Surgical procedure of extracting teeth for obtaining dental pulp for regenerative medicine in swine

Materials and methods

Four healthy 7–8-month-old male NIBS miniature pigs (12.2–21.9 kg) (Nisseiken, Ome, Japan) were used for this study. Mandibular deciduous right incisors 1 and 2 (Di1 and Di2) were extracted to obtain dental pulp tissue. Animal care and handling procedures were performed in accordance with the ‘Principles of Laboratory Animal Care’ of the Tokyo Women’s Medical University Animal Experimentation Committee. Harvested dental pulp cells were used for the autologous peripheral nerve regeneration study. The pigs were given enrofloxacin at a dose of 5 mg/kg by subcutaneous injection after surgery and fasted for a minimum of 12 h after surgery. The pigs were given only pain-relief via local anaesthesia, because they underwent further surgery in an autologous peripheral nerve regeneration study several hours after the extraction of their teeth. The pigs were sacrificed at the end of this project by an intravenous injection of potassium chloride under sevoflurane-induced anaesthesia. No adverse effects were observed.

Surgical procedure

The miniature pig was first premedicated by a combined intramuscular injection of medetomidine (80 µg/kg), ketamine (5 mg/kg), and butorphanol (0.2 mg/kg), and then anaesthetized by an intravenous injection of 1% propofol (induction: 20–30 mg), and maintained by repeated intravenous injections of 10 mg propofol that were given every 10 min.

Because the root of the swine incisor is extremely long, routine closed extraction caused root fracture. Therefore, an open extraction was needed. A full-thickness mucoperiosteal flap was created on the buccal side of the gingiva (Figure 1a). The flap included the surface mucosa, submucosa and periosteum. The base of the flap had to be broader than the free margin to preserve an adequate blood supply. A narrow flap at the base might have led to flap necrosis. The margin of flap had to be located on the intact bone that would remain after the surgical procedure was complete in order to promote healing. ¹²

Local anaesthesia was injected into the subperiosteal using 1% lidocaine HCl and hydrochloride epinephrine at a dilution of 1:100,000 through a 23-gauge needle for (1) inducing local analgesia, (2) reducing the amount of haemorrhage during extraction, and (3) facilitating subperiosteal dissection by hydrodissection.

A gingival envelope or sulcular incision with vertical-releasing incision was performed using a No. 15 scalpel, and a trapezoidal full-thickness mucoperiosteal flap was made. The alveolar bone was exposed by peeling the subperiosteal with a periosteal elevator. The mental nerve was observed at the alveolar bone of the apical region (Figure 1b).

The alveolar bone of the buccal root surface was removed by green stick fracture, which was performed by osteotomy using a bone chisel and hammer. Osteotomy was performed at (1) the buccal surface of the root of Di1 and Di2, (2) the distal of root surface of Di2, and (3) the medial of the root surface of Di1. The buccal surface of the root of Di1 and Di2 was then exposed (Figure 1c).

Each tooth was extracted using dental extraction forceps. The crown and root were grasped as deeply as possible, and luxated by shaking the tooth, which was removed toward the buccal side (Figure 1d).

When the green stick fractured alveolar bone and mucoperiosteal flap were repositioned, the wound was sutured by simple suture with 3–0 degradable surgical sutures. If any free alveolar bone was observed, it should be removed to prevent bone necrosis. The first suture was placed on the papilla of vertical-releasing incision, the second suture was placed on the other papilla, the third suture was placed on the socket, and finally the vertical incision line was sutured (Figures 1e and 1f).

Dental pulp tissue was split and collected from the extracted teeth by splitting using bone cutting forceps or bone rongeurs (Figure 2).

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Figure 1.

Extraction of two mandibular deciduous right incisors (Di1 and Di2) from a 22 kg, eight-month-old miniature swine. (a) A full-thickness mucoperiosteal flap was created on the buccal side of the gingiva. (b) A gingival sulcular incision with vertical-releasing incision was performed, and the alveolar bone (AB) was exposed by peeling the subperiosteal with a periosteal elevator. Mental nerve (MN) was observed at the alveolar bone of the apical region. (c) The alveolar bone of the buccal root surface was removed by green stick fracture, which was performed by osteotomy using a bone chisel and hammer. Osteotomy was performed at the buccal surface of the root of incisors, distal of root surface of Di2, and the medial of root surface of Di1 (white dotted line). The buccal surface of the root of incisors (Di1 and Di2) was exposed. Asterisk shows green stick fractured alveolar bone. (d) The incisor (Di2) was extracted using dental extraction forceps. The crown and root was as deeply grasped as possible, and luxated by shaking the tooth, and removed toward the buccal side. (e) When green stick fractured alveolar bone and mucoperiosteal flap were repositioned, the wound was sutured by simple suture using 3-0 degradable surgical sutures. First suture was placed on the papilla of vertical-releasing incision (1 and 2), the second suture was placed on the other papilla (3). (f) After the closure of the wound.

