The patient’s course suggested important clinical issues. Although a well-designed SI can function successfully for many years, chronic mechanical irritation caused by the mobility of an improperly designed SI can be a physical factor in carcinogenesis. More importantly, prolonged peri-implantitis without regular maintenance can act as a biological factor in carcinogenesis, although tobacco and alcohol are often considered the major risk factors for oral cancer.
SIs fitting the shape of the jaw are integrated into the subperiosteal layer of the alveolus and held in place by a fibrous connection generated by the overlying periosteum [16]. A properly designed SI can function successfully for a significant period [17], with bone morphology and texture playing major roles in determining its success [18]. In the case of a maxillary SI, the anterior nasal spine, zygomatic buttresses, canine pillars, and tuberosities are the main struts of the SI for stability against vertical and lateral forces [18]. Previous studies have suggested that the higher failure rate of maxillary SIs than mandibular SIs could be due to the following: (1) the negative effect of gravity on the maintenance of a good bone–implant contact zone and (2) difficulty in positioning maxillary implants on a cortical bone basis [8, 17]. Furthermore, the use of SIs is contraindicated when the opposing teeth are natural [16]. In this case, the main struts of the SI were not designed to accommodate anatomic landmarks, and the bone–implant contact area was small. In addition, one of the two retaining screws fell off immediately, and the opposing teeth had a lower molar with a large masticatory force. These factors resulted in the deterioration of the retention and mobility of the SI.
Available evidence indicates that chronic mechanical irritation could act as a tumor promoter [5]. Recurrent persistent inflammation due to chronic mechanical irritation may induce or promote carcinogenesis by DNA damage, inciting cell proliferation, and the release of cytokines and growth factors [5, 19]. Further, chronic mechanical irritation can be considered a factor affecting carcinogenesis when all of the following conditions are registered: (1) objective clinical cancerous lesions compatible with a mechanical origin; (2) mechanical factors present before the onset of the cancerous lesion, and (3) mechanical agent in direct contact with the cancerous lesion during functional/parafunctional movements [3, 4]. In this case, the remaining screw and metal framework with prolonged mobility were present before the onset of the SCC. The screw and framework placed on the maxilla frequently moved during mastication and were in direct contact with the palatal tissue after the formation of the fistula.
The characteristics of peri-implantitis, which is the most common peri-implant lesion, include swelling, erythema, suppuration, and bone loss [20]. In addition, chronic infections associated with a failing SI can manifest as extraoral/intraoral fistulas [16]. Various mechanisms underlying carcinogenesis related to persistent inflammation, which is regarded as a possible risk factor, have been suggested [15]. The concept of inflammation-induced cancer was explored with respect to free radicals, such as reactive oxygen and nitrogen species, generated by the inflammatory cells, which are suspected to play a pivotal role in causing mutations and carcinogenesis [12, 21]. In this case, mucous perforations around the six struts were a direct gateway for organisms from the oral cavity, and the organisms invaded the subperiosteal space expanded by the mobility of the metal framework. Eventually, the peri-implantitis accompanying the fistula occurred around the remaining loose screw.
Previously hypothesized factors regarding carcinogenesis associated with dental implant include the following: (1) dental implant corrosion; (2) the possible association between corrosion products and cancer; (3) the possible association between particulate titanium and cancer, and (4) the hypothesized carcinogenic effect of sustained metallic ion release after implant placement [22]. Conversely, titanium is one of the most inert metallic ions that is especially resistant to corrosion (with a corrosion rate of 0.003 µA/cm2) owing to the stability of the layer of titanium dioxide; however, inflammatory reactions, such as peri-implantitis, could upset the layer favoring possible corrosion [22,23,24]. In this case, EDX analysis showed no titanium in the resected specimen; conversely, SEM showed a few microcracks, suggesting pitting corrosion. In addition, the patient had no history of smoking or drinking; therefore, the influence of chemical factors on carcinogenesis was considered relatively small.
Ultimately, based on these clinical courses, it is conceivable that SCC can emerge through persistent inflammation caused by chronic mechanical irritation and peri-implantitis as physical and biological factors, respectively. Therefore, this case indicates that lesions associated with an exposed SI should be considered for possible malignancy.