Effect of Anchor-Guiding Sleeve Length on the Accuracy of Computer-Guided Flapless Implant Surgery

Computer-guided implant surgery involves the use of a surgical guide that reproduces a virtual implant position designed from digital data.^1–4  The most important step in guided surgery is precisely positioning and stabilizing the guide. Inaccurate placement of the template can lead to implant deviation.⁵  To minimize the potential inaccuracy of guided flapless implant surgery in edentulous patients, Cassetta et al⁶  measured the accuracy of implant placement with and without fixation screws using muco-supported surgical guides in edentulous jaws. The results revealed that implants placed with fixed guides had a higher accuracy where the angular deviation was statistically significant (bias: with, −4.09°; without, −5.62°). They concluded that surgical guide fixation allowed greater transoperative stability, which reduced the potential inaccuracy between the planned and performed treatments. Vasak et al⁷  proved a correlation between mucosal thickness at the implant site and the degree of deviation, thus demonstrating the negative impact of mucosal thickness on guide stability and reproducibility of positioning. This study suggested that the accurate and stable positioning of surgical guides can be impeded in the edentulous jaws b7y mobile mucosal tissue. In addition, ridge atrophy makes placing the guide in the correct position difficult.^3–7  Stübinger et al⁸  used bone-supported templates with the open-flap surgical approach. Among all studies examined related to the placement of dental implants in edentulous ridges, the Stübinger study showed the lowest deviation and noted that the use of muco-supported templates resulted in a higher deviation than those using direct bone-supported templates. However, Stübinger used the mucoperiosteal flap procedure with some disadvantages for computer-assisted implant surgery, particularly regarding transoperative and postoperative morbidities. In contrast, the muco-supported guide has clinical advantages for flapless surgery of simplicity, causing less trauma to bones, less bleeding, shorter chair time, postoperative pain, and less bone loss around the implant surface.⁹  In that regard, previous studies have advocated the use of a muco-supported surgical guides for placing implants in edentulous patients.^2,5,6,10,11  However, to overcome the limitations of guided flapless surgery the surgical guide must be (1) accurately designed, (2) precisely fabricated, and (3) accurately positioned.

In an attempt to help seat the surgical guide more precisely on the edentulous mucosal tissue in flapless guided implant surgery, this experimental study was performed to determine whether the use of long anchor-guiding sleeves to fix anchor screws leads to a more precise guide placement, thus minimizing the inaccuracy in guided flapless implant surgery. To the best of our knowledge, no reports have been published on the role of anchor-guiding sleeves in improving the accuracy of implant placement when implants are placed on an edentulous ridge. Such a study would provide comprehensive information for the development of successful strategies to increase the accuracy of implant placement in edentulous patients. The purpose of this study was to evaluate the effect of anchor-guiding sleeve length on the accuracy of computer-guided flapless implant surgery in edentulous cases.

Twelve identical polyurethane edentulous mandibular models with a soft-tissue replica were used. The soft-tissue replica made of a silicone material was discriminated from the polyurethane model on cone-beam computerized tomography (CBCT). They were divided equally into 2 groups according to the type of anchor-guiding sleeve. The short sleeve group contained 2.0- × 4.0-mm anchor-guiding sleeves (DIO Implant Co, Pusan, Korea; Figure 1a). The long sleeve group contained 2.0- × 8.0-mm anchor-guiding sleeves (DIO Implant Co; Figure 1b).

