Sumiya Hobo

Porcelain fused to metal: Tooth preparation and coping design

Abstract A brief history of the development of the currently popular porcelain-fused-to-metal restoration has been presented. A tooth preparation design has been described with labial reduction of 1.2 mm. done in two planes to allow a bulk of porcelain for esthetic appeal while protecting the integrity of the pulp. A beveled shoulder finish line is advocated on the labial to aid in minimizing distortions which can occur during the fabrication of the restoration. Occlusal contact on metal is recommended, especially for posterior teeth. A bridge design has also been presented, with an exposed metal strip on the lingual and a small pontic tissue contact in porcelain.

Articulator selection for restorative dentistry

There are many types of articulators that can be used for making fixed restorations. While unquestionably the most accurate, the fully adjustable instrument is not feasible or necessary for many forms of simple occlusal treatment. The effects of tooth-hinge axis radius, intercondylar distance, and condylar inclination on occlusal morphology have been discussed. Guidelines have been proposed for matching the type of articulator with the extent of treatment anticipated. The use of small, nonadjustable hinge articulators is not recommended.

Twin-tables technique for occlusal rehabilitation: Part I—Mechanism of anterior guidance

Anterior guidance and the condylar path have been considered independent factors. In a recent study, it was revealed that the anterior guidance influenced the working condylar path and even changed when the lateral incisal path deviated from the optimal orbit. This supports the hypothesis that anterior guidance and condylar path are dependent factors. When setting anterior guidance, it is recommended to set the working condyle so that it moves straight outward along the transverse horizontal axis. The angle of hinge rotation created by the angular difference between anterior guidance and condylar path assists the posterior disclusion, but is not solely responsible. The anatomy of the cusps is created by establishing the appropriate form of the posterior cusps aligned to the condylar path so that it also contributes to posterior disclusion. Posterior disclusion is crucial in controlling harmful lateral forces. The molars must disclude slightly more than the deviation in the condylar path to avoid occlusal interferences.

A kinematic investigation of mandibular border movement by means of an electronic measuring system. Part II: A study of the Bennett movement.

Fifty adults with orthognathic maxillomandibular relationships were selected as the test group. The movement of the working condyle was examined during lateral movement of the mandible with an electronic system capable of measuring six degrees of freedom of the mandible with an accuracy of +/- 0.06 mm. The results were: Orbits of the points on the terminal hinge axis d mm from the midpoint during lateral movement of the mandible varied appreciably as the value of d changed. The orbits more affected were the working side on the horizontal and frontal planes. The point on the working side moved forward, laterally, and downward as the value of d was small; and it moved backward, laterally, and upward as the value of d increased. Depending on the value of d, orbits moved in opposite directions. As the value of d varied from small to large, the point on the working side moved only laterally along the terminal hinge axis when d was 55 mm. From this observation, the Bennett movement is explained as the lateral shift of the working condyle along the terminal hinge axis. Reports concerning the directions and mode of motion of the Bennett movement differ because of the lack of specification of location of the positions (targets) assigned for measurement.

A kinematic investigation of mandibular border movement by means of an electronic measuring system. Part III: Rotational center of lateral movement

M any theories regarding the rotational center of the mandible have been propounded. Spee,’ Hall,’ Monson,’ Needle? and others, formulated theories based on anatomic observations. Gysi” reported a theory based on two-dimensional geometric principles, while McCollum and Stuart” developed a theory based OJ~ their experimentations. Although the rotational center concept of the mandible was discussed, the theories were not proven three dimensionally by means of modern kinematics. ‘l’arget orbits 55 mm from the midpoint of the terminal hinge axis were discussed.” It was found that the orbits shift straight laterally on the terminal hinge axis and do not show any deviation in the sagittal plane. Based on these findings, it was believed that the Bennett movement was a straight lateral shift of the working condyle along the terminal hinge axis. If a point 55 mm from the midpoint is considered the kinematic center of the lateral movement of the mandible, it may be defined as the three-dimensional rotational center of the mandible because it is situated on the terminal hinge axis, which is the rotational center of the mandible in the sagittal plane. In other words, the mandible may rotate in three-dimensional space around a specific point during lateral movement and shift in the lateral direction an amount equivalent to the Bennett movement. The purpose of this study is to explain and illustrate the rotational center of the lateral movement of the mandible by means of an electronic measuring system8 that measures 6 degrees of free movement.

Twin-tables technique for occlusal rehabilitation: Part II—Clinical procedures☆

Molar disclusion is determined by the cusp-shape factor and the angle of hinge rotation. A new twin-tables technique has been introduced for developing molar disclusion by using two incisal tables. It is a relatively uncomplicated technique and does not require special equipment. The final prosthesis with the twin-tables technique ensures a restoration with a predictable posterior disclusion and anterior guidance in harmony with the condyle path.

Distortion of occlusal porcelain during glazing

The amount of vertical height distortion of porcelain cusps and fossae during glazing was investigated, and the following conclusions can be drawn. 1. Vertical height loss was minimized by using opaque cones to support cuspal porcelain. 2. The amount of vertical distortion was less after the second glaze than the first. 3. Adjustment of the occlusal surface after the first glaze followed by reglazing minimized distortion. Glazing distortion can, in fact, be minimized to an acceptable level.

Castable apatite ceramics-its development and physical properties.

水酸 アパ タイ ト(HYDROXY APATITE)は 歯や骨 を 構成す る主な無 機成分で,1970年 代の初頭 よ り生 体 材 料 として応用 され るようになった.こ の硬物質 がエナ メ ル質の主成分 であることか ら,も し水酸アパタイ ト(以 下 アパ タイ トと記す こととす る)を 歯冠修 復材 に利 用で きれば天然歯 に最 も近 い人工歯冠が誕生す る こ と に な る.し か し,現 実 にアパ タイ ト結晶体 を鋳造 しようとし ても溶解が困難で,し か もそ の溶湯の流動性が温度 によ って大き く変化す るため,そ の鋳造 は事実上不可能で あ る.現 在開 発されているアパ タイ トの人工歯根 や人工骨 はいずれ も原料の粉体 を成形 した後に焼結する ことによ り製造 され てい る1~3).この方法は焼結条 件や焼成 収 縮 などの関係でオーダーメー ド的には実行できない.そ し て,仮 にそれ を行 った として も,製 作 される歯冠修 復物 は焼結による収縮が大 きいため良好 な適合が得難 く,白 濁 しているため審美性 に問題があ り,ま た多孔性で ある ため吸湿性 が残存 しやす いとい った欠点 をもつ ことにな る.こ うした理由のためアパ タイ トを歯冠修復材 として 利用する手 だては閉 ざされた ままになってい る. 保母 らと京 セラ ・バ イオセ ラム ・グループは,ア パ タ イ トと類似 の組成を有す る鋳造可能 なセラ ミックスを合 成 し,ロ ス トワ ック ス法 で成形 した のちアパ タイ ト結晶 を析出 させる方 法 を 検 討 し た。そ し て,CaO-P2O5 MgO-SiO2系 セ ラミックスの組成 を適切 に設定 す れ ば 鋳造時の流動性 が改善 されて鋳 造可能 とな り,し かも鋳 造後一定条件下 で再加熱する と材 料中にアパ タイ トの微 結晶が均一 に,し か も安定 して析 出す るこ とを見い出 し た.こ れによ リアパ タイ ト結 晶か らなるクラウンやイ ン レーを製 作す ることがは じめて可能 になった.