Purpose: The aim of this article is to standardize the designs of zirconia copings and abutments from the viewpoint of fracture mechanics, by using a 3-D finite element method.
Materials and methods: 3-D finite element models were created in which single standing screw implants were placed in the alveolar bone, having zirconia coping crowns and zirconia abutments. Three zirconia copings were designed and designated as follows: RP0, no porcelain support; RP1, 1 mm porcelain support; RP2, 2 mm porcelain support. Similarly, three abutments were designed: RP0, having a regular diameter neck (as 4 mm implant platform); NP0, having a narrow concave curved neck; and WP0, having a wide 5 mm diameter abutment neck. Oblique static loading 100 N (30-degree buccolingal direction) was applied, and the maximum principal stress was analyzed under the different coping and abutment designed models to estimate the structural integrity of their designs.
Results: Maximum principal stress of zirconia copings was measured in RP0 as 60.2 MPa; in RP1 as 14.1 MPa; and in RP2 as 3.8 MPa. Wider porcelain support models tended to decrease principal stress. The maximum principal stress of zirconia abutments was measured in NP0 as 189 MPa;in RP0 as 94.7 MPa;and in WP0 as 91.0 MPa. The maximum principal stress in NP0 was twice that of RP0 and WP0.
Conclusion: It is suggested that a zirconia coping design having a wide porcelain support would be effective to reduce stress in the implant crown. However, a narrow concave curved neck zirconia abutment might be unfavorable from the viewpoint of fracture mechanics.
This study aimed to clarify the influence of zirconia on the sliding-wear behavior of titanium. Two-body wear tests were performed using tetragonal zirconia polycrystal (TZP), cp-titanium (JIS type 4), and titanium alloy (Ti-6Al-4V) in distilled water. In addition, a surface roughness measurement, surface hardness test, optical microscopic observation and electron probe micro-analysis were carried out to speculate on the wear mechanism. The hardness of TZP (Hv=1,356) was much higher than that of cp-titanium (Hv=177) and titanium alloy (Hv=256). The cp-titanium and titanium alloy were quite susceptible to wear by both TZP and titanium. On the other hand, the TZP was less susceptible to wear by both materials. It is considered that cp-titanium and titanium alloy subjected to wear by TZP had the mechanism of abrasive wear, whereas titanium subjected to wear by titanium had the mechanism of adhesive wear.
In summary, though the titanium was susceptible to wear by zirconia, the wear amounts by the zirconia were equivalent to those by titanium. These results suggested that zirconia abutments were acceptable for the clinical application.
Dexmedetomidine (DEX), a highly selective α2-adrenergic agonist, produces a sedative effect through a central mechanism. However, DEX may not be the sedative agent of choice in dental procedures, in which its plasma half-life is 2 to 3 hours. Sedation by a combination of DEX and other select drugs may reduce the total dosage of DEX and strengthen the amnesic action. We evaluated the amnesic action and recovery process under DEX sedation combined with butorphanol and midazolam for implant surgery.
Fifteen patients scheduled for implant surgery were enrolled in this study. They were administered with 0.01 mg/kg butorphanol and 0.05 mg/kg midazolam, and DEX infusion started. They were managed with 0.2 to 0.7μg/kg/h DEX to keep patientʼs level of being within 2 or 3 according to the Richmond Agitation-Sedation Scale Score. DEX infusion discontinued at the end of each operation. To assess an amnesic action, memory of local anesthesia, auditory memory (calling at the start of suture) and visual memory (watches or syringes etc. shown at the end of each operation) were verified at three hours after the operation. To determine psychomotor function and recovery of equilibrium in the recovery process, Trieger Dot Test (TDT) plot error ratio and a one-leg standing test with eyes closed (OL-test) were performed. Before leaving the hospital, a questionnaire regarding patientʼs feeling during and after the operation and a questionnaire regarding the management of sedation were taken.
