Optimizing post-space drying: insights from an in vitro study using standardized three-dimensional-printed models

Article information

Restor Dent Endod. 2026;.e43

Publication date (electronic) : 2026 August 3

doi : https://doi.org/10.5395/rde.2026.51.e43

Takanori Sakai^1,, Hideshi Sekine¹, Syuntaro Nomoto¹, Mamoru Yotsuya¹, Esha Mukherjee², Hidehiko Watanabe²

¹Department of Fixed Prosthodontics, Tokyo Dental College, Tokyo, Japan

²Department of Restorative Dentistry, Oregon Health & Science University School of Dentistry, Portland, OR, USA

^*Correspondence to Takanori Sakai, DDS, PhD Department of Fixed Prosthodontics, Tokyo Dental College, 2-9-18 Kandamisaki-cho, Chiyoda-ku, Tokyo 101-0061, Japan Email: hayashibaratakanori@tdc.ac.jp

Citation: Sakai T, Sekine H, Nomoto S, Yotsuya M, Mukherjee E, Watanabe H. Optimizing post-space drying: insights from an in vitro study using standardized three-dimensional-printed models. Restor Dent Endod 2026;51(4):e43.

Received 2025 October 18; Revised 2026 April 16; Accepted 2026 June 4.

Abstract

INTRODUCTION

Core build-up restores a caries- or trauma-damaged coronal portion to an abutment form suitable for a fixed prosthesis. An abutment tooth requiring a core build-up is often reinforced with a post-and-core, which provides structural support for the definitive restoration [1,2]. Traditionally, cast post-and-core build-ups have been widely used owing to their mechanical strength [3,4]. However, vertical root fractures have been reported to occur more frequently in teeth restored with cast post-and-core systems than in those restored with fiber posts [5].

Root fractures are associated with external factors such as excessive occlusal loading and bruxism [6], and internal factors such as post length and residual tooth morphology [5]. Because metal posts have a much higher elastic modulus than dentin, stress is not efficiently dissipated, leading to interfacial stress concentration and initiation of root fracture [4,7,8]. The integrity of the adhesive interface between the post/core and dentin also influences fracture behavior [9].

Advances in adhesive dentistry have enabled the use of resin cements and core materials with high bond strength; consequently, post-and-core build-ups using fiber-reinforced composite (FRC) posts, whose elastic modulus approximates that of dentin, have become common [1,4,7]. Studies indicate that resin cores with FRC posts lead to fewer root fractures than metal cores [7,10,11]. When a translucent ceramic crown is placed, the abutment shade influences the final color and optical properties, making resin composite cores more esthetic than metal cores [12].

Despite these advantages, the flexibility of FRC posts may induce cervical microleakage with potential for secondary caries [7]. Debonding has also been reported, most commonly as loss of retention at the resin-cement–dentin interface [7,10,13]. Long-term sealing is therefore essential to prevent microleakage, secondary caries, and post/core debonding [7], which can be exacerbated by the presence of endodontic sealer remnants or poor interfacial quality [14,15]. Factors that inhibit adhesion include a high configuration factor [16,17], limited visibility in the post space, remnants of root canal filling or temporary materials [14,17], saliva contamination, and technical errors during the adhesive procedure [2,15,17]. Adhesion to apical dentin is more challenging compared to coronal dentin, due to morphological differences, including lower tubule density and altered collagen expression, which results in significantly lower bond strengths in the apical third of the root canal [14].

Furthermore, the post space is long and narrow, making complete removal of remaining moisture difficult. Remaining moisture in the post space can inhibit bonding to root canal dentin [15], degrade resin core properties via water uptake, and cause time-dependent hydrolytic deterioration of adhesion [18]. In clinical practice, drying the abutment tooth and post-space after post-and-core preparation is a prerequisite for adhesive cementing; however, few studies have examined how different drying methods and post-space geometries affect remaining moisture [19]. Therefore, this study investigated four drying methods across a range of post-space diameters using standardized three-dimensional (3D)-printed resin tooth specimens.

METHODS

Specimen preparation

The maxillary right central incisor typodont tooth was prepared with diamond burs (BRIO856.31.016 [M] RD TAP 25P and BRIO8856.31.016 [F] RD TAP 25P; Brasseler USA, Savannah, GA, USA) for a core build-up, using a high-speed handpiece (WA-99LT; W&H Impex Inc, Bürmoos, Salzburg, Austria). The margin was placed at the gingival level, and a 1.0-mm deep chamfer was prepared around the entire margin for an all-ceramic crown restoration. In addition, a 2.0-mm-high ferrule was incorporated around the entire tooth to support the core build-up.

