Implant surface modifications
There are four methods for modifying surface properties on implants: topographical, chemical, physical and mechanical. The first two have been the subject of particular interest in recent years.
The market is now dominated by moderately rough isotropic surfaces. But now a question arises: could they also be harmful? The speaker posited two questions:
- Is it true that the rougher the surface, the higher the risk for peri-implantitis and therefore the greater the difficulty to clean it?
- Could rough surfaces be a cause of peri-implant inflammation, since rougher surfaces are associated with a higher release of particles/ions?
The speaker explained that oxidised surfaces can improve protein adsorption and, consequently, cell adhesion (Hing et al., 2004, Anselme et al., 2000). At the same time, however, they may also facilitate bacterial colonisation (Derks et al., 2015).
A recent meta-analysis compared turned and oxidised surfaces, and verified that turned implants have a significant risk ratio of 2.82, which puts them at a higher risk of failure than oxidised implants. However, the study found no difference in marginal bone levels between the two surfaces (Chrcanovic et al., 2016) (fig 1–2).
Another systematic review compared the long-term clinical results of different implant surfaces. Implants with anodised surfaces showed the fewest failures; whereas turned and blasted implants showed the lowest levels of marginal bone loss (Wennerberg et al., 2018). We can conclude that modern implants with a moderately rough surface have improved primary stability and can promote faster osseointegration, although mechanical and biological complications do still exist.
In 2007, the concept of nanoroughness (10–9 mm) was introduced to implant surface research (Webster et al., 2007). Nanoroughness was achieved by coating the implant surface (HA, TiO2). However, it was soon found that nanoroughness can spontaneously form over time, and was reported two weeks after surface contact with water or saline (Wennerberg et al., 2013) (fig 3–4).
An experimental study of rabbit tibias found a correlation between nanostructure surfaces and pull-out forces (Wennerberg et al., 2014). The potential benefits of this finding on the clinical environment has not yet been demonstrated.
Chemical modifications (fig 5–6)
Current research on implant surfaces primarily focuses on combining chemical and topographical modifications. One such example is the EISA technique (evaporation induced self-assembly), which aims to chemically coat implant surfaces with a thin, mesoporous TiO2 film which configures structures of 6nm. Apatite grows in the nano pores (Karlsson et al., 2012). These nano pores can be loaded with Mg ions, and this kind of modified surface was found to increase the strength of the bone-implant interface in an experimental study on rabbits (Galli et al. 2016).
Another research focus has been local drug delivery. In a recent study, a titanium mesoporous film covered with a polymer and with gold nano rods incorporated was found to be capable of delivering drugs to the surrounding area (Alenezi et al., 2019) (fig 7).
The speaker described another ongoing study in biodegradable implants which use Mg10Gd (magnesium gadolinium alloy). These implants could provide a useful alternative in large-scale bone reconstructions, where it would be advantageous to not have to remove them after bone healing.
Although polyetheretherketone (PEEK) alone doesn’t achieve osseointegration, it is susceptible to surface modifications. PEEK surfaces can be modified with hydroxyapatite (HA) nano coating to improve bone reactivity (Johansson et al., 2017). However, this method is still in an experimental stage, and PEEK needs to be researched and evaluated further before being introduced to clinical use (fig 8).
It is worth emphasising the recent investigation conducted by Duddeck and colleagues on the impurities and contamination of commercial implants. Implant surfaces from several well-known brands and copycat products from others were examined with SEM imaging and energy-dispersive X-ray spectroscopy. The study found many more impurities than expected. More contamination from both organic and inorganic sources was found in the copycat products compared with the originals, such as organic residue and unintended metal particles (e.g. iron or aluminium) (Duddeck et al., 2019). The true clinical significance of unclean implants remains unknown (fig 9–10).