The use of 3D additive manufacturing technology in autogenous dental transplantation

Background In medicine and dentistry, 3D technology allows the virtual planning and printing of surgical replicas of anatomical structures that can facilitate certain transplant procedures. In dentistry, 3D technology is useful in autogenous tooth transplantation. Case presentation We present a clinical case of an ectopic mandibular second premolar, describing the preoperative planning with dental replicas and the autotransplantation surgery. 3D prints of the surgical replica of the tooth to be transplanted was made using an Objet30 Prime® Printer, PolyJet. Clinical controls performed at 3, 6 and 12 months indicated the satisfactory evolution of the transplanted tooth. Conclusion 3D additive manufacturing technology allows the preparation of a new recipient socket with the aid of a surgical replica of the tooth to be transplanted, thus minimizing handling and extraoral time.


Background
Ectopic tooth eruption is an alteration of the eruptive process that most frequently affects the third molars and the upper canines [1][2][3].
The mandibular second premolar (2 PM) is one of the last permanent teeth to erupt. Its eruption may be delayed, especially when associated with the agenesis of a permanent tooth [4]. In this situation, the 2 PM lacks sufficient space in the arch and so there is an increased risk of malocclusions such as crowding, ectopy or impaction. The etiology of ectopic 2 PM remains unknown and its frequency is estimated at around 0.2-0.3%. Dental abnormalities such as agenesis, microdontia, developmental delay, canine palatal ectopia, and ankylosis often coincide with the distal angulation of 2 PM [5][6][7]. In 2009, all these combinations of alterations were grouped together under the umbrella term "dental anomaly patterns" and they are found in almost 25% of orthodontic patients [8,9].
The treatment of ectopic tooth eruption aims to relocate the tooth in its anatomical position. It involves surgical exposure and subsequent orthodontic traction. In severe cases with poor prognosis, autogenous tooth transplant is considered as a therapeutic alternative to surgical removal of the ectopic tooth.
In autogenous tooth transplantation, a recipient socket must be created for the insertion of the donor tooth. The tooth to be transplanted is extremely vulnerable, especially during the examination of its fit inside the new alveolar bed, when injury to the periodontal ligament is inevitable.
Furthermore, whenever possible, the extraoral handling time of the donor tooth should be kept to a minimum, since it negatively affects the tooth's viability [10].
In order to minimize damage to the tooth to be transplanted, helical CT/cone beam-computed tomography is used in combination with computer-aided rapid prototyping [11]. The process named 3D additive manufacturing technology consists of building a three-dimensional object directly using a 3D model in any type of file (e.g., STL, 3MF, STP), usually by the successive addition of material layer upon layer [12]. This technology allows the printing of dental replicas for use as guides during surgery, thus minimizing extraoral time and limiting probable damage to the periodontal tissue of the donor tooth [11]. All of this favors the creation of a new recipient socket, avoiding risks to the donor tooth and, more generally, helping to standardize the procedure [13,14].
Here we describe a case of autogenous tooth transplant using replicas of the donor tooth, 3D printed with Polyjet technology, for use as a surgical guide in the creation of the recipient socket.