Before obtaining digital data, a flowable composite resin (Charmfil Flow, Dentkist Inc, Seoul, Korea) was injected at 4 different sites as half-ball shapes with a diameter of 1 or 2 mm on the soft-tissue replica to use as markers for image fusion of the intraoral scan data and CBCT data. The injected resin was then polymerized by light curing. After the CBCT scan was performed on the model, it was scanned using an intraoral scanner (TRIOS, 3 Shape A/S, Copenhagen, Denmark). CBCT and intraoral scanning were performed for each model. Both digital files of STL generated by intraoral scanning and Digital Imaging and Communications in Medicine (DICOM) data obtained from the CBCT scan were imported into the software (Implant Studio, 3Shape A/S). The images of the CBCT data and digital STL files were fused. After performing image fusion, the implant position was planned using virtual implant-planning software. The implant positions determined were the cuspid, first premolar, and first molar regions bilaterally. Virtual implant planning was performed for each model. Once the implant location was determined, the surgical guide was designed on the intraoral scan, which provided information on the implant position (Figure 2). Holes for long and short anchor-guiding sleeves were also designed. The designed surgical guide was fabricated using a 3D printer (3D Printer Probe, DIO Inc; Figure 3a and b). After fabricating the surgical guide, a bite registration putty was made for surgical guide fixation using a vinylpolysiloxane material and surgical guide, together with the maxillary and mandibular models at the occlusal vertical dimension (OVD). The OVD was determined by artificial teeth arrangement using denture-designing software (3 Shape A/S). The bite registration putty was fabricated by seating the surgical guide on the edentulous mandibular model followed by placement of vinylpolysiloxane interocclusal recording material (Bite Registration Creme of EXABITE II NDS, GC America Inc, Alsip, Ill) between the surgical template and the occlusion surface of the opposing teeth, and then further guiding the model to the OVD that was stabilized before the impression material was completely polymerized (Figure 4).

The surgical guide was placed on the edentulous model using the bite registration putty. Subsequently, drilling with an anchor drill was performed through the anchor-guiding sleeve, and the surgical guide was fixed in place with an anchor screw (2.0 × 15 mm). After fixing the surgical guide, flapless implant surgery was performed using a surgical guide. The drilling was guided by a drill key (DIO Navi Guide, DIO Inc). All drillings were performed at low speed (50 rpm) without irrigation. Implants (UFII, DIO Inc) were placed under the guidance of the surgical template. Implant placement was performed by an experienced surgeon (B.-H.C.).

After implant placement, the scan body (DIO Inc) was connected to each implant, and a digital impression was obtained using an intraoral scanner (TRIOS, 3 Shape A/S). The obtained STL files were imported into the software for file editing (3Shape Designer, 3 Shape A/S). The STL files of the corresponding inserted implants were then attached to each implant by perfect matching of the scan body, applying the best-fit algorithm. The planned treatment and digital impression data were imported into the file-editing software. To measure the deviation between the planned and placed positions of each implant, the objects in both data were overlapped automatically using file-editing software. Using the software's measurement function, the following deviation parameters were calculated between the planned and placed implants: angular deviation, linear deviation at the implant apex, linear deviation at the implant shoulder, and depth deviation (Figure 5).

The differences between the 2 groups were calculated using the Mann-Whitney U test. A P value <.05 was considered statistically significant. The methodology was reviewed by an independent statistician.

A total of 72 implants were inserted. Thirty-six implants were placed on 6 edentulous mandibular models using surgical guides that contained long anchor-guiding sleeves, whereas the other 36 implants were placed on 6 other edentulous mandibular models using surgical guides with short anchor-guiding sleeves.

In the short anchor-guiding sleeve group, the median angular deviation was 4.05° (range, 2.87°–7.55°). The median linear deviation amounted to 1.17 mm (range, 0.24−2.17 mm) for the implant apex and 0.82 mm (range, 0.43−1.67 mm) for the implant shoulder. The median deviation of the depth was 0.31 mm (range, 0.20−0.79 mm; Figure 6a). In the long anchor-guiding sleeve group, the median angular deviation was 2.70° (range, 1.77°−4.08°). The median linear deviation amounted to 0.88 mm (range, 0.21−1.77 mm) for the implant apex and 0.63 mm (range, 0.11−1.97 mm) for the implant shoulder. The median deviation of the depth was 0.24 mm (range, 0.09−0.53 mm; Figure 6b). There were significant differences between these 2 groups in angular and depth deviations as well as in the linear deviation at the implant apex and shoulder (Table 1).

