The accuracy of custom-made metal posts manufactured using selective laser sintering versus conventional casting techniques: a laboratory study

Abstract

Background

The purpose of this study was to evaluate the efficacy of selective laser sintering (SLS) against the traditional casting method in fabricating customized Co-Cr dental posts, employing 3D coordinate metrology for analysis.

Methods

A 10 mm post space was prepared in a transparent acrylic block using a red ParaPost XP drill (1.25 mm diameter). An impression of this cavity was taken with a 1.143 mm diameter ParaPost impression post and auto-polymerizing acrylic resin. The resin patterns obtained were digitized with a Straumann Cares scanner, generating STL files, which were forwarded to Renishaw for the production of 10 Co-Cr posts through SLS. Simultaneously, the original resin patterns underwent investment and casting in Co-Cr alloy. The dimensional accuracy of the posts produced by both methods was evaluated using the triple scan method.

Results

The mean discrepancy was − 0.048 mm when comparing the dimensions of scanned resin posts to those of the conventionally cast posts, and − 0.067 mm between the scanned SLS-produced posts and the original resin patterns. Statistical analysis indicated no significant difference between the two sets of mean values (P = 0.107).

Conclusion

SLS technology is a viable alternative to the conventional casting technique for the manufacture of customized Co-Cr posts. Furthermore, SLS offers advantages in terms of cost and time efficiency without compromising the accuracy of the end product.

Background

In the current modern era, digitization become a daily routine in everyday dental practice [1]. The computer-aided design (CAD) and computer-aided manufacture (CAM) technology innovation exhibited an excellent ability to produce different types of dental restorations with different materials [2], showing several advantages over conventional techniques such as reducing cost, time, and possible human errors associated with conventional methods [2, 3]. Moreover, CAD/CAM allows for the development of more conservative, esthetic, creative, and novel dental treatment approaches [3, 4].

CAD/CAM utilizes mainly two production methods: subtractive manufacturing (milling: SM) and additive manufacturing (3-D printing: AM). SM technology prevailed in the dental field in the last four decades, which is utilized to produce dental parts. However, AM technology possesses several advantages over SM technology such as the ability to produce complex geometrical parts and hollow structures, lower cost, time efficiency, and lower material waste, besides being an environmentally friendly technology [5]. Although both SM and AM technologies share the designing step using CAD software, they are different in many aspects such as the production technique, the materials composition, and the post-production processing [5]. There are several 3D printing technologies used in dentistry, however, not all can be used to produce metal parts, those technology utilized to manufacture metal parts are vat photopolymerization (Stereolithography (SLA), digital light processing (DLP)) [6, 7], material jetting; MJT (ink-jet printing (IJP), nanoparticle jetting (NPJ)) [8], material extrusion; MEX (e.g. direct ink writing (DIW), and fused filament fusion or fused deposition modeling (FDM)) [9], 3D gel deposition/printing (3DGP)) [10], selective laser melting (SLM) [11] and selective laser sintering (SLS) [12], three-dimensional slurry printing (3DSP) [13], binder jetting; BJT [14], and direct energy deposition; DED [15]. Although these technologies have been known to 3D print metal parts, most of them fabricate soft resinous-metal products, and some others are still under development that work on innovative materials for metal 3D-printing. However, DED, and before all SLM and SLS have been used successfully to produce hard metal parts, in particular metal alloy for dental applications [16]. SLM/SLS printing technologies revealed efficient production of dental restorations that are more accurate than the conventional lost cast method [17].

Post-and-core as a treatment option for root canal-treated teeth is recommended when the remaining tooth structure is inadequate to support the overlying restoration [18, 19]. Different metal alloys such as gold (Au/Pd), nickel/chromium (Ni/Cr), and cobalt/chromium (Co/Cr) were used to fabricate cast posts. According to literature, Co-Cr alloys are the most widely used non-precious alloys in the dental field [11]. Non-metal ceramic post-and-core crowns are widely used to restore endodontically treated teeth, however, many practitioners still use the Co-Cr post-and-core crowns either due to particular indication, especially for the non-esthetic posterior area of the mouth [20, 21], or personal preference referring to their superior mechanical properties [21].

