Implant Surgical Guides: State of the Art

Abstract

Positioning dental implant is vital in both prosthetic and aesthetic perspectives. The optimal three-dimensional placement not only reduces biomechanical complications but also imprecates the odds of implant failure. Advancements in the field of implantology such as three-dimensional imaging, implant-planning software, computer-aided-design/computer-aided-manufacturing (CAD/CAM) technology, computer-guided, and navigated implant surgery have led to the computerization of implant-dentistry. This three-dimensional computer-generated implant-planning and surgery has not only enabled accurate preoperative evaluation of the anatomic limitations but has also facilitated preoperative planning of implant positions along with virtual implant placement and subsequently transferring the virtual treatment plans onto the surgical phase via static (guided) or dynamic (navigated) systems aided by CAD/CAM technology.

Keywords: Dental implants, surgical guide, CBCT, software, computer-aided-design/computer-aided-manufacturing (CAD/CAM), Computer guided implant surgery (CGIS), Computer navigated implant surgery (CNIS), Robotic-implant-dentistry

Introduction

Dental implants have been used for the replacement of missing teeth for many years. Since the mid-20^th century, there has been an increase in interest in the implant process. The anatomic limitation and restorative demands challenge the surgeon to gain precision in planning and surgical positioning of dental implants. Ideal placement of the implant facilitates the establishment of favorable forces on the implants and the prosthetic component as well as ensures an aesthetic outcome. High accuracy in planning and execution of surgical procedures is important in securing a high-success rate without causing iatrogenic damage.[1] This can be achieved by computed tomography (CT), three-dimensional implant planning software, image-guided template production techniques, and computer-aided surgery.

Misaligned placement of the implant is a very common dilemma that frequently complicates the clinical laboratory procedures intended for the fabrication of superstructures.

Therefore, it is important to establish a logical continuity between the planned restoration and the surgical phases. This can be achieved by means of a surgical guide template that fits on to the existing dentition or on to the edentulous span at the time of surgery, providing adequate information regarding implant placement [2-5]

In this his paper, we provide a general summary of the state of the art in the dental implant guides and explore possible advantages and limitations for further improvements.

Surgical Template

By definition, surgical template is a guide used to assist in proper surgical placement and angulation of dental implants.[6]

The desired configuration of the surgical template is determined by the prescribed prosthesis. The surgical template dictates to the surgeon the implant body placement that offers the best combination of (1) support for the repetitive forces of occlusion, (2) esthetics and (3) hygiene requirements[7,8]

Several types of surgical guides have been reported in the literature. Some are designed for placement of a single implant, while other reports present designs for implant fixed partial dentures, multiple single implants, and implant-retained overdentures.

The template requirements are more significant than the options of fabrication. The template essential requirements are:

• The template should be stable and rigid when in the correct position.
• The template should fit over and/or around the available teeth on the treated arch, to be stable in position.
• The template should extend onto un-reflected soft tissue regions, if the treated arch has no remaining teeth, this way the template may be used after the soft tissues have been reflected from the implant site.[9,10]
• The template should not be bulky and difficult to insert or obscure surrounding surgical landmarks. [11]
• The surgical template must not contaminate a surgical field during bone grafts or implant placement.
• It should be transparent, hence the bony ridge and drills can be observed more easily when the template is in place.
• The surgical template should be related to the ideal facial contour.

i.e; many edentulous ridges have lost facial bone, and the template can determine the amount of augmentation required for implant placement or support of the lips and face.)

• The template should permit re-sterilization and use for several procedures.[12-14] i.e.; may be used in conjunction with a bone graft, insertion of implants and again for implant uncovery.

Surgical Template Fabrication

There are various methods of fabrication for the surgical template.

The Customized Conventional Radiographic Surgical Template

Diagnostic casts of the dental arches are made from irreversible hydrocolloid impressions. A diagnostic wax up of the proposed in case of an implant supported FPD is done. A silicone impression of the cast with the waxed FPD is made as a mold. A clear, chemically activated acrylic resin is poured into the mold space and cured. Access holes are made according to information obtained from the cast model for initial surgical drill. Stainless steel guide sleeves of uniform length is cut and placed in access holes and cured. [2,16,17]

Another method to prepare a radiographic guide is from vacuum formed templates. After the diagnostic wax up of the final restoration is completed, duplication is made and a cast is poured. The vacuum formed template fabricated is placed over the cast and the edentulous space is filled with radio opaque material (Barium sulphate, lead strip, gutta percha). [18-19]

In another method, it makes use of two vacuum formed templates, one over the blocked out diagnostic cast and other over the duplicate cast of the diagnostic wax up with a clear plastic sheet is made.[3] Both the templates are returned to the unaltered diagnostic cast. The edges of the two templates are trimmed to make them coincident. The diagnostic wax template is removed and filled with clear orthodontic resin or radio opaque material. The filled template is placed over the template of the unaltered diagnostic cast.[3] Holes are made according to information obtained from the radiograph for placement of implants, followed by placement of drill guides.