Subsequently, the prepared typodont tooth was removed from the typodont model, scanned with the Primescan, and the scan data were saved as an STL file. Next, multiple post-space designs were incorporated into the abutment teeth using FreeCAD. The coronal opening diameter of each post space was specified, and the apical end was designed as a hemispherical shape. The overall post-space geometry was created using a rotational design approach. Nine post-space conditions were established by combining three coronal opening diameters (1.4, 1.7, and 2.0 mm) with three lengths (6.0, 9.0, and 12.0 mm). These dimensions were selected based on the clinical range of commercially available fiber post systems, which typically vary between 1.0 and 2.1 mm in diameter [1]. The 1.4-mm diameter was used as a baseline consistent with established experimental models [13], while 1.7-mm and 2.0-mm diameters were included to cover the spectrum of standard and larger clinical preparations [1,17]. The total lengths were determined to reflect a comprehensive range of clinical configurations by integrating a consistent 2.0-mm ferrule height [20,21] with intra-canal depths that correspond to the range evaluated in previous literature for mechanical performance (approximately 5.0 to 10.0 mm) [20-22]. This range ensures the inclusion of both conservative preparations, where less gutta-percha is removed to preserve internal radicular dentin, and the clinically recommended depth of at least 10 mm or two-thirds of the root length [21,22] (Figure 1).

Figure 1.

CAD-based designs of post-spaces used in this study.

The abutment teeth with the designed post-spaces were printed with the 3D printer. The printing conditions were as follows: a layer thickness of 50 µm, with the build orientation starting from the root side and the tooth axis tilted at 40°. Supports were placed to avoid the finish line. Following the manufacturer’s instructions, the printed specimens were cleaned in a washing unit (SprintRay Pro Wash/Dry; SprintRay) using 91% isopropyl alcohol, after which the supports were removed. Post-curing was then performed for 15 minutes using a light-curing unit (ProCure Model SRP1811A; SprintRay) at 405 nm. After post-curing, a periodontal probe was used to verify that the post spaces of the 3D-printed resin tooth specimens had the intended lengths and coronal opening diameters. Representative 3D-printed specimens with designed post-spaces are shown in Figure 2.

Figure 2.

Representative 3D-printed maxillary right central incisors with designed post-spaces. (A) Representative examples of the 3D-printed artificial teeth fabricated in this study. (B) Typodont model with a specimen mounted.

A total of 81 standardized 3D-printed resin tooth specimens were fabricated, with nine specimens assigned to each of the nine post-space conditions (n = 9 per condition).

Test procedure

In this study, remaining moisture was evaluated gravimetrically. First, the mass of the dry specimens was measured using an electronic balance (Pioneer PX224; Ohaus, Parsippany, NJ, USA), which served as the reference mass. Next, the post spaces of the 3D-printed resin tooth specimens were filled with distilled water, ensuring that the water surface at the coronal opening was flush with the specimen surface. This filling procedure was performed under a stereomicroscope (SM-1(B/T)-PL; AmScope, Irvine, CA, USA) to ensure that no air bubbles were present within the post-spaces, and the mass was measured again. The specimens were mounted on the typodont model, and the post spaces were dried using the following four methods (Figure 3).

Figure 3.

Four post-space drying methods used in this study. The post-air-blow method (PAB) and paper-point method (PP) were performed consecutively after the air-syringe method (AS), whereas the root-canal-suction method (RCS) was performed independently.

• Air-syringe method (AS): The post-spaces were dried by directing an air blow from a position 10 mm above the coronal opening for 10 seconds using a three-way syringe (A-dec 332; A-dec Inc., Newberg, OR, USA). The air was delivered by fully depressing the syringe lever to provide maximum pressure consistently across all specimens.

• Post-air-blow method (PAB): After drying with the AS method, an air blow was applied directly into the post-spaces for 10 seconds using a post-space air-blowing device (Bond Air Ease; Morimura Co., Ltd., Tokyo, Japan).

• Paper-point method (PP): After drying with the AS method, the post-spaces were further dried using paper points (Dental Endodontic Paper Points F4; Meta Biomed, Cheongju, Republic of Korea). The same paper point size was used for all specimens across the three post-space diameters (1.4, 1.7, and 2.0 mm). Each paper point was inserted to full depth and gently rotated under magnification to facilitate wall contact. The number of paper points was not predetermined; drying was continued until no visible moisture was detected.

