. The science and practice of dental surgery. clination ofsides. Sometimes complicated shapes occur,which may be looked upon as two or moresimple cavities jomed into one; such are thecombmation crown and approximal cavitiesof molars and premolars, forming isthmus , saddle-shaped , and dovetail all differ from the simple type ratherin their extent than in the principles of theirpreparation. In aO cases the cavity must, if possible, be so shaped that dislodgement is resisted by some definite keying of the porcelain into the tooth substance. In saddle-shaped cavities, involving the
. The science and practice of dental surgery. clination ofsides. Sometimes complicated shapes occur,which may be looked upon as two or moresimple cavities jomed into one; such are thecombmation crown and approximal cavitiesof molars and premolars, forming isthmus , saddle-shaped , and dovetail all differ from the simple type ratherin their extent than in the principles of theirpreparation. In aO cases the cavity must, if possible, be so shaped that dislodgement is resisted by some definite keying of the porcelain into the tooth substance. In saddle-shaped cavities, involving the medial, crown, and distal aspects of a molar or premolar, the natural shape affords ideal retention and resistance to dislodgement by the bite. 407 As good a result is also obtained in isthmus and dovetail cavities, whose shape wUl onlyallow the insertion or withdrawal of tlie inlayfrom the cro«ii surface. In cavities exposed to the bite, and in wliichno dovetail or post can be arranged, the bestplan is to make the floor and wall meet at a. BFig. 471. (A) Approximal cavity in canine, opening on to lingual surface. (B) Transverse section of san^e tootli witli inlay in position. (C) Longitudinal section of same tooth with inlay in position. sharp angle at the point opposite the directionof the prevailing force. In such a position asthe cervical part of a premolar or molar, or thelabial margm of a front tooth cavity that ojJenson the lingual surface, the angle may for a shortdistance be less than a right angle, so that thepressure tends rather to diive the uilay furtherinto the cavity than to dislodge it. A different mode of preparation is often, al-though not always, adojited, wlien a inn, post,or staple is used for the retention of the more the retention depends on the metal results in actual practice, or by experience ofthe analogous arrangement in the ordinary pinteeth used in j)rosthetic work. In preparing a cavity for this kmd of reten-tion, it is necessary to de
Size: 2337px × 1069px
Photo credit: © The Reading Room / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookdecade1910, booksubjectdentistry, bookyear19
