Introduction

Early childhood caries (ECC) is the most prevalent oral pathology among children globally, impacting both developed and developing nations. This disease transcends mere oral health, potentially influencing craniofacial development and, in severe instances, the overall physical and mental progression of young patients1. The preservation of primary dentition is paramount, not only for mastication but also for maintaining the necessary spacing for permanent teeth, thus necessitating their retention until natural shedding occurs2. The management of carious primary teeth has, therefore, emerged as a critical concern, drawing considerable research focus in recent years3. International guidelines from esteemed pediatric dental associations, such as the American Association of Pediatric Dentistry (AAPD) and the European Association of Pediatric Dentistry (EAPD), advocate for the utilization of prefabricated crowns in managing extensive carious lesions in young patients’ molar teeth4,5. These restorations offer a more durable solution with a reduced risk of recurrent caries and failure when compared to traditional restorative options like composite resins6,7. Furthermore, they are especially beneficial in restoring extensively damaged primary molars, including those affected by traumatic injuries8,9. Consequently, the advancement of pediatric dentistry is closely linked to ongoing research into prefabricated crowns for primary teeth.

Preformed metal crowns (PMCs), such as stainless steel crowns (SSCs) for primary molars, have a longstanding history of clinical use, demonstrating consistently favorable outcomes in terms of treatment efficacy10,11,12. Nevertheless, despite their durability, performance, and cost-effectiveness, PMCs are often criticized for their lack of aesthetic appeal, a factor of considerable importance to both young patients and their parents13,14. Additionally, there are concerns about potential health risks associated with exposure to metals such as nickel and chromium allergies15. Parents are also hesitant to have metal in their children’s mouths due to perceived health risks. Recent innovations, such as open-faced and pre-veneered stainless steel crowns, have been developed in response to the demand for aesthetic improvements. These, however, have been met with challenges as their resin facings are prone to detachment, increasing the risk of plaque accumulation8,16. Zirconia pediatric crowns represent another alternative, boasting superior mechanical strength, resistance to wear and corrosion, and enhanced biocompatibility and aesthetics17,18,19. Yet, their widespread adoption is hindered by high costs and the need for precise technical handling during placement20. Thus, the dental industry is faced with the formidable task of developing prefabricated crowns that encapsulate robust mechanical properties, cost-efficiency, and visual appeal.

In the quest for suitable materials, the dental and medical fields have turned their attention to polymers, with polyether ether ketone (PEEK) emerging as a particularly promising candidate. As a member of the polyaryletherketone (PAEK) family, this colorless thermoplastic polymer is produced through step-growth polymerization from bisphenolate salt dialkylation21. PEEK’s attributes—excellent resistance to creep and wear, chemical stability, affordability, and superior biocompatibility—make it an attractive option for various applications within both biomedical and dental domains22,23,24. Despite the advantageous characteristics of this thermoplastic material, the suboptimal adhesive and shear bonding strengths associated with PEEK’s relatively inert surfaces present considerable obstacles when integrating it into dental clinical research, such as for prefabricated crowns25. To mitigate these limitations, various surface treatment methodologies, including sandblasting, plasma treatment, and acid etching, have been explored26,27,28. Acid etching and plasma treatments mainly function as chemical modifications, altering the surface morphology to a degree and introducing surface-active functional groups that facilitate better interaction with adhesives29,30On the other hand, sandblasting acts as a physical modification strategy, enhancing the mechanical interlock effect, which in turn increases surface roughness, elevates surface free energy, and ultimately improves adhesive properties31.

It is also critical to consider that certain manufacturing techniques may inadvertently introduce defects such as microcracks, potentially altering the structural integrity of the material. These defects can adversely affect the PEEK crowns by diminishing their fatigue resistance, fracture toughness, compressive strength, and other vital mechanical properties32. To date, the literature regarding the application of PEEK preformed crowns in pediatric dentistry, particularly for the restoration of primary teeth, remains scarce.

