The use of custom surgical guides in inferior alveolar nerve repairs: a clinical practice review

Introduction

Background

The late-1980s marked the advent of the use of three-dimensional printing technology in the medical setting when custom models and prosthetics were first utilized (1). Since then, there have been tremendous advancements made in virtual surgical planning (VSP) guides, custom implants, and anatomical models, which are now extensively used in multiple fields, including orthopedics, ophthalmology, neurosurgery, dentistry, and maxillofacial surgery (2-8). Custom surgical guides created via virtual planning increase surgical accuracy for more predictable outcomes, especially when preservation of vital structures is essential (2,9-17). The use of guides also decreases surgical time, thereby reducing both the time spent under general anesthesia and the longevity of wound exposure leading to a potentially shorter recovery with fewer post-operative complications (2,3,10-17). Within the field of oral and maxillofacial surgery, custom guides are used widely from oncologic ablation and reconstruction to dental implant placement (3,9,11,14).

Rationale and knowledge gap

Recently, computed tomography (CT) and intraoral scans have been employed in the fabrication of custom guides for the repair of the inferior alveolar nerve (IAN) (17-20). The IAN can be injured in a variety of surgical procedures to the mandible including third molar removal, dental implant placement, bone grafting, endodontic therapy, ablative procedures, trauma, and orthognathic surgery (21-24). These neurosensory injuries may result in a lack of sensation and/or neuropathic pain of the lower lip, chin, and gingiva that affects the patient’s ability to eat, drink, and speak (21,22). When IAN function has failed to recover spontaneously during the first several months post-operatively and the deficit significantly affects the patient’s quality of life, exploratory trigeminal nerve microsurgery can be considered (21). Access to the injured nerve is achieved via removal of the lateral cortex of the mandible using a saw, ultrasonic, and rotary instruments (19,25). Creating a blind osteotomy to access the nerve can lead to excessive bone removal and significant post-operative sequelae, including increased edema, pain, and even pathologic fracture of the mandible (19,26).

Objective

This clinical practice review will describe cases in which 3D-printed custom guides were used in IAN exploratory microsurgery, as our experience both with and without them suggests that they allow for a more precise and conservative osteotomy in accessing the inferior alveolar canal. As a result, we have observed fewer post-operative complications, decreases in surgical time, and more favorable surgical outcomes.

Case 1

In April 2021, a 39-year-old female was referred to the Rutgers Oral and Maxillofacial Surgery Faculty Practice in Newark, NJ for persistent numbness of the left lower lip and chin, which began after implant placement at site #19 three months prior by an outside surgeon. The patient reported that she had experienced minimal spontaneous improvement in sensation since the procedure and that no attempt at implant removal had been made. Neurosensory testing, which included pinprick, hot and cold testing, two-point discrimination, vibration, brush, and Von Frey filaments revealed hypoesthesia in the left IAN distribution. According to the medical research council scale (MRCS), the patient had grade S2 sensation in the left IAN distribution (27). Both panoramic radiograph, as seen in Figure 1A, and CT demonstrated an osteotomy in close proximity to the inferior alveolar canal with the implant at site #19 in place superior to the canal. Sensory retraining exercises were recommended to the patient along with daily vitamin B12 supplementation. The patient was seen again in several months for nerve monitoring which revealed minimal improvement in sensation. The patient felt that the sensory deficit affected her quality of life such that she decided to proceed with exploratory microsurgery of the left IAN. Using a digital imaging and communications in medicine (DICOM) file from the CT scan and an standard tessellation language (STL) file from an intraoral scan, a surgical guide was designed in a VSP session (Figure 1B). The procedure was performed under general anesthesia in September 2021. A buccal vestibular incision was made in the left posterior mandible, a full thickness mucoperiosteal flap was reflected, and the mental nerve was identified, skeletonized, and protected. The surgical guide was then seated passively both on the occlusal surfaces of the mandibular teeth and the buccal surface of the mandible, as seen in Figure 1C. With a round carbide bur, the osteotomy was marked using the guidance of the window representing the location of the nerve injury. The guide was then removed, and the osteotomy, as seen in Figure 1D, was taken to a depth of about 5 mm, as measured pre-operatively. A diamond bur and hand instruments were then used to expose the IAN, which was found to be compressed. External neurolysis was then performed, releasing the nerve to allow for expansion. Gel foam was placed in the osteotomy site with platelet-rich plasma, and the osteotomy was covered with a collagen membrane, and the incision was closed. The procedure lasted for about two hours, and there were no intra- or post-operative complications noted. The patient was seen at one-month, five-month, and one-year visits post-operatively, where nerve testing was again performed with final sensation graded as S3+.