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Figure 2.

(a) Two extracted mandibular incisors (Di1 and Di2) from a 22 kg, eight-month-old male miniature swine. Crown and root of Di1 were 5 and 27 mm, respectively. Crown and root of Di2 were 8 and 30 mm, respectively. Scale bars are in mm. (b) Separated mandibular incisor (Di2) and its dental pulp tissue from a 20 kg, six-month-old male miniature swine. (c) Dental pulp tissue from Di1 and Di2 from a 12 kg, seven-month-old male miniature swine.

Results

A total 9.8 ± 2.5 × 10⁵ dental pulp cells were obtained from the two mandibular deciduous incisors using this procedures (n = 4). All the experimental procedures were found to have minor complications such as facial swelling and non-active bleeding.

Discussion

In cell replacement therapies for various nerve disorders with tissue engineering, autologous transplantation is better than allograft, because allograft causes immune rejection, and the patients need immunosuppressants throughout their lives. Therefore, the cell source should be easily accessible with a low complication for autologous transplantation. Dental pulp, derived from neural crest, contains Schwann and stem cells.^5,14–16 The third molar is an easily accessible source of human dental pulp. The extraction of the third molar is one of the most common basic procedures performed in an oral and maxillofacial surgery unit. The overall complication rate is low, and most complications are minor. ¹⁷ The extracted third molar is routinely discarded as medical waste. Previously, the authors have isolated rat dental pulp cells by an enzymatic method and have incorporated them into collagen gel, which could then be placed into a nerve guide. A nerve guide containing collagen gel with dental pulp cells has been found to regenerate nerve cells more effectively than the control nerve guides containing the gel alone. Furthermore, transplanted dental pulp cells contain dental pulp-derived endothelial cells and Schwann cell-like supportive cells. These results suggest that axon and blood vessel regenerations are promoted by the dental pulp-derived supportive cells, which are produced by neurotrophic factors, including nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and glial cell line-derived neurotrophic factor (GDNF).^3,4

Miniature swine can be used as an experimental or pre-clinical model in maxillofacial surgery, including mandibular reconstruction ¹⁸ and distraction osteogenesis, ¹⁹ dental implantology, ²⁰ bisphosphonate-related jaw osteonecrosis, ²¹ and face transplantation, ²² (1) because of their anatomical structure, (2) because their size is similar to that of humans,^8–11 and (3) because they are suitable for surgical training and for teaching new innovative transplantation methods.^10,23 This report outlines a surgical procedure of tooth extraction in swine and using dental pulp as a cell source for autologous cell transplantation by regenerative medicine. The miniature pig has both deciduous and permanent dentitions, which are similar to those of humans. The dental formulae of the deciduous dentitions of a miniature pig are Di3/3, Dc1/1, Dp2/3, Dm2/1, and those of the permanent dentitions are I3, C1, P4 (3), M3 (maxillary) and I3, C1, P3 (4), M3 (mandible). ¹¹ In this study, Di1 and Di2 were used for harvesting dental pulp tissue, because (1) mandibular incisors can be better accessed than molars, (2) a single root incisor can be easier to extract than the molar which has four or six roots, ¹¹ and (3) Di1 and Di2 have a larger dental pulp than Di3 or Dc1 at the age of 7–8 months. Di1 and Di2 are erupted at 1–3 weeks and 2–3 months, respectively, whereas permanent I1 and I2 are erupted only at 12–16 and 16–20 months, respectively. ¹¹ All incisors in miniature pigs have a very short crown and a long simple root, and the length of the root is two to three times than that of the crown. ¹¹ Therefore, an open extraction is needed to prevent root fracture. It was possible to harvest approximately 10 × 10⁵ dental pulp cells from the mandibular incisors Di1 and Di2. Although the required number of dental pulp cells for regenerative medicine is still unclear, the authors have previously reported that 1.0 × 10⁵ transplanted dental pulp cells can regenerate a 7 mm nerve gap in a rat model. ³ If a larger amount of dental pulp cell is needed, a bilateral Di2 could be extracted and a vertical-releasing incision could be made bilaterally distal to the Di2. Our method could contribute toward the progress of regeneration research using dental pulp tissue in miniature swine.