In this study, we examined the effects of surface treatments on the bonding and mechanical properties of prefabricated PEEK crowns for primary molars. Two surface treatment methods were explored: Al2O3 sandblasting and a combination of sandblasting with acid etching. We measured the shear bond strength of pretreated PEEK when bonded with different types of dental adhesive materials. Additionally, the mechanical performance and tooth preparation requirements of custom-made PEEK pediatric crowns were evaluated and compared to standard prefabricated crowns on the market, including stainless steel and zirconia crowns. This study aims to provide comprehensive experimental evidence for the potential clinical use of PEEK prefabricated crowns for deciduous teeth.

Materials and methods

Materials

The details and characteristics of materials used in this research were summarized in Table 1.

Table 1 List of materials used and their characteristics.
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Surface characterization

The surface and interface characteristics of the PEEK materials, subjected to various treatments such as untreated, sandblasting, and a combination of sandblasting and acid etching, were examined using a Scanning Electron Microscope (SEM, Hitachi S4800, Hitachi, Tokyo, Japan). Additionally, Confocal Laser Scanning Microscopy (CLSM, LSM799 MAT, ZEISS, Oberkochen, Germany) was utilized to assess the surface topography and roughness of different pre-treatment of PEEK materials. For this, a single random specimen from each surface treatment category was scrutinized under a 6144 × 6144 pixels scanning area. The resultant data was methodically analyzed with the aid of MATLAB software.

The hydrophilic and hydrophobic properties of both untreated and treated PEEK were analyzed using a JC2000DM goniometer (Zhongchen Company, Beijing, China) by measuring contact angles (n = 3 per group). The test fluids included deionized water and artificial saliva (Shaanxi Yinuo Biology Technology). A microsyringe ensured consistent droplet volume (0.2 µL) across the experiments. Each group underwent three separate measurements under a controlled temperature of 25 ℃.

Shear bond strength

For the assessment of shear bond strength, PEEK plates measuring 100 mm x 25 mm x 2 mm were precisely cut using equipment from Shenzhen Xiangtong Optoelectronic Technology Co., Ltd, China. Prior to any surface treatment, these specimens underwent ultrasonic cleaning in absolute ethanol for 30 min and were subsequently dried at 60 °C for 3 h. All specimens except for no treatment group were uniformly sandblasted with 110 μm Al2O3 particles under the pressure condition of 0.2 MPa for 10s, followed by rinsing with absolute ethanol and ultra-pure water for 30 min again.

The specimens were divided into four groups, each containing five samples, to undergo four distinct surface treatments: (1) no surface treatment, bonded with RelyX Ultimate Clicker adhesive resin cement (PEEK-A); (2) treated with 98% concentrated sulfuric acid for two minutes33 and bonded with glass ionomer cement (PEEK-B); (3) treated with 98% H2SO4 for two minutes, followed by bonding with Dentex adhesive resin cement (PEEK-C); and (4) treated with 98% H2SO4 for two minutes and bonded with RelyX Ultimate Clicker adhesive resin cement (PEEK-D). The specific distribution and information of each group were demonstrated in Table 2. All groups, except for PEEK-B which did not require light curing, were exposed to an LED curing light (Elipar DeepCure-S, 3 M, USA) for 10 min for complete curing.

Table 2 Groups distribution with all clinical procedures.
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Shear bond strength was quantitatively measured using the Instron 5982 Materials Testing Instrument (INSTRON, USA) at a crosshead displacement rate of 1 mm/min. The shear bond strength calculation was based on the formula σ = F/S, where ‘F’ represents the maximum force before failure, ‘S’ is the adhesive film’s area, and ‘σ’ denotes the shear bond strength. Each experimental group consisted of 5 specimens.

After the shear bond strength test, the failure modes according to the fracture surfaces were classified into three categories: (1) Interface failure, which means that the fracture interface only occurs at the bonding interface between the PEEK plate and the cement, with no adhesive remaining on the surface of the PEEK material; (2) Cohesive failure, which refers that the fracture interface only occurs inside the adhesive, and the surface of the PEEK material is covered by cement.; (3) Mixed failure, which means that the fracture interface is partly located at the bonding interface between the PEEK plate and the cement, and partly located inside the adhesive, with cement residue in some areas of the PEEK surface. Post-testing, the deboned surfaces were analyzed using both a Stereomicroscope (Model 15JE, Shanghai, China) and an SEM (FE-SEM S-4800, Hitachi, Japan) to determine the failure modes of the specimens.

