Bone Reduction to Facilitate Immediate Implant Placement and Loading Using CAD/CAM Surgical Guides for Patients With Terminal Dentition

Oral rehabilitation of patients diagnosed with terminal dentition can be challenging, especially if the treatment plan involves using dental implants immediately placed after extraction of the remaining failing dentition. In such cases, extensive bone reduction (alveoloplasty) may be indicated. Bone reduction in such cases is usually performed to create the interarch space needed for the planned future prosthesis and/or eliminate a knife-edge thin crestal ridge that can impede accurate implant placement. Such bone reduction creates a more favorable bony architecture, with a wider ridge, for the placement of the implants.¹  In such situations, precise reduction of bone is critical to avoid unfavorable outcomes, as over- or under-reduction can result in the implants being placed in an undesirable location and/or compromised restorative space.

With recent advances in digital computer-aided design/computer-aided manufacturing (CAD/CAM) technology, it may now be possible to provide a more accurate treatment outcome in such cases. In the last decade of development in these technologies, 3 types of CAD/CAM surgical guides have been described: tooth supported, mucosa supported, and bone supported, with the latter 2 being used mostly for completely edentulous cases.²  A major disadvantage of using mucosa- or bone-supported surgical guides for completely edentulous cases is the lack of guide stability.^3,4  To improve the stability of such guides, anchor pins are used to minimize movement of the guide during surgery. However, even with the use of anchor pins, mucosa- and bone-supported guides are still reported to be less accurate than teeth-supported guides.² 

Computer-aided design/CAM technology has recently expanded not only to improve dental implant placement but also to allow more accurate bone reduction. This study describes the use of CAD/CAM tooth-supported bone reduction guides to more accurately achieve the desired amount of bone reduction, and a method to improve guide stability in patients diagnosed with terminal dentition. The aim of this study is to present a method, using 3 CAD/CAM surgical guides, to accurately obtain the desired bone reduction followed by immediate implant placement and loading for patients diagnosed with terminal dentition. We coin this method as the “3-guide technique."

Patients who had bone reduction, implants placed, and immediate loading using Anatomage Invivo 5 (Anatomage, San Jose, Calif) CAD/CAM surgical guides between the period 2013 and 2015 were evaluated retrospectively. Patients diagnosed with terminal dentition and treated using the 3-guide technique were identified. The patients were informed about the procedures and signed a consent form prior to treatment.

All patients had preoperative cone beam computerized tomography (CBCT) scans performed. The treatment plan was executed on the digital DICOM images using Anatomage Invivo-5 software. This software is a volumetric 3-dimensional (3D) interactive imaging program that manages DICOM data created by CBCT devices.⁵  It is able to use 3D tools to segment and view anatomical landmarks of interest.⁵  In addition, it can immediately perform virtual implant placement and bone reduction planning after opening DICOM data of CBCT images.⁵  This software also fabricates surgical guides from stereolithographic (SLA) models by combining rapid prototyping and traditional laboratory techniques.⁵  Using a digital intraoral scanner (iTero, Align Technologies, San Jose, Calif), standard triangulation language (STL) data files were obtained to provide the dental and soft-tissue information. Then this STL data was sent to Anatomage for superimposition and merging with CBCT DICOM data files, which represent the data of the simulated implant.

First, the locations of 3 anchor pins were planned, and then a tooth-supported guide was fabricated using an SLA model generated by the manufacturer (Figure 1a). This step is considered unique to this proposed method because it takes advantage of the support of the remaining dentition and accurately replicates the position of the anchor pins for subsequent guides, hence ensuring an accurate replication of the guide position. Then, the desired bone reduction is virtually performed on the CBCT DICOM images using the software, and a second surgical guide is fabricated on an SLA model replicating this bone reduction (Figure 1b and c). It should be noted that the locations of the anchor pins are maintained identical to the first surgical guide. Finally, implant positions are virtually planned in the CBCT DICOM image based on the desired treatment plan. Then a third (and final) CAD/CAM surgical guide is fabricated to accommodate this plan with the exact locations of the anchor pins generated from previous guides (Figure 1d and e). For all patients, an immediate complete denture was fabricated with the intention of immediate loading after achieving primary stability of all implants placed. Holes in the immediate dentures were created based on the implant positions replicated in the final SLA model (Figure 1f).

Prior to surgery, the 3 guides were examined on the Anatomodels carefully (Figures 1 and 2a). The Anatomodels are the SLA models provided by Anatomage on which the surgical guides were made. Surgery was performed under local anesthesia with intravenous sedation. The first tooth-supported CAD/CAM surgical guide (Figure 1a) was placed, and complete seating was ensured (Figure 2b). Using surgical drills, 2.0-mm osteotomies were created through the soft tissue to place the anchor pins as dictated by the guide and a prescription provided by the manufacturer indicating the length and diameter of drills. These 3 anchor holes would be used to fixate the location of all subsequent guides after all the teeth were extracted. The first guide was then removed, teeth were extracted, and the second bone-supported surgical guide was placed and stabilized by the same anchor pins placed at exactly the same locations as the first tooth-supported surgical guide (Figure 2c). Bone reduction was performed with the second guide in place as a reference to where the desired level of bone will be. After leveling the bone with the window in the second guide, bone reduction was considered to be complete (Figure 2d). The second guide was then removed, and finally, the third bone-supported guide was placed and stabilized with the same anchor pins placed at exactly the same locations as the osteotomies created using the first guide (Figure 2e). This third and final guide is the implant placement guide. Implants were then placed using a guided implant kit, according to the prescription provided by the manufacturer using the final/third bone-supported guide (Figure 2f). At this stage, autogenous bone grafting may be performed if required around the neck of the implants and socket walls. After confirming primary stability of all the implants using Osstell (Integration Diagnostics AB, Sävedalen, Sweden) and traditional torquing, the implants were immediately loaded. Multiunit abutments were placed on the implants (Figure 2g), and temporary titanium cylinders were secured (Figure 2h). The prefabricated immediate complete denture was then retrofitted and converted to a fixed provisional prosthesis (Figure 2i and j). Postsurgical CBCT images were made for all 5 patients.

