Integration of 3D anatomical data obtained by CT imaging and 3D optical scanning for computer aided implant surgery
- Gianni Frisardi†1, 2Email author,
- Giacomo Chessa†2,
- Sandro Barone†3,
- Alessandro Paoli†3,
- Armando Razionale†3 and
- Flavio Frisardi†1
© Frisardi et al; licensee BioMed Central Ltd. 2011
Received: 12 September 2010
Accepted: 21 February 2011
Published: 21 February 2011
A precise placement of dental implants is a crucial step to optimize both prosthetic aspects and functional constraints. In this context, the use of virtual guiding systems has been recognized as a fundamental tool to control the ideal implant position. In particular, complex periodontal surgeries can be performed using preoperative planning based on CT data. The critical point of the procedure relies on the lack of accuracy in transferring CT planning information to surgical field through custom-made stereo-lithographic surgical guides.
In this work, a novel methodology is proposed for monitoring loss of accuracy in transferring CT dental information into periodontal surgical field. The methodology is based on integrating 3D data of anatomical (impression and cast) and preoperative (radiographic template) models, obtained by both CT and optical scanning processes.
A clinical case, relative to a fully edentulous jaw patient, has been used as test case to assess the accuracy of the various steps concurring in manufacturing surgical guides. In particular, a surgical guide has been designed to place implants in the bone structure of the patient. The analysis of the results has allowed the clinician to monitor all the errors, which have been occurring step by step manufacturing the physical templates.
The use of an optical scanner, which has a higher resolution and accuracy than CT scanning, has demonstrated to be a valid support to control the precision of the various physical models adopted and to point out possible error sources. A case study regarding a fully edentulous patient has confirmed the feasibility of the proposed methodology.
Over the last few years, dental prostheses supported by osseointegrated implants have progressively replaced the use of removable dentures in the treatment of edentulous patients. The restoration of missing teeth must provide a patient with aesthetical, biomechanical and functional requirements of natural dentition, particularly concerning chewing functions. When conventional implantation techniques are used, the clinical outcome is often unpredictable, since it greatly relies on skills and experience of dental surgeons.
The placement of endosseous implants is based on invasive procedures which require a long time to be completed. Recently, many different implant planning procedures have been developed to support oral implant positioning. Number, size, position of implants must be related to bone morphology, as well as to the accompanying vital structures (e.g. neurovascular bundles). Complex surgical interventions can be performed using preoperative planning based on 3D imaging. The developments in computer-assisted surgery have brought to the definition of effective operating procedures in dental implantology. Several systems have been designed to guide treatment-planning processes: from simulation environments to surgical fields . The guided approaches are generally based on three-dimensional reconstructions of patient anatomies processing data obtained by either Computed Tomography (CT) or Cone-Beam Computed Tomography (CBCT) . These methodologies allow more accurate assessments of surgical difficulties through less invasive procedures and operating time reductions. In particular, radiographic data (depth and proximity to anatomical landmarks) and restorative requirements are crucial for a complete transfer of implant planning (positioning, trajectory and distribution) to surgical field . Virtual planning processes provide digital models of drill guides, which are typically manufactured by stereo-lithography and used as surgical guidance in the preparation of implant receptor sites.
In the past decade, a methodology based on the use of two different guides and a double CT scan procedure, has been introduced  and later commercialized as NobelGuide® by NobelBiocare (Zurich, Switzerland). This procedure involves an intermediate template (radiographic template) that is used to refer the soft tissues with respect to the bone structure derived from patient CT scan data. The guide is manufactured on the basis of diagnostic wax-up reproducing the desired prosthetic end result. The diagnostic wax-up is obtained starting from the dental cast, produced from the impression of the patient's mouth, and helps in the definition of a proper dental prosthesis design. Moreover, the radiographic template is made of a non radio-opaque material, usually acrylic resin, to avoid image disturbs when CT scans of patients are carried. Then, the template is separately scanned changing radiological parameters in order to visualize the acrylic resin. The computer-based alignment of the prosthetic model with respect to the maxillofacial structure is obtained by small radio-opaque gutta-percha spheres inserted within the radiographic template. These gutta-percha markers are visible in both the different CT scans and can be used as references to register the two data sets through point-based rigid registration techniques .
Specific 3D image-based software programs for implant surgery planning, based on CT scan data, have been recently developed and clinically approved by many manufacturers. These software applications allow surgeons to locate implant receptor sites and simulate implant placement . The planned implant positions are then transferred to the surgical field by means of a surgical guide made by stereo-lithographic techniques. Surgical guides can be bone-supported, tooth-supported or mucosa-supported depending on the specific patient's conditions. Bone-supported guides are designed to fit on the jawbone and can be used for partially or fully edentulous cases, while tooth-supported guides are tailored to fit directly on the teeth. The latters are mostly effective for single tooth and partially edentulous cases. Mucosa-supported surgical guides are rather designed for placement on soft tissues and are recommended for fully edentulous patients when minimally invasive surgery is required.
The surgical guide is then placed within the patient's mouth and can be anchored, especially when mucosa-supported guides are used, to the jawbone by stabilizing pins (Anchor Pins).
The weak point of the whole procedure relies on the accuracy in transferring information deriving from CT data into surgical planning. Geometrical deviations of implant positions between planning and intervention stages could cause irreversible damages of anatomical structure, such as sensory nerves. The surgical guide should closely fit with the hard and/or soft tissue surface in a unique and stable position in order to accurately transfer the pre-operative treatment plan. If the surgical template is not accurate, the fit will be improper, compromising the implant placement. Even small angular errors in the placement of perforation guides can, indeed, propagate in considerable horizontal deviations due to the depth of the implant.