Figure 1 A left inferior alveolar nerve injury treated via exploratory microsurgery using a 3D-printed custom guide. (A) A panoramic image reveals an osteotomy extending apically and in close proximity to inferior alveolar nerve canal at site #19, where the dental implant was placed. (B) The custom surgical guide created via a virtual surgical planning session is seated on the 3D printed stereolithographic model. (C) The custom surgical guide is passively seated on the occlusal surfaces of the teeth and the buccal cortex of the mandible. (D) A conservative osteotomy was made using the custom surgical guide to access the canal and expose the inferior alveolar nerve.

Case 2

In the fall of 2021, a 59-year-old female presented to Faculty Practice with a chief complaint of lower left lip numbness. Her symptoms began when three dental implants were placed by a general dentist at sites #18, 19 and 20 three months prior. The implants were removed a week after placement. Nerve testing showed a significant deficit in sensation in the left IAN distribution, with a grade of S2. A CT scan, as seen in Figure 2A, revealed the implant osteotomies to be in close proximity to the left IAN canal. After another three months of minimal improvement, the patient opted for exploratory microsurgery with simultaneous placement of dental implants. A VSP session was held to design a custom surgical guide that outlined the area of the presumed IAN injury to facilitate osteotomy placement (Figure 2B). The custom surgical guide, as seen in Figure 2C, was passively seated both on the occlusal surfaces of the remaining mandibular teeth and the lateral surface of the jaw. A round carbide bur was used to initiate the buccal osteotomy, which was completed using an ultrasonic bone scraper and hand instruments until the nerve was fully exposed. The nerve, as seen in Figure 2D, was found to be atrophic as a result of the implant osteotomies. The osteotomy was extended anteriorly to the mental foramen and the incisive branch was sacrificed in order to lateralize and protect the nerve in a tension-free manner. A separate implant guide was seated, osteotomies were made, and the three implants at sites #18, 19 and 20 were placed under direct observation medial to the IAN (Figure 2E). Bone putty and a collagen membrane were placed around the implant fixtures, and a conduit was placed around the nerve. The procedure lasted for approximately two and a half hours. A post-operative panoramic radiograph (Figure 2F) demonstrates placement of the implants. The patient underwent an uneventful post-operative course with no complications noted. Final sensory recovery for the patient was graded as S3+. The patient underwent restoration of the implants four months after surgery.

Figure 2 Left inferior alveolar nerve exploratory microsurgery, nerve lateralization, and dental implant placement performed with a custom surgical guide created via virtual surgical planning. (A) A sagittal view of the computed tomography scan shows the dental implant osteotomy created is in close proximity to the inferior alveolar nerve canal. (B) The surgical guide, which consists of a window for the corticotomy and two fixation holes, was seated on the mandibular model. (C) The surgical occlusal- and bone-borne guide was seated on the patients left mandible to facilitate in making the osteotomy. (D) The damage to the nerve as a result of the initial implant surgery can be appreciated with nerve lateralization and sacrifice of the incisive nerve branch. (E) After the left IAN was lateralized, three dental implants were placed and noted to have primary stability. (F) A panoramic X-ray taken post-operatively shows the three dental implants to be intact and placed at the planned positions. IAN, inferior alveolar nerve.

Discussion

While most IAN injuries are not severe enough to require intervention, some are so functionally debilitating that patients elect to undergo exploration and microsurgical repair (28). There are many options available to the microsurgeon for surgical reconstruction of the nerve, including external decompression, neuroma excision, primary neurorrhaphy, or indirect neurorrhaphy via either autogenous or allogeneic grafting (21,29). All of these techniques require exposure of the nerve via the removal of the mandibular buccal cortex. With a guide, the corticotomy to access the nerve is confined to the area as planned on a VSP session, thereby decreasing unnecessary bone removal and risk of iatrogenic damage to healthy structures (19,20). With the use of custom guides, we have found that adequate exposure of the nerve is possible with an osteotomy extending 2 cm anterior and 2 cm posterior to the site of nerve injury. The precision of the guide also allows for an osteotomy with a width of just 5 to 6 mm. Excessive bone removal can lead to an increased frequency of complications, such as pathologic fracture of the mandible (26). Figure 3A shows a radiograph taken of a 71-year-old male post exploratory microsurgery of the left IAN, which was injured five months prior due to dental implant placement at site #18. There was no surgical guide utilized in the procedure. The patient reported resuming normal diet and activities and returned ten days later with a pathologic fracture. This required a return to the operating room for open reduction and internal rigid fixation of the fracture, along with re-exploration and repair of the allogeneic nerve graft (Figure 3B). A post-operative CT scan was obtained (Figure 3C), and he was discharged from the hospital two days later. The result of this case highlights the importance of accuracy in surgery and how custom adjuncts, like surgical guides, can be used to increase precision via a smaller osteotomy and therefore decrease the risk of post-operative morbidity. As a result, the senior author opts to use custom surgical guides whenever possible for exploratory IAN microsurgery.