Limitations of the Conventional Radiographic Surgical Template :

1. i) Diagnostic limitations, such as expansion and distortion, setting error, positional artifacts and lack of information regarding the dimension of bone in bucco-lingual direction. [20] ii) Templates are fabricated on dental casts, which is a rigid, nonfunctional surface without the knowledge of underlying soft tissue resiliency and bone topography. [18, 21] iii) Anatomical landmarks are not precisely located, it does not show the lingual blood vessels, and approach is always two dimensional.[21,22] iv) More chances of malpositioning the implants during placement. v) There is less stability during surgery. vi) The success of the final outcome always depends on clinician skill and alertness.[4] vii) Requires more chair time, leads to stress on the dentist and patient. viii) They do not provide exact 3D guidance.[23]

To overcome these limitations, many advancements have taken place, which have computerized the implant-dentistry. These include:

• Cone-beam computed tomography (CBCT) imaging
• CBCT-based implant-planning software
• Computer-aided-design/computer- aided-manufacturing (CAD/CAM) technology
• Computer guided implant surgery (CGIS)
• Computer navigated implant surgery (CNIS)
• Robotic-implant-dentistry.

Computer-Aided Surgical Template

To overcome the limitations associated with conventional radiographic surgical template, computer-aided surgical template have been evolved. Computer-aided design/computer-assisted manufacturing (CAD/CAM) technology uses data from cone-beam computed tomography (CBCT), digital scans of current oral tissues, and a template of the definitive prosthetics to produce a computer-aided design and computer-aided manufacturing (CAD-CAM) guide. [24-34] This guide controls the drill angulation, depth, and location of the implant. [24-34] The CAD-CAM guide is usually milled or additively manufactured (3D-printed). [35]

Advantages of (CAD/CAM):

1. Precise placement of implants
2. Conservation of anatomic structures
3. Three-dimensional technology allows precise evaluation of anatomic points such as the size of the maxillary sinus in the upper jaw and location of the alveolar nerve in the lower jaw
4. Precise analysis of osseous topography
5. Provides information about size, direction, and bone location for accurate positioning of implants
6. High observed accuracy of 0.1 mm
7. Reduced surgical exposure time
8. Less invasive, flapless surgery, and, therefore, less chance of swelling
9. Less postoperative strain on dentist and patient
10. Transparency of material which allows seeing through the model
11. To summarize stereolithography fabrication process
12. A CT scan procedure is performed with a radiographic template fabricated using radio-opaque marker in place.

Limitations of (CAD/CAM):

1. Lack of visibility and tactile control during the surgical procedure
2. Insufficient mouth opening jeopardizes surgical procedure
3. A risk of damage to vital anatomical structures.

A milled guide is dimensionally stable and usually less brittle.[36]

However, the cost of the material and milling machine plus the material waste that results from the milling process are drawbacks.

Dentsply Sirona Azento^®

Azento^® is a convenient start-to-finish implant solution for single tooth replacements streamlining planning, purchasing, and delivery and enabling dentists to provide consistently excellent implant treatments. It can use digital images and data from CEREC systems and most other digital dentistry platforms to deliver a custom patient solution. The dental specialist easily Scans the patient and upload the order in the Azento^® case management portal. Within one business day a precise, customized digital treatment plan based on the patient’s digital scans will be received. After approving the Azento^® treatment plan, all components and instruments necessary to complete the implant treatment are delivered within five business days.

The Azento^® planning service will:

• Merge the CBCT and intraoral scan data.
• Create a surgical and restorative plan.
• Return the plan to the dentist within 24 hours for review and approval.

Each patient receives a precise, custom treatment plan recommendation based on their CBCT and intraoral scanning images. The planning service contains both a surgical and restorative plan.

