direction of the long axis of each tooth is changed relative to the horizontal plane and the change in the position of the survey line changes the relative location of the infrabulge and suprabulge areas on each tooth. Tilting can be used to advantage in surveying to make the most of desirable infrabulge areas on abutment teeth and to minimize the effects of those which are undesirable. Changing the position of the survey line will, in some cases, help to determine the type of direct retainer which will be used (fig 9-18). The dental officer, not dental prosthetic specialist, determines the type of direct retainer to be used. The specialist, however, must be familiar with the procedure. Undesirable soft tissue undercuts or undercuts on teeth can be minimized to better position the rigid parts of the removable partial denture (fig 9-19). Yet it must be emphasized that tilting will not produce an undercut on a tooth or tooth surface that does not already have one.

A

B

TISSUE UNDERCUT

b. Path of Insertion. The path of insertion and removal is the route traversed by the removable partial denture as it is inserted or removed from the cast or the mouth. As the cast is tilted in various planes on the surveyor table, the path of insertion is changed relative to the occlusal plane of the teeth on the cast. When the cast is tilted to a given plane, the survey lines marked, and the prosthesis designed to those survey lines, it can only be inserted and removed from the cast in the same direction as it was surveyed. The path of insertion should always be parallel to the surveyor spindle.

9-15. Control of Stresses Induced by

Removable Partial Dentures

The success of a removable partial denture depends directly upon the control of stresses that can be induced by the partial denture. Uncontrolled stresses can cause irreparable damage to the remaining oral structures supporting the prosthesis. Even though full masticating function may be restored to the patient for a time immediately after the insertion of the prosthesis, deterioration of the supporting structures will almost certainly follow. The stresses that can be exerted and methods to control, prevent, or eliminate them are described below.

a. Stresses Resulting From Inaccurate Pros

They are not as damaging as those caused by an illfitting prosthesis, because there is no occlusal contact while the stress is being applied and, therefore, no malocclusion.

(1) Stress is exerted by the rigid part of prosthesis passing over a surface bulge of a tooth. This is prevented by the dental officer establishing as much parallelism as possible between the tooth surfaces which the prosthesis will contact.. When enough parallelism cannot be obtained during preparation of the patient's mouth, any remaining undesirable undercuts must be eliminated during the final survey of the master cast. Relief of the finished prosthesis itself is a measure of last resort done by the dental officer. Excessive grinding can dangerously weaken the metal. It can also result in space between the prosthesis and tooth surfaces, which would allow an undesirable accumulation of food particles.

(2) Stress is exerted when the retention arm of a clasp is temporarily distorted as it passes over the bulge of an abutment tooth. The force generated by the springing of this flexible arm must be balanced by a reciprocal clasp arm on any tooth which will be stressed in this manner.

c. Stresses Arising From Impingement on the Gingiva.

(1) Excessive pressure contact can cause inflammation and edema of the gingiva. Proper rests will prevent the prosthesis from moving toward the gingiva. Appropriate areas on the master cast must be relieved.

(2) Sharp borders of metal can cut into the soft tissues. All metal edges must be smooth and rounded and there should be no sharp angles.

d. Stresses Caused by Placing Rests on Inclines. (1) This produces an unfavorable leverage on the abutment tooth, especially if it is an anterior tooth. Rest preparations on the abutment teeth must be properly formed and located so that occlusal pressure is directed along the long axes of the teeth. If necessary, a tooth can be reshaped by a metal restoration with the rest area prepared

in the metal. In extreme cases, an auxiliary rest can be used to compensate for an undesirable direction of the pressure exerted by the principal rest.

(2) The prosthesis moves toward the gingiva when an occlusal force is applied. The proper shape and location of the rest preparation will prevent any such movement and its consequent impingement damage to the underlying soft tissue.

e. Stresses Caused by Major Connectors.

(1) A major connector can cause pressure if it is too flexible. This is a common problem when lingual bars are used. It can be prevented by increasing its bulk as its length is increased, adding a second bar, or using a lingual or palatal plate instead. A bar should be half-pear shaped rather than half-round or flat. When two bars are used, they should lie in different planes so that they act like the two sides of an angle-iron brace. (2) Lateral movement of the prosthesis can arise from the weakness of the supporting teeth or flat alveolar ridges which cannot adequately resist this movement. The resulting impingement on soft tissue can cause inflammation, edema, and necrosis of tissue. Such stresses can be prevented by relieving the master cast in appropriate areas, reducing occlusal interferences, splinting abutment teeth to obtain better support and properly extending the denture base flanges. In the dental laboratory, the case can be designed with more rigid stabilizing units, such as reciprocal clasp arms, auxiliary rests, and indirect retainers. It is better to prevent this type of stress than to try to correct it after the prosthesis is finished. Relieving the connector by grinding is hazardous.

(3) The connector may settle. Adequate occlusal rests must be provided. If they lie upon restorations in the abutment teeth, those restorations must be hard enough to resist abrasion. Denture base flanges should be extended to give maximum tissue coverage and the occlusal table can be reduced in width and length.

10-1. Impressions

Irreversible hydrocolloid impression material (alginate) is used more than any other type for the fabrication of removable partial dentures.

a. Manipulation of Irreversible Hydrocolloid. Manufacturer's directions must be followed for mixing irreversible hydrocolloid. The water temperature should be 70° to 75° F. The impression material should be mixed for about 1 minute before making the impression. When the powder and water are first combined, the amount of water will appear insufficient. As spatulation is continued, the mass will assume the correct creamy-smooth consistency. The dental officer has previously selected and modified a rim lock impression tray to fit the patient's arch. If not, he uses a customized acrylic resin tray to make the final impression.

b. Pouring the Impression. The impression is rinsed carefully in slurry water and flushed with clear water. The excess water is shaken off and an artificial stone cast is poured immediately.

10-2. Care of Master Cast

The precautions outlined in paragraph 5-10 for master casts used in the fabrication of complete dentures apply equally in the fabrication of removable partial dentures.

10-3. Occlusion Rims

a. Occasionally, enough natural teeth are pres

ent in the maxillary and mandibular arches to permit the casts to be placed together in centric occlusion. In this case, the dental officer draws vertical pencil lines across the facial surfaces of at least three pairs of opposing teeth to be a guide in occluding the casts in centric occlusion. No other registrations need be made.

b. More frequently, occlusion rims will have to be constructed to obtain the registrations required to occlude the cast properly. These are made the same as complete dentures except for obvious modifications caused by the presence of teeth on the casts (A and B, fig 10-1). The cast is soaked 1 minute in water to keep the baseplate material from sticking to the cast. The baseplate portion of the occlusion rim must be contoured so that it does not scrape against the teeth when it is taken off or placed on the cast. The wax portion of the occlusion rim should be made slightly wider and higher than the adjacent natural teeth. The occlusion rim is stabilized as described for complete denture fabrication (para 5–15).

c. Reinforcement of the weaker section of a mandibular removable partial denture occlusion rim may be required. A piece of paper clip or coat hanger wire is embedded in the baseplate material to give the desired strength (C, fig 10-1).