Figure 3 A pathologic fracture of the left mandible was observed secondary to inferior alveolar nerve exploratory microsurgery performed without the aid of a custom surgical guide. (A) A panoramic image shows a pathologic fracture of the left body of the mandible that occurred as a result of excessive bone removal during previous inferior alveolar nerve microsurgery. (B) The patient was placed in maxillomandibular fixation and the left mandibular body fracture was rigidly fixated. The left inferior alveolar nerve was re-explored and repaired using allogeneic nerve graft and a conduit. (C) A 3D reconstruction of the post-operative CT scan shows the mandibular fracture to be adequately reduced and the fixation hardware to be intact. CT, computed tomography.

Our experiences using custom 3D-printed adjuncts for trigeminal nerve repair suggest that their benefits, in addition to decreased chance of post-operative complications, include a reduction in surgical time to approximately two hours, lending to potentially less intra-operative blood loss and reduced exposure time of the wound. Across multiple surgical subspecialties, increased operation time is associated with a higher incidence of post-operative complications, specifically surgical site infection. This is suspected to be due to prolonged exposure of the wound to microbes, increased risk of breaks in sterility, and increased tissue handling leading to tissue damage and higher susceptibilities (17,30). In addition, extended time under general anesthesia positively correlates with the frequency of other post-operative complications, like deep venous thrombosis, prolonged intubation, increased length of hospital stay, and need for return to the operating room (31,32).

The disadvantages of 3D-printed adjuncts in surgery include higher hospital costs and a longer waiting time until surgery to allow for planning, manufacturing, and shipping of the custom guides (2,4,10,12,33,34). To many surgeons, these limitations are justifiable when the benefits of decreased operative time and more ideal clinical outcomes are considered (17). This is especially true as certain complications require a return to the operating room, making treatment more financially and medically costly than if a custom surgical guide was used initially.

This review demonstrates the rationale behind a preference for custom-guided microsurgery over non-guided surgery through the experience of the senior author, who has performed a vast array of IAN microsurgical procedures. The imaging, diagnostic testing, and virtual planning typically done in the pre-operative planning stage of a guided trigeminal nerve exploration are outlined in detail in this clinical practice review. Even so, this review is opinion-based and there is no statistical data to prove the benefits of custom surgical guides. This hypothesis could be tested by comparing post-surgical neurosensory recovery outcomes in patients whose nerves were repaired utilizing custom guides with those whose nerves were explored without them.

Conclusions

In conclusion, the advantage of using custom cutting guides for IAN repair are demonstrated in this clinical practice review, and future studies with controls will test these benefits. As technology continues to evolve, 3D-printed guides created via VSP will likely become increasingly integral to the armamentarium of oral and maxillofacial surgeons, ultimately enhancing the quality of care delivered to patients undergoing trigeminal nerve exploratory procedures.

Acknowledgments

None.

Footnote

Peer Review File: Available at https://fomm.amegroups.com/article/view/10.21037/fomm-23-75/prf

Funding: None.

Conflicts of Interest: Both authors have completed the ICMJE uniform disclosure form (available at https://fomm.amegroups.com/article/view/10.21037/fomm-23-75/coif). V.B.Z. serves as an unpaid editorial board member of Frontiers of Oral and Maxillofacial Medicine from November 2025 to October 2027. The other author has no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. All clinical procedures described in this study were performed in accordance with the ethical standards of the International Review Board at Rutgers University (study ID: Pro2023001115) and with the Declaration of Helsinki and its subsequent amendments. Written informed consent for publication of this article and accompanying images was not obtained from the patients or the relatives after all possible attempts were made.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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