Background
This study aims to evaluate the fundamental material properties of ink-jet–printed zirconia (IPZ) after vibratory polishing and analyze the frictional behavior and dimensional accuracy of the resulting orthodontic brackets.
Methods
IPZ was used to fabricate customized brackets, followed by a 3-phase vibratory polishing. Vickers hardness, fracture toughness, surface roughness, scanning electron microscopy morphology, and linear reduction rate were measured on IPZ cubic samples (n = 20). IPZ brackets (n = 80) of 0.022 × 0.028-in slot were assessed for frictional resistance, scanning electron microscopy morphology, and slot dimensions. Control groups comprised stainless steel brackets, ceramic brackets, and stainless steel self-ligating brackets.
Results
Hardness and fracture toughness of IPZ were 1374.34 ± 34.12 Vickers hardness and 9.01 ± 0.68 MPa·m 1/2, respectively. The average surface roughness value of our final polished cubic samples was <0.2 μm. Frictional resistance of polished IPZ brackets (eg, phase 3: 0.507 ± 0.153 N) was comparable to steel self-ligating brackets (0.573 ± 0.153 N; P >0.05) from phase 1 onward, with no further significant reduction in phases 2 and 3. The slot depth and height conformed to the German DIN 13996 standard. Substantial reduction of stair-stepped deposition layer lines was observed on the surface of polished cubic samples, whereas the morphologic changes were conspicuous in the slot base and the labial third of slot walls.
Conclusions
IPZ demonstrated promising in vitro mechanical and surface properties, with the vibratory polishing manifesting its efficacy in reducing the stair-step effect, though the clinical performance of IPZ brackets requires subsequent validation. Further optimization of 3-dimensional printing techniques and slot-specific polishing methods is warranted.
Highlights
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We fabricated brackets by ink-jet printing zirconia with a layer height of 10 μm.
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We developed vibratory polishing against the surface defect of additive manufacturing.
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Surface roughness achieves a clinically friendly level of <0.2 μm.
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Frictional resistance of slots aligns with that of metal self-ligating brackets.
The efficiency, accuracy, and predictability of orthodontic treatment are concerning issues. Customized orthodontic brackets with personalized torque, angulation, and slot dimensions offer a promising approach for enhancing treatment efficiency and effectiveness, potentially improving patient comfort and reducing clinical workload. , Although 3-dimensional (3D) printing has become routine for surgical guides, occlusal splints, and crowns, their application to customized brackets remains largely experimental. ,
Various printable materials, including metal, ceramics, and resins, have been evaluated for brackets. , Among them, zirconia is a ceramic material widely used in fixed prosthodontics because of its high flexural strength, esthetic performance by coloring, biocompatibility, low plaque affinity, and resistance to staining compared with resinous materials. , Preliminary research on 3D-printed zirconia brackets suggested higher fracture toughness and lower hardness compared with alumina, which may minimize wing fractures and enamel abrasion.
In contemporary dentistry, 3D printing, or additive manufacturing (AM), encompasses various technologies, such as stereolithography (SLA), digital light processing (DLP), ink-jet printing (IJP), and powder binder printing. AM offers advantages over traditional subtractive manufacturing, mainly including design freedom and material usage efficiency. The printing accuracy depends on the choice of AM technology, associated process parameters, and postprocessing. IJP has demonstrated superior accuracy compared with DLP, ,, and the printed bracket slot dimensions can meet the German DIN 13996 standard. , However, an inherent challenge of AM is the stair-step effect from layer deposition (typically 25-300 μm), which affects color properties, dimensional accuracy, and surface roughness (Ra). ,,, This roughness influences the frictional resistance (FR) of bracket slots, opposing orthodontically applied forces, thus making postprocessing polishing essential. Current ceramic polishing techniques primarily target flat geometries and are unsuitable for intricate bracket slots. Manual polishing is impractical for mass production. Industrial micromanufacturing employs methods, such as vibratory polishing, for complex parts, suggesting a viable solution for 3D-printed brackets.
Although 3D printing technology has found clinical applications in orthodontics, critical challenges remain in achieving the qualified surface quality, dimensional accuracy, and mechanical properties required for customized orthodontic brackets. This study, therefore, aimed to assess the fundamental material properties of 3D-printed zirconia fabricated by IJP combined with vibratory polishing for surface quality improvement. The morphology, FR, and dimensional accuracy of the resulting zirconia brackets were evaluated in comparison with commercial metal and ceramic brackets.
