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.