Conventional versus CAD/CAM unilateral space maintainers: a fracture resistance test

Introduction

Primary teeth play an important part in directing the eruption of permanent teeth [1]. Premature extrac­tion of deciduous teeth, which is a common consequence of untreated dental caries or inadequate treatment, can lead to migration of adjacent teeth mesially, resulting in loss of space for a permanent tooth, loss of arch integ­rity, dental misalignment characterized by crowded teeth, unerupted permanent teeth, and overeruption of opposing teeth [2].
The most certain way to prevent future malocclusions due to early tooth loss is to place a space maintainer (SM) to, as the name suggests, maintain a space until successive eruption of permanent teeth [2]. There are different types of SMs, i.e., unilateral, bilateral, fixed, or removable, each presenting its own manufacturing procedure as well as advantages and disadvantages [1, 3]. The choice of appliance relies on multiple considera­tions, such as the extent of tooth loss and status of exi­sting teeth, type of occlusion, patient’s age, patient’s cooperation, parental motivation, dexterity, and practitioner’s preferences. Of the various fixed SMs, band and loop SM is the most frequently used for premature loss of a single primary molar.
However, weld failure, caries formation along band margins, and long manufacturing time, are some of the drawbacks associated [2]. Current SM manufacturing methods have certain limitations, hence the interest in moving towards computer-aided design and manu­facturing (CAD/CAM). This technique, used since the 1980s, is now increasingly common in dentistry, particularly in the field of cosmetic dentistry. As an example, endocrowns are performed using the CAD/CAM technique [4]. Studies have shown that the stability and durability of a material are more important when a material is used with the CAD/CAM technique, rather than with traditional techniques [5]. Digital technology has also made it possible to reduce human errors by automating the manufacturing process of prostheses and dental appliances [2, 5].
In this context, one of the challenges for a dentist these days is the transition adopting a digital workflow, and incorporating emerging technologies and equipment into daily clinical practice, which enable dentists to enhance creativity and deliver treatments that are more predictable, minimally invasive, and cost-efficient over time [6]. At present, digital design and rapid prototyping techniques are commonly applied in the design of metal frameworks for removable partial dentures (RPDs), either by creating resin prototypes or by directly fabricating metal structures. A study by Guo et al. [5] developed a digital method for creating partial acrylic prostheses for children using 3Shape® software, comparing polyetheretherketone (PEEK) and conventional manufacturing techniques. Qualitative assessments revealed no significant differences between the two groups, but quantitative analysis showed that PEEK spacers had significantly smaller mean gaps (44.32 ± 1.75 μm) compared with conventional spacers (137.36 ± 18.63 μm), indicating superior fit. These findings suggest that digitally designed PEEK spacers using CAD/CAM offer enhanced precision and are well-suited for clinical applications in pediatric prostho­dontics.
To our knowledge, during the last few years, pilot studies and clinical cases have been carried out on space maintainers made by CAD/CAM. Several shapes and materials were mentioned.

Objectives

The aim of this study was to compare two kinds of unilateral space maintainers in terms of resistance to fracture: the conventional metallic spacer made from chrome cobalt alloy and a new resin material used for spacer, Saremco Print Crowntec composite resin shade A2, manu­factured by addition (3D printing) of CAD/CAM.