Table 3 Classification of failure modes at interfaces.
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Preparation of pediatric crowns

The prefabricated PEEK crowns, supplied by Xi’an Noble & Novel Dental Technology Co., Ltd., had specific dimensions: an occlusal surface thickness of 0.7 mm, proximal and buccal/lingual surfaces at 0.5 mm, and a margin thickness of 0.2 mm (refer to Fig. 1a). A skilled pediatric dentist consistently performed the tooth preparations. Baseline tooth morphology was initially scanned using Aoralscan 3 (3Shape TRIOS, Denmark). After grouping for preparation and placing the prefabricated pediatric crowns34, digital scans of the prepared teeth were executed (see Fig. 1b). The preparation of the tooth must ensure that the prefabricated crowns fit perfectly, fully seated on the abutment tooth without any rotational movement.

The model data, both pre and post tooth preparation, was analyzed using Geography Wrap 2021 software. This analysis facilitated the precise quantification of tooth preparation, including measurements of occlusal reduction, buccal-lingual reduction, and mesio-distal reduction. Subsequently, the stainless steel crowns were affixed onto the primary abutment molar teeth using glass ionomer cement. In contrast, the zirconia crowns and prepared PEEK prefabricated pediatric crowns were secured using RelyX Ultimate Clicker adhesive. These were then cured with an LED curing light for 1 min, during which any excess cement at the margins of the crowns was meticulously removed. The different prefabricated crowns used in the study are illustrated in Fig. 1c-f.

Fig. 1
figure 1

(a) The dimensions and schematic representation of Prefabricated PEEK crowns. (b) The image of the specimen under digital scanning after tooth preparation. (c-f) The specimens with different prefabricated crowns: (c) Without prefabricated crown; (d) With prefabricated PEEK crown; (e) With prefabricated Zirconia crown and (f) With prefabricated metal crown.

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Compressive performance test

To evaluate the compressive performance of the PEEK primary molar prefabricated crowns, a universal testing machine was utilized. The cemented crowns were segregated into three groups - metal, zirconia, and PEEK, with five crowns for each group. Drawing from previous research35, a vertical load was applied using a 4 mm diameter stainless steel ball that was aligned at the center of the crowns’ occlusal surfaces. The experiment proceeded at room temperature (T = 25℃) until fracture, employing a crosshead displacement rate of 0.5 mm/min and pressure from an 8 mm steel spindle head (Fig. 2a). Fracturing was denoted by observable cracks, audible events, or noticeable load reductions in the stress-strain diagram. All data were documented, with the highest force value recorded before fracture denoting the breaking load to intuitively illustrate the specimens’ compressive performance.

Fig. 2
figure 2

(a) The universal testing machine used to test the compressive performance. (b) M-500 electric test which was used to conduct the fatigue measurement. c Stress loading head of the fatigue testing machine.

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Fatigue performance test

For the fatigue test, 15 specimens, each sported with three different crowns (n= 5 per group), were put under test with an oral fatigue wear testing machine (M-500 electric test system, Care, Tianjin, China). Maximizing the load at 100 N allowed for 160,000 cycles, simulating the expected wear and tear on prefab crowns from a child’s one-year chewing regime36. The fatigue frequency was set at 1.2 Hz to mimic children’s typical chewing rhythm. To generate natural occlusion features, fatigue tests were undertaken 0.5 mm lingual to the cusp tip and adjusted 0.7 mm lingually along the dentofacial cusp via a 4 mm diameter loading head. After completing the loading cycle steps, the specimens were analyzed on the universal testing machine to identify the maximum breaking load, following the previously described procedure. The fatigue performance of PEEK deciduous prefab crowns was compared against the other two material groups. The configuration of the compressive and fatigue performance tests is illustrated in Fig. 2b and c. The fractured surfaces’ morphology was examined using Scanning Electron Microscopy (SEM).

