4.2.2 Integrated Digital Workflow

New technologies combining CT/CBCT data (DICOM) with information on the soft tissues and crown morphology, obtained through digital high-resolution optical scanners, should be encouraged. The superimposition of the DICOM and optical surface scanning (STL) files requires the identification of at least three dental units or points in the jaw to allow for proper matching between the two data sets (Fig. 4.4 and 4.5). Mutual landmarks on both data files are needed, and this represents the main limitation to the use of the integrated digital workflow in the full edentulous patients.

Ritter et al. [21] assessed the accuracy of this newly developed digital workflow on 16 patients through 1792 measurements. All data pairs were matched successfully and mean deviations between CBCT and optical surface scanning data were between 0.03 (±0.33) and 0.14 (±0.18) mm. According to the results of this study, they concluded that registration of 3D surface data and CBCT data works reliably and is sufficiently accurate for implant planning. Therefore the digital integrated workflow using tooth-supported surgical templates, having a higher degree of accuracy when compared to the bone- and mucosa-supported surgical templates, may reduce the 3D discrepancy between the virtual plan and the real-life insertion of the implant [14, 22–24].

The recently introduced 3D software program (NobelClinician, Nobel Biocare, Kloten, Switzerland) automatically overlays the DICOM data from CT/CBCT of the patient with the STL data from the extra-oral (EOS) and/or intraoral (IOS) high-resolution optical surface scanning of the patient anatomy with or without tooth set-up, using a proprietary algorithm process (SmartFusionTM, Nobel Biocare). Therefore the patient’s dentition is scanned and integrated with the craniofacial (hard tissue) model to create a more accurate 3D model of the patient’s hard- and soft-tissue anatomy [25, 26].

Technically, the accuracy of this automatic matching workflow is 1 voxel size below (internal data, Nobel Biocare) manual segmentation workflow, based on pairing at least three points on the surface of the patient CT/CBCT anatomy with the equivalent ones of the patient anatomy achieved by the digital high-resolution optical scanning. An additional benefit to streamline the workflow comes from the use of an intraoral optical scanner to retrieve the surface scanning of the residual dental arch and soft-tissue architecture. The combination of intraoral scanners and cone-beam computerised tomography images, by mutual superimposing and use of planning software, provides an almost complete three-dimensional representation of soft and hard tissues.

The availability of tooth morphology through virtual digital libraries within the planning software will streamline the digital planning further by providing a virtual digital wax-up used to visualise the ideal prosthetic setup. The virtual diagnostic waxing can be performed on the stereolithography (STL) files of the patient digital models, assisting the digital planning of the implant surgery (Fig. 4.6). However it is still challenging to make a three-dimensional (3D) model of the patient face, with the dentition in the anatomically correct position in terms of function and aesthetics.

4.2.3 The Smiling Scan

In order to reduce the numbers of 3D data sets and simplify a more comprehensive facially driven digital treatment plan for cross-arch implant-supported restorations the authors started to investigate the possibility of obtaining the CBCT scan while the patient displays a broad smile on their face for the duration of the scan. The authors named this protocol the “smiling scan”. The smile is a facial expression initiated by flexing the muscles of both ends of the mouth (orbicularis oris) without vocalisation, thus displaying the front teeth. These instructions are given to the patient before taking the scan. All the patients were scanned with high-speed CBCT device (Scanora 3Dx, Soredex, Tuusula, Finland) with the following parameters: FOV (140 mm height, 100 mm width), high resolution (voxel sizes 0,25 mm), kV 90, mA 10, and scan time 18 s with an effective exposure time of 6 s. The patient’s head was properly secured to the CBCT scanner chair through the head-frame positioner. The chin rest and support were not used to avoid any restrictions to the muscle movements during the smile (Fig. 4.7). The smiling CBCT scan allows the clinician to import all the information related to the patient’s 3D facial anatomy using only one data set. The residual teeth are kept in contact during the smile in order to record the patient’s existing occlusion. This includes the relationships between the upper, mid, and lower thirds of the face, the lines of symmetry, the position and design of the lips during a broad smile, and the overall extra-oral and intraoral bone anatomy and the residual dentition. In cases of complete edentulism of one or both dental arches the patient is scanned with the prosthesis in the mouth. In this case the smiling scan will allow the clinician to visualise within the software the relationship between the lips, cheeks and current prostheses during the smile to properly evaluate the aesthetic zone. In case of patients with a terminal dentition (at least three residual teeth per arch to provide the proper number of landmarks for the digital superimposition of the DICOM and STL data sets) the smiling CBCT scan is performed without any removable prostheses in the mouth (Fig. 4.8, 4.9 and 4.10).