To evaluate the accuracy of the 3-guide technique, the accuracy of the bone reductions and the implant placements were evaluated using the presurgical and postsurgical CBCT with the simulated and the actual implants superimposed. To evaluate the accuracy of the bone reductions, 3 sites were selected: right, middle, and left side on each arch. Within every site, 3 points were selected to measure deviations of the planned reduction from the actual bone reduction; these are as follows: buccal/labial, mid-crestal, and lingual/palatal. The 3 points were then averaged and presented as a single deviation at the site. Then the total deviation was calculated based on the average of the 3 sites. The dimensions evaluated for implant placements were deviation at crest (point C), deviation at apex (point A), and deviation of axis (angle A).

Five patients (2 women and 3 men; age range 49–67 years old) diagnosed with terminal dentition were included in this study. Three patients had mandibular arches with terminal dentition and 2 patients had maxillary terminal dentition. A total of 26 implants were placed in 5 arches. All implants placed were Nobel Biocare conical connection (Nobel Biocare, Göteborg, Sweden). All patients were medically healthy and no contraindication was noted prior to treatment.

The overall deviation means measured for bone reduction using the 3-guide technique was 1.98 mm. The overall deviation means measured for implant placement at the crest, apex, and angle were 1.43 mm, 1.90 mm, and 4.14°, respectively. Results are presented in Tables 1 and 2.

The present study describes a method to perform bone reduction and immediate implant placements, immediately loaded, using CAD/CAM surgical guides in patients diagnosed with terminal dentition. The study proposes a method to improve support and stability of the surgical guides, hence possibly improving accuracy, by taking advantage of the remaining dentition prior to extractions using the proposed 3-guide technique. Due to the absence of support for guides after extractions, anchor pins act to stabilize the guide during surgery. Research shows that tooth-borne surgical guides provide the most accurate and predictable results when placing implants using the guided techniques.²  Hence, it was elected to take advantage of the terminal dentition in stabilizing the surgical guide and improve the accuracy of the bone reduction guide and implant placement guide. This study describes this approach as the 3-guide technique: the 3 guides with anchor pins provide the exact same positioning for all subsequent guides. It is important to note that after the bone reduction, both the bone quantity and quality are reduced due to the removal of the cortical layer. This technique provides a more predictable way to place the implants in their desired and most optimal position based on the bone quality noted on the presurgical CBCT. In addition, the bone reduction will remove some anatomical landmarks that may have been used by the clinician to place the implants. These changes may raise challenges in accurately placing implants in the desired position and avoid violation of vital structures, hence influencing the overall treatment outcome.

Multiple studies have reported an accuracy assessment of CAD/CAM surgical guides for implant placements in completely edentulous patients.^2,6,7  To the knowledge of the authors, no report described the use of such CAD/CAM technology for bone reduction prior to implant placement in completely edentulous patients and/or patients with terminal dentition. In this study, an overall average deviation of the implants at the crest of 1.43 mm, at the apex of 1.9 mm, and an angular deviation of 4.14° was found. In addition, a deviation in the amount of bone reduction was found to be an overall average of 1.98 mm.

D'Haese et al⁶  used AstraTech's Facilitate software package and evaluated the accuracy of mucosal-supported SLA guides in the edentulous maxillae (without bone reduction). The deviation at the crest ranged between 0.29 and 2.45 mm, with a mean of 0.91 mm. Average angular deviation was 2.60° (range, 0.16°–8.86°). At the apex, the deviation ranged between 0.32 and 3.01 mm, with a mean of 1.13 mm.⁶  They reported significant angular and linear deviations with short implants showing lower apical deviations compared with longer implants.⁶  D'Haese et al⁶  suggested that connecting abutments for immediate loading protocols may cause additional deviations due to the torque applied during abutment fixation in less-stable implants. It is suggested that these deviations may be multifactorial. Furthermore, another factor that needs to be considered a possible cause for deviation in our study is the lack of cortical plate after bone reduction. Bone reduction not only decreases the quantity of bone, but also the quality because bone reduction is performed mainly on the crestal aspect of the jaw, which is where a significant amount of cortical bone is present. After bone reduction, this cortical bone will either be eliminated or significantly minimized, hence exposing lower bone quality for implant placements. Lower bone quality may also contribute to higher implant deviations.⁶ 

When immediate loading is considered in such cases, multiple advantages and disadvantages have been described. Advantages include short treatment time and maximum comfort for patients.⁸  Some reports indicated multiple disadvantages especially with removable implant-supported prosthesis causing biological disturbances between mucosa and abutments and significant pain for patients.⁹  In addition, a microgap might form between the implant and the abutment that may influence remodeling of crestal bone.⁸ 

CAD/CAM surgical guide fabrication is an emerging tool that may facilitate the surgical process and aid in safe and predictable execution of bone reduction and immediate implant placement. A method is presented, using 3 CAD/CAM surgical guides, to obtain the desired bone reduction followed by immediate implant placement and loading for patients diagnosed with terminal dentition. This method may improve guide stability for patients with terminal dentition undergoing complete implant-supported treatment by taking advantage of the teeth to be extracted.

Abbreviations

3D

3-dimensional

CAD/CAM

computer-aided design/computer-aided manufacturing

CBCT

cone beam computerized tomography

SLA

stereolithographic

STL

standard triangulation language