• Root canal suction method (RCS): The post-spaces were dried for 10 seconds using a root canal suction device (Multi Suction E-type; Neo Dental Chemical Products Co., Ltd., Tokyo, Japan).

The specimens were then removed from the model, and their masses were measured again. The remaining-moisture amount and remaining-moisture content of the post spaces were calculated with the following formulas (n = 9 per condition):

Remaining-moisture amount (mg) = M(dry) − M(ref)

Remaining-moisture content (%) = [M(dry) − M(ref)] / [M(fill) − M(ref)] × 100

where M(ref), mass of the dried specimen (reference); M(fill), mass after filling the post space with distilled water; and M(dry), mass after drying the post space.

RESULTS

Across all conditions, remaining moisture depended on the drying method and the post-space geometry. Figures 4 and 5 summarize the principal patterns, with detailed statistics provided in Tables 1 and 2.

Figure 4.

Remaining moisture amounts of post-spaces. (A–C) Remaining moisture amounts of post-spaces with lengths of 6 mm (A), 9 mm (B), and 12 mm (C). Identical lowercase letters indicate no statistically significant differences among the conditions (p > 0.05). Definitions of the drying methods are provided in Figure 3. AS, air-syringe method; PAB, post-air-blow method; PP, paper-point method; RCS, root-canal-suction method.

Figure 5.

Remaining moisture contents of post-spaces. (A–C) Remaining moisture contents of post-spaces with lengths of 6 mm (A), 9 mm (B), and 12 mm (C). Identical lowercase letters indicate no statistically significant differences among the conditions (p > 0.05). Definitions of the drying methods are provided in Figure 3. AS, air-syringe method; PAB, post-air-blow method; PP, paper-point method; RCS, root-canal-suction method.

Table 1.

Results of two-way analysis of variance for remaining-moisture amount

Table 2.

Results of two-way analysis of variance for remaining-moisture content

DISCUSSION

This study investigated the effects of four different post-space drying methods on remaining moisture using standardized 3D-printed resin tooth specimens. Among the four techniques, the AS method consistently showed the lowest drying efficiency regardless of post-space geometry. The PP method was mainly affected by post-space length, with longer post-spaces retaining more moisture. The RCS method demonstrated a diameter effect only in short post-spaces. In contrast, the PAB method showed a strong dependence on diameter, with long and narrow post-spaces tending to retain more remaining moisture.

These results indicate that drying efficiency is not determined solely by the selection of technique, but rather by the interaction between method-specific mechanisms and post-space geometry. For instance, the AS method relies on coronal air blowing, where air pressure decreases with distance, resulting in insufficient drying in deeper regions. In line with previous studies, our findings indicate that air drying alone is insufficient to remove remaining moisture from post spaces fully, consistent with the results of Aziz et al. [23]. The PAB method showed distinct trends in specific post-space geometries, with particularly long and narrow post-spaces (e.g., 12 mm × 1.4 mm diameter) showing higher remaining-moisture levels and a significant interaction between diameter and length. This can be attributed to the PAB mechanism, in which air delivered from the coronal opening is directed toward the apical end: larger diameters allow air to flow more efficiently, reducing remaining moisture, whereas smaller diameters restrict airflow and, combined with surface tension, tend to retain moisture. The PP method depends on direct absorption, which becomes less effective in longer post-spaces where contact with the entire surface is limited. The RCS method uses suction to remove moisture from wall surfaces, showing relatively stable performance except in short, narrow post-spaces.

In this study, 3D-printed resin tooth specimens representing maxillary central incisors were used. The interior of the post space in natural teeth is composed of dentin, but its surface properties vary, which makes it challenging to achieve consistent experimental conditions. In contrast, CAD/CAM technology enables precise standardization of both post-space geometry and internal surface properties. This allows for reproducible simulation of clinical conditions.

The DLP method was selected to fabricate the specimens with post-spaces. The DLP method allows high-resolution fabrication with smooth surface quality and excellent dimensional accuracy, which are essential for standardizing post-space morphology. Particularly, producing narrow and elongated shapes such as post-spaces is difficult with subtractive milling. Moreover, its relatively fast printing speed and compatibility with dental resins make it suitable for efficiently producing multiple experimental specimens.

