Asier Eguia del Valle, Senior Lecturer at the University of the Basque Country (UPV/EHU). Member of the Spanish Society of Oral Surgery (SECIB). 

Javier Alberdi Navarro, Acting Lecturer at the University of the Basque Country (UPV/EHU). Member of the Spanish Society of Oral Surgery (SECIB). 

ABSTRACT 

The presence of metallic and other particles in the tissues surrounding dental implants has been associated with the aetiopathogenesis of peri-implantitis and peri-implant related reactive lesions. Biotribocorrosion, deficient decontamination of prostheses prior to their placement, or release of the surface of implants during insertion may be their primary sources. 

INTRODUCTION 

In recent years, interest has increased in learning about the source and possible effects of metallic microparticles (MM) and those of other origins—cements, resins, etc. that may sometimes be detected in hard and soft tissues that surround dental implants⁽¹⁾. Although lacking definitive conclusions, several studies have indicated recently that these particles may be relevant in the aetiopathogenesis of peri-implantitis and peri-implant related reactive lesions⁽¹⁾. 

MATERIALS DETECTED 

By applying different methodologies, both ions and microparticles of titanium (Ti), (either commercially pure cp-Ti, grade II or IV), titanium dioxide (TiO2), or cements^(1-3) have been detected in bone and soft tissue surrounding some implants. Other types of particles have also been identified, such as chromium-cobalt (Cr-Co) alloys, ceramics, polishes or resins, although they have been much less studied and their possible local effects are even less well known than those of Ti particles^(1-3). 

SOURCE OF THE PARTICLES 

The mechanisms responsible for the release of particles are only partly known. Friction at the implant-abutment interface from load-related micromotion, especially in cases of poor fits, biotribocorrosion, and detachment from the implant surfaces during insertion or during therapeutic procedures—scraping, implantoplasty, and others—may be the main causes^(1,4-6). 

Biotribocorrosion is the degradation of a material produced by the synergistic effect of the action of microorganisms and different cells, chemical or electrochemical reactions, and mechanical stress. 

Different studies indicate that the bacteria present in the biofilm on implants and prosthetic attachments may lower pH, damage the passive protective layer of surface oxides, and facilitate the release of microparticles and ions^(5,6). Similarly, the action of corrosive substances present in the mouth (citric acid, lactic acid, chlorides), toothpastes and other hygiene and tooth-whitening products, or the solutions used for decontamination in peri-implantitis treatments, may alter the oxidation layer⁽¹⁾. 

BIOLOGICAL EFFECTS 

The actual effect of MMs released on peri-implant tissues remains a controversial issue. In vitro and in vivo studies conducted in the fields of dentistry and orthopaedics have demonstrated various biological effects of MMs on the tissues surrounding implants and other alloplastic devices. Micro- and nano-particles may be identified as foreign bodies by the immune system, stimulating the release of pro-inflammatory mediators^(7,8). 

Certain studies^(7-10) have shown that Ti particles derived from dental implants can stimulate the production of pro-inflammatory cytokines such as IL-1, IL-6 and TNF-α, activating or enhancing the inflammatory response. MMs have also been seen to produce direct effects on fibroblast proliferation (including cytotoxicity), changes in neutrophils and macrophages, and effects on regulation of the RANKL/RANK/OPG system, which controls the resorptive activity of osteoclasts. 

METALLIC MICROPARTICLES AND PERI-IMPLANTITIS 

Peri-implantitis is an inflammatory process that can be triggered and modulated by numerous local and systemic factors. The literature available suggests that MMs or ions released from the surface of implants, abutments and prostheses and at their respective interfaces may negatively influence the pathogenesis of peri-implant bone loss^(1,4,6,7). 

However, most of the research on the biological effects of MMs has been conducted in vitro and has focused on the effects of Ti ions/particles (or its alloys), and there is little conclusive data in vivo and in relation to other materials used in implant dentistry. 

Ti MMs might increase the secretion of IL-1ß, IL-6, and TNF-α in cultured human macrophages, and alter the RANKL/RANK/OPG ratio (the system regulating osteoclastic activity) in gingival tissues in the presence of lipopolysaccharides (LPS) from Porphyromonas gingivalis, or might contribute to the interruption of epithelial homeostasis, activating the response to DNA damage (CHK2 activation and BRCA1 recruitment), or increase the rate of early osteoblast apoptosis(1,9, 10), amongst other consequences. All these phenomena may have negative synergistic effects with other aetiopathogenic factors in peri-implantitis. However, many of these results have not yet been substantiated in vivo. 

PERI-IMPLANT RELATED REACTIVE LESIONS 

Some studies^(11,12) have analysed reactive lesions around implants and teeth by polarised light microscopy, showing the presence of a notable concentration of metallic foreign bodies in two of the most common reactive lesions (pyogenic granuloma and peripheral giant cell granuloma), and even a significantly greater presence of particles in lesions surrounding implants than in lesions surrounding teeth. Despite this, the precise role of these MMs has not yet been fully determined. 

EVIDENCE IN OTHER FIELDS 

Regarding the effects of MMs, there is a significant amount of scientific literature available on various prosthetic devices used in trauma, such as plates or total joint replacement prostheses^(8,13). Various studies of hip replacement prostheses have linked the release of metallic and ceramic wear particles with the origin of the so-called aseptic loosening (failure of osseointegration in the absence of infection) and with different types of benign tumours, cysts and pseudo-tumours^(8,13). 

DETECTION OF PARTICLES IN PERI-IMPLANT TISSUES 

Although its clinical effect is uncertain, the presence of MMs and ions in hard and soft peri-implant tissues has been amply documented using different techniques, such as scanning electron microscopy (SEM), energy-dispersive X-ray spectrometry (EDS), polarised light microscopy (PLM) and inductively coupled plasma mass spectrometry (ICP-MS), amongst others^(1-5). 

On many occasions, they may be detected in biopsies and smears of healthy patients and patients with peri-implantitis^(10-12,14), and in some studies, a greater number of MMs has been observed in patients with peri-implantitis than in healthy patients⁽¹⁴⁾. 

CONCLUSIONS 

Although the role of MMs in the aetiology of peri-implantitis and peri-implant related reactive lesions has not been conclusively determined, there are indications suggesting that research in this field and use of clinical protocols and procedures would be warranted in order to abate the release MMs.