Accuracy Evaluation of the Computer-Designed Selective Laser Sintering Surgical Guide for Flapless Immediate Loading Dental Implants Surgery in Edentulous Jaws Open Access

Several systems using rapid prototyping technology, such as stereolithography^1–3  and selective laser sintering (SLS), are available for the fabrication of surgical guides.^4,5  Some variables, such as mobility of the mucosa or precarious fixation of the surgical guide over the jaw, may interfere with the accuracy of guided dental implant surgery and consequently positioning failure of fixed total dental prosthesis produced before the surgeries.^6–10  In patients with a totally edentulous maxilla and/or mandible, the limiting variables are as follows: (1) stability of the radiographic guide during preoperative cone-beam computed tomography (CBCT), (2) patient movement during CBCT,¹¹  (3) quality of the CBCT images, (4) segmentation of images—manipulation of the images of both the prosthesis and the tissues of the patient, (5) stability of the surgical guide on the mucosa during surgery,³  (6) accuracy of the drill kit of the dental implant system,^12,13  (7) dimensional distortion of the surgical guide,¹⁴  and (8) iatrogenic interference in the final positioning of the implants or no previous training of the surgeon.^15–17 

Several studies have evaluated the precision and drawbacks of surgical guide systems in relation to flapless dental implants surgery and definitive immediate loading prostheses.^18–21  In a previous study,⁴  60 dental implants and 12 prostheses were placed in 12 patients, and the results showed that the mean angular standard deviation (SD) between the long axes of the planned and placed dental implants was 4.31°–6.53°, with a mean lateral deviation of 1.35–0.65 mm in the crestal region and 1.79–1.01 mm at the apex of the dental implants. These differences were statistically significant (P < .001). Coronal and apical deviations ≤2 mm were observed in 82.67% and 58.33% of the dental implants, respectively. Some of the possibilities that may have influenced the results are as follows: (1) micromotion of the titanium guide tubes, which is clinically imperceptible, may have occurred inside the guide perforations; (2) the diameters of the titanium guide tubes were 0.2 mm larger than those of the drills, which may have resulted in an angle deviation of 2.3°; (3) use of an older type of CBCT device, which yielded low-resolution diagnostic images and had low segmentation accuracy; (4) improper placement of radiographic templates during scanning and improper placement or instability of surgical guides may have resulted in deviations to some extent; or (5) the dental implants were placed freehand.

The aim of the present study was to evaluate the accuracy of a refined computer-designed, selectively laser sintered surgical guide for flapless dental implant surgery in edentulous jaws, followed by immediate loading with temporary fixed total dentures. This study used (1) CBCT equipment with improved images, (2) the double scanning technique during CBCT, (3) a drill kit with a lower degree of freedom between the drills and guide tubes, and 4) at least 3 fixation pins for the surgical guides. We hypothesized that these modifications would reduce the angular and linear deviations, resulting in an improvement in the technique.⁴ 

This clinical study was approved by the Committee of Ethics of the Hospital São Paulo of the Federal University of São Paulo (Process 01619312.4.0000.5505) in accordance with international human protection guidelines, and the methodology was reviewed by an independent statistician.

The sample consisted of 11 patients (4 males and 7 females) aged between 41 and 76 years who received a total of 50 dental implants and 9 fixed total prostheses with immediate loading. The inclusion criterion was patients with totally edentulous maxilla and/or mandible with no parafunctional masticatory habit. The exclusion criteria were patients with systemic diseases, such as diabetes; history of smoking, drug use, alcohol abuse, or head and neck radiotherapy; insufficient amount of bone tissue or patients requiring bone graft for the placement of the dental implant; and limited mouth opening.

All surgeries were performed by a single dental surgeon, with expertise in the virtual planning of dental implants and computer-assisted surgeries for dental implant placement.

A denture that met the esthetic and functional requirements and a standard interocclusal acrylic index spacer to provide stability to the total prosthesis were prepared for each patient. Then CBCT was performed using the I-CAT 3D imaging system (I-CAT Cone Beam, Imaging Sciences International LLC, Hatfield, Pa) using the double scan technique.²⁰  The image files were opened in the surgical planning software ImplantViewer (ImplantViewer 1.9, Anne Solutions, São Paulo, Brazil), and the dental implant positions were planned virtually, considering the availability of bone tissue and the best prosthetic position to respect the occlusion and masticatory biomechanics. Image files with information on bone tissue, virtual dental implants, and prosthetic abutments were saved in the standard triangle language (STL) format (Figure 1).

