Question:

In choosing the major connector for a maxillary removable partial denture framework, what factors should I consider?

Answer:

The most important factor to consider in choosing a major connector for a maxillary RPD is to evaluate the case classification. There are seven different case classifications and five different types of maxillary connector designs.

My recommendations are as follows: ANTERIOR TOOTHBORNE (Kennedy Class III): 1. palatal strap 2. horseshoe

POSTERIOR TOOTHBORNE (Kennedy Class III): 1. palatal strap 2. ant-post palatal strap

COMBINATION TOOTHBORNE (Kennedy Class III): 1. horseshoe 2. ant-post palatal strap 3. closed horseshoe

UNILATERAL DISTAL EXTENSION (Kennedy Class II): 1. horseshoe 2. ant-post palatal strap 3. closed horseshoe

BILATERAL DISTAL EXTENSION (Kennedy Class I): 1. ant-post palatal strap 2. closed horseshoe 3. full palatal plate

ANTERIOR EXTENSION (Kennedy Class IV): 1. horseshoe 2. ant-post palatal strap 3. closed horseshoe

COMBINATION EXTENSION (Kennedy Class I or II): 1. ant-post palatal strap 2. closed horseshoe 3. full palatal plate

Question:

Can