In the dental literature, a significant number of studies assessed the accuracy of different metal-ceramic prostheses produced using the SLS technique, but there is a lack of published studies to assess custom-made posts manufactured using this method. The bulk of studies were on subtractive manufactured ceramic posts and core crowns [22, 23] that have been applied in dental practice for more than two decades [24]. However, 3D-printed customized post and core crowns studies are very limited. The additive manufacturing of post-and-core crowns can be accomplished indirectly by printing resin-wax or resin patterns for the lost wax technique or directly through 3D printing the post-and-core crown utilizing a variety of materials. Two studies were found focusing on direct 3D printed posts and evaluated the mechanical behavior of 3D printed posts compared with ready-made fiber posts and composite cores [25, 26]. Two studies were found in the literature comparing the accuracy of 3D printed wax resin and resin patterns for cast metal post-and-core crowns to the conventional manual fabrication methods [27, 28]. Therefore, the purpose of this in-vitro study was to compare the accuracy of custom-made Co-Cr posts manufactured via SLS technology and the manual conventional casting method using a 3D Coordinate Metrology Analysis. The null hypothesis was that there would be a significant difference in the dimensional accuracy between the conventionally fabricated custom-made posts and the SLS-manufactured posts.

Materials and methods

Post space preparation

A clear acrylic block that is used to teach pre-clinical endodontics (Endo-Block, Dentsply-Maillefer, Switzerland) was utilized as a standardized block for post-space preparation. A low-speed handpiece (KaVo Bella Torque Mini; KaVo, Lake Zurich, Ill) and a red ParaPost XP drill size 1.25 mm (Coltene-Whaledent Inc., Cuyahoga Falls, OH, USA) were used to prepare 10 mm length post space in the block (Fig. 1). The post space was cleaned and dried using copious air-water spray. A plastic laboratory burnout impression post (ParaPost) with a 1.143 mm diameter and an auto-polymerizing acrylic resin (DuraLay, Reliance Dental Mfg Co, Worth, Ill, US) were utilized to take an impression of the post space. The ‘bead on’ mixed resin was carried by the plastic post into the post space and held until polymerization of the resin material. This was repeated to produce 10 resin post patterns, Fig. 1.

Fig. 1

From left to right; ParaPost XP system, clear acrylic block for post space preparation, and an auto polymerized resin post pattern

Scanning of the resin post patterns

The resulting resin patterns were scanned using a laboratory dental scanner (Straumann Cares 7 series, dental wings, Switzerland) the scanned files were transmitted into the Cares CAD software (CARES VISUAL version 6.2, Straumann, Switzerland), stored as STL files, labeled from 1 to 10 and marked as “scanned patterns”, Fig. 2,

Fig. 2

From left to right; the FTL file of the scanned resin post pattern, the AM250 metal additive manufacturing system, conventional manual fabricated cast metal post

Printed post-additive manufacturing

The STL files of the scanned resin patterns were sent to the 3D printing company for production (Renishaw plc. Wotton-under-Edge, Gloucestershire, UK). The posts were manufactured from Co-Cr alloy powder containing about 60% cobalt, 30% Chromium, and 5–7% Molybdenum (CoCr-0404 powder, Renishaw plc.) through AM250 metal additive manufacturing system (AM250, Renishaw plc.). Ten metal posts were produced from the scanned resin post patterns using SLS technology with the following parameters: a layer thickness of 30 μm, a laser power of 200 W, and a scan speed of 700 mm/second. The produced posts were tagged according to their corresponding STL file names, Fig. 2.

Conventional manual fabricated cast metal post

The actual resin post patterns were sent to a commercial dental laboratory for conventional production (Woodlands Dental Laboratory, Beamond End, England). Each resin post was gently sprayed with a surfactant agent (Degudent Waxit, Dentsply, Germany). A phosphate bonded investment (Fujivest Premium, GC, USA) was mixed in a vacuum mixer (Iris Advance, Mestra, Spain). and poured into the investment ring containing the resin post patterns, according to the manufacturer’s recommendation. The burnout procedure was carried out using a dental laboratory furnace (Bifa MS8, ShenPaz Dental, Israel) applying a gradual increase of the temperature up to 900 °C according to the manufacturer’s instructions. Titanium bearing Co-Cr alloy (CoCrMoTi, Crutanium, Köln, Germany) was the applied material to cast the patterns using an induction-casting machine (Millennium-R, Manfredi, Italy) at 1329 °C following the manufacturer’s instructions. The cast posts were divested and checked for defects, any mild casting nodules were removed by sandpaper disks. Each cast post was tagged according to its corresponding pattern name, Fig. 2.

