the interference of hard or soft tissues with insertion and removal of the prosthesis.

(2) Modifications may be needed to provide nearly parallel guiding planes on the abutment teeth to guide clasp arms and minor connectors to their terminal positions and to resist dislodgement.

(3) Contours of abutment teeth may be modified to improve the guiding planes or reposition survey lines.

(4) Retentive areas may need to be redistributed. Retention is usually the last element considered in the analysis of a cast. The amount of retention on an abutment tooth can be increased or decreased by slight changes in the path of insertion. This will not create a retentive area where none exists, but retention can be increased in one segment of the arch at the expense of the opposite segment. This is true only when adequate guiding planes have been developed.

(5) If there is conflict between esthetic and mechanical factors which requires a compromise, the dental officer must decide which shall take precedence. He may reevaluate the treatment plan to change certain mouth preparations.

10-5. Marking Survey Lines

After the proper tilt has been established, the surveyor table screw is locked, the analyzing rod attachment replaced with the carbon marker (A, fig 10-2), and the survey lines traced on the abutment teeth (B, fig 10-2). Survey lines generally fall into three general types characterized by their relation to the occlusal rest on the side of the tooth adjacent to the edentulous space (fig 10-3).

a. Class 1 Survey Line. The class 1 survey line is low on the occlusal rest side of the tooth and high on the opposite side. This indicates the retentive infrabulge area is on the side of the tooth opposite the edentulous space (A, fig 10-3).

b. Class 2 Survey Line. The class 2 survey line is high on the occlusal rest side of the tooth and low on the opposite side. This indicates the retentive infrabulge area is on the same side as the occlusal rest, next to the edentulous space (B, fig 10-3).

c. Class 3 Survey Line. The class 3 survey line is high on the occlusal rest side and on the opposite side of the tooth. This indicates there are two retentive infrabulge areas, or one continuous retentive area from one side of the other (C, fig 10-3).

methods for tripoding a master cast: point tripod and line tripod. The point tripod method uses three separate small spots marked on the master cast with the carbon marker. They are marked as far as possible from each other in the horizontal plane and at definite landmarks. If possible, spots are marked in areas which will not be covered by the prosthesis so that they may be located on any duplicate cast and the cast may be properly positioned (fig 10-4). The line tripod method uses three widely separated lines drawn on the land area of the cast (fig 10-5).

10-7. Maxillomandibular Relations and Prosthodontic Prescription

Following the construction of the occlusion rims, the dental officer uses them to make a centric jaw registration. This procedure orients the casts in the same position as the teeth are in the patient's mouth. He places tripod marks on the master cast. He also fills out a DA Form 2868 (Prosthodontic Prescription and Consultation Request) (fig 10-6) for the cast. Both prescription and cast are mailed to a Regional Dental Activity for fabrication unless the fabrication is to be done at the home station.

C. CLASS III.

Figure 10-3. Types of survey lines.