• Surgical guide
• All necessary drills
• Case-specific implant
• A custom Atlantis Healing Abutment
• Atlantis temporary restoration including an abutment, dental crown and abutment core file (optional)

Azento^® shortens the treatment process to only 3 major steps – scan, approve, treat.

The high cost of the service, materials and milling machine is considered as a limitation.

Recently developed, affordable, high-quality 3D printers offer an alternative that can produce a guide with limited material waste and minimal polymerization shrinkage.[37]

Despite being the most accurate, CAD-CAM guides are the least used, in part because of the cost of the systems. [38-41] The time involved with planning, the turnaround from the laboratory, and the learning curve for the technique can also be limiting factors. [29,37]

Affordable desktop 3D printers with high precision can allow a dental office to produce anatomic casts and implant drilling guides.

Some of the software programs commercially available are: [8]

• Procera-Software^® (Nobel Biocare, Göteborg, Sweden)
• coDiagnostiX^® (IVS Solutions AG, Chemnitz, Germany)
• Easy Guide (Keystone-Dental, Burlington, MA, USA)
• SICAT (SICAT GmbH and Co. KG, Brunnenallee, Bonn, Germany)
• Virtual Implant Planning (BioHorizons, Birmingham, USA)
• ImplantMaster TM (I-Dent Imaging Ltd., Hod Hasharon, Israel)
• Simplant^®, SurgiCase^® (Materialize Inc., Leuven, Belgium)
• Implant3D Media Lab Software (Media Lab Srl, Follo (SP), Italy)
• DentalSlice (Bioparts, Brazil)
• Scan2Guide or S2G (iDent, Ft. Lauderdale, Florida)
• Tx Studio software (i-CAT, Imaging Sciences International, Hatfield, PA) etc.

The implant-planning software has the following advantages:

• Digital planning and fabrication of a virtual wax-up, implant position, abutment design, surgical guide, provisional restoration, and as well as final restoration
• Allows predetermination of the size of the implant, the abutment and the provisional restoration
• Averts any possible complications by highlighting the inaccuracies in the selection of implant size or position, during virtual planning, which can then be easily rectified using the softwareAssists in anticipation, guiding and planning of procedures like alveolectomy, alveoplasty, implant positioning in situations with anatomical limitations, visualization of the amount of available bone in each area and aids in selecting the ideal donor site for osseous grafts, graft location, shape and volume of graft, sinus lift technique and placement of implants in one step surgery, treatment of atrophic maxillae as well as placement of transzygomatic implants etc.
• Allows for the storage of the treatment plan and all other data of the patients on the computer
• Demonstration of the virtual treatment plan to the patient is possible.

Though these software programs have facilitated accurate implant placement, yet, there are certain limitations associated with them:

• Requires time to understand how the software functions
• High investment cost
• Requires an exact localization of natural or fiducial markers in image data and reality for an accurate registration of the patient.[41]

Computer Guided Implant Surgery (CGIS) (Static System)

The static system, which employs the use of a static surgical template/guide to reproduce virtual implant position in the surgical field, can be categorized into two types based on the CAD/CAM technology used for the surgical guide production.

The computer guided implant surgery employing surgical template has the following advantages: [41-43]

• It precisely guide the osteotomy drills
• Directs the surgeon in the exact location and angulation to place the implant based on virtual treatment plan
• It allows flapless surgery, which entails less bleeding, less swelling, decreased healing time and postoperative pain
• Aids in the preservation of hard and soft tissue and maintains blood circulation to the surgical site
• Considerably increased accuracy of implant placement
• Avoidance of vital structures
• Shorter period required for surgery.

Though CGIS technique has been proven to be an accurate and viable technique, it also has certain drawbacks and limitations. The most common drawbacks and limitations associated with CGIS include:[11,44-46]

• Error in data acquisition or incorrect processing of the image
• Deviations from planned implant positions especially in the coronal and apical portions of the implants as well as with implant angulation
• Inaccurate fixation of the guide resulting in displacement during perforation
• Mechanical errors caused by angulation of the drills during perforation
• Changed positioning of surgical instruments due to reduced mouth opening
• Fracture of the surgical guide
• Complexity of the whole system
• The total cost of tools needed including the software program and surgical templates
• The potential for thermal injury secondary to reduced access for external irrigation during osteotomy preparation during flapless implant placement with surgical guides
• Does not allow intraoperative modification of implant position.