Material and methods
Specimens were manufactured via IJP of advance customized jetting (ACJ) system (Thales Medtech, Hangzhou, China), with the layer height of 10 μm and the speed of 1 min/layer. The printing ink was composed of zirconia slurry (zirconia nanoparticles: 50-100 nm, 40-60 wt%) and a water-soluble support slurry (sodium carbonate nanoparticles: 50-100 nm, 40-60 wt%). After printing, the support material was dissolved in a circulating soft-water bath (20°C-40°C, 20-100 L/h). Sintering adopted a gradient temperature profile from room temperature to 1450°C. By multistage heating and temperature holds, this gradient sintering protocol enables grain growth and porosity reduction in the zirconia green body. The linear shrinkage rate of sintered ink-jet-printed zirconia (IPZ) cubic samples was equal in all 3 dimensions, averaging 17.8% ( Supplementary Material ). Digital bracket designs were based on the virtual teeth arrangement in patient dental arches and adjusted by computer-aided design software to optimize the contact between the bracket base and tooth surface. A total of 20 cubic samples (10 × 10 × 10 mm) and 80 IPZ brackets of 0.022 × 0.028-in slot for maxillary central incisors were printed.
Vibratory polishing was performed using a vibratory polisher (ZHM-6L; Zhengxing Co, Guangdong, China) with a 6 L chamber operating at 180 W and 0.3-8 mm amplitude. The chamber was loaded with 1.2 kg of cylindrical high-frequency ceramic abrasives (Φ 1.5 × 5 mm; Zhongwei Co, Guangdong, China) and 250 mL of water-based diamond slurry (Zhongwei Co). The polishing followed 3 sequential 72-hour phases ( Fig 1 , A ): phase 1 with 1300# slurry (median particle size D50: 10.0-12.0 μm), phase 2 with 6000# slurry (D50: 2.6-3.0 μm), and phase 3 with 12000# slurry (D50: 1.1-1.3 μm). Between phases, specimens underwent ultrasonic cleaning in deionized water at 40 kHz for 10 minutes, followed by drying with oil-free air.
Study design: A, Polishing protocol schematics; B and C, Tests for IPZ cubic samples and brackets during each polishing phase.
IPZ cubic samples were measured on 3 orthogonal planes (xy, yz, and xz) defined by the printing coordinate system (x, y, and z) ( Fig 1 , B ). The xy-plane was parallel to the print bed, whereas the z-axis was parallel to the building orientation. A linear infill pattern was deposited with the material jet traversing along the y-axis.
The Vickers hardness and fracture toughness of 3D-printed zirconia were measured using an HVS-1000Z instrument (Suliang Instrument Technology Co Ltd, Suzhou, China) with the HV1 (Vickers Hardness with a 1 kilogram-force load) test parameter. A diamond indenter was pressed into the surface of each sample under the specified load for 10 seconds. Measurements were taken automatically and checked by the operator.
Ra was measured using a contact roughness meter (TR200; Beijing TIME High Technology Ltd, China; range: 0.005-16 μm, resolution: 0.001 μm). The sampling length was set to 0.8 mm with 3 equally spaced measurement traces on the surface ( Fig 1 , B ).
The dimensional measurements were conducted using a digital caliper with a precision of 0.01 mm. Linear reduction rate (LRR) was calculated as follows:
Measurements of Ra and dimensions were taken before polishing and repeated at 24-hour intervals thereafter on the consistent 17 samples.
For scanning electron microscopy (SEM) observations, 4 samples were randomly selected from 4 polishing phase groups. Samples were sectioned by a high-precision cutting instrument (IsoMet low-speed saw; Buehler, Lake Bluff, Ill) to expose all planes and edges.
For comparative analysis, 3 types of commercial brackets for maxillary central incisors with 0.022 × 0.028-in slot dimensions were included as control groups (each n = 15): (1) ceramic conventional brackets (3M Clarity; Monrovia, Calif); (2) stainless steel conventional brackets (SS brackets) (MIM Standard Series; Puterbaugh, China); and (3) SS self-ligating brackets (SSS brackets) (Damon Q2; Ormco, Brea, Calif).
Slot depth (buccal-lingual direction; d) and height (incisal-apical direction; h) of 60 IPZ brackets (15 of each polishing phase) were quantified using a stereo microscope (Model S6D; Leica Microsystems, Wetzlar, Germany; resolution: 0.01 μm) according to ISO 27020:2019 ( Fig 1 , C ).