4.3 Surgical Workup

Once the planning is completed and approved by the clinician, the digital information is used to produce the surgical template with CAM rapid prototyping (milling or 3D printing) that will be tooth supported in the case of a dentate patient or mucosa supported in case of a fully edentulous patient. Based on three-dimensional imaging and a three-dimensional design, the computer-assisted surgical templates are commonly produced using photopolymerisation techniques after which the metal drill/guide sleeves are bonded by hand into the openings found in the surgical template.

The bone-supported surgical guides showed the highest inaccuracy [24], while a high level of accuracy could be achieved when reference mini-implants were used to support the radiographic template during the computer tomography scan and the final surgical guide, leading to the immediate loading of a prefabricated implant-supported prostheses. However positioning mini-implants prior to the main surgical procedures results too cumbersome, complicated and not easily applicable in daily practice. Tooth-supported guides showed significantly smaller deviations compared with mucosal- and bone-supported guides: 0.87 ± 0.40 mm (coronal deviation), 0.95 ± 0.60 mm (apical deviation) and 2.94° (angular deviation) [27]. However a wider variation of values was reported for sites with only a few teeth remaining. Larger deviation was also found for templates relying on unilateral anchorage (free-ending tooth-supported templates such as Kennedy Class I or II partially dentate patients) due to tilting and bending of the template [28]. The use of a rigid material for fabricating the surgical template or the relining of the templates in order to obtain sufficient stiffness to prevent such tilting should be advocated.

4.3.1 Type of Guided Surgery (Fully Guided Placement, Semi-guided/Pilot Drilling and Freehand Implant Placement)

The fully guided protocol permits the preparation of the implant site and placement of the implant through the surgical template (Fig. 4.11). A semi-guided protocol permits the use of the initial twist drill to start the osteotomy (pilot drilling); however the larger diameter final twist drills and the implant installation are performed with a freehand technique (Fig. 4.12 and 4.13). The semi-guided protocols can be very useful in cases where the implant site requires augmentation, in the fresh extraction sockets or in cases where the mesial-distal spacing is too narrow to accommodate the diameter of the drill guide sleeves (Figs. 4.14 and 4.15).

4.3.2 Minimally Invasive Surgical Technique: Flapless vs. with Flap

Surgeons, when operating using freehand technique, commonly elevate mucoperiosteal flaps to better visualise the recipient site. This may become unnecessary when fully guided implant placement is performed since the implant positions are predetermined and surgical guidance is provided intraoperatively by the surgical template. CAD-CAM methods may help clinicians to perform successful implant-based rehabilitation while avoiding elevation of large mucoperiosteal flaps or eliminating them altogether, resulting in less pain and discomfort for the patient [29–32]. Flapless or mini-flap surgical procedures preserve blood circulation in the soft tissues, enhancing the recovery of the soft-tissue architecture [33]. Flapless approach may lead to a more favourable bone resorption pattern [34] but also enhanced papilla regrowth and hence the aesthetic outcome of single implants [35].

Avoidance of flap elevation seems to benefit peri-implant mucosal outcome, particularly in terms of maximum preservation of peri-implant papillae and reduced mucosal recession [36]. Furthermore, a flapless approach avoids elevation of the mucoperiosteal flap and keeps the periosteum in contact with the bone and the supraperiosteal plexus intact, hence preserving the osteogenic potential and the blood supply to the underlying bone and/or implant. Flapless surgery in patients with newly grafted bone may also reduce the bone resorption associated with interruption of the periosteal blood supply [37]. Bone denudation causes increased bone loss [38]. Flapless computer-guided surgery may allow implant treatment in medically compromised patients who would be excluded due to the stress related to the length of the surgical intervention and the higher risk of intraoperative and postoperative complications [39]. Another benefit from having implants placed using a fully guided approach is the potential for loading the implants immediately [16].