This study found a significant difference between implants placed with short anchor-guiding sleeves and those placed with long anchor-guiding sleeves in terms of angular deviation and deviation in position at the apex and platform. Our results suggest that the increase in anchor-guiding sleeve length has a considerable positive influence on preventing deviation during implant insertion. Some studies have demonstrated that the accuracy of muco-supported guides is significantly lower than that of the bone-supported guide for implant placement in edentulous patients. Di Giacomo et al⁵  used guides adapted to the mucosal surface and reported an angular deviation of 6.53°. Cassetta et al⁶  measured the accuracy of muco-supported surgical guides and reported an angular deviation of 4.09° that is almost identical to the short sleeve group of the current study. Valente et al¹²  reported an angular deviation of 7.9° with muco-supported surgical guides. Compared with those previous studies, the muco-supported surgical guides with long anchor-guiding sleeves in our study showed significantly greater implant placement accuracy. The median angular deviation of implant placement with the long anchor-guiding sleeve was 2.7°.

The higher accuracy of implant placement in this study may be due to the long anchor-guiding sleeve that guided the anchor drill within the surgical guide. The anchor drill is important because it plays a determinative role in the axis of the anchor screw. If any error occurs in the axis of drilling inside the bone, it becomes impossible to fix the surgical guide in the correct position. Consequently, the drilling conditions for the anchor screw must be optimized to decrease the drill deviation. In our study, the long anchor-guiding sleeve provided long guidance for the anchor screw inside the implant guide, thus minimizing the lateral movement of the drill. These findings are supported by the study by Choi et al¹³  that evaluated the effect of the surgical guide channel length on implant placement error in an in vitro study. They defined that the length of the channel was the main determinant in reducing the angular deviation of the implants and recommended using the longest possible channel to minimize deviation.¹⁴ 

Compared with bone-supported surgical guides, muco-supported surgical guides are disadvantaged for fixing surgical guides because mucosal resiliency in edentulous ridges can cause inconsistent guide adaptation.^8,15,16  However, the use of a bite registration putty and long anchor-guiding sleeve provides a technique that allows the surgical guide to be seated and fixed on the edentulous ridges as precisely as bone-supported surgical guides. Stübinger et al⁸  and Pettersson et al¹¹  used bone-supported surgical guides and reported mean angular deviations of 2.39° and 1.90°, respectively. Therefore, it can be concluded that surgical guide fixation with long anchor-guiding sleeves along with a bite registration putty can provide more accurate fixation of surgical guides, thus reducing errors between the planned and placed implants.

There is concern that surgical guides that contain long anchor-guiding sleeves may interfere with the effective use of surgical instruments because the top of the hole holding a long sleeve is raised.¹⁶  In our study, interference between the surgical handpiece and the hole part of the surgical guide did not occur when drilling was performed. This may be because there was enough space between the anchor screw and the implant placement sites in the edentulous case.

Despite the accuracy and the potential benefit of long screws, there are negative factors when using long anchor screws, including iatrogenic effects such as higher postoperative bleeding, pain, swelling, and possible nerve damage. Moreover, long anchor screws may interfere with the proper application of the handpieces and anchor screws inside the mouth. The properties of the new 8.0-mm-long screws in the current study make it less harmful to bone with fewer adverse effects and increase the accuracy of the entering direction through the longer screw-sleeve contact area.

The findings of this study were derived from a model experiment. However, a philosophical analysis may render problematic issues found in model studies. Limitations found with model-based studies are (1) models do not completely incorporate the complex details of natural phenomena and (2) models do not always generate possible uncontrolled cause-and-effect outcomes that may occur under natural conditions. These are considered critical limitations of in vitro research studies. In particular, the thickness of the mucosa may not illustrate variability in the clinical conditions found in patients. Therefore, further in vivo investigations on the effect of long anchor-guiding sleeves are required to determine whether the results of this study will be consistent with clinical findings. Our aim is to provide the study in this sphere and determine whether these results can be replicated under clinically natural conditions.

The results of this study in edentulous models indicate that the accuracy of the muco-supported surgical guide was improved by using a long anchor-guiding sleeve, thus providing more accurate flapless implant placement.

Abbreviations

CBCT:

cone-beam computerized tomography

OVD:

occlusal vertical dimension