２本連結したインプラント体の破折機序に関する研究

抄録

Titanium is now widely used for dental implants as the most reliable material. Although titanium has excellent mechanical properties, some clinical cases of fracture of titanium implants have been reported. This study examined the cause and mechanism of splinted implant fracture by observing the fracture surface.
Seven structured implants in four cases that had been splinted and placed in the posterior molar region were investigated. The fracture surfaces were examined with stereoscopic microscopy, scanning electron microscopy (SEM), and electron probe microanalysis (EPMA). Longitudinal sections of those implants were also prepared to observe their structure in detail.
Stereoscopic microscope observations revealed that a crack started at the lingual or buccal site. SEM analyses showed a striation-like geometry suggestive of metal fatigue. Corrosion of titanium could not have been relevant to the fracture, as no corrosion product or pitting corrosion was detected. Panoramic radiograph showed that the fracture occurred at the border between the osseointegration zone and bone resorption zone. The longitudinal sections and SEM photos demonstrated that the thinnest part of the implant body was 500 μm, at which the fracture line started.
Based on these results, the following was speculated. The direction of fracture of the splinted implants was not in the mesio-distal direction but in the lingual-tobuccal direction. The level of alveolar bone was positioned in the vicinity of the fractured site and was considered to have a mechanical influence on the structure. Therefore, it is suggested that bending stress was concentrated at the abutment screw tip when an occlusal force was applied to the superstructure. The pitch of the external threads and internal threads of the implant was different, and so the implant thickness changed periodically and a structurally thinnest part appeared. When such a weak part is located at the tip of the abutment screw, it will become the fulcrum of bending stress and thereby promote the concentration of stress, where fracture is the most likely to occur.