The results were summarized as follows;
1. In intravenous sedation, the amnesic action of memory of local anesthesia, auditory memory and visual memory was 87%, 80% and 80% respectively.
2. TDT plot error ratio (24.5±25.3%) at one hour after the end of each operation showed a significant increase and all patients were successful in the OL-test at three hours after the end of the operations.
3. In a questionnaire for patients, 14% had uncomfortableness during and after sedation. However, all patients hoped that they would be managed with sedation in the present study if they were sedated again. In a questionnaire for Surgeons, they could operate smoothly. For this reason, 67% of them answered that patients hardly choked on irrigation saline. All surgeons hoped to operate under such a sedation environment in our study if they had an opportunity to repeat this procedure.
These results indicated that sedation with butorphanol, midazolam and DEX has a quality amnesic action, produces a smooth operation and satisfies both surgeons and patients. Therefore, this combination is available for sedation on the day surgery.
For imaging diagnosis targeting dental implant placement, panoramic radiography, computed tomography (CT), multi-slice computed tomography (multi-slice CT), and recently, cone-beam computed tomography (conebeam CT), are used.
However, the use of CT in routine clinical practice is difficult in some cases for various reasons, and the buccolingual alveolar width may be wide in others.
Therefore, to evaluate the usefulness of panoramic radiography, we investigated the mandibular canal wall-imaging ability of digital radiography, and the dimensional accuracy of panoramic radiography.
In 100 patients for whom mandibular implant placement was planned, 224 molar regions were investigated. The mandibular canal wall-imaging ability of panoramic radiography was investigated, and measurements of the distance between the alveolar crest and upper wall of the mandibular canal were compared with those on double-oblique imaging. Regarding the measurement on double-oblique imaging, one measurement was performed vertically for the slice surface, and the other was bucco-lingually along the direction of the aluminum tube.
The mandibular canal wall-imaging rate on panoramic radiography was 62.5%. Furthermore, there were no differences in the measurement on the double-oblique imaging. Differences in measurements between panoramic radiography and double-oblique imaging were 1.0 mm or less in approximately 50% of the measurements.
In conclusion, although digital panoramic radiography was performed for implant imaging diagnosis, only about 30% of the mandibular molar regions showed clear imaging of the upper wall of the mandibular canal along with a measurement accuracy of within 1 mm, suggesting that the accuracy of panoramic radiography is insufficient.
We report a case of a patient who had undergone functional reconstruction of the mandible with revascularized free bone transplantation and dental implants and had recovered from a fracture of the fixture after a six-year functional period. A 39-year-old male, who developed a recurrent ameloblastoma of the mandible, underwent mandibular segmental resection and immediate reconstruction with the revascularized scapular osteocutaneous flap. Eight months after surgery, six fixtures (diameter, 3.75 mm;length, 15 mm) were installed in the reconstructed mandible. Six months after implantation, the abutments were connected to the osseointegrated fixtures, and the superstructure extending from the right first molar to the left first premolar was seated for occlusal reconstruction. However, after functioning for six years, the right-most posterior fixture suffered a fracture at the level of the abutment screw apex. The part of the fixture retained in the bone was retrieved using a trephine bur. The bone-implant contact ratio of the retrieved fixture was 79.3%. The surrounding bone density was approximately 830 Hounsfield units as revealed by computed tomograms. New bone of 10～15 mm in thickness formed lingually to the osseous flap was also observed. This might have occurred due to unintentional periosteal distraction osteogenesis, which was caused by the suprahyoid muscles attached to the periosteum on its lingual aspect. The main reason of the fracture appeared to be a large lateral force element acting on the inclined fixture, which was amplified due to the lever action. The additional bone allowed the installation of another fixture of 5 mm in diameter in a position that was 7 mm posterior and 13 degrees inferior to that of the retrieved fixture. Three months after implantation, the abutment was connected to the additional osseointegrated fixture, and the superstructure modified to fit the abutment was seated. Approximately 10 years of the functional course have been uneventful and the bone surrounding the additional fixture has been stable.