Case presentation
A 16-year-old patient with no medical history of interest attended the Orthodontic Service of the Hospital St Joan de Déu in due to transposition of teeth 13 and 14 (FDI notation) and persistence of tooth 75. Panoramic radiography showed tooth 35 to be impacted, in lingual direction, and oriented distally, close to the mesial root of tooth 36. Orthodontic treatment of the discrepancy consisted of the extraction of tooth 14 to allow eruption of canine 13, and extraction of tooth 75 to allow spontaneous eruption of tooth 35. However, 4 months after the extraction, tooth 35 remained in its ectopic position (Fig. 1).
An alveolotomy was then performed in tooth 35 to facilitate its eruptive process and orthodontic traction. Four months later, an X-ray revealed a dehiscence of the lingual cortical of tooth 35, but no change from its original position.
Before surgical excision, it was decided to plan the autotransplant of tooth 35 to a new recipient socket with the help of a 3D printed dental replica. This replica was to be used as a surgical guide to prepare the alveolar bed, thus minimizing the handling of the donor tooth during its relocation.
To obtain the replica, helical CT images (Philips iCT256) with 58 mAs, kV 100 were used; radiation dose: DLP; 29.5 mGy-cm; CTDI vol (mGY): 4.9. After obtaining the image in DICOM format, it was segmented using the IntelliSpace Portal 11 program (Phillips®). This process allows the separation of a digital image into various structures, selecting the anatomical elements to be operated uponin our case, the lower jaw and the ectopic tooth. Once the images were digitally segmented, in STL format, the surgery was planned using the Meshmixer program (Autodesk®), a 3D general design freeware that allows virtual planning of surgery on the basis  of a post-processed model and the removal of the artifacts present in the CT caused by the brackets. It was also checked virtually means that the space available for the recipient socket was sufficient for the placement of the donor tooth (Fig. 2). The Objet30 Prime® printer provided the dental replica and the Sigmar R19 (BCN 3D® Technologies) the mandibular segment, using PolyJet and FDM technologies respectively (Fig. 3). Med610 material (Stratasys®) was used for 3D printing of the dental replica and polylactic acid (PLA) for the printing of the mandibular segment and adjacent teeth. Before surgery, both the surgical replica of the ectopic tooth and the mandibular segment underwent low-temperature hydrogen peroxide plasma sterilization (< 50°C; VH202 Matachana®).
Given the difficulty of the surgery required to extract the donor tooth, it was decided to perform autogenous tooth transplant under deep sedation. A crestal and intrasulcular incision was made of adjacent teeth 34 and 36, without any discharge. Mucoperiosteal detachment of the lingual flap and bone fenestration were performed to allow careful extraction of tooth 35, which was immediately immersed in sterile saline solution (Fig. 4). Next, using a sequence of implant drills (MIS® Iberia), the recipient socket was created, 10 mm long and 5 mm in diameter. Throughout the process to check the position and stability, only the dental replica was used. We were obliged to perform minimal selective grinding of the replica tooth to correct a small mesiodistal discrepancy between teeth 34-36, which was also transferred to the donor tooth. After 15 min of extraoral time, tooth 35 was placed in its new receptor socket and stabilized with a Vicryl®4/0 cross suture (Fig. 5).
The patient received 7 days' treatment with amoxicillin (500 mg every 8 h) and twice-daily chlorhexidine 0.12% rinses were indicated. Two weeks after the autotransplantation, endodontic treatment of premolar 35 was started (Fig. 6). Periodical clinical-radiological controls were performed at 1 month and at 3, 6 and 12 months (Fig. 7).