Supplementary Table 1 summarizes the characteristics of the alloys utilized in the study.

Scanning of the specimens

The produced 3D printed and cast metal posts were scanned via laboratory dental scanner (Straumann Cares 7series scanner, dental wings) and saved as STL files and named according to the production technique and specimen number, making the specimens ready for the 3D coordinate metrology test, Fig. 3.

Fig. 3

The figure on the left shows the STL file of a random cast metal post, while the figure on the right shows an STL file of a random 3D printed specimen

3D coordinate metrology analysis

Each scanned resin post pattern and its corresponding produced specimen were imported into 3dmdvultus 64 software (3dMD Inc., Atlanta, GA, USA). Then, the digital 3D objects of the scanned resin pattern and its corresponding scanned specimen were manually aligned and a surface-based registration method was undertaken; to increase the accuracy of the superimposition, an automated surface registration method was performed. The main objective of the last method is to align the two superimposed objects as close as possible to each other by utilizing an iterative closest point (ICP) algorithm. Since each scanned object is represented digitally as thousands of triangles, the ICP algorithm looks for similar topographical characteristics of the superimposed objects through these triangles accompanied by computation of the square root distance between every single triangle in the scanned resin pattern and its corresponding in the scanned specimen. The automated surface registration between the scanned objects was continuously performed until the minimum value of the square root distance was reached, Fig. 4. For all produced posts either cast or printed, the distances between the triangles of the scanned posts and their corresponding triangles in the scanned resin patterns were measured. In addition, the mean distance and the standard deviation were also calculated. The statistical analyses were performed using Statistical Package for the Social Sciences (SPSS), Version 20.0. The Levene’s Test for Equality of Variances was used to test whether the variances of the two specimens are approximately equal. The t-test for equality of means provides the results for the actual independent t-test of the specimens. A significance level of α = 0.05 was used. A p-value less than 0.05 was considered statistically significant.

Fig. 4

Surface-based registration of the scanned specimen and its corresponding design

Results

The mean distance between the scanned resin posts and their corresponding scanned conventional cast posts ranged from − 0.02 to -0.08 mm with a mean of -0.048 mm. The mean distance between the scanned resin posts and their corresponding scanned printed posts ranged from − 0.02 to -0.1 mm with a mean of -0.067 mm. The highest deviation was associated with one of the printed specimens at -0.1 mm. On the other hand, the least deviation was recorded at -0.02 mm with both methods, Table 1.

Levene’s Test for Equality of Variances tests whether the variances of the two specimens are approximately equal. The p-value of the Leven’s test is 0.048 (P < 0.05). This means that the equal variance between the two groups is not assumed. T-test for equality of means provides the results for the actual independent specimens t-test. The p-value is 0.107 (P > 0.05), this implies that there is no significant difference in the means between the specimens that were fabricated using SLS and conventional casting techniques, Table 1.

Table 1 Results of the surface-based registration test, and T-test statistics

Discussion

The current research compares the accuracy of the SLS method and the conventional casting technique and their capability to produce metal posts that exactly match the acrylic resin patterns by using a 3D coordinate metrology analysis. There are many studies in the literature comparing the accuracy of conventional lost wax technique with SLS manufacturing technology regarding the production of different dental restorations such as single crowns [12, 29], fixed partial dentures [30], and removable partial [31] and complete dentures [32], SLS technology exhibited an outstanding and efficient results by producing delicate dental parts such as RPD clasps [33]. However, there is a lack of evidence about the capability and reliability of the SLS technique to produce custom-made metal posts. The main bulk of data investigated indirect lost wax casting of digitally manufactured resin patterns through CNC milling or 3D printing.

