dental implants

Intraoral scanning accuracy can be improved with these techniques: l Keep the teeth dry, especially the occlusal surfaces. Scanners have difficulty differentiating between teeth and saliva. l Use a proper fulcrum or manufacturer-specified support. l Visually focus on the computer monitor and not the intraoral scanner itself. l Capture a few millimeters of soft tissue past gingival margins of the teeth, especially in the edentulous space where the implant will go. l Capture the interproximal tooth surfaces of the teeth adjacent to the edentulous space. This requires tipping the scanner head mesially or distally to capture tooth structure cervical to the height of contour. l More scanning does not mean more accuracy. The data should be captured completely, but efficiently. If certain areas need to be rescanned multiple times to get the data completely, there is likely an error or discrepancy somewhere in the scan, and it is best to start over.

Digital bite registrations can be improved with these techniques: l Make sure the patient is biting properly in their maximum intercuspal position (MIP) without moving or quivering. l Ensure the teeth are quite dry.

Merging the CBCT and Intraoral Scan Data

Once the CBCT DICOM and intraoral scan STL files have been created, they can be imported into the implant planning software. Many different implant planning software packages are available. Common examples are Planmeca Romexis, 3D Diagnostics 3DDX, 3Shape Implant Studio, Dentsply Sirona Simplant and BlueSkyPlan by Blue Sky Bio. Implant planning software have similar capabilities; the choice depends on the subscription model and the ease with which the software integrates into the existing hardware of a particular office. The images in this section are from BlueSkyPlan by Blue Sky Bio.

The first step is to align the DICOM data containing bone and tooth surfaces with the STL data containing tooth and soft tissue surfaces. Some software merge the two datasets automatically. Additional manual refinement can be done by shifting the model in any of three axes to better align with the CBCT image. Aligning CBCT and tooth surface data can result in higher accuracy of implant placement. 18 If the software does not align automatically, you can manually do so by merging with points (Fig. 2). In this method, the software user must select a series of corresponding points on the model and the CBCT image, such as grooves or cusp tips, which are easily identifiable in both data sets. The software will then align the two datasets together based on the points selected.

Regardless of the alignment method, the accuracy of the alignment needs to be verified. If the models are well aligned, tooth surfaces from both the model and the CBCT should be intimately adapted on individual CBCT slices (Figs. 3).

Once the two data sets are merged, a digital wax-up of the tooth for the implant sites can be created. This is done by inserting a tooth shape from the software and adjusting its size and position along the mesiodistal, buccolingual and apicocoronal axes (Fig. 4). The tooth should be positioned and sized exactly as the final restoration will be because the implant will be planned according to this digital wax-up. In most programs, the digital wax-up can be locked in so that it is not inadvertently altered later in the implant planning process.

Digital Implant Planning

The implant surgeon should review the entire volume of the CBCT to get acquainted with patient-specific local anatomy and look for bone abnormalities and/or presence of any other pathology. If implants in the mandible are placed, the inferior alveolar nerve should be marked in the volume by identifying its course distal from the mental foramen.

Likewise, in the maxilla, the position of unusually prominent neurovascular bundles in the bone should be marked (Fig. 5).

The next step is to determine the appropriately sized implant from the collection of implants in the chosen implant system. The available bone for implant placement provides the possible size for an implant at a given site and often dictates which implant size can be placed. To measure available bone accurately, the sectional views must be lined up so that the buccolingual and mesiodistal sections are perpendicular to t

Dental Implants

the bone surfaces and the view is centered on each implant site (Fig. 6).

Each implant must also meet the following requirements: l At least 2.65 mm superior to the inferior alveolar nerve 19 and 5 mm mesial to the mental foramen. 20 l At least 1.0 mm (platform-switched implants) to 1.5 mm (nonplatform switched implants) from the implant platform to adjacent teeth21 and at least 3.0 mm between adjacent implant platforms.22 l A 2 mm thick shield of facial bone to the facial implant surface is advised.23

Most implants with completely rough surfaces need to be placed with the implant platform flush or slightly apical to the crestal bone. Placing implants with machined collars apical to the crestal bone may result in bone loss. These requirements typically provide maximum implant dimensions for the available bone. However, available implant dimensions and restorative design considerations including desired emergence profile and support may dictate different dimensions. For example, even if a posterior maxilla implant site features a 12 mm-wide ridge allowing placement of an 8 mm-wide implant, the largest implant size available from a given manufacturer may only be 6 mm in diameter.24 Likewise, if available bone is less than required for restorative needs, site development procedures such as ridge augmentation need to be done.25 Short implants (less than 10 mm length) may help overcome limitations in available bone height, although concerns have been raised about possible mechanical disadvantages from a poor crown implant ratio. Yet, crown implant ratio does not seem to be associated with enhanced peri-implant bone loss and may not affect implant survival.26,27 Small diameter implants (less than 3.5 mm) may have similar bone loss and survival rates than standard diameter implants28 when placed in atrophic ridges, but may have higher complication rates and potential for fracture if placed in posterior areas.29,30

Once the appropriately sized implant has been selected, it typically needs to be placed at the center of the restoration for posterior teeth and canines and palatal to the restoration center for incisors. The goal for incisors is to place the implant so that the facial platform edge is just lingual to the planned incisal edge to allow for a screw-retained restoration and allow for easier development of the facial emergence profile. The overall goal for implant placement is to achieve an implant axis perpendicular to the occlusal table of the restoration to minimize off-axis loading and avoid prosthetic complications31 (Fig. 7).

