Outline of the case
A 28-year-old patient was referred to the Department of VIP Dental Service, School and Hospital of Stomatology, Fujian Medical University, with a missing right first mandibular molar (tooth 46). The main complaint was the loss of right lower posterior tooth for 4 months. The patient's right lower posterior tooth was extracted 4 months prior due to "cavities". The patient had no history of cardiovascular disease, diabetes, infectious diseases or allergies. Tooth 46 was absent, the occlusal-gingival distance was 7 mm, there was no obvious absorption in the alveolar crest, the adjacent teeth were not loose, and the mandibular right second molar was mesialized.
The workflow diagram followed the sequence shown in Fig. 2. The treatment plan involved a single implant treatment guided by an implant surgical guide for tooth 46 to support the fixed restoration design. The steps were as follows: (1) CBCT (iCAT, KaVo, Germany) was performed, and a mandibular impression was made; (2) the CBCT data were imported into medical image processing software (Mimics 10.0, Materialise, Belgium), and the implant design was simulated; (3) the implant design on CBCT was transferred to the mandibular model using TPT, and the implant surgical guide was made to guide the dental implant operation.
The research protocol was reviewed and approved by the Research Ethics Committee at the School and Hospital of Stomatology, Fujian Medical University (No. 2020-CX-32).
Materials for TPT-based implant guide
The materials used were as follows: (1) a cylindrical bar (Hengrun, China) with a diameter of 2.80 mm and a length of 35.00 mm, extending 5.00 mm to a tip from the centre of one end and ground to a shallow concave depth of 0.50 mm at the centre point of the other end; a home-made titanium guide ring with an outer diameter of 5.00 mm, a length of 7.00 mm, and an inner diameter of 2.85 mm (Fig. 3); (2) two compasses (H2030, Hero, China); (3) electronic digital callipers (ARTPOL, Jiangsu Jingjiang, China) with an accuracy of 0.01 mm; and (4) light-cured temporary crown resin (Revotek LC, GC, Japan).
CBCT and virtual implant planning
CBCT was used to scan the mandibular dentition and alveolar bone of the patient. CBCT was performed at a voltage of 120 kV, scan current of 5.00 mA, exposure time of 7.0 s, resolution of 0.2 mm, image matrix size of 800 × 800 pixels, and axis layer thickness of 0.2 mm. A total of 304 two-dimensional tomography images were obtained. The data were saved in digital imaging and communications in medicine (DICOM) format and imported into Mimics 10.0 for processing to establish a three-dimensional morphological model including the tooth and mandibular alveolar bone. A cylindrical structure was established to simulate the implant diameter (4.1 mm), length (12 mm), position and orientation (Fig. 4).
TPT-based guide production
Production of the TPT-based guide consisted of two main steps: software operation and dental stone model operation. The goal was to transfer the important design in the software to the dental stone model through TPT.
The first step was performed in Mimics 10.0 software. The diameter of the cylindrical implant was reduced to 0.1 mm to accurately locate its junction with the gingival mucosa, which was designated as point P1 (Fig. 4). The position of point P2 was determined by extending the cylindrical implant from P1 to the occlusal side by 39 mm. The central fossa of tooth 37 and the distal fossa of tooth 34 were designated as P3 and P4.
The three-dimensional coordinate values of the four points read by Mimics 10.0 software were as follows: P1 (60.06, 65.50, 46.95); P2 (70.77, 57.90, 83.63); P3 (105.50, 76.68, 49.56); and P4 (97.70, 50.62, 47.73). According to the formula for the distance between two points [8], if the coordinates of two points are \(A(x_{1} ,y_{1} ,z_{1} )\) and \(B(x_{2} ,y_{2} ,z_{2} )\), then
$$|AB| = \sqrt {(x_{1} - x_{2} )^{2} + (y_{1} - y_{2} )^{2} + (z_{1} - z_{2} )^{2} }$$
where P1–P3 = 46.86 mm, P1–P4 = 40.47 mm, P2–P3 = 52.15 mm, and P2–P4 = 45.46 mm.
The second step was performed on the dental stone model. A mandibular impression was taken with an alginate impression material and poured into a stone cast. A compass was used on the dental stone model with the central fossa of tooth 37 (P3) and the distal fossa of tooth 34 (P4) as the centre and 46.86 mm (P1–P3 distance) and 40.47 mm (P1–P4 distance) as the radius. Two arcs were drawn on the model of missing tooth 46, and the intersection point was the corresponding position of P1 on the model (Fig. 5A). At the P1 position of the model, a shallow recess with a depth of 0.5 mm was ground. The tip of the cylindrical guide bar was placed in the shallow recess, and the other end was fixed with P3 and P4 using two compasses (Fig. 5B). The length of the two compasses was set to 52.15 mm and 45.46 mm, respectively, to locate the implantation site and direction of the implant. A titanium guide ring with an inner diameter of 2.85 mm was sleeved into the cylindrical bar. A partial-arch tooth-supported implant surgical guide was designed for the missing right first mandibular molar. The surgical template was designed with a thickness of 2–3 mm. The light-cured temporary crown resin was fixed to the adjacent teeth, and the light-cured guide was completed (Fig. 5C). The distance from the upper edge of the guide ring to P1 was 10.69 mm, the thickness of the mucosa was 4.00 mm, and the implantation depth was 12.00 mm. The guiding depth is equal to the sum of these three lengths: 10.69 mm + 4 mm + 12 mm = 26.69 mm.
Guide implantation and accuracy
The guide was sterilized and inspected in place (Fig. 6A). After making an incision to create the flap, the 2.8-mm bar was guided by the TPT-based guide, and then preparation and implant placement were performed freehand. A bone lever implant (4.1 × 12 mm, BL, Straumann, Switzerland) was placed at the location of the right first mandibular molar (Fig. 6B). Postoperative CBCT was performed. Mimics 10.0 software was used to superimpose the postoperative implant position (Fig. 6C) with the virtual dental model used for preoperative planning (Fig. 6D). Differences in the position of the centre of the implant head and apex between preoperatively and postoperatively, as well as in the angular deviation of the implant axis, were calculated by the software. The total spatial deviation was obtained by calculating the differences between the planned and final positions on the x-, y-, and z-axes. Using the formula 3D deviation = √x2 + y2 + z2, a total spatial deviation value was obtained for this case that reflected the difference between the planned and postoperative implant positions. The central deviation of the implant head was 0.31 mm, the central deviation of the implant apex was 0.93 mm, and the implant angular deviation was 2.45°.