered the most frequent diagnostic tool in daily dental practice, with more than one quarter of all medical radiographs in Europe being made by dentists. Dental radiographs have been the predominant source of diagnostic information in the oral and maxillofacial complex. Yet, two-dimensional (2D) imaging techniques are unable to depict complicated three-dimensional (3D) anatomical structures and related pathologies.

There was a growing tendency in using 3D information as an aid for dentomaxillofacial diagnosis and treatment. These developments went hand in hand with the increasing use of 3D imaging applications for presurgical planning and transfer of oral implant treatment. While the required 3D acquisition for dental applications was initially realized by medical computed tomography (CT), dental CBCT rapidly took over. For the clinicians involved in implant rehabilitation, the power of a dental 3D dataset is not only situated in the diagnostic field, but also in the potential of gathering integrated patient information for presurgical and treatment applications related to oral implant placement. Nowadays, rapid advances of digital technology and computer-aided design/computer-aided manufacturing (CAD/CAM) systems are indeed creating challenging opportunities for diagnosis, surgical implant planning and delivery of implant-supported prostheses. While there is still a huge demand for maximised integration of 3D datasets acquired from various imaging sources, there is also a call for simplified solutions. Yet, when striving for optimized patient-specific implant rehabilitation, the ultimate goal remains to fully integrate the available 3D imaging data thus creating the virtual patient, aiding presurgical simulation and peroperative transfer to the surgical field with further prosthetic rehabilitation.

The overall advantage of using CBCT in implant dentistry is related to its ability to acquire detailed volumetric image data of the maxillofacial region for diagnostic and presurgical planning purposes. This includes hybrid or so-called multimodal systems for combined 2D (panoramic and/or cephalometric) and 3D (CBCT) imaging apart from less expensive and primary panoramic machines with a small detector size for scanning narrow field-of-views with a 3D button. CBCT machines are used for diagnostic indications, yet also for presurgical planning and transfer to implant surgery and rehabilitation. A direct consequence of this CBCT revolution and the exponential rise in equipment remains the creation of a clinically significant gap between the existing scientific literature and available hardware and software. Despite the dedicated properties of CBCT for dento maxillofacial examinations and its growing use over the last decade, more specifically in implant dentistry, there is an enormous variation in radiation doses and image quality and attributed to machine- and protocol-dependent variables.

Image quality performance of CBCT devices may vary widely, similar to, but not only related to exposure protocols and radiation dose ranges. CBCT images are usually considered offering a high spatial resolution with voxel sizes of reconstructed CBCT datasets ranging between 0.08 and 0.4 mm. Small voxel sizes could be diagnostically useful for cases in which small structures such as root canals and periodontal tissues need to be depicted. Variation is also observed when it comes to segmentation accuracy. The latter is a crucial factor when going for an integrated virtual planning including jaw bone models, fabrication of radiographic and surgical guides as well as further prosthetic models. Another shortcoming of CBCT is the lack of diagnostically distinct soft tissue contrast, narrowing down the diagnostic potential and hampering applications for soft tissue integration in the presurgical planning. Furthermore, Hounsfield units do not apply to CBCT images, yielding it impossible to compare grey values among or within patients over time. This lack of standardized grey value distribution is complicating the use of CBCT for clinical bone density assessment and follow-up of bone density changes. Hounsfield units (HU) have been designed for medical CT, but do not apply for CBCT. Compared to HU units for medical CT, the reliability of CBCT-based jaw bone density assessment has been found unreliable over time and with significant variations influenced by CBCT devices, imaging parameters and positioning. This lack of HU standardization is a major problem for most CBCT devices. Hitherto, one may question the relevance of this problem when it comes to actual implant dentistry, considering that nowadays a healthy vascularized bone may be more beneficial for implant placement than a sclerotic dense and poorly vascularized bone. What one might thus need instead is a structural bone analysis, like that available in dedicated μCT software. Such structural analysis has already been validated to be used for CBCT imaging, and thus might even have clinical potential for presurgical assessment of bone quality.