Statistical analysis

Data were analyzed using GraphPad Prism software (v9.0, GraphPad, USA). The Shapiro-Wilk test assessed statistical normal distribution, while the Levene test evaluated the equality of variances. A One-way ANOVA determined the shear bond strength’s statistical significance among the four groups (PEEK-A, PEEK-B, PEEK-C, PEEK-D). Comparisons of fracture loads before and after fatigue testing of the prefabricated crowns employed Two-way ANOVA for paired or repeated measures data within the software. A p-value of less than 0.05 was set as the threshold for statistical significance in all tests.

Results

Surface analysis of PEEK with different treatments

Surface morphology and roughness

SEM micrographs showcasing the surface morphology of untreated PEEK, PEEK post-sandblasting, and PEEK following sandblasting and concentrated H2SO4 treatment are presented in Fig. 3. The surface of untreated PEEK specimens, typically produced by conventional injection molding, was relatively smooth, as shown in Fig. 3a, d. After undergoing Al2O3 particle sandblasting, the specimens exhibited minor holes and irregularities (Fig. 3b, e). The subsequent concentrated sulfuric acid treatment further enhanced surface irregularity, notably increasing the porosity of the specimens (Fig. 3c, f).

The surface roughness and 3D micromorphology, measured by CLSM and illustrated in Fig. 3g-i, revealed that untreated PEEK had the smoothest surface with a Sa value of 1.60 μm. In contrast, the sandblasted specimens demonstrated the highest roughness (Sa = 11.00 μm). The sandblasted PEEK samples treated with 98% H2SO4 showed a significant reduction in surface roughness to Sa = 6.10 μm, despite the acid etching creating a density of pores on the surface. The influence of PEEK surface morphology and roughness on adhesion and shear bond strength is further explored in Sect. 3.2.

Fig. 3
figure 3

SEM and three-dimensional CLSM images of the surface morphology and roughness. (a, d) untreated PEEK under SEM; (b, e) PEEK with sandblasting under SEM; (c, f) PEEK with sandblasting followed by concentrated H2SO4 treatment under SEM. (g-i) untreated PEEK, PEEK with sandblasting and PEEK with sandblasting followed by concentrated H2SO4 treatment under CLSM.

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Contact angle

Contact angle measurements were conducted to evaluate the hydrophilicity and hydrophobicity of PEEK materials following three distinct pretreatments. Deionized water and artificial saliva served as the test liquids for assessing surface wettability. The contact angle, a key indicator of wettability, inversely correlates with hydrophilicity; a lower angle signifies greater hydrophilicity. As shown in Fig. 4a-c, untreated PEEK had a water contact angle of 76.53°±2.47°. This angle reduced to 66.73°±3.33° for sandblasted PEEK. When treated with concentrated sulfuric acid, the contact angle further decreased to approximately 57.43°. These results demonstrate that both sandblasting and acid etching effectively enhance the hydrophilicity of the PEEK surface, with their combined use yielding a more substantial effect (Fig. 4g, h). The contact angle measurements with artificial saliva mirrored this trend across the differently treated PEEK variants (Fig. 4d-f). This suggests that the combined approach of sandblasting and concentrated H2SO4 treatment optimizes the PEEK surface for environmental adaptability.

Fig. 4
figure 4

The surface wettability of PEEK specimens before and after sandblasting and acid etching treatments. (a-c) Water contact angle; (d-f) Artificial saliva contact angle, respectively; (g, h) Water contact angle and Artificial saliva contact angle of three pretreatments. Values have been expressed as mean ± SD (*, P ≤ 0.05; **, P ≤ 0.01; ***, P ≤ 0.001; one-way ANOVA).

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Shear bond strength

The mean shear bond strength of PEEK with two distinct surface treatments, using three different luting cements as adhesives, is displayed in Fig. 5a. Specimens subjected solely to sandblasting (PEEK-A) exhibited the lowest shear bond strength, approximately 0.872 ± 0.3 MPa. The shear bond strength notably improved following combined treatments of sandblasting and 98% concentrated H2SO4. It increased to 1.72 ± 0.2 MPa, 2.27 ± 0.7 MPa, and 4.25 ± 0.9 MPa with the use of glass ionomer cement (PEEK-B), Dentex adhesive resin cement (PEEK-C), and RelyX Ultimate Clicker adhesive resin cement (PEEK-D), respectively. This enhancement underscores the positive impact of acid etching with post-sandblasting.