To replicate a clinically relevant environment, 3D-printed abutment teeth with post-spaces were mounted in the typodont, rather than using isolated post-space specimens. The length of the post-spaces was determined based on clinical evidence, including post retention and the dimensions of natural teeth, as well as mechanical and biological considerations, such as the amount of remaining tooth structure, protection of the apical portion, and stress distribution [20-22]. The influence of post length on the strength of endodontically treated teeth has been previously reported [22]. It has also been reported that a large gap between the post-space and the post increases the thickness of the cement layer, thereby reducing bond strength [24]. Therefore, in the present study, the post-space diameters were set assuming the use of fiber posts for core build-up, covering a range (1.4, 1.7, and 2.0 mm) that reflects the standardized diameters of most commercially available post systems [1]. By including the 2.0-mm diameter, which represents the upper limit of clinical post-space preparation, we were able to evaluate the drying efficacy even in cases where extensive dentin removal occurs, which is known to potentially reduce fracture resistance [17].

To simulate the abutment during post-and-core restoration, a 2-mm ferrule was incorporated into the abutment tooth design. A ferrule height of 1.5-2 mm is recommended to enhance the fracture resistance of endodontically treated teeth [25]. This design was adopted to more accurately reflect the morphology of teeth restored with post-and-core, thereby reproducing conditions close to those in clinical practice. Creating post spaces required crown preparation of the abutment teeth for subsequent core build-up, making experiments without crown preparation unfeasible. Crown preparation provided realistic post-space access and enhanced the clinical relevance of the drying evaluation, without directly affecting the intrinsic drying performance of each method.

The experimental settings for each drying method were also carefully designed to reflect clinical procedures while ensuring reproducibility. For the AS method, air blowing was performed from a position 10 mm above the coronal opening using a dental unit syringe, reproducing the clinical application of coronal air drying. Lee et al. [26] reported that within their experimental conditions (1, 5, and 10 seconds), longer drying resulted in higher bond strength of self-etch adhesives. Consistent with this, Iwashita et al. [19] set the drying time at 10 seconds in their study on post-space dentin bonding. Five seconds may not be sufficient in long post-spaces or under high-moisture conditions, so 10 seconds was selected to provide a safety margin to remove moisture sufficiently. Ten seconds is unlikely to cause excessive drying or dentin damage under clinical conditions. For the PAB method, a nozzle tip with an outer diameter of 1 mm was used, which was small enough to be inserted into the coronal opening and deliver air to the apical end under all conditions. The PP method was intended to replicate conventional clinical drying using absorbent paper points. For the RCS method, suction was applied with a gentle pumping motion rather than fixing the tip, to promote effective moisture removal and prevent water scattering. In accordance with their clinical application, the PP and PAB methods were performed following preliminary air blowing because they are typically used as adjunctive procedures rather than standalone techniques. In contrast, the RCS method was evaluated independently, as it is designed as a self-contained suction-based system that does not require prior air blowing. These procedural designs ensured that the drying techniques were clinically relevant while maintaining standardized experimental conditions.

In this study, the remaining moisture in the post-spaces was evaluated gravimetrically. Other methods commonly employed for water measurement, such as staining-based or spectroscopic techniques, were not used [27,28]. The staining method was deemed impractical due to the long and narrow post-spaces, making non-destructive assessment impossible. Spectroscopic techniques are generally limited to surface-near information and thus unsuitable for non-destructive evaluation of moisture inside long and narrow post spaces.

For these reasons, the gravimetric method was adopted. This method allows direct and objective quantification of the remaining moisture of the specimens as a continuous variable. It is less susceptible to operator- and technique-specific bias and was the most suitable approach for comparing the drying methods in this study. Although the precision of the electronic balance was limited to 0.1 mg, and the local distribution of moisture cannot be evaluated, finer measurements were not feasible in our setup. Nevertheless, this method is considered appropriate because it allows the detection of significant differences and trends between conditions.

Although adhesive cementing and bond strength testing were not performed, the quantification of remaining moisture provides critical insight into the role of drying as a pretreatment step in adhesive procedures. Previous investigations have demonstrated that variations in remaining moisture can significantly influence resin infiltration and interfacial integrity [19,23]. By using a standardized 3D-printed resin tooth model, this study isolates the drying variable from other confounding factors such as dentin heterogeneity and post-space geometry, thereby providing reproducible data that can inform optimal pretreatment protocols prior to adhesive cementing.

CONCLUSIONS

Remaining moisture, both in terms of total amount and relative content, is influenced by the drying method, post-space length, and diameter. The AS method exhibited the lowest drying efficiency, while the PP method was primarily affected by post-space length. The RCS method showed a significant effect of diameter only in short post spaces. The PAB method demonstrated a strong dependence on diameter, with long and narrow post-spaces tending to retain more moisture. These findings indicate that drying efficiency is determined by the combined effects of the drying method, post-space length, and diameter, and should be considered when selecting a drying protocol for clinical practice.

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