Use of Computer-Guided Surgery to Avoid Severe Buccal Defect and Lingual Concavity in Mandibular Implant Placement

Treatment planning of a dental implant involves the analysis of existing bone to determine the ideal surgical position of the fixture. Typically, this planning involves the interpretation of a 2-dimensional (2D) radiograph and the analysis of a stone model. Although this method historically has been used with success, limitations that are of concern do exist.

The use of radiographs for the treatment planning of dental implants has proven to be valuable. Vital structures such as the inferior alveolar nerve or maxillary sinus can be avoided during the surgical planning phase through analysis of radiographs. However, 2D periapical or panoramic images may present with a variety of limitations, such as magnification error.¹ For instance, a panoramic image can have a magnification range of 1.09 to 1.28 depending on the positioning of the patient.²

In addition to radiographs, stone models typically also have been used to verify the position of the implant. Because a stone model offers a rigid, nonfunctional surface, the model may not accurately reflect the true morphology of the underlying alveolar bone concealed under the resilient soft tissue.³ Stone models also do not allow visualization of the location of vital anatomy such as the lingual artery or inferior alveolar nerve.⁴ Moreover, the accuracy of a stone model is based on the dimensional stability of the final impression; if the impression is inaccurate or presents with distortion, this could negatively affect the proposed position of a dental implant.⁵

Compared to traditional 2D radiographs or stone models, cone-beam computed tomography (CBCT) can greatly enhance the diagnostic and planning capabilities of a dental implant surgeon.⁶ CBCT offers visualization of the alveolar crest in three dimensions and enables more accurate positioning of the appropriate fixture. If, for example, a horizontal defect is present in the alveolar crest, the surgeon can preoperatively decide whether to place the implant or perform grafting.

Vital structures such as the inferior alveolar nerve, mental foramen, and incisive canal may be readily identified through CBCT. One such anatomical structure that must be identified and avoided during implant placement is the mandibular lingual concavity. Perforation of the lingual plate concavity can have serious consequences for the patient. Arising from the external carotid artery, the lingual artery loops around to pass between the genioglossus and hyoglossus muscles.⁶ A large lingual concavity in the anterior region of the mandible can pose a risk for perforation in this area, and piercing of the vasculature on the ventral side of the tongue can lead to arterial hemorrhage.⁷

Consideration of anatomical structures is important for successful planning of implant cases and is particularly critical for cases involving the mandible. This article presents a case in which CBCT was utilized for computer-guided surgical placement of a dental implant in an atrophic mandible with severe lingual concavity.

A 27-year-old Asian male presented for an implant consultation to replace tooth No. 19. His medical history was unremarkable. Clinical examination revealed a significant defect on the facial aspect of the proposed implant site (Figure 1). Finger palpation of the lingual aspect revealed a substantial undercut. The maximum incisal opening was approximately 39 mm.

A CBCT was taken (Orthophos XG 3, Dentsply Sirona, dentsplysirona.com). Imaging parameters for all scans were as follows: area dose 693 mgy x cm², tube current 6 ma, and tube voltage 85 kvp. The field of view was set at 8 cm x 8 cm for the patient. Total radiation received was 166 microsieverts (µSv).

The CBCT was uploaded to the dental laboratory (Precision Guided Surgery/Dorsey Dental Lab, dorseydentallab.com) as uncompressed multiformat DICOM media. The stone models were scanned and imported into the software (coDiagnostiX^®, Dental Wings, dentalwings.com) for interpretation and dental implant planning. The CBCT confirmed a bony defect on the facial surface of the edentulous site of tooth No. 19 as well as a large lingual undercut (Figure 2). The site was planned to receive a 4.5 mm x 8.5 mm implant (Hiossen ETIII, Hiossen, hiossen.com), which would be slightly offset to the lingual to avoid a mandibular undercut. A digital crown was placed on the scan to verify that the implant would be in appropriate occlusion with the opposing arch (Figure 3). From the CBCT and scanned models, a computer-generated guide (OneGuide, Hiossen) was fabricated based on the analysis and proposed position of the implant. Because the implant was to be placed in the posterior region, an open slot was used for the guide (Figure 4).

The patient was anesthetized with 2% lidocaine with 1:100,000 epinephrine. The surgical guide was placed in the patient's mouth. To ensure an accurate fit, the incisal edges of the teeth on the contralateral side were inspected to confirm that they were in complete contact with the surgical guide (Figure 5). A 4.5 mm tissue punch was used to create an incision and facilitate removal of the crestal gingiva (Figure 6). The gingiva was removed via a #9 Molt periosteal elevator. An osteotomy was created following the Hiossen OneGuide 1 - 2 - 2 drilling sequence. An initial drill was used to create the osteotomy, which was enlarged using a 3.5 mm x 8.5 mm drill and then, finally, a 4.5 mm x 8.5 mm drill (Figure 7). To ensure that the lingual undercut was not violated, a depth gauge was used to palpate the apex of the osteotomy site (Figure 8).