Fig. 5
figure 5

(a) The shear bond strength of PEEK-A, PEEK-B, PEEK-C and PEEK-D; (b) the stereomicroscope images of PEEK-A, PEEK-B, PEEK-C and PEEK-D, the yellow line indicates the boundary between the fracture; (c-f) The SEM images of fractured surface in PEEK-A, PEEK-B, PEEK-C, PEEK-D, respectively. Values have been expressed as mean ± SD (*, P ≤ 0.05; **, P ≤ 0.01; ***, P ≤ 0.001; ****, P ≤ 0.0001, one-way ANOVA).

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Fracture mode analysis was conducted on four different PEEK specimens to assess shear bond strength. Which involved both interface and bulk of the PEEK samples, as summarized in Table 3. It was observed that the majority of specimens exhibited fractures predominantly in the resin adhesive layer by utilizing the stereomicroscope (Fig. 5b). Furthermore, the SEM results provided detailed imaging of the fractured surfaces across variously treated PEEK samples, as depicted in Fig. 5c-f. The analysis revealed that, apart from PEEK-B specimens which predominantly experienced interfacial fractures, the other three groups exhibited mixed-mode fractures (Table 4). This indicates less effective bonding ability and mechanical interlocking of glass ionomers with PEEK materials compared to resin adhesive cements. Consequently, for PEEK prefabricated crowns, resin-based adhesives, especially RelyX Ultimate Clicker adhesive resin cement, are recommended for clinical dentistry applications.

Table 4 Adhesive failure modes after different surface treatments (unit = numbers).
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Compressive strength and fatigue property

Our study reveals that while the compressive strength of prefabricated PEEK crowns is lower than that of PMC, they exhibit better compressive and fatigue properties comparable to zirconia crowns (Fig. 6a, b). The failure mode and fracture surface for each group are illustrated in Fig. 6c-e.

Fig. 6
figure 6

(a, b) Compressive and fatigue strength of 3 different prefabricated pediatric crowns. (c-e) The failure mode and fracture surface of three different kinds of prefabricated pediatric crowns: (c) Prefabricated PEEK crown; (d) Zirconia crown and (e) Performed metal crown.

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Volum of tooth preparation

The volume of tooth preparation was analyzed using Geography Wrap 2021 software, which processed digital scans taken before and after tooth preparation (Fig. 7a). The study revealed a significant reduction in the extent of dental abutment preparation needed for PEEK preformed crowns compared to zirconia preformed crowns across all surfaces. Notably, when compared with the preparation required for metal preformed crowns, no significant differences were observed on the occlusal and mesial-distal proximal surfaces. However, the preparations on the buccal-lingual surfaces showed a marked difference (Fig. 7b).

Fig. 7
figure 7

(a)The volume of tooth preparation was analyzed by the Geography Wrap 2021 software. (b) Tooth preparation volume of occlusal, buccal-lingual and mesio-distal. Values are reported as mean ± SD (*, P ≤ 0.05; **, P ≤ 0.01; ***, P ≤ 0.001; ****, P ≤ 0.0001, two-way ANOVA).

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Discussion

The rising standard of living and advancements in biomaterial technology have heightened the demand for aesthetic dental restorations in pediatric dentistry, both among children and their parents. Preformed zirconia crowns, in recent years, have emerged as a reliable choice for aesthetic restorations in deciduous teeth, as evidenced by various studies10,34,37. Despite their effectiveness, the high cost of zirconia crowns has limited their widespread adoption, particularly in economically disadvantaged countries and regions. Moreover, zirconia crowns are known for their technique sensitivity, which can lead to issues such as crown fractures or bonding failures if not properly handled38. In contrast, PEEK (Polyetheretherketone) offers an attractive alternative as an aesthetic restoration material, characterized by its relative affordability. However, the potential of PEEK prefabricated crowns in the restoration of dental defects in deciduous teeth warrants further exploration.

