Photoelastic stress analysis of mandibular molars moved distally with the skeletal anchorage system.

INTRODUCTION: The purpose of this study was to analyze photoelastically the stress distribution around teeth in the simulated distal movement of mandibular molars with the skeletal anchorage system. METHODS: Two types of the photoelastic mandibular dentition models were used, 1 before and 1 after distal movement of the second molar. The experiment was performed with 3 forms of traction--first-molar single traction, second-molar single traction, and simultaneous first- and second-molar traction. The direction of traction was set parallel to the occlusal plane and at an angle of 30 degrees downward to the occlusal plane. RESULTS: In the first-molar single traction model, extremely high stress was generated around the first molar with traction parallel to the occlusal plane. With the traction 30 degrees downward to the occlusal plane, all models showed the stress around the molars extended distally and downward. CONCLUSIONS: Simultaneous traction of the first and second molars might be preferable to the sequential traction of each molar to prevent the unfavorable distal tipping of the first molar. Regardless of whether simultaneous or sequential traction is used, the downward traction to the occlusal plane seems to induce intrusion of the molars as well as their distal movement.

Neuromagnetic evidence that gingiva area is adjacent to tongue area in human primary somatosensory cortex.

The somatotopic organization of the human primary somatosensory (SI) area in the cerebral cortex has been intensively studied for the hand, lip, and tongue, but little is known about the gingiva. Penfield concluded that the gingival SI area was above the tongue area, as shown in his famous homunculus map. However, our recent study suggested that the lingual gingiva area was not so different to the tongue area. To delineate the fine SI somatotopy of the gingiva area, evoked magnetic fields were measured in 6 healthy subjects for the stimulus of the anterior or posterior and upper or lower parts of the lip, buccal and lingual gingiva, and tongue. Source position was estimated by a current dipole model at the first peak of the posterior-oriented current in a total of 12 cerebral hemispheres contralateral to the stimulation side. No significant difference was found between the positions of anterior and posterior or upper and lower parts of each structure. Both buccal and lingual gingiva areas were localized adjacent to the tongue area, but significantly lower than the lip area. We believe that the fine SI somatotopy of the human oral structures should be reconsidered.

Biomechanical comparison of straight and staggered implant placement configurations.

The effect of buccolingual staggered implant placement on stress distribution within the supporting structure was examined photoelastically. Two photoelastic models of a human mandible, edentulous distal to the canine, were fabricated. Three screw-type implants were embedded into the edentulous region of each model. The implants were placed in a straight line in one model and in a buccolingual staggered configuration in the other. Vertical and lateral loads were applied to a fixed partial denture superstructure. No clear biomechanical advantage to a staggered 1.5 mm buccal and lingual offset placement configuration was observed.