Patient selection

A total of 15 children who attended the Pediatric Dentistry Department at the Third Affiliated Hospital of the Air Force Military Medical University between April and June 2022 were included in the study. Inclusion criteria were as follows: (1) age range of 4–8 years with the ability to cooperate with clinical treatment; (2) premature loss of a single first primary molar with earlier development of permanent successor than Nolla7 and its surface covered with bone; (3) no mobility or root resorption, or if present, the root resorption was less than 1/3 of the second primary molar and primary canine (abutment tooth); (4) no caries of the abutment tooth or the abutment tooth underwent caries/pulp therapies; (5) informed consent obtained from both the children and their parents and willingness to undergo regular follow-up. Exclusion criteria included: (1) poor compliance and inability to maintain good oral hygiene by the children and their parents; (2) root resorption of the abutment tooth exceeding 1/3. The new technology of the digital SM was approved by the School of Stomatology, the Fourth Military Medical University, under registration number IRB-REV-2019023.

Digital design and fabrication of semi-rigid bridge SM

Example patient: An 8-year-old girl presented with premature loss of tooth 74 due to periapical periodontitis. Tooth 75 underwent root canal therapy and standardized tooth preparation for a composite resin crown, including 1.5 mm of occlusal surface preparation and 1.0 mm of buccal, lingual, and proximal surface preparation, with a shoulder of 1.0 mm (Fig. 1). The treatment plan was discussed with the patient’s parents and proceeded with a semi-rigid bridge SM.

Fig. 1

Preoperative view (1a mandibular occlusal view; 1b lateral view)

Digital data acquisition of the primary teeth involved scanning the plaster cast of the dentition without teeth or dentition defects using the CEREC CAD/CAM system (Sinold, Germany). The resulting files were saved in STL format and imported into CAD software (Exocad DentalCAD, Exocad GmbH Company, Germany). Each scanned primary tooth model was then copied one by one based on its tooth position, and the copied tooth data were trimmed and annotated for the mesial, distal, buccal, and lingual aspects to create separate tooth morphology models.

Digital model:

The CEREC CAD/CAM system was used to obtain digital maxillary and mandibular models of the side with premature loss and occlusion relationship, which were saved in STL format (Fig. 2).

Fig. 2

The digital model (2a mandibular model; 2b maxillary model)

Design of semi-rigid bridge SM: The semi-rigid bridge SM consisted of three parts: the retainer, pontic, and connector. The files in STL format were imported into the CAD software, and the path of insertion was determined.

Retainer design: The retainer had two parts. One end was a rigid connection, and the other end was a non-rigid connection. The retainer for the rigid connection was designed as a full crown or band based on the defect range of the abutment teeth. In this case, the retainer of the rigid connection was designed as a full crown. The placement of the edge line was determined by considering the position of the abutment tooth’s shoulder, pre-aligned tooth morphology from the database, and available bonding space. The occlusion, adjacent relationship, and other variables were also designed. Finally, the occlusal relationship was verified to avoid excessive occlusal contacts (Fig. 3a and b). The non-rigid connection retainer was designed as a two-arm clasp with a thickness of 1.5 mm and a width of 2.5 mm, avoiding the occlusal contact area (Fig. 3d).

Fig. 3

Digital design of semi-rigid bridge SM (full crown) (3a determined edge line; 3b full crown design; 3c pontic design; 3d non-rigid connection retainer design; 3e connector design; 3f check the occlusal relationship

Pontic design: In order to establish the appropriate orders within the software, the primary tooth data obtained by scanning the plaster cast were utilized to select the anatomical missing tooth that corresponds to the losing tooth position. Within the design module of the software, the pontic design should be personalized based on the morphology of the primary tooth and the occlusal relationship. To minimize the burden on the abutment teeth, it is recommended to appropriately reduce the buccolingual width of the pontic and the inclination of the cusp. Additionally, the pontic gingival types should be designed to be suspended with a 2–3 mm gap to the gingiva in the area of the missing tooth to facilitate food passage without accumulation (Fig. 3c).

Connector design: The connector should be positioned in the middle third of the partial occlusal surface, with a cross-sectional area of approximately 4 mm2, to create a normal embrasure and interproximal space (Fig. 3e). After designing all components, it is important to verify the occlusal relationship (Fig. 3f).

When the abutment tooth has no defects, the retainer can be designed as a band. The band’s form should be designed according to the appearance of the abutment and avoid any occlusal contact spots. The thickness should be designed as 1.5 mm, and the width should be designed according to the crown length of the abutment, stopping at a position 1 mm above the gingival margin. Other variables, such as abutment undercut depth of the band and adjacent relationships, should also be designed. The design of other components should be the same as described above (Fig. 4).

Fig. 4

Digital design of semi-rigid bridge SM (band)

Fabrication of semi-rigid bridge SM:

The designed data files of the digital semi-rigid bridge SM were imported into the milling equipment (Ceramiall Matik, Amann Girrbach Company, Germany). The SM should be cut and formed using PEEK (PEEK disc, Sino-Dentex Co., Ltd., China) (Fig. 5).

Fig. 5

PEEK digital semi-rigid bridge SM

Semi-rigid bridge SM try-in and bonding

Tried in the semi-rigid bridge SM and checked the marginal adaptation, retention, occlusion, and the distance between the pontic gingival surface and the gingiva. If any adjustment was necessary during the try-in, we modified the bridge and then polished it again. Finally, the semi-fixed bridge SM was bonded with resin cement (Clearfil Sa Luting Dual Cure Dental Adhesive System; Kuraray Noritake Dental Inc., Okayama, Japan) under a rubber dam (Kerr Optidam; 3-Dimensional Rubber-Dam System, USA) isolation (Fig. 6).

Fig. 6

PEEK semi-rigid bridge SM view (6a mandibular occlusal view; 6b lateral view)

The patient and their parents were duly informed of the critical significance of maintaining proper oral hygiene and adhering to scheduled follow-up appointments. Furthermore, they were counseled on the implementation of the Bath method and encouraged to incorporate dental floss or water floss for effective removal of food debris within the diastemata. The mesiodistal width of the edentulous space was measured immediately after bonding the SM (T0) and then at 1 (T1), 3 (T2), and 6 months (T3) post-treatment. The periodontal condition and the mobility degree of the SM and abutment were also examined. Additionally, the children and parents satisfaction with the aesthetics and masticatory function of the SMs were survied though oral questioning.

Statistical analysis

Statistical analysis was performed using Statistica software for Windows (SPSS 20.0, IBM, USA). Normally distributed measurement data were represented with \(}\)±S. The SNK-q test was used to analyze intragroup comparisons between observation times T0, T1, T2, and T3 for parametric data. P < 0.05 was considered statistically significant.