Among different technologies, available data on metal 3D printing in the dental literature accentuated more selective laser sintering. However, accuracy and data reproducibility do not present consistent results and there appears room for improvement in 3D-printed dental restorations before such technologies are favored over conventional casting methods.

The results of the current study showed no significant difference in the accuracy of the two production methods; SLS and conventional casting that were used to fabricate custom-made posts. Although the results rejected the assumption that the two groups will significantly differ. The disparity, in fact, was minor and unexpectedly favored conventional casting.This may be attributed to the fact that SLS can be affected by factors such as laser efficiency, material quality, and the specifics of post-production treatments, which can influence the final accuracy of the printed products.

When all specimens were subjectively tried in the actual endo block at which the impressions were made, the SLS produced showed more passive fit in comparison to the cast posts which exhibited more resistance in the insertion and more retention in the removal. This could be referred to as the dimensional changes or positive artifacts of the conventional casted posts in any step of manual production, while the SLS manufactured posts are less impacted by human-induced errors and require no adjustment; this was reflected in the passivity of fit of SLS posts compared with the conventional ones. The passivity of fit was confirmed by previous studies, for larger prosthetic appliances, screw-retained implant framework fabricated by SLS technology exhibited superior accuracy and passivity of fit to the partial digital approach using SLA and milled frameworks [34].

The evidence-based in the literature confirmed the findings of the current study, a systematic review concluded comparable accuracy of the single coping restorations fabricated via SLS technology and conventional lost wax technique [12]. Moreover, comparable or better marginal fit has been documented with SLS-produced frameworks and crowns when compared in vitro to lost wax and milling techniques. SLS-manufactured frameworks reported a 43.9 μm marginal gap. On the other side, conventionally fabricated frameworks showed a marginal gap of 47.5 μm [35].

However, metal copings fabricated using SLM showed the best result compared with subtractive manufacturing and the conventional lost wax technique [29]; the lower accuracy of copings produced using the conventional lost wax method and particularly the CNC milling technology could be referred to the limitations of the conventional lost wax technique and the CNC milling technology [5]. Meanwhile, the superiority of SLM technology could be referred to the possibility of using high-energy deposition for production that can melt and build the metal powders without the need for a secondary binder, or post-production debinding resulting in less influence of the printing procedures on the metal copings properties and accuracy [36], the printing without resin binder is also feasible for SLS as developed by Miners et al. 2001 [37] and shown by Bertrand et al. 2007 [38].

Dimensional precision, physical and mechanical properties, costs, and time must all be taken into account when producing dental prostheses. Although SLS technologies reduce manufacturing time and costs, minimize human errors, and prevent possible defects in the cast objects compared to conventional casting method, there are many challenges associated with this technique beginning with the digitization of data using different scanners; which differ in technologies and quality of scanning. Various parameters such as object orientation, the thickness of the slices, and structural support must be specified. In addition, the quality of the surface of the printed prosthesis is influenced by the particle size, shape, and melting temperature. All these variables are interrelated and affect accuracy, mechanical and physical properties, cost, and time of manufacture of the produced prostheses [5].

The presented study might have some limitations including, the post-space preparation was carried out on an acrylic block rather than real human teeth, the configuration and anatomical variations of natural teeth might exhibited different results, relatively small sample size; increasing the sample size might result in more powerful statistic, the printing technology used and the printing parameters: studying different parameters might conclude the proper parameters required for more accurately produced SLS dental restorations. Further investigations of these variables and parameters might provide further evidence, nevertheless the current study has reached good insight on the feasibility of 3D printing customized post-and-core crowns, concluding the promising outcomes of the technology and its comparable accuracy to the conventional lost wax method, leaving room for further investment and development in the SLS technology, with the aim to encounter more precise, true, and accurate dental restorations compared to the conventional lost wax method or the subtractive manufacturing technology.

Conclusion

Based on the results of this study, there were no statistically significant differences between the SLS and conventional casting techniques when producing custom-made metal posts in terms of dimensional accuracy. Hence, the SLS technique is capable of producing custom-made metal posts that are dimensionally comparable to those fabricated using conventional casting methods and are much more cost and time effective.