Discussion
The use of 3D print technologies in medicine and dentistry is expanding rapidly. The range of applications is not limited to medical devices or prosthetics: as the development of bioprinting continues to evolve, the implantation of living tissues in regenerative medicine is now a possibility. 3D printing allows the production of a wide range of devices, from hearing aids to Invisaling® aligners to prosthetic limbs, all tailored to meet the specific needs of the patient. 3D printing has several applications in the different dental subspecialties. In oral and maxillofacial surgery, it offers a new approach to simulation and surgical planning. Prior to surgery, anatomical models provide a realistic impression. The concept "touch to understand" has caused a paradigm shift in the representation of anatomical models, which are now not just visual aids but visuo-tactile aids [14].
The treatment of ectopic tooth eruption varies according to the degree of severity: i) extraction of the deciduous tooth to facilitate the spontaneous eruption of the ectopic tooth, in cases with the depth of impaction < 5 mm and the inclination < 55º; ii) fenestration in combination with orthodontic traction when the depth of impaction is < 5.5 mm and inclination < 95º; and iii) autotransplantation, as the final therapeutic alternative to excision, if the depth of impaction is > 5.5 mm and without any inclination. Other aspects to consider are the patient's age and ability to collaborate, the space available in the arch, and the presence of keratinized gingival tissue [16]. In our case, when analysing the impaction level of tooth 35, the distance was calculated using the 3D print of the mandibular segment. As it was above 6 mm, we decided to perform an autogenous dental transplant. In our case, the printing of the mandibular segment was necessary to design the shape of the recipient bed. Soon, using the mandibular replica, it would be interesting to prepare a surgical guide for these procedures. With this device resting on the adjacent teeth the surgeon is able to carry out guided surgery with better orientation and greater precision during the preparation of the new socket. Kafourou et al. [17] reported the results of autogenous dental transplant according to two concepts, success and survival ( Table 2).
A systematic review of the success of autotransplantation in teeth with incomplete root formation found that the donor tooth with the highest success and survival rates was the premolar (98.1%-98.4%), and that the most successful recipient area was the anterior area of the maxilla (98.5%) [18]. Other researchers analysing the same parameters, but in teeth with complete root formation, found the highest 5-year survival rates in the anterior teeth (96.9%), followed by the premolars (92.3%) and molars (84%) [19].
In our clinical case, the transplanted tooth was viable, with normal adjacent hard and soft tissues, and in stable occlusion; it was therefore considered successful. Other criteria assessed were the absence of progressive root resorption, discomfort, infection, physiological tooth mobility and a crown-root ratio ≤ 1.
The development of the 2 PM root is completed between the ages of 12 and 14. However, the ideal time for autotransplantation is when the apex of the tooth is still open with Moore's stage 3-4, which favors its revascularization and complete root formation [19]. In our clinical case root development was already complete, and so endodontic treatment was required, as in the case of an avulsed tooth with a closed apex. Some authors recommend root canal treatment 14 days after tooth stabilization [19,20]. Others, however, report a lower success rate of autotransplanted teeth with complete root formation [21].
In the literature, several researchers have reported factors that may influence the long-term results [22,23]. Most agree that there are three variables that have a particularly negative influence on the survival of the donor tooth to be transplanted: i) damage to the periodontal ligament of the donor tooth by trying to fit it into the new alveolus in the recipient site; ii) excessive extraalveolar time, which traumatizes viable periodontal ligament cells on the donor tooth; and iii) distance between the new alveolus and the root of the donor tooth [22][23][24][25][26][27][28].
According to Kafourou et al., the preservation of the periodontal ligament is the key factor in the prognosis of autotransplantation [17]. Whether or not an autotransplanted tooth requires splinting is a controversial issue. On the one hand, some authors consider that the mobility between the socket-tooth should be minimal in order   Successful endodontic treatment performed (with absence of periapical pathogenesis): -teeth with necrotic immature root, -teeth with complete root.
Ankylosis: root surface merged with alveolar bone.
Favorable periodontal healing: -no evidence of external root resorption -resorption controlled with endodontic treatment.
The tooth is still present in its transplanted position at the final followup visit. to accelerate cell proliferation, reduce osteoclastic activity, and avoid possible occlusal trauma on the transplanted tooth [29]. Others, however, argue against splinting, in order to avoid pulp necrosis and inflammatory root resorption and to favor periodontal ligament repair [30,31]. In our practice, we do not splint the transplanted tooth with the adjacent ones, but merely stabilize it with a surgical suture. Several authors agree that teeth transplanted immediately or within 15-30 min of tooth extraction have the best prognosis [32,33]. Lee et al. [11] were pioneers in applying additive manufacturing for the 3D printing of surgical dental replicas, in a study of 22 adult patients with clinical follow-up of 18 months. These authors highlighted the importance of reducing the extraoral time in donor teeth (mean 7.7 min) and of minimizing handling in order to preserve the fibers of the periodontal ligament [11]. In our case, the extraoral time was extended to 15 min because we had to correct a size discrepancy caused by the delay in performing surgery after the CT scan (a period of 4 months). In fact, the time between the CT and the surgery must be as short as possible, since slight orthodontic movements may cause small variations in the residual space. For all these reasons, orthodontic overcorrection is recommended to increase the space allocated to the recipient socket. Verweij et al. [34] recommend performing the cone beamcomputed tomography scan between 2 months and 2 weeks before the autotransplantation procedure, because the time taken to create a 3D replica of the donor tooth is approximately 2 weeks. Table 3 displays the 3D technique systems used in previous studies and the results obtained [11,[34][35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50][51]. Although resin is normally used as printing material for the replicas, some researchers have recently reported the use of titanium or cobalt-chrome alloys to make metallic replicas in order to prevent the development of deformities during the sterilization process [34,48].
The most frequent complications in autogenous tooth transplant are ankylosis, root resorption (inflammatory resorption) and pulp necrosis [18]. If present, ankylosis usually appears within 12 months of autogenous transplantation [31]; but some long-term follow-up studies indicated that this complication may also occur at a later time point [22,52].
It is important to stress that the presence of pulp necrosis does not indicate surgical failure. In fact, endodontics is part of the autogenous tooth transplantation treatment plan [18]. For its part, progressive root resorption may appear due to injuries to the periodontal ligament and/or pulp tissue, which are usually seen on periapical x-rays between 2 months and 3 years after the autotransplant [32,53].

Conclusion
Autogenous tooth transplantation is a valid therapeutic alternative to extraction for resolving certain severe cases of ectopic tooth eruption. 3D additive manufacturing technology allows the preparation of a new recipient socket with the aid of a surgical replica of the tooth to be transplanted, thus minimizing handling and extraoral time.