To compare FR levels of IPZ brackets of different polishing phases with other commercial brackets, 105 brackets (15 of each group) and 105 rectangular SS archwires of 0.019 × 0.025-in (Model 380-251; TP Orthodontics, Wuxi, China) were used. The conventional brackets were ligated using elastic ligatures (640-1264, Short Sticks; Ormco) to ensure a consistent ligation force, whereas SSS brackets required no external ligature. FR tests were performed on a universal testing machine (Model WDW-10C; HUALONG, China) at a unidirectional sliding speed of 1 mm/min over a 5 mm displacement distance under dry conditions. A custom fixture secured the parallelism between the wire and the bracket slot axis. The maximum static FR was automatically captured.
For SEM observation, brackets (5 IPZ brackets of each polishing phase, 5 SS brackets, and 5 ceramic brackets) were cut by a high-precision cutting instrument to expose slot walls and slot base.
All measurements were conducted by blinded operators to eliminate observational bias.
Statistical analysis
Statistical analysis was performed using GraphPad Prism (version 9.5.0 for Windows; GraphPad Software, Boston, Mass) and R statistical software (version 4.4.2; The R Project for Statistical Computing, Vienna, Austria). Descriptive statistics are reported as mean ± standard deviation. Data normality and variance homogeneity were assessed using the Shapiro-Wilk test and the Brown-Forsythe test, respectively. Parametric data were analyzed using analysis of variance or repeated-measures analysis of variance, whereas nonparametric data were analyzed using Kruskal-Wallis or Friedman tests, as appropriate. Post-hoc comparisons were conducted using the Tukey honest significant difference test or the Dunn test. P < 0.05 was considered statistically significant.
Results
Hardness and fracture toughness of the IPZ specimens were 1374.34 ± 34.12 Vickers hardness (VHN) and 9.01 ± 0.68 MPa·m 1/2, respectively. Ra was higher on yz- and xz-planes than on the xy-plane before and after polishing ( Table Ⅰ ; Fig 2 , A-C ), but all finished planes achieved a mean Ra <0.2 μm ( Fig 2 , G ), the recognized clinical threshold Ra for intraoral materials with respect to biofilm formation. LRR was positively correlated with abrasive particle size ( Fig 2 , D-F and H ). The aggregate linear reduction over 9 days was 110.7 μm, promoting a further 55 μm (half of the total linear reduction) compensation in the designed bracket slot depth in addition to the sintering shrinkage compensation.
Table I
Ra and dimensions of cubic samples (n = 17)
| Polishing phase | Ra (μm) | Edge length (mm) | ||||||
|---|---|---|---|---|---|---|---|---|
| xy | yz | xz | x | y | z | LR (μm) | LRR (μm/d) | |
| Prepolishing | 1.235 ± 0.372 | 3.538 ± 1.172 | 2.007 ± 0.653 | 10.02 ± 0.011 | 10.04 ± 0.009 | 10.00 ± 0.010 | ||
| Phase 1 | 0.479 ± 0.111 | 1.580 ± 0.559 | 1.283 ± 0.289 | 9.92 ± 0.013 | 9.95 ± 0.010 | 9.96 ± 0.007 | 77.1 | 25.7 |
| Phase 2 | 0.135 ± 0.079 | 0.537 ± 0.361 | 0.355 ± 0.243 | 9.90 ± 0.012 | 9.92 ± 0.008 | 9.94 ± 0.006 | 21.5 | 7.2 |
| Phase 3 | 0.062 ± 0.046 | 0.181 ± 0.196 | 0.103 ± 0.092 | 9.88 ± 0.012 | 9.91 ± 0.008 | 9.93 ± 0.005 | 12.1 | 4.0 |
| Total | 110.7 | 36.9 | ||||||
LR , linear reduction; LRR , linear reduction rate.
Ra and dimensions of cubic samples: A and D, Changes in Ra and edge length of every polishing phase; B and C, Ra values across planes before and after polishing; E and F, Edge lengths before and after polishing; G and H, Decreasing trends of Ra and edge length over time. ∗ P <0.05, ∗∗∗ P <0.001, ∗∗∗∗ P <0.0001.