One of the challenges with PEEK is its low surface energy, which complicates achieving effective chemical bonding with veneering composites22. Acid etching and plasma treatments are the most prevalent chemical methods used to introduce surface active functional groups, thereby facilitating efficient adhesive interactions29,30. On the other hand, sandblasting serves as a physical modification technique. It enhances surface bonding by creating a mechanical interlocking effect, ultimately improving adhesion31.

In this research, the surface texture of PEEK was altered using both physical and chemical methods. Observation revealed that PEEK surfaces treated with sandblasting alone developed prominent uneven structures. However, when sandblasting was combined with concentrated sulfuric acid, the irregularities were somewhat mitigated, resulting in the formation of circular micro-pores on the surface layer. Analytical results from Confocal Laser Scanning Microscopy (CLSM) and Scanning Electron Microscopy (SEM) indicated that the sandblasted samples exhibited the highest surface roughness (Sa = 11.00 μm), whereas untreated PEEK had the smoothest surface. The group treated with sandblasting and sulfuric acid displayed an intermediate level of roughness. The adhesion of resin to PEEK primarily depends on the resin’s ability to infiltrate these micro-pores, thereby creating a mechanical interlocking effect39. Furthermore, the water contact angles of sandblasted + H2SO4 dramatically decreased compared to that of the group with only sandblasting treatment, which means that the combination of both has a greater impact on the hydrophilicity of PEEK specimens. In addition, the artificial saliva contact angle demonstrated a similar trend on untreated PEEK and PEEK materials with different pretreatments (Fig. 4), which means that concentrated H2SO4treatment with sandblasting was also the most effective modification for the surface wettability of PEEK materials under an environment that simulates the human oral cavity. This study aligns with prior findings, demonstrating a consistent decrease in contact angle with an increase in surface roughness and mechanical interlocking properties, and the improvement of hydrophilicity will further enhance the shear bonding strength of the material40. Sandblasting was particularly effective in augmenting surface roughness, while sulfuric acid treatment promoted the creation of micro-pores, enhancing the mechanical anchoring of adhesives40,41. Hence, the acid etching pretreatment after sandblasting has a critical significance on prefabricated PEEK crowns for clinical application.

Additionally, RelyX Ultimate Clicker resin cement emerged as a superior adhesive, attributed to its dual-curing properties that bolster mechanical interlocking. Intriguingly, SEM and CLSM observations, along with shear bond strength data, suggest that the adhesive efficacy of PEEK post-treatment is not solely reliant on surface roughness. Despite the highest roughness observed in the sandblasted PEEK samples, their adhesive properties were the weakest among all groups (PEEK-A), indicating that PEEK materials treated solely with sandblasting are prone to debonding and thus, unsuitable for primary prefabricated crowns.

Contrastingly, the application of RelyX Ultimate Clicker adhesive resin cement significantly improved the shear bond strength of PEEK samples subjected to sandblasting followed by concentrated sulfuric acid etching (PEEK-D). Specifically, there was an impressive 387.4% increase in bond strength compared to the samples treated solely with sandblasting (PEEK-A). This enhancement can be attributed to the fact that, despite a reduction in surface roughness post-acid etching, the creation of numerous pores on the surface (as depicted in Fig. 3) enlarged the contact area with the adhesive. Consequently, the resin adhesive was able to penetrate these pores more effectively, thereby substantially bolstering the mechanical interlocking between PEEK and the adhesive resin cement. This resulted in a marked improvement in shear bond strength. Given the critical importance of shear bond strength in clinical settings, the combination of PEEK treated with sandblasting followed by concentrated H2SO4 etching, along with RelyX Ultimate Clicker adhesive resin cement, is highly significant for the application of prefabricated crowns in pediatric dentistry.