A previous in ex vivo study to assess the accuracy of 10-15 mm-long implant positioning using CBCT, revealed a mean angular deviation of 2° (SD ± 0.8, range 0.7° ÷ 4°) and a mean linear deviation of 1.1 mm (SD ± 0.7 mm, range 0.3 ÷ 2.3 mm) at the hexagon and 2 mm (SD ± 0.7 mm, range 0.7 ÷ 2.4 mm) at the tip .
Sarment et al.  compared the accuracy of a stereo-lithographic surgical template to conventional surgical template in vitro. An average linear deviation of 1.5 mm at the entrance, and 2.1 mm at the apex for the conventional template, as compared with 0.9 and 1.0 mm for the stereo-lithographic surgical template was reported.
Di Giacomo et al.  published a preliminary study involving the placement of 21 implants using a stereo-lithographic surgical template, showing an angular deviation of 7.25° between planned and actual implant axes, whereas the linear deviation was 1.45 mm.
In a recent study , the accuracy of a surgical template in transferring planned implant position to the real patient surgery has been assessed. The mean mesio-distal angular deviation of the planned to the actual was 0.17° (SD ± 5.02°) ranging from 0.262° to 12.2°, though, the mean bucco-lingual angular deviation was 0.46° (SD ± 4.48°) ranging from 0.085° to 7.67°.
These studies confirm that the error could be high, especially in neurovascular anatomical districts, such as the mandibular nerve. In this anatomical area, a moderate damage may also result in severe symptoms. For example, the lesion of the mandibular nerve is of the Wallerian degenerative type , which is a slow degenerative process and the diagnosis by laser-evoked potentials and trigeminal reflexes would allow early decompression .
Deviations between planning and postoperative outcome may reflect the sum of many error sources. For instance, CT scan quality and processing of DICOM (Digital Imaging and Communication in Medicine) images affect the creation of the corresponding 3D digital models. Misalignment errors can also be introduced during the arrangement of the radiographic template within the maxillofacial structures by the gutta-percha markers. Moreover, further inaccuracies can be introduced in manufacturing physical models by stereo-lithographic techniques.
This paper concerns the development of an innovative methodology to evaluate the accuracy in transferring CT based implant planning into surgical fields for oral rehabilitation.
The proposed methodology is based on the combined use of CT scan data and a structured light vision system. In particular, the data acquisition phase regards two different scanning technologies: radiological scanning and optical scanning.
A clinical case, relative to a fully edentulous patient, has been used as test case to assess the feasibility of the proposed methodology. The ethics approval was obtained by Human Research Ethics Committee at the Sassari Hospital (n° 971) and written form approval was obtained by the patient.
CT scan data
CT scanning of maxillofacial region is based on the acquisition of several slices of the jaw bone at each turn of a helical movement of an x-ray source and a reciprocating area detector. The acquired data can be stored in DICOM format.
Statistical data relative to different DICOM reconstructions
Statistical data relative to discrepancies between cast and dental models
The analysis of the results allows the detection of possible errors occurred in manufacturing surgical guides. Low discrepancy values between the impression and cast models prove the correctness in the manufacturing process of the gypsum cast. The almost perfect superimposition between the radiological template and the study cast should have been expected since the radiological template is customized by manually fitting it on the cast. The transfer from the radiological to the surgical guides involves two distinct processes: the reconstruction of the radiological guide model by CT scanning and the manufacturing of the surgical guide starting from this digital model. The accuracy of the first step has been verified aligning the model obtained by processing the DICOM images with the gypsum cast. The fine adjustment of the threshold value in the segmentation process, using the model obtained by optical scanning as the anatomical truth, has allowed the minimization of the deviations with respect to the cast. For this reason, the high misalignment errors regarding the surgical template can be attributed to the stereo-lithographic process, which has been used to manufacture the surgical guide. The geometrical differences of the surfaces mating with the gypsum cast, certainly affect the overall accuracy in the implant placement positions. As a further proof, the surgical guide has demonstrated to improperly fit the physical model of the dental gypsum cast. This could lead the surgeon to anchor the template in the wrong way, compromising the desired implant placement.
A thorough study of the effect of these discrepancies on the maximum deviations obtained between the planned positions of the implants and the postoperative result should be done.
In this paper, a methodology to evaluate the transfer accuracy of CT dental information into periodontal surgical field has been proposed. The procedure is based on the integration of a structured light vision system within the CT scan based preoperative planning process. The use of the optical scanner, having a higher resolution and accuracy than CT scanning, has demonstrated to be a valid support to evaluate the precision of the various physical models adopted and to point out possible error sources. Optical scanning of the radiological guide, mounted on the gypsum cast, could be furthermore helpful for the integration of the prosthetic data within the bone structure. In case of not fully edentulous patients, the acquisition of teeth's shape could be used, in addition to gutta-percha markers, to optimize or verify the positioning of the radiological guide with respect to the maxillofacial structure. Moreover, the accurate digital model of the mouth impression could be the base for the direct design of the radiological guide using CAD/CAM technologies, without passing through manufacturing the gypsum cast, drastically reducing errors and planning time.
Written consent was obtained from the patient for publication of present study.
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