Material and methods

A total of 40 spacers were included in the study: 20 were fabricated from chrome cobalt alloy and 20 were made from a CAD/CAM resin. Spacers were manufactured according to standardized specifications and were designed to fit the required dimensions for unilateral space maintenance. They were divided into four groups: 1) upper left (UL), 2) upper right (UR), 3) lower left (LL), and 4) lower right (LR).
Conventional space maintainers were fabricated directly on stone models after taking conventional impression. The size of a ring was chosen according to the size of a tooth, while the thickness of a loop was set to 0.9 mm and welded to the stainless steel 3M band by a lab technician (Figure 1) [6].
Resin space maintainers were obtained by additive CAD/CAM method using the Saremco Print Crowntec (Saremco-Crowntec Dental AG, Switzerland) composite resin shade A2, made on a base of methacrylic acid ester; they were designed virtually and then printed [7]. The first step was to obtain the study model, on which the design and manufacture of various unilateral space maintainers were made. The second step was designing the innovative maintainers using a design software, and then printing them in resin (Figure 2).
• Step 1: optical impression (iTero) and obtaining the virtual model on a computer (Ceramill Mind software).
• Step 2: the virtual model was instantly integrated into the CAD software system (computer-aided design from 3Shape appliance designer software), in order to manipulate the numerical data and acquire a model of the spacer. By using zoom, rotation, and panning functions, the model could be examined from various perspectives and magnifications, facilitating analysis and design of a customized device. This technology supports the development of such devices, and enables precise evaluation of multiple factors, including material thickness, retention, and undercuts case gap.
• Step 3: the file was transferred to begin the computer-aided manufacturing. This was an addition manu­facturing method. The Saremco composite resin was employed with a A2 shade, and the spacers were printed using Rapident 3D printer (Figures 3 and 4).
After obtaining 20 resin space maintainers and 20 conventional ones, they were glued onto resin models with Choice 2 light-cure resin cement (Variolink Esthetic) using Ivoclar Vivadent glue (Figures 5A-C). The conventional space maintenance was mounted on the model as follows:
• try in;
• restoration and resin model etching with 37% phos­phoric acid for 30 seconds;
• bonding, rubbing for 20 seconds without light-curing;
• cementation by applying Variolink Esthetic LC directly into the restoration;
• removing the excess with micro-brush;
• light curing for 40 seconds.
Band parts of spacers were cemented on the second primary molars of models with missing first primary molars, while loops were holding on the adjacent canines.
All tests were carried out at the Biomaterials Laboratory of the Campus of Innovation and Sport of the Saint Joseph University of Beirut. Universal testing machine was used to assess the resistance to fracture (Figure 5). Head speed was set to 1 mm per minute [8]. Each spacer was mounted onto the universal testing machine, and a force was gradually applied until a fracture or bending occurred. The force at the point of fracture or bending was recorded for each spacer (Figures 6-8).

Statistical analysis

Data were analyzed using IBM SPSS Statistics for Windows, version 26 (IBM Corp., Armonk, NY, USA). Descriptive statistics of fracture resistance and flexure resistance (N) were calculated and presented as means ± standard deviations (SD), medians (interquartile ranges), and minimum/maximum values. Shapiro-Wilk test was applied to evaluate normality of distribution of the quantitative variables. Since fracture resistance was not normally distributed, Mann-Whitney U test was used to compare values between two different designs (maxillary and mandibular) within the resin material. In contrast, flexure resistance was normally distributed, and independent Student’s t-test was employed to compare means between the two different designs within the metal material. To compare medians and means with theoretical value (289 N), which represented the maxi­mum occlusal force in early mixed stage, one-sample t-test and Wilcoxon signed-rank tests were employed [9, 10]. Level of significance was set at 5%, and all tests were two-tailed.

Results

As mentioned earlier, the space maintainers were divided into four groups: UL, UR, LL, and LR. The resistance to fracture of the resin-manufactured space maintainers ranged between 256 N to 714 N for the lower right spacer and the upper left spacer, respectively. The mean resistance to fracture was 395.5 (± 135.03 N). The resistance to fracture of the metallic space maintainer ranged between 1,279 N and 1,949 N for the lower left spacer and the upper right spacer, respectively. The mean resistance to fracture was 1,579.35 (± 218.21 N). The lower space maintai­ners (resin and metallic) showed less resistance to fracture compared with the upper space maintainers (Table 1).
• Maxillary resin space maintainers demonstrated significantly higher fracture resistance compared with mandibular ones (p < 0.001) (Tables 2 and 3).
• Maxillary metal space maintainers presented significantly higher flexure resistance compared with mandibular ones (p < 0.001).
• Resin space maintainers had significantly higher fracture resistance compared with the theoretical value 289 N (p = 0.002).
• Maxillary resin space maintainers showed significantly higher fracture resistance compared with the theoretical value 289 N (p = 0.005).
• The fracture resistance of mandibular resin space maintainers was not statistically greater than 289 N (p = 0.506) (Table 4).
• Metal space maintainers had significantly higher fracture resistance compared with the theoretical value 289 N (p < 0.001).
• Maxillary metal space maintainers showed significantly higher fracture resistance compared with the theoretical value 289 N (p < 0.001).
• Maxillary metal space maintainers had significantly higher fracture resistance compared with the theoretical value 289 N (p < 0.001).