Because the lingual plate was intact, a 4.5 mm x 8.5 mm implant was placed and torqued to 35 Ncm. An internal cover screw was placed and torqued to hand tight. A final periapical radiograph was taken to confirm placement (Figure 9). The patient was dismissed in stable condition and given appropriate postoperative instructions and prescriptions.

After 2 weeks, the patient returned for a routine postoperative appointment. There was minimal inflammation, and the patient reported no discomfort. The gingival tissues appeared healthy, and the gingival tissue punch had nearly closed (Figure 10). Approximately 8 weeks after this appointment the implant was uncovered and a provisional crown was fabricated to help with soft-tissue remodeling. The final restoration was completed approximately 1 month following the temporization (Figure 11 and Figure 12).

This case highlights the use of computer-guided surgery to place a dental implant in the presence of both a buccal defect and significant mandibular lingual undercut. Additionally, the guide used for this surgery featured a unique open design that allowed for implant placement in a reduced maximal incisal opening.

A main advantage of implant placement using a computer-generated guide is a high degree of accuracy. Geng et al evaluated the accuracy of 111 dental implants placed using computer-guided technology.⁸ Their study revealed that after placing 52 implants via tooth-supported computer guide, the total deviation at the neck of the implant from the original planned position was 0.27 mm +/- 0.24 mm. The total angular deviation was 1.72 mm +/- 1.67 mm. Vermeulen also confirmed a high level of accuracy through testing implants placed by experienced dental surgeons using surgical guides.⁹ He found that the total deviation at the coronal level was 0.42 mm with surgical guided placement versus 1.27 mm deviation with freehand placement.

These studies indicate that computer-guided surgery can offer an advantage when placing a dental implant in situations with minimal bone or in anatomically challenging areas. The use of guided surgery in this case report was crucial to avoid both the buccal defect and the mandibular undercut.

The mandibular lingual undercut has been a topic of extensive study in an effort to avoid complications in implant placement. Yoon et al analyzed 104 CBCT scans for the presence of a mandibular undercut and found it in 60% of mandibles; the average angulation of the undercuts was 75.48 degrees.¹⁰ These findings were confirmed by Chan et al, who found that lingual undercut presented in 66% of all mandibular cases.¹¹ The patient in the present case report had a significant undercut that immensely affected the surgical treatment planning of the implant.

Violation of the mandibular undercut space during implant placement can lead to catastrophic consequences due to the presence of the lingual artery. Arising from the external carotid artery, the lingual artery loops around to pass between the genioglossus and hyoglossus muscles.¹² Piercing of the vasculature on the ventral side of the tongue can lead to potentially fatal arterial hemorrhage.¹³

In addition to careful consideration of the anatomical structures, the available amount of buccal and lingual bone also should be analyzed during implant treatment planning. Ideally, in an adequate biotype, at least 1 mm of bone both buccal and lingual is necessary to retain soft-tissue contours.¹⁴ The patient in this case presented with a mandibular buccal defect, which was evident both during clinical examination and on the CBCT. Treatment options for this patient included ridge augmentation with delayed implant placement, ridge splitting with immediate implant placement, or a fixed prosthesis.

In this case, the utilization of CBCT-guided surgery allowed for an implant to be placed without any additional ridge augmentation. To avoid the mandibular buccal defect, the implant was angled to the lingual. Although this was not the ideal position, literature supports the placement of lingualized implants. Peñarrocha et al investigated the survival rate of implants placed in the mandible with a lingual inclination.¹⁵ Their study revealed a 94.9% success rate over 12 months with marginal bone loss of less than 0.59 mm. The loss of marginal bone was thought to be directly related to the angulation of the implant.¹⁶ De Faria et al, however, showed through finite element analysis that an internal hex connection can vastly reduce the amount of crestal bone strain on implants angulated up to 30 degrees.¹⁷ Thus, the lingualized placement of the implant in the present case was justified and should provide for a successful long-term restoration.

This case demonstrates the use of computer-guided surgery to effectively place an implant in the presence of a buccal bone defect and significant lingual concavity. Through the utilization of advanced implant surgical techniques the patient was able to receive a final implant restoration without the need for bone grafting or potentially violating critical anatomical structures. From treatment planning to surgical placement, computer-guided implant surgery can simplify even extremely difficult cases. This, in turn, can lead to better results and safer surgery compared to traditional treatment planning.

Thomas Y.H. Yoon, DDS, MS, MHSA
Faculty, Lake Erie College of Osteopathic Medicine, School of Dental Medicine, Bradenton, Florida

Thanhphuong N. Dinh, DMD
Faculty, Lake Erie College of Osteopathic Medicine, School of Dental Medicine, Bradenton, Florida

Hyun-Jun Kong, DDS
Private Practice, Winter Park, Florida