Preparation and In Vitro Analysis of Craniofacial Titanium Implants Surfaces Produced by Additive 3D Printing and Conventional Manufacturing

Abstract

Study Design:

Surface finish of titanium patient-specific craniofacial implants is known to affect their acceptability and durability and relevant literature still inconclusive on the best surface finishing protocol.

Objectives:

This study investigated surface topography of three-dimensionally (3D) printed and conventionally manufactured craniofacial titanium implants following non-contact 3D laser profile-meter analysis.

Methods:

Seven groups of titanium specimens (n = 10) were prepared and their surfaces were treated differently and included sole or combined treatment of mechanical polishing, gritting with 50 micron AL₂O₃, cold acid treatment using nitric acid for 20 hours (70% w/w), etching using acidic solution (69% nitric and 48% hydrofluoric acids) for 10 minutes and then electro-chemically anodized in another acidic solution (85% orthophosphoric and 98%sulphuric acid). Eighth group included specimens that were 3D printed. 3D micro-roughness parameters Sa, Sp, Sv, and Sz were determined (μm) for each specimen. Data was analyzed using one way ANOVA and Dunett T3 post-hoc tests (p < 0.05).

Results:

There were statistically significant effects of surface finishing protocols (p < 0.05). Sa values were 2.72-13.75 and specimens which were electroplated or mechanically polished and acid treated were the smoothest (p < 0.05). Sp was in the range 9.07-43.56 as sandblasting significantly roughened surfaces (p < 0.05). The same inferior effect was evident for the Sv (p < 0.05). The Sz values were 19.46-107.05 and was the highest for sandblasted surfaces (p < 0.05) and the lowest for surfaces of electro-chemical treatment (p < 0.05).

Conclusion:

Titanium surfaces are affected by the finishing procedure and electro-chemical treatment or mechanical polishing combined with acid treatment produced clinically-favorable smooth surfaces.

Introduction

Titanium alloys are widely used in reconstructing missing bony structures in head and neck region due to their favorable biocompatibility, mechanical and electrochemical properties.^1,2 Their use is successfully increasing³ reaching over 1 million titanium implantation every year, mostly intra-oral implants.⁴ The titanium craniofacial implants can be produced conventionally in which a titanium plate is pressed to the shape of missing bone.^3,5–8 Alternatively, it is manufactured following computer-aided design/computer-aided manufacture (CAD/CAM) principle utilizing computerized novel additive manufacturing (AM) technologies such as Direct Metal Laser Sintering (DMLS) or Electron Beam Melting (EBM) technologies that use lasers to sinter the metal powder layer by layer.^3,6,9 This aids to restore bony defects of skull (cranioplasty plate),^8,10,11 orbital floor,^12–14 and mandibular jaw.^9,15–18 Other bodily uses may include orthopedics, fixing plates and hip replacements. The implant surface composition, hydrophilicity, and micro-topography are key factors to determine effective interaction between the implant and surrounding tissue, and subsequently its success.^4,19–21 Success of titanium implant depends on effective biomaterial-tissue interaction, which is affected by the implant surface composition, hydrophilicity, and morphology including micro-geometry and roughness. Different surface treatments are followed,^19,22–24 and include abrasive smoothening and polishing, gritting with micro-particles (i.e. Al₂O₃), cold treatment using hydrofluoric acid, or passivation and anodization.^4,24–27 However, each treatment leaves distinctive surface characters and there is no ideal one treatment that can produce clinically favorable implant surface (Figure 1). Recent study showed that these treatments significantly change the surface topography and affect its micro-hardness and biocompatibility.²⁸

Figure 1.

An example of skull implant showing 4 different surface treatments.

On the other hand, 3D printed implants (i.e. produced by DMLS or EBM technologies) are increasingly used but claimed to be rough and necessitate surface polishing for smoothness and removal of microstructural flaws, thus reducing chances of fatigue failures and compromised clinical performance.²⁹ Although surface roughness of EBM processed titanium was reported to be higher than that of DMLS processed; still the surfaces were non-cytotoxic in processed and polished forms.³⁰

The aim of the current work is to characterize surface topography of craniofacial titanium implants manufactured following conventional finishing protocols and DMLS technique. Our null hypothesis indicates that finishing protocol and DMLS will not affect the surface topography of titanium implant.