Advancing this approach, we developed PEEK preformed crowns for the maxillary first deciduous molar. Considering the thickness requirements of a deciduous tooth and the necessary restoration strength, the occlusal surface of these PEEK prefabricated crowns was designed with a maximum thickness of 0.7 mm. The buccal, lingual, and proximal surfaces were set at a thickness of 0.5 mm to balance durability and fit. To ensure a seamless integration with the tooth, the margin thickness was meticulously calibrated to a minimal 0.2 mm. For comparative purposes, standard resin crowns were prepared on the abutment teeth, followed by restoration using metal, zirconia, and PEEK prefabricated crowns. To comprehensively evaluate the tooth preparation volume, we performed intraoral digital scans before and subsequent to the preparation of each type of prefabricated crown. Intriguingly, our findings indicated that despite the PEEK crown’s thicker profile compared to the zirconia crown, the volume of tooth preparation required for the PEEK crown was markedly lower. This phenomenon is predominantly due to the unique properties of PEEK material, which possesses a lower elastic modulus and higher flexural strength. This allows for a marginally greater deformation at the crown’s margin upon seating, unlike zirconia crowns where margin stress can lead to fracture due to their rigidity34. Considering the typically broader pulp chamber and elevated pulp horn in primary teeth, minimizing tooth preparation is advantageous in pediatric dentistry, as it reduces the risk of inadvertent pulp exposure during abutment preparation.

Our research also highlighted that although the compressive strength of the PEEK crown is lower than that of the preformed metal crown (PMC), it performs better in terms of compressive and fatigue strengths compared to the zirconia crown. Notably, the maximum bite force exerted by children on deciduous molars during normal chewing activities is around 380 MPa, a pressure well within the PEEK crown’s capacity to withstand, as referenced in our study42. Furthermore, multiple clinical studies have corroborated the adequacy of zirconia crowns for pediatric patients, thereby underscoring the suitability of PEEK crowns for clinical use3738. While PMCs have been a staple in clinical practice for nearly three decades, their susceptibility to long-term restoration failure due to wear has been documented43. Our study revealed a contrast; despite stainless steel crowns possessing high compressive strength, they tend to lose strength significantly under fatigue loading. In contrast, both PEEK and zirconia crowns maintained their strength post-fatigue loading, demonstrating their durability and reliability in clinical applications. In brief, considering various specific factors in pediatric prefabricated crowns, PEEK prefabricated crown is a good selection among plenty of alternatives. Firstly, the performed metal crowns have already triggered unsatisfactions in the group of potential patients due to their poor aesthetic performance, even though they are equipped with greater mechanical properties. As for all-ceramic prefabricated crowns, both the compressive and fatigue performance are more deficient than that of PEEK crowns, as well as the toughness and flexibility, which will lead to more abutment preparation and the waste of resources in the manufacturing process. Given the outstanding wear resistance, low elastic modulus, and high fracture resistance of PEEK, it is plausible to consider the PEEK crown as a highly promising option for aesthetic restoration in deciduous teeth.

However, this study is still equipped with several limitations for clinical applications. The toxicity and corrosion of concentrated sulfuric acid indicate potential security risks for clinicians and researchers, which require more stringent safeguards such as protective eyewear in operation44. Furthermore, the research also lacked an analysis of aesthetics in the former study, which is one of the most crucial factors that should be considered in the design of pediatric prefabricated crowns19. Therefore, it is significant for us to explore alternative methods for acid etching to enhance the surface roughness of PEEK materials and conduct further experiments on aesthetics in the follow-up studies.

Conclusions

With the limitations of current research, the following conclusions were drawn under the experimental conditions:

  • It is proven that the surface roughness and hydrophilic properties of PEEK material have a positive correlation with shear bond strength. After Al2O3 sandblasting and 98% concentrated sulfuric acid treatment, the adhesion between PEEK and cement enhanced significantly, which provides a prerequisite for the clinical application of pediatric prefabricated crowns.

  • When compared to other commercial prefabricated crowns available on the market, prefabricated PEEK crowns demonstrate greater compressive and fatigue strengths than zirconia crowns, while requiring remarkably less dental abutment preparation.

  • Considering the aesthetic factors and according to the robust experimental support in this study, the prefabricated PEEK crowns should be another choice in the restorations of primary teeth in children over the performed metal and zirconia crowns.