SEM results showed pronounced layer lines on xz- and yz-planes, with greater layer thickness visible on the yz-plane ( Fig 3 ). Sharp edges displayed material overdeposition at vertices, consistent with AM’s inherent edge-enhancement phenomenon. Postpolishing, lamination textures transitioned to scattered shallow microscratches on xz- and yz-planes, whereas the xy-plane remained scratch-free. All edges uniformly progressed to rounded chamfers, despite an accidental crack defect observed on the y-edge.
Micromorphology of cubic samples at 100x magnification.
Table II and Figure 4 summarize the FR of IPZ brackets across polishing phases and 3 commercial bracket systems. Phase 1 polishing reduced FR by 38.3% relative to original brackets, with no further significant reduction after phases 2 or 3 ( P >0.05; Fig 4 , A ). Prepolished IPZ brackets (1.220 ± 0.291 N) exhibited FR comparable to that of SS brackets (1.020 ± 0.218 N; P = 0.2415), but significantly higher FR than both ceramic (0.947 ± 0.245 N; P = 0.0312) and SSS brackets (0.573 ± 0.153 N; P <0.0001; Fig 4 , B ). After 3-phase polishing, IPZ brackets (0.507 ± 0.153 N) demonstrated significantly lower FR than ceramic and SS brackets ( P <0.0001), whereas they matched with SSS brackets ( P = 0.9867; Fig 4 , C ).
Table II
FR of all bracket types
| Brackets | n | FR (N) | F value | P value | |
|---|---|---|---|---|---|
| IPZ brackets | Prepolishing | 15 | 1.220 ± 0.291 | 18.88 | <0.0001 |
| Phase 1 | 15 | 0.753 ± 0.323 | |||
| Phase 2 | 15 | 0.620 ± 0.208 | |||
| Phase 3 | 15 | 0.507 ± 0.153 | |||
| Ceramic brackets | 15 | 0.947 ± 0.245 , | 18.88 | <0.0001 | |
| SS brackets | 15 | 1.020 ± 0.218 | 18.88 | <0.0001 | |
| SSS brackets | 15 | 0.573 ± 0.153 | 18.88 | <0.0001 | |
Slot FR and dimensions of IPZ brackets: A, FR changes during polishing; B and C, FR comparisons among bracket groups; D and E, Slot depth and height changes during polishing. ∗ P <0.05, ∗∗∗ P <0.001, ∗∗∗∗ P <0.0001.
SEM examination indicated that ceramic brackets had the most even surface ( Fig 5 ). SS brackets showed dotted and reticular indentations. The IPZ brackets displayed clear layers perpendicular to the printing axis because of the stair-step effect on slot walls and uneven patches across the slot base. Polishing markedly reduced the layered texture at the labial third of slot walls and progressively planarized the slot base, particularly near the slot entrance, with phases 1 and 2 providing the most improvement.
Micromorphology of ceramic, SS, and IPZ brackets across polishing phases.
Slot dimensions changed significantly after polishing phase 1 in both depth (mean difference 23.96 μm; P <0.0001) and height (mean difference 15.43 μm; P <0.0001), whereas no significant additional dimensional alterations were observed in phases 2 or 3 ( P >0.05) ( Table Ⅲ ; Fig 4 , D and E ). Final slot dimensions (d: 777.5 ± 42.23 μm; h: 587.2 ± 12.15 μm) of all brackets fitted the DIN standard (d: 720 − 0/+ 100 μm; h: 560 − 0/+ 40 μm).
Table III
Slot dimensions of IPZ brackets before and after polishing
| IPZ brackets | n | d (μm) | h (μm) | ||||
|---|---|---|---|---|---|---|---|
| Mean ± SD | F value | P value | Mean ± SD | F value | P value | ||
| Designed parameter | 775 | 560 | |||||
| Prepolishing | 15 | 812.5 ± 31.02 | 17.52 | <0.0001 | 569.4 ± 8.630 | 27.78 | <0.0001 |
| Polishing phase 1 | 15 | 788.6 ± 28.51 | 17.52 | <0.0001 | 584.8 ± 9.737 | 27.78 | <0.0001 |
| Polishing phase 2 | 15 | 781.2 ± 37.79 | 17.52 | <0.0001 | 585.2 ± 10.92 | 27.78 | <0.0001 |
| Polishing phase 3 | 15 | 777.5 ± 42.23 | 17.52 | <0.0001 | 587.2 ± 12.15 | 27.78 | <0.0001 |
| DIN standard | 720 − 0/+ 100 | 560 − 0/+ 40 | |||||
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