Materials and Methods

Readily available titanium sheets (300 mm × 300 mm × 0.7 mm thick) ASTM F136 (TiAL6V4; Titanium International; Birmingham, UK) were cut in small square shaped specimens (10 mm × 0.7 mm). A total of 70 specimen was produced and allocated to 7 groups (n = 10) of different surface treatment. Group 1 was control and received no treatment. Group 2 specimens were polished using abrasive powder (pumice and lathe) for 10 seconds. Group 3 specimens were submerged in nitric acid for 20 hours (70% w/w). Specimens of group 4 were held about 5 cm from the nozzle head and air abraded for 10 seconds with micro-particle Al₂O₃ (50 μm) at controlled air pressure (3-4 bar). Group 5 specimens received 2 different treatments of mechanical polishing and then immersed in acid as described earlier. Group 6 specimens received 2 different treatments of air abrasion then acid immersion as previously mentioned. Group 7 specimens were etched in an acidic solution (69% nitric and 48% hydrofluoric acids) for 10 minutes and then anodized electro-chemically in another acidic solution (85% orthophosphoric and 98% sulphuric acid) at 12 mV. An additional 10 specimens were manufactured using Direct Metal Laser Sintering Technology (DMLS) from a Ti64 ELI alloy (Electro Optical Systems-EOS GmbH, Krailling, Germany) and then polished and prepared per manufacturer instructions and these were assigned to group 8. Specimens of each group were cleaned in ultrasonic cleaning bath for 10 minutes before commencing three dimensional Micro-roughness (3D micro-Roughness) measurements. A non-contact surface profilometry (Talysurf CLI 1000, Taylor Hobson precision) was used. Bi-directional canning (X and Y axes) was performed at scanning speed of 500 µm/sec. The instrument consisted of 2 cross-slides on the X and Y axes. Scanning area was 4 × 4 mm and estimated scanning time for the full 16 mm² area was 60 minutes. Scanning resolution was 1 and 10 (in μm) at X and Y axes respectively.

Statistical Analysis

Statistical Package for Social Sciences software (SPSS) was used (IL, USA). Data was analyzed using parametric test one way Analysis Of Variance (ANOVA) to check for statistically significant difference between the 8 groups. And Dunett T3 post-hoc was used to run paired comparisons between each 2 groups (p < 0.05).

Results

Results and statistical significances are presented in Table 1. 3D images of the groups were created using the non-contact surface profilometry (Figure 2).

Table 1.

Mean and SD Values of Micro-Roughness Results (µm).

Group (n = 10)	Sa	Sp	Sv	Sz
1	12.55a	29.74a	30.61a	60.35a
	4.03	7.32	8.20	15.37
2	6.00b	14.26b	14.90b	29.16b
	1.69	3.23	3.71	6.82
3	4.88b	14.86b	15.88b	30.74b
	1.72	4.56	3.27	7.65
4	11.98a	34.91c	56.64c	107.05d
	4.95	9.54	13.24	51.82
5	2.72c	12.02b	9.59b	23.83b
	0.88	5.71	2.86	13.40
6	13.95a	43.56c	62.67c	106.23d
	4.33	15.42	8.03	18.59
7	2.31c	9.07b	10.39b	19.46b
	1.16	3.26	2.73	5.55
8	4.29b	23.23a	19.55b	42.75c
	2.09	6.18	2.72	7.78

* Sp (maximum value of peak height); Sv (maximum value of valley depth); Sz (sum of Sp and Sv); and Sa (Arithmetical mean height of the surface).

* Similar superscript letters within the same measurement indicate homogeneous subsets (P > 0.05).

Figure 2.

3D non-contact profilometer used and close up view of the scanning table (top row). Scanning parameters followed and schematic diagram of scanning topography (bottom row).

The 3D surface roughness parameters were (ISO 25178) Sp (Maximum height of peaks); Sv (Maximum height of valleys); Sz (Maximum height of the surface); and Sa (Arithmetical mean height of the surface). Surface images of the specimens under different treatments were captured using SEM at various magnifications (1000 and 5000) (Figure 3). Mean surface images of surface roughness (3D) and surface topography (2D) are shown in Figure 4.

Figure 3.

SEM images of the specimen surfaces after 4 major surface treatments and at 2 different magnifications (1000 and 5000).

Figure 4.

2D and 3D reconstruction of mean images of titanium surfaces of selected groups (1, 2, 6, 7, and 8). The 3D images show average surface with deep cavities colored in red and observed in the mechanical polishing group.

There were statistically significant effects of surface finishing protocols on micro-roughness parameters (p < 0.05). Statistically significantly smoothest surface was evident after following electro-chemical treatment as micro-roughness parameters were the lowest (p < 0.05).

Height parameter (amplitude mean in the height direction) Sa value (μm) was in the range 2.72-13.75. Specimens which were electroplated or mechanically polished and then acid treated were the lowest (P < 0.05) while sandblasting had no effect (p > 0.05). For the height parameters (peaks and valleys) Sp peak height values (μm) were in the range 9.07-43.56 and specimens that were sandblasted (with or without acid treatment) inferiorly affected this parameter (P < 0.05); while the remaining surface treatments had smoothening effect (p < 0.05) except for the 3D printed specimens which had no effect (P > 0.05). The same inferior effect was clear for the valleys depths (Sv) (P < 0.05) and values (μm) were in the range 9.59-62.67.