Surgical guides were designed using computer-assisted design (CAD) software (Rhinoceros 4.0, Robert McNeel and Associates, Seattle, Wash) regardless of the surgical planning software system. The occlusal surfaces of the abutments were extended (elongated) up to the occlusal surface of the image of the prosthesis using image manipulation. The images were edited to obtain a 3D image of the total dental prosthesis positioned on the alveolar bone and 3 occlusal points for stabilization of the prosthesis (distal third of the last tooth on each side and the incisal edges of at least 2 anterior teeth).

Thus, by virtual planning, an image set of the bone tissue (mandible or maxilla), dental implants and their abutments, mucosa relief, and dental prostheses was obtained. The above-mentioned images were used to generate a virtual surgical guide. SLS technology (Sinterstation HiQ, 3D Systems Corp, Pella, Iowa) was used to print the 3D prototypes for each patient.

Surgeries were performed under local anesthesia, with appropriate sterilization protocols. The surgical guides (mucosa supported) were placed over the alveolar ridge mucosa along with a standard interocclusal acrylic index spacer for stabilization of the total prostheses and fixed using 3 anchor pins (Figure 2) such that they covered the maxillary tuberosity or retromolar area of the mandible. Next, a drill kit (Slice Guide, Conexão Sistema de Prótese Ltda, Arujá, Brazil) with sequential drills was used for the guided placement of auto-tapping external hexagon dental implants (Master Torq Porus, Conexão Sistemas de Prótese) with diameters of 3.75 or 4.0 mm and lengths between 10 and 13 mm with a minimum torque of 35 Ncm (4–5 dental implants in each patient). At the end of the guided dental implant placement, the surgical guide was removed, and the conical and cylindrical abutments were positioned (Figure 3).

Temporary fixed prostheses were placed over the alveolar ridge mucosa (Figure 4) and attached to the metal cylinders with acrylic resin. The total dental prosthesis was stabilized with the standard interocclusal acrylic index until the final setting of the resin.

Subsequently, the fixation screws and prosthesis were removed from the mouth of the patient. The ridge laps were removed, the concavities were filled, and finishing and polishing were performed. The temporary fixed prosthesis was replaced into the mouth of the patient, interocclusal relationships were checked, and adjustments were performed.

Each patient was prescribed amoxicillin (875 mg BID for 7 days, starting 1 h before the surgery), ibuprofen (400 mg TDS for 1 day PRN), and chlorhexidine mouthwash (0.12%, BID for 7 days).

Follow-up examinations were performed at 2 weeks and 3, 6, and 12 months after the surgery.

CBCT was repeated within 15 days of surgery to obtain postoperative images. CAD software (Rhinoceros 4.0) was used to overlap and compare the virtually planned position with the surgical position of the dental implants.

During the follow-up visits, oral hygiene was evaluated and reinforced, and occlusion and stability of the screws of the dental prostheses were evaluated.

The images of the post-surgical and planned dental implant positions were superimposed and compared using the CAD software as previously described.⁴  In brief, the angular deviation was measured as the 3D angle between the longitudinal axes of the planned and placed dental implants. The coronal and apical lateral deviations were calculated as the distance between the center of the planned dental implant and the intersection point of the longitudinal axis of the placed dental implant and a reference line (perpendicular to the longitudinal axis of the planned implant) at the coronal and apical centers of the dental implant, respectively⁴  (Figure 5).

Statistical analyses were performed by an independent statistician using the SPSS software, version 19.0 (IBM Corp, Armonk, NY). Fifty dental implants were subjected to the 1-sample t test to compare the planned and final dental implant positions. The null hypothesis was that the mean deviation between the planned and actual dental implant positions was zero. Differences were considered statistically significant if the P value was <.05. Furthermore, lateral deviation measurements were categorized as clinically negligible, ≤1 mm deviation; probably clinically irrelevant, >1 to ≤2 mm deviation; and potentially clinically relevant, >2 mm deviation. Descriptive statistical analyses were performed.

Eleven patients were enrolled in the study, and 9 immediate prostheses were placed. In 2 patients, a torque of 35 Ncm was not achieved in the distal implants, and temporary dental prostheses were not placed immediately.

Deviations between the planned and actual postoperative dental implant positioning were calculated for all 50 implants (Table 1). The mean ± angular SD between the long axes of the planned and final dental implant positions was 4.58° ± 2.85° in the coronal and 0.87 ± 0.49 mm in the apical region of the dental implants, with a mean apical deviation of 1.37 ± 0.69 mm. These differences were statistically significant (P <.001).

Figure 6 indicates the coronal and apical deviations of dental implants. In the apical region, 80% of the dental implants had mild and moderate deviations (up to 2.0 mm), and 20% had relevant deviations (>2.0 mm). In the coronal region, the percentages for mild and moderate deviations were 98% and 2.0%, respectively.

This study showed that the linear and angular deviations of the final positions from the planned positions, although statistically significant, were clinically insignificant in 98% of the dental implants. The mean linear deviation was approximately 1 mm at the implant neck and implant apex, and the angular deviation was approximately 4°.