Computer Navigated Implant Surgery (CNIS) (Dynamic System)

Computer navigated implant surgery involves the use of a surgical navigation system that reproduces virtual implant position directly from CT data with the optical bur tracking system without the requirement of an intraoral surgical guide. There are several navigation or positional tracking systems available in implant-dentistry,[44] but few meet the computer-aided-surgery requirements in terms of accuracy (about 1 mm in 1 m³), reliability, and clinical usability.[9] Sensors attached to both the patient and the surgical hand-piece transmit three-dimensional positional information to a camera or detector that allows the computer to instantaneously calculate and display the virtual position of the instruments relative to the image data and also allows the visualization of the movements of the instruments in real-time to the surgeon via side-viewers or advanced see-through viewers.[24,34,47]

Computer navigated implant surgery has many advantages over CGIS in that:[34]

• It allows intraoperative changes in implant position that is, the virtual surgical plan can be altered or modified during surgery and the clinician can use the navigation system to concurrently visualize the patient’s anatomy, permitting the surgeon to steer around obstacles, defects etc., that were not apparent on the presurgical scan
• Bur tracking allows the drill to be continuously visualized on a computer screen in all three-dimensions (x, y and z)
• It overcomes other limitations of CGIS like secondary thermal injury, displacement or fracture of guide etc.

Though CNIS technology (using optical tracking systems) has been widely used with superb accuracy but it also suffers from the following limitations:[48-50]

• They are sensitive to reflections and interference with the line of sight between the sensors and the cameras that is, a line-of-sight between the tracking device and the instrument to be tracked has to be maintained, which is not always convenient especially with the typical seating arrangement of dental surgeon and assistant and hence may preclude tracking of instruments
• More expensive and requires an expensive hardware
• Requires rigorous intraoperative referencing
• Significant learning curve.

However, compared with CNIS that can cost about $60,000-$200,000, CGIS is less expensive and outsourcing is possible with CGIS, as a remote company can fabricate surgical template, omitting the need to purchase expensive hardware by the clinician.[50] The authors also validated that CNIS is a promising technology, already successfully tested in routine clinical application, that can substantially contribute to an increase in quality and intraoperative safety for the insertion of implants.[51] Computerization of implant-dentistry has opened up new vistas and has eased implant placement in patients with complex problems following a significant alteration of the bony anatomy as a result of benign or malignant pathology of the jaws or trauma and in patients with physical and emotional problems (that limited the amount of time a patient could sit in a dental chair). Computerized-implant-dentistry being minimally invasive in nature has also enabled implant placement in patients with medical comorbidities (e.g. radiation therapy, blood dyscrasias).[52]

Robotic-Implant-Dentistry

Robots are expected to be more accurate and more reliable than a human being and can work as part of an interactive system, are immune to radiation and can be automatically programmed for documentation, evaluation and training protocols. As such, in the cranial area, robotic systems have already been considered for defined drilling of holes or implant beds with an automatic stop, for milling of the bone surfaces, for performing deep saw-cuts for osteotomies and allowing for the precise three-dimensional transportation of the subsequent bone segments or CAD/CAM transplant etc.[53] In fact, a partial section of robot-assisted dental surgery project has already been attempted to develop fully aided system on navigated, guided and assisted surgical performance.[54]

Conclusion

Based on the available data, CT scanning and interactive software programs for the treatment planning and fabrication of CAD/CAM-based stereolithographic surgical guides (SSGs) appear to be viable and promising tools that may improve the predictability, safety, and precision of implant placement compared to conventional freehand placement. The high accuracy of computer-guided implant placement may facilitate the clinical management of complicated cases. Moreover, the high accuracy and precision of transferring the treatment plan to the patient’s mouth makes the immediate delivery of a prefabricated final prosthesis a reliable option. However, computer-guided implant placement is not without limitations. It is a technique-sensitive procedure, involving the creation of studying models, di-agnostic wax-ups, and radiographic templates, CT scanning, interpretation of CT data, treatment planning, and the fabrication and stabilization of a surgical guide in the patient’s mouth during surgery. An error at any of these steps may affect the accuracy and precision of implant placement.

Although computer- guided implant placement is a promising technology, its performance must be critically evaluated, particularly because this technique is already commercially available and used by many clinicians. Given the limited data, relatively short observation periods, and lack of randomized controlled trials available in the literature, prospective clinical studies with long-term follow-ups should be performed. Such studies should strive to improve the systems and procedures with respect to their accuracy, predictability, and reproducibility of implant placement, as well as the surgical and prosthetic outcomes.

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