Discussion

The aim of this study was to focus on a new resin material (Saremco) used for the first time for the fabrication of a unilateral space maintainer by additive CAD/CAM method. Also, a comparison between two kinds of unilateral space maintainers was carried out: the conventional metallic spacer and the resin spacer manufactured by additive CAD/CAM. The resistance to fracture was investigated for the two types of space maintainers.
Unilateral space maintainers are widely used in pediatric dentistry to preserve the space of a permanent tooth when the respective baby tooth has undergone an early extraction. Conventional metallic space maintainers are usually made of stainless steel or nickel-titanium alloys. They are known for their stiffness and durability, and are found to be more resistant to fracture than CAD/CAM-printed resin space maintainers. However, nowadays, the demand for esthetic restorations in children is no longer limited to the anterior teeth exclusively. Patients and their parents are increasingly expecting improved posterior esthetic options both in temporary and permanent dentition, which can apply to any treatment involving metal: from crowns to space maintainers. Nevertheless, aesthetics are not the only criteria for comparison, especially when such appliances aim to support chewing forces. Therefore, our research was carried out to compare the mean fracture forces of esthetic resin SMs with the mean masticatory force in children to assess their clinical potential.
The findings of our study provide important insights into the mechanical performance of unilateral resin and metallic space maintainers. The metallic spacers demonstrated remarkable resistance to fracture, with none of them breaking under the applied force. These results agree with Kulkarni et al. [11], who also noticed that the spacers exhibited bending, indicating their potential for deformation under high loads. This bending beha­vior may have practical implications, as it may lead to patient’s discomfort or injury if not managed properly. Another systematic review on the survival rate of bonded space maintainers by Deshpande et al. [12] reported that the bonded space maintainer’s average survival rate was 11.2 months, but more studies were needed to reach a higher level of evidence, leaving this systematic review inconclusive. Therefore, bonded space maintainers demonstrated a life-span similar to that of banded space maintainers, suggesting they can serve as a viable alternative in pediatric dental care [12].
On the other hand, the resin spacers displayed a wider range of resistance to fracture, with some spacers breaking at lower forces. The variation in fracture resistance may be attributed to differences in material composition, manufacturing techniques, or designs of spacers depending on its location in the oral cavity, since the same material was used to manufacture the entire studied sample. Similar results were found by Kulkarni et al. [11], Beldiman et al. [13], and Souror et al. [14].
The range of occlusal bite forces in children varies across different dentition stages: early primary dentition averages around 176 N, late primary dentition appro­ximately 240 N, early mixed dentition 289 N, and late mixed dentition up to 433 N [10, 15, 16]. These values are significant in evaluating the performance of dental appliances; for instance, a resin-fabricated space maintainer with a fracture resistance of 395.5 N can withstand masticatory forces in early mixed dentition, while metallic spacers with a bending force of 1,579 N offer substantial durability across all stages.
The flexural strength and resistance to fracture not only depend on the composition of resin, but also on the preprinting, printing, and postprinting technique [20]. According to a systematic review conducted by Gad et al. [20], in the preprinting phase, the addition of filler or nanofiller enhances the strength of 3D-printed resins. During the printing phase, different studies showed increased strength with decreased layering thickness, but with contradictory results regarding printing orientation, whether 0°, 45°, or 90° was considered the best choice. In the postprinting phase, the results obtained by Bayarsaikhan et al. [21] showed that water storage reduced the flexural strength, whereas increasing postcuring time and temperature could increase the flexural strength. Nevertheless, further studies combining these factors are recommended.
The current study’s design holds significant clinical relevance, as it assesses the fracture resistance of space maintainers fabricated from various 3D printable materials, such as metal and resin, after simulating oral conditions through thermal aging. This approach reflects real-life scenarios, where space maintainers are subjected to thermal fluctuations and mechanical stresses in the oral cavity, providing insights into their durability and performance in pediatric patients. While it is anticipated that metal space maintainers exhibit higher fracture resistance due to their inherent material properties, the rationale for testing lies in evaluating whether alternative materials, such as resin, can offer sufficient strength to withstand masticatory forces in children. This is crucial because, despite their lower fracture resistance compared with metal, resin space maintainers offer advantages, such as improved aesthetics and potentially better patient acceptance.