During conventional freehand surgery the true topography of the underlying available bone cannot be observed without elevation of a flap. Clinical palpation alone is not advisable in complex cases because thick epithelium and thick mucosa may hide a narrow ridge, perforation of the cortical bone on the buccal or lingual side or improper implant location [40]. Flapless freehand surgery can only be recommended in select and appropriately planned situations by experienced surgeons and when there is adequate bone volume (Fig. 4.16). A computer-guided approach may overcome these drawbacks [24]. However the quality and quantity of the soft tissue at the implant recipient sites need to be addressed when choosing between flapped and flapless surgery. Soft-tissue punching and removal, generally associated with a flapless approach, may not be indicated in patients with a narrow zone of keratinised mucosa and limited soft-tissue volume or mesial-distal space. In some cases, flapless surgery using a soft-tissue punch may result in the removal of too much well-needed keratinised soft tissue around the implants. When the width of keratinised mucosa is limited, specific surgical soft-tissue management may be favoured. In such instances, surgical modifications, such as repositioning the area where soft-tissue punch is used, or limited flap technique may be favoured [41, 42].

4.3.3 Drilling Protocol

Stereolithographic surgical templates are fabricated with hollow metallic cylinders to guide implant placement in the virtually planned position. A surgical occlusion index fabricated in silicone (Exabite II NDS, GC Europe, Haasrode-Leuven, Belgium) enables the accurate seating of the surgical template during the intervention.

The precise fit of the surgical template has to be visually and manually checked before surgery. The surgical occlusal index is helpful for the treatment of both edentulous and dentate patients, particularly when the remaining teeth are not adequately positioned in the residual dental arch to assist in mechanically stabilising the surgical guide. Once the surgical template is positioned the template is stabilised with three to four pre-planned anchor pins for the fully edentulous and at least one anchor pin for the dentate arch.

Implant sites are to be prepared according to the manufacturer’s guidelines. However, in the presence of poor-quality bone, or fresh extraction sockets, alteration of the guidelines may be necessary. The implant sites may have to be underprepared without screw tapping, in order to achieve the required primary mechanical stability especially when planning for the immediate provisionalisation.

Immediate loading of the implants is recommended when the insertion torque is greater than 40 Ncm or an implant stability reads higher than 70 ISQ value. If the insertion torque is less than 35 Ncm or ISQ value is less than 55, the authors prefer to leave the implants submerged for 3 months. The implant insertion torque has to be measured with the manual torque wrench after the surgical template is removed to avoid inaccurate measurements due to possibility of binding of the implant carriers (implant mounts) as they are passed through the template.

The neck of the implants should be placed flush with the bone, with exception of immediate post-extraction implants which are placed approximately 1.5 mm below the most coronal bony peak or in situations where the prosthetic requirements demand for deeper placement. Whenever possible implants are engaged bicortically.

4.4 Prosthetic Protocol

4.4.1 Interim Prostheses

A metal-reinforced, screw-retained, acrylic resin interim restoration without any cantilever for immediate implant loading can be conventionally pre-fabricated in the laboratory, using the stereolithographic surgical templates to generate the preoperative master casts after attaching the implant replicas to the surgical template, or milled from poly-methyl methacrylate blank with a CAD-CAM digital integrated workup (NobelClinician Software, NobelDesign software, Nobel Biocare AG).

If titanium temporary abutments are used the pre-fabricated metal-reinforced, screw-retained acrylic resin interim prostheses are relined on non-engaging titanium temporary abutments using an auto-polymerising polyurethane resin (Voco, Cuxhaven, Germany). Immediately before provisional restoration delivery, the marginal precision, retention and stability are checked for accuracy. Occlusion is carefully evaluated by means of a 40 μm articulating paper (Bausch Articulating Paper, Köln, Germany), until light occlusal contacts, uniformly distributed on the entire prosthetic arch, are obtained. Lateral excursions should provide posterior disclusion with the absence of non-working-side interferences. Patients are usually recalled 3 days following delivery to evaluate occlusion and to address any concerns the patient may have. 7 days after implant placement the patient is again recalled to recheck occlusion, remove suture and educate the patient in oral hygiene measures (Fig. 4.17).

Four months after initial loading, implant stability can be checked by tightening the abutment screws with a 20 Ncm torque using a torque control device (Torq Control, Anthogyr, Sallanches, France) and definitive impressions can be taken at implant or abutment level, using an open, customised impression tray (Elite LC tray, Zhermack SpA, Badia Polesine, Rovigo, Italy) (Fig. 4.18).

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