Compared with previous studies,⁴  all deviations were significantly reduced in the present study (in 98% of dental implants, the coronal portion showed mild to moderate deviation, and in 70% of implants, it was ≤1.0 mm). In a previous study,⁴  only 31% of implants showed mild deviations. It was also noted that only 2.0% of the dental implants in the present study, compared with 17% of the dental implants in the previous study, presented deviations >2.0 mm (relevant). Only 20% of the dental implants in the current study had a critical or relevant apical deviation (>2 mm), while this number was 42% in the previous study.⁴ 

Besides an improvement in accuracy, a reduction in the variability (heterogeneity) of the results was observed in comparison to the previous study.⁴  In this study, the mean of deviations was within the variances found in studies conducted by other groups.^18,19,22 

Clinically acceptable linear and angular deviations are debatable.¹¹  A precision of <0.5 mm seems extremely difficult to achieve.²³  Considering the imprecision that surgical guide systems have today, clinicians should be aware of this limitation during their planning.²⁴ 

The changes proposed in the current study, such as the use of double scanning technique, a drill kit, and placement of dental implants using a surgical guide with at least 3 fixation pins, were made considering the limitations of the previous study.⁴  In the previous study, the main factors that contributed to the deviations in the final positions of the dental implants were micromotion of the titanium guide tubes, freehand placement of the implants into the site of the guided osteotomy, and the low-resolution diagnostic images and low segmentation accuracy²⁵  obtained due to the type of CBCT device used. Thus, we cannot identify the individual effects of each change, but the set of changes provided a significant improvement in accuracy.

The CBCT equipment used in the present study and the double scan technique facilitated segmentation of images, the definition of the limits of the bone tissue and total dental prostheses, and, consequently, the preparation of the surgical guide, which may have influenced the accuracy of the system.

Despite the instructions given to the patients, errors in the positioning of the prosthesis (radiographic guide) were observed when CBCT was performed. These problems occurred particularly in patients whose mandibular alveolar bone was greatly resorbed and those who had not used total dental prostheses previously or were not regular users of total dental prostheses. The scans were repeated in these patients.

In this study, the new drill kits provided less freedom of movement between system components (tube/guide drill/drill) compared with the system used in the previous study, where the guide tubes were 0.2 mm wider than the drills, which may have resulted in an angle deviation of ≤2.3°. Similar to a previous study¹³  that clinically compared drill kits with different degrees of freedom, this study showed that the accuracy of guided surgery was increased by limiting the degree of freedom of the drill kit.

Interestingly, when the dental implant suffered angular deviation, the insertion torque was increased at the end of the placement, probably because of the friction between the drill kit assembler and the guide tube of the surgical guide. Surgeons must be aware of this variable and verify that the implant conforms to the requirements established in the protocol of immediate loading, that is, a torque >35 Ncm.

In the present study, 3 anchor pins were used for fixation of the surgical guide in the bone tissue in contrast to the 2 pins used in the previous study. This resulted in a greater stability of the guide during instrumentation. Thus, the system features and other variables resulted in an improved accuracy with a margin of error within clinically acceptable limits.¹¹  All dental implants placed with the surgical guides were used to support temporary dental prostheses; no angled abutments were used, and no major adjustments were performed. In other words, the angular or positional variations verified were not clinically significant. In agreement with this assertion, the use of prosthetic guides based on prosthetic surgical planning prior to the surgery provided temporary fixed prostheses that required minimal occlusal adjustments after the union in the mouth in centric occlusion.²¹ 

The results indicate that computer-guided surgery reduces the differences between the planned and final results, although the surgeon must still consider possible deviations when using this technology. The present study can be improved with a larger sample size and new computational tools to simplify the clinical stages and optimize the results.

The proposed modifications reduced deviations, resulting in an improvement of the technique. Using the present protocol, the surgeon was able to place the implants and temporary prostheses, taking into account the differences between the planned and final positions of the dental implants.

3D:

3-dimensional

BID:

twice a day

CAD:

computer-aided design

CBCT:

cone-beam computer tomography

PRN:

when necessary

SD:

standard deviation

SLS:

selective laser sintering

STL:

standard triangle language

TDS:

thrice daily

The authors would like to thank Conexão Sistemas de Prótese Ltda for providing implants and prosthetic components, the technical team of the Department of 3-Dimensional/3-D Technology of the Renato Acher MCT Center for information technology–related support and technical support, and Patrícia Cury, PhD, DDS, from the Department of Periodontics, School of Dentistry, Federal University of Bahia, Salvador, Brazil, for the independent statistician review.

The authors declare that they have no conflict of interest regarding the publication of this article.