In the study, only Crowntec Saremco resin was tested. This resin contains Bis-EMA trimethylbenzoyldiphenylphosphine oxide, a monomer lacking hydroxyl groups, which reduces the formation of strong intermolecular hydrogen bonds between polymer chains [17-19]. This structural characteristic can lead to decreased flexural strength, potentially compromising the material’s durability under masticatory forces [17, 19]. Hata­mleh et al. [19] demonstrated that resin mixed with a 70% concentration of Bis-GMA containing hydroxyl groups, exhibit higher flexural strength due to enhanced hydrogen bonding and a more rigid core structure. Therefore, testing resins with higher Bis-GMA content, such as polymethylmethacrylate mixtures, can provide improved fracture resistance. Additionally, other studies have shown that permanent bridge resin (PB) (not commercially available; Saremco, Dental AG, Rebstein, Switzerland) possesses superior mechanical properties compared with Crowntec Saremco and other resin materials, making it a promising candidate for 3D-printed space maintainers [17]. Therefore, it might be interesting to examine other resin mixtures for 3D printing, which can show higher mechanical properties, such as PB resin, temporary crown and bridge (Formlabs), acrylate photopolymer bis-acrylic, Enlighten AA temp, NextDent, DentaBASE, 3D-printed composite resin from VarseoSmile Crown Plus, etc. [17, 18].
The comparison of fracture resistance between metallic and resin based appliances goes beyond space maintainers. A study conducted by Abo-Elsoud et al. [22] comparing restorative crowns for temporary teeth, showed promising results for resin-based biomaterials in terms of fracture resistance, where Bioflx crowns, which are resin based crowns, demonstrated greater results than zirconia crowns, while stainless steel crowns had the highest fracture resistance. Stainless steel and Bioflx crowns presented deformation of the occlusal surface and microperforations, while zirconia crowns exhibited fracture lines.
Resin based biomaterials show promising results. Biocompatible hybrid resin polymer present in appliances have more flexibility and better elasticity, and is designed to improve ductility, adaptability, and durability when compared with the more fragile esthetic zirconia [22, 23].
This study has some limitations. The Crowntec Saremco resin was exclusively tested, which restricts generalizability of the findings. Also, our study as in vitro in nature have some constraints: the research was conducted under controlled laboratory conditions, which may not accurately replicate the complex environment of the oral cavity. Factors, such as temperature fluctuations, pH variations, salivary enzymes, microbial activity, and masticatory forces, can significantly influence the performance and degradation of dental materials over time. Finally, the study did not assess the long-term effects of aging on the mechanical properties of the tested resin. Aging processes, including thermal cycling, can impact the durability and performance of dental materials.
3D-printed appliances have become popular with the emergence of new technologies, and are becoming more accessible and affordable for clinicians with a variety of resin biomaterials available [17]. Clinically, the significance of these results is profound. The study demonstrated that all tested materials showed fracture resistance values exceeding the average masticatory forces in children, suggesting their suitability for clinical use. However, it is worth noting that the resistance to fracture is just one aspect to consider when selecting a space maintainer. This opens avenues for personalized treatment plans, where material selection should be tailored based on individual patient’s needs and balancing factors, such as strength, aesthetics, and comfort. Other aspects, including biocompatibility, variety of materials, aesthetics, ease of placement, and patient compliance, should also be taken into account. Therefore, a comprehensive evaluation of the benefits and limitations of each material is necessary to informed clinical decision-making, and further studies are needed to obtain a conclusive statement on this matter.

Conclusions

Despite the limitations of this study, we can conclude that 3D-printed unilateral space maintainers made from resin may still be a suitable choice for cases, where aesthetics are of primary concern and occlusal loads are relatively low, thereby expanding treatment options in pediatric dentistry. In vitro studies provide a promising foundation for innovation in this field. However, future clinical trials are essential to confirm the efficacy of these devices in real-world settings and diverse clinical contexts. Additionally, the selection of space maintainers should consider the actual masticatory forces of young patients, esthetic factors, and patient and parental pre­ferences. Patient follow-up and parental education remain crucial for the success of treatment. Further studies are needed with different types of resins to compare and evaluate the best kind of material to fabricate an esthetic and resistant space maintainer.

Disclosures

1. Institutional review board statement: This study was approved by the Ethics Committee of the Saint Joseph University of Beirut (ref. No. USJ-2023-179).
2. Assistance with the article: None.
3. Financial support and sponsorship: None.
4. Conflicts of interest: None.

References