It is possible that the position of the virtual implant dictated by the restoration results in facial perforation of the cortex at the implant apex. In this case, the choice is either to accept a more difficult restoration by adjusting the implant position or to address the perforation with grafting during implant placement. The decision depends on which method can be more predictably achieved for a given case.

If the appropriate virtual implant length results in perforation of the sinus, appropriate sinus augmentation procedures should be planned along with implant placement. If the existing bone width is not sufficient for implant placement, ridge augmentation or alternatives to implant therapy should be considered. When working as a team, the implant surgeon and restorative dentist must agree on the desired implant position.

A benefit of digital implant planning is the ability to try out different implant sizes and positions and quickly see the outcome in terms of screw-hole position and relationship to the restoration (Figs. 8).

Guided Surgery Kits

Guided implant placement requires the use of specialized guided surgery kits. The design of the kits varies by manufacturer, but in general the kits can have either guided drills (Fig. 9) or conventional drills (Fig. 10) with a series of adapter keys.

With the latter, the keys engage the guide tube, and each key has a hole in the center, of varying diameter, which allows the conventional drill to pass through. Using the key during the osteotomy can be challenging because it needs to be held in place inside the guide tube. The advantage of these systems is that the cost to transition to guided surgery is lower as they make use of the conventional drill kit.

With guided drill kits, every drill has a guiding surface built into it. These kits are much simpler to use than keys because the drill goes directly into the guide. These kits often come with guided implant carriers that for allow fully guided placement.

Dental Implants

The disadvantage of these systems is the higher cost.

Designing the Guide

Once the implant position is finalized, the next step is to design the surgical guide. Each implant manufacturer’s guided implant surgical kit is different and has its own parameters to create the guide tube (Figs. 11), which directs the guiding portion of the drill. The following are the parameters needed: l Diameter: This is the inner diameter of the guide tube. It should be as narrow as possible to still allow the guiding portion of the drill to pass through yet reduce lateral movements of the drills. 32,33 Some systems have a separate drill for each step with a single diameter guiding portion that does not change. Other systems use a key system where the drill diameters are different, so a key is needed as an adapter between the drill and the guide. This controls the implant placement in the buccolingual and mesiodistal axes. l Offset: This determines the position of the top of the guide tube. It is the vertical distance between the stopper on the drill and the platform of the implant. This controls the implant placement in the apicocoronal axis. l Height: This determines the position of the bottom of the guide tube. It is the height of the guide tube from the top to the tissue level. This should be as tall as possible to minimize lateral movements of the drills.34

Once this information is programmed into the software, the extensions of the guide need to be marked. In dentate areas, it is best to go just past the height of contour to allow retention of the guide during implant placement (Figs. 11). In edentulous areas where implants will be placed, it is best to go 3 mm to 4 mm past the neighboring gingival zenith to help retract the elevated tissue during implant placement using a flap approach.

Adding windows at selected cusps and incisal edges is helpful, as it allows verification of complete guide seating during implant placement (Fig. 12).

Once satisfied with the guide, the guide can be exported as an STL file (Fig. 12) to be fabricated in a 3D printing machine. A benefit of the BlueSkyPlan software is that digital implant planning is free and a cost is only incurred once the STL file is exported.

Surgical Guide Fabrication

The STL file from the planning process can be used to print a guide in office with a 3D printer capable of printing surgical guides with resins approved by the FDA for intraoral use or to send to a dental laboratory for guide fabrication. Some implant systems require a metal sleeve to be inserted into the guide tube after fabrication, while others are sleeveless (Fig. 13).

3D Printing

3D printers work through a process known as additive manufacturing: 3D models are cut into many digital layers and then built up in the printer layer by layer. The quality of the object improves with thinner layers. This is akin to CT scanning software that creates a 3D image of part of the body by combining slices together. While many 3D printers exist, the most common printers in dentistry utilize resin as their material. Two of the common methods for 3D resin printing are: l Material extrusion (FDM): This method involves extruding a plastic filament material through a heated nozzle. The printer extrudes the material back and forth along a predetermined path to create a 3D object, 35 similar to dot-matrix printers for paper (Fig. 14). This method has lower dimensional accuracy, as the thinnest layer possible is 0.5 mm. FDM printing is not recommended for dental purposes due to insufficient level of detail and the lack of FDA-approved materials. l Vat polymerization (SLA, DLP): This method involves using a light to cure individual layers from a vat of resin (Fig. 15). This method has much higher dimensional t