Evaluation of a novel fixed-space maintainer made of light-cured acrylic resin: an in vitro study

Premature loss of primary molars can cause tooth movement, space loss, and subsequent malocclusions. Space maintainers preserve arch integrity; removable types are easy to fabricate but prone to loss, while fixed types like band-and-loop are effective yet have drawbacks (cement failure, solder breakage, caries risk, unaesthetic metal appearance, and lengthy fabrication). Fiber-reinforced composites offer better esthetics but may lack rigidity compared to band-and-loop. Light-cured acrylic resins (LCAR) are widely used in dentistry for trays, temporaries, and orthodontic appliances. Fundamental in vitro metrics for clinical acceptance include flexural strength (resistance to bending) and shear bond strength (adhesion to enamel). The study aimed to evaluate a novel fixed-space maintainer made of LCAR with respect to flexural strength and shear bond strength when bonded to enamel using different adhesive systems.

The introduction reviews consequences of early primary molar loss on arch space and malocclusions, highlighting greater mandibular space loss and significant losses following early second primary molar loss. Fixed space maintainers like band-and-loop have high success but mechanical and caries-related limitations and are unaesthetic. Fiber-reinforced composites provide esthetics yet are more flexible and less rigid than metal. LCAR materials are established in various dental applications. Adequate flexural strength is needed to withstand masticatory stresses, and high bond strength is required to resist polymerization and functional stresses at the tooth-restoration interface. Prior studies cited indicate Transbond XT often yields higher shear bond strengths compared to flowable composites and resin-modified glass ionomers (RMGICs), and that acceptable orthodontic bond strengths range roughly from 6–8 MPa for clinical use while avoiding excessively high values that risk enamel damage.

Design: In vitro study assessing shear bond strength of LCAR to enamel using three adhesive systems and evaluating LCAR flexural strength. Sample size was based on previous similar studies. Shear bond strength testing:

• Specimens: 45 extracted lower second primary molars with intact buccal/lingual surfaces; no decay, cracks, or defects. Cleaned with tap water and soft brush (1 min), examined under stereomicroscope for defects, pumice-cleaned, stored in 0.5% chloramine-T at 4 °C for 24 h.
• Mounting and surface preparation: Roots removed with diamond saw; crowns mounted horizontally in cold-cured acrylic cylinders exposing facial surface. Buccal enamel superficially prepared with diamond fissure bur under water spray for a smooth surface.
• Grouping (n=15/group) by bonding system after 37% phosphoric acid etching (15 s) and rinse (15 s): • Group 1 (Transbond XT): Moist enamel; two layers of Adper Single Bond 2 applied and light-cured 20 s; thin coat of Transbond XT paste applied and light-cured 40 s. • Group 2 (Tetric Flow): Same etch/adhesive protocol; thin layer of Tetric Flow flowable composite applied and light-cured 40 s. • Group 3 (Fuji Ortho LC): After etch/rinse leaving enamel wet, Fuji Ortho LC RMGIC triturated 10 s, dispensed via capsule/gun onto enamel and light-cured 40 s.
• LCAR application: Triad VLC placed into a Teflon mold (internal diameter 3 mm, height 4 mm) centered on the prepared enamel; inserted incrementally, each increment light-cured 2 min. After removing the mold, additional curing 2 min performed. Specimens stored in water at 37 °C for 24 h and thermocycled 5–55 °C.
• Testing: Shear bond strength measured with LLOYD LR 5K universal testing machine; blade aligned to resin–enamel interface; crosshead speed 0.5 mm/min. Maximum load at failure (N) divided by bonded area to yield MPa. Adhesive remnant index (ARI, 0–3) scored under magnification. Statistics: Normality by Kolmogorov–Smirnov; one-way ANOVA and Tukey post hoc for bond strength (p<0.05); chi-square for ARI distributions (p<0.05). Flexural strength testing:
• Specimens: Ten LCAR bars (approx. 16 × 5 × 4 mm) prepared by pressing uncured LCAR in a PTFE split mold between glass slabs within an adjustable frame; light-cured 2 min per side (450 mW/cm²), then demolded and cured an additional 2 min. Stored in water at 37 °C for 48 h.
• Testing: Three-point bending on LLOYD LR 5K with 1 mm/min crosshead speed; span length per fixture; load at fracture recorded. Flexural strength S (MPa) calculated as S = 3·P·L / (2·b·d²), where P is load at fracture (N), L support span (mm), b width (mm), d depth (mm). Statistical reporting of mean ± SD.

• Flexural strength (LCAR/Triad VLC): Minimum 75.78 MPa; median 82.83 MPa; maximum 90.07 MPa. Mean ± SD = 82.83 ± 5.2 MPa.
• Shear bond strength to enamel (mean ± SD, MPa): • Transbond XT: 18.43 ± 2.93 (highest). • Tetric Flow: 11.47 ± 2.74 (significantly lower than Transbond XT; higher than Fuji Ortho LC). • Fuji Ortho LC: 8.02 ± 1.37 (lowest).
• ANOVA showed significant differences among groups (p < 0.05); Tukey post hoc confirmed pairwise differences as reported.
• ARI distributions showed no significant differences among groups (chi-square p > 0.05); ARI score 0 predominated across groups, indicating little to no adhesive remained on enamel after debonding.

LCAR demonstrated flexural strength well above the ISO threshold for polymer-based crown and bridge materials (>50 MPa), indicating adequate rigidity to withstand intraoral flexure. Shear bond strength varied by adhesive system, with Transbond XT outperforming Tetric Flow and Fuji Ortho LC. The superior performance of Transbond XT is attributed to its viscosity and handling properties that favor stable placement and adaptation compared to low-viscosity flowable composites, which can slump under gravity. RMGIC (Fuji Ortho LC) exhibited the lowest bond strength, consistent with its ionic bonding mechanism and sensitivity to phosphoric acid etching protocols, which can reduce bond efficacy; RMGICs are generally not considered the gold standard for orthodontic bonding. All measured bond strengths exceeded the commonly cited minimal clinical range (approximately 6–8 MPa) for orthodontic applications, while remaining below levels associated with increased risk of enamel damage during debonding. ARI outcomes did not parallel bond strength differences, as most failures left no adhesive on enamel across groups, a common observation in orthodontic bonding studies. Overall, the results support the feasibility of an LCAR-based fixed space maintainer with appropriate adhesive selection, particularly Transbond XT, to achieve reliable enamel bonding alongside satisfactory mechanical properties.

Within the limitations of this in vitro study, the light-cured acrylic resin (LCAR) tested exhibited acceptable flexural strength and shear bond strength to enamel, supporting its potential use as a fixed, chair-side space maintainer in primary dentition. Further in vitro and in vivo investigations are recommended to refine the design and validate long-term clinical performance, including biological considerations.

1. Bond strength to permanent teeth was not evaluated. 2) Biological factors such as plaque accumulation and bacterial colonization were not assessed. 3) As an in vitro study, clinical longevity and performance were not directly measured; additional biological and long-term clinical evaluations are needed to optimize the LCAR space maintainer design.