To weigh the merits of various designs, Dr Frank J. Kratochvil conducted studies in 1962, which visually demonstrated a variety of benefits from the mesial rest, guide-plane and I-bar design. The mesial location of a rest tends to direct occlusal forces in the edentulous segment more perpendicular to the residual ridge.
This has the benefit of reducing pile-up of the gingiva into the small space between the denture base and the terminal abutment. Since the terminal abutment frequently shows a loss of alveolar support, the occlusal rest is often本论文由英语论文网/ placed even farther away from the edentulous space upon a tooth which has better alveolar support and can better accept additional load. However, when a rest like this is located remote from the edentulous segment, it is important to realize that all parts of the denture, from the rest to the terminus of the denture base, would move tissue ward during occlusal loading. Under these circumstances there are two points to remember:
l . If the lingual alveolus in the bicuspid region of the mandible has a flat slope rather than a steep vertical fall, and the mucosa lies thin over the prominent bony contours, then any rigid metal covering the area must be relieved during construction. Adding a thin layer of wax or foil during the block-out phase of framework fabrication attains relief. The resulting cast metal framework will have freedom to function without traumatizing the thin mandibular alveolar mucosa.
The amount of relief prescribed will depend upon the clinical evaluation of the quality and quantity of the denture base support and the efficiency of the occlusion.
2. Rigid metal, which contacts rigid tooth structure between the rest location and the denture base must not rise above the survey line. An error in design here causes elements such as plating and the body of clasps to function as rests on acutely inclined coronal contours, a regrettable occurrence.
With a rest located on the mesial occlusal of an abutment, the I-bar retainer will move in function in all removable partial dentures that depend upon the residual ridge for support. Since this motion can not be prevented, it must be permitted to occur without torquing the remaining teeth. This functional motion may be very small in framework elements located close to the terminal rest (the rest nearest the edentulous space). The magnitude is related to gross movement of the denture base and the distance that the particular element lies from the terminal rest. For example, if an I-bar is placed on the buccal surface of an abutment having a mesial occlusal rest, functional motion at the tip of the retainer is minimal. Even so, it is best to position the I-bar tip in an undercut, which lies at the greatest mesial distal curvature of the buccal surface.
In addition to the traditional occluso-gingival height of contour (survey line) which is recorded in a coronal plane, there is a height of contour in a frontal plane. It is a height of contour whose prominence lies on the buccal and lingual surfaces running from cusp tip to apex and back to the opposite cusp tip本论文由英语论文网/ (the vertical line of contour scribed in a frontal plane). In Class I, II, and IV dentures I-bars should be located in 01 inch of undercut on the line of prominence of，英语论文题目，英语毕业论文 |