The Sz parameter which expresses the sum of the maximum value of peak height and the maximum value of valley depth was in the range 19.46-107-05. It was the highest for surfaces treated with sandblasting (with or without acid treatment) (P < 0.05) and was the least for surfaces treated with acid treatment with or without mechanical polishing, or electroplating (P < 0.05) and in the range (19.46-2916).

Discussion

Titanium implant surface topography plays an integral role toward its acceptance by the body, post-implantation healing time, and the implant/body interface. It determines implant’s surface quality and its chemical, physical, and mechanical properties. The different properties interact with each other, and a change in surface topography may change surface energy, thickness of oxide layer and surface chemical composition. The topography relates to the degree of roughness of the surface and the orientation of the surface irregularities. Smooth surfaces are advisable for out of load implants such as craniofacial implants and plates, and rough surfaces are required to promote osseointegration when they are under functional load such as dental implants and fixing screws. Current study showed that the roughness of the implant surfaces varied with the surface finish protocol, hence we rejected our null hypothesis.

The implant surface should warrant a good corrosion and wear resistance, anti-galling properties as well as an optimal biological response. Its treatment encompasses wide range of protocols that modify the surface topography by means of chemical deposition, grinding, polishing, or etching.^24–26 Current study showed that micro-roughness parameters are affected by these protocols. Smooth implant surfaces are always desirable especially in craniofacial implant applications where implant surface plays an interactive role with overlying soft and hard tissue structures and significantly affects implant durability, functionality, stability and its overall success. Reconstructed 3D images obtained by the non-contact profilometry confirms the waviness of the surfaces but with different magnitudes as shown in Figure 4. At micro level, these surfaces exhibited significant variation in surface topography at 2 different magnifications 500 and 1000 as shown in Figure 3.

The 3D surface parameters calculated highlight a surface’s waviness, micro-roughness, wear ability and lubricant retention.³¹ These are defined through a set of standard “S Parameters” in 4 general categories: amplitude, spatial, hybrid and functional. The results for the titanium specimens included only the amplitude data of Sv, Sz, and Sa,³¹ that are clinically relevant. Sa represents amplitude mean in the height direction and the lesser the value the better the surface finish. Electroplating or mechanical polishing along with acid treatment exhibited the lowest value (P < 0.05). This is purely because polishing removes surface irregularities at micro level and anodization seals the surface be depositing titanium oxide layer of controlled thickness. Sp represents the maximum peak height value on the surface in the measured area and usually used for evaluation of frictional force and electrical contact resistance. The value decreased after mechanical polishing with or without acid treatment. The same was evident for the Sv parameter, which represents an area below the average level of the surface and it is often used for evaluation of surface strength and corrosion resistance. The Sz parameter expresses the sum of Sp and Sv on the surface within the measured area and lower value indicates a glossy and luster surface. The Sz values were highest after sandblasting (with or without acid treatment). The results showed that air abrasion produced matt finish and rough surfaces. Generally, sandblasting should be avoided in treating craniofacial implants surface though it is preferable in other applications like dental implants. Regardless, earlier work showed that cell attachment and cell proliferation are not affected by the surface finishing protocol employed.²⁸

On the other hand, our results showed that DMLS produced titanium implants are comparable in surface topography to machined (custom made) titanium surfaces. This emphasizes the wide spread of Direct Metal Laser Sintering of titanium (DMLS) to manufacture customized mesh like bone screws, orbital implants, mesh for bone augmentation, and acetabular cup and femoral knee implants.^2,3 However, microscopic view showed metal beads that are not affected by the laser from the DMLS process. This is in harmony with an earlier research³⁰ and it might cause flaws within the structure and cause fatigue fracture if the implant is loaded under function.

For a careful topographical characterization it is necessary to use measuring methods that provide numerical and visual images. Specimens’ roughness was determined using non-contact 3D profilometry. It provides a reliable methodology in reporting surface topography in controlled reliable protocols as shown in Figure 2. The appropriate method must be chosen with respect to the desired measuring range, height range, resolution and material to be measured. No method is optimal for every purpose. A “true” surface roughness value does not exist as it varies among other things with the capability of the measuring equipment.³¹ This can be a limitation to our methodology.

Conclusion

Within the limitations of this study, it can be concluded that craniofacial titanium surfaces are affected by the finishing procedure. The surface roughness was comparable among conventionally and DMLS produced titanium craniofacial implants. Electro-chemical treatment or Mechanical polishing combined with acid treatment produce favorably smooth surface and should be followed in preparing surfaces of titanium craniofacial implants.