Artificial dental implants are the most reliable restoration option available for mature tooth loss. Despite their long potential lifespan, poor integration into the jaw and a slow healing process often create implant failure. Ensuring longevity requires adequate protection of the abutment-implant interface, protection achieved with surface functionalization by biocompatible coatings or advanced surface plasma treatments. The research summarized in this article examines the effect of using titanium nitride (TiN) and diamond-like carbon (DLC) thin-film coatings for implants.
The Problem with Implants
Dental clinical research studies conducted in the United States and Europe have reported more than a 90 percent success rate for dental implant cases.1 A survey from the American Academy of Implant Dentistry shows that 9 percent of the American population have successful dental implants and estimated the future demand for implants to increase by 500,000 annually. Traditional dentures used for restoration driven dentistry have historically lasted for 7–10 years, while artificial dental implants can have an extended lifetime of about 15-20 years.2 However, several issues can reduce implant lifespan. Major factors include insufficient osseointegration, bacterial invasion on the implant, and bio-mechanical and biochemical instabilities existing within the implant system.
Dental implant components are made of various metal alloys (Co-Cr, Ni-Cr, Ti, Ti6Al4V) and consist of an implant, an abutment, and a screw. The screws hold the implant and abutment together inside the jaw. Most clinical case studies conducted at laboratories and on patients have suggested a need for improvement of the biochemical and biomechanical quality of the implant-abutment interface. The improvements would be designed to minimize any associated complications during and after implant surgery. Biochemical failure caused by insufficient bio-inertness at the implantgum-bone interface leads to loosening of the implants. This issue accounts for about 17 percent of reported implant failures.3,4 However, the failure rate caused by mechanical complications investigated are even higher at 31 percent.3
Studies on mechanical failure of implants from various sources reveal that higher friction forces and abrasive wear at the abutment-implant interface increase the loss of pre-load during the abutment-implant screw tightening procedure.5,6 Furthermore, sequential micro-movement at the abutment-implant interface during chewing and biting leads to fatigue fractures of the abutment and paves the way for bacteria.7,8 Figure 1 shows the critical functional requirements at the abutment-implant-screw interfaces and necessary coating properties needed to protect the implant-abutment-screw interface.
Technological Advances Create New Options
In the past decade, advanced plasma-based physical vapor deposition (PVD) and plasma-enhanced chemical vapor deposition (PE-CVD) nanostructured coatings have become more frequently considered for biomedical applications. Such coatings can be made very attractive for aesthetic appeal while meeting application-specific functional needs, particularly for dental implants. Using plasma coating technology, these combined possibilities can be a very effective and economical option available for dental OEM manufacturers.
Recent advances in PVD and PE-CVD technologies have also stimulated development of nanostructured coatings with various compositions and microstructures designed to achieve a wide range of properties for multisectoral applications, including biomedical and dental. These coatings are fabricated in high-vacuum chambers using a plasma-enhanced chemically reactive environment.
Figure 2 shows the VaporTech® Cadence™ system on which this research was conducted. The system is designed to fabricate a range of nanostructured coatings using Remote Anode Assisted Magnetron Sputtering (RAAMS™), a unique enhanced magnetron sputtering PVD technology for metal-based coatings. The system is also designed to operate in PE-CVD mode for diamond-like carbon (DLC) coatings. The blue space in Figure 2 is the coating zone where coating deposition on parts takes place. Table 1 shows the functional features of the coating system in PVD and PE-CVD mode.
Promising Results for Dental Implant Longevity
Potential coatings for dental implants can be classified as wear-resistant and solid lubricant coatings. The aesthetic appearance and biofunctionality of TiN and DLC coatings make them a natural choice for biomedical applications. Both coatings show promise for their wear resistance, biocompatibility, and low-friction properties compared with uncoated components in in-situ biomedical conditions.
During natural human oral movement, fatigue load generated at the jaw during biting and chewing forces occurs at between 20 and 400 N in both the lateral and vertical directions. Coated implant-abutment surfaces used for restoration purposes must be stable during high fatigue load and must act as a protective layer on implants against a corrosive attack in the natural oral fluid environment.
Figure 3 shows TiN-Gold and DLC-Black coatings on Ti6Al4V substrate, a potential dental implant material. Table 2 lists the basic characteristics evaluated for these coatings. In PVD coatings, residual stress is a critical factor that influences the adhesion of coating at the interface and the thickness to which coating can be grown, as well as the tri-bological, fracture, and fatigue properties.9–12 The standard Daimler-Benz Rockwell indentation test (VDI 3198) offers a quick method for evaluating the residual stress level and adhesion of the coating to the substrate.
In this test, a conical-shaped diamond tip was used to create an indentation on the coated component at a load of 150 kg. After the test, the indentation is evaluated to understand the nature of any coating fracture or delamination.13 According to the above standard, both TiN and DLC coatings in this case have shown HF1 (the best possible) adhesion ratings on Ti6Al4V substrate (see Figure 3) without any delamination across the coating-substrate interface.
Nano hardness testing (at 20 mN; Berkovich tip) and biotribological tests evaluated the structural reliability of the deposited coatings. Biotribological tests were conducted per ASTM F732 and ISO 14242. These test methods standardize the motion, load, and speed profiles relevant to oral biomechanical conditions. Evaluated test results were compared to the properties of similar coatings evaluated in literature.
Table 3 shows the nanomechanical properties of uncoated and coated TiAl6Al4V substrate. Both TiN and DLC coatings deposited in the test system have shown high hardness and elastic recovery. This indicates potential for providing better wear resistance and higher load-bearing capacity. These attributes are essential for mechanical durability at the implant-abutment interface.
How to Get Coatings “Hard Enough” but Not “Too Hard”
Recently, in-situ dental studies have shown that very high hardness is not desirable on implant surfaces as it tends to damage any opposing tooth during occlusal contact. However, the system used in this study is equipped with adequate plasma process control tools to modify the mechanical properties of coatings and provide optimum biomechanical functionality to match internal oral tooth dynamics.
Tribological test performance data for TiN-Gold and DLC-Black is shown in Tables 4 and 5. Data are compared with previously reported findings for similar coatings deposited by other industrial and research sources. For each test, the biological liquids used, as well as the dynamic contact pressure measured, are shown to compare the relative performance of each coating.
The tested PVD TiN coating showed a comparable friction coefficient to the other coatings tested. The amount of wear was also comparable, despite the much higher contact pressure used during the test. The data show that in using this coating system, deposited TiN has similar or slightly improved performance compared with the other coatings. The DLC-black coatings show comparable or lower friction coefficients, as well as significantly decreased wear despite the higher contact pressure during the test.
The coatings evaluated in this study showed improved mechanical and biotribological properties under high stress cyclic loading conditions. They show the required adaptability to address the wear and friction issues associated with dental implants. The TiN coating tested could be a good coating for the bottom of the abutment, where it can provide a smooth surface, fretting wear resistance, biocompatibility, and load-bearing capacity.
DLC may be a good coating on the screw and implant surfaces. Its low friction would be useful during surgical procedures. DLC also has the necessary bone compatibility as well as providing a barrier that prohibits metal ion release at the implant-bone interface. The coating system used in this study can produce thick nanostructured coatings with high deposition rates and low residual stress.
This article was written by Ganesh Kamath, PhD, an R&D scientist with Vapor Technologies, Inc., manufacturer of the VaporTech Cadence thin-film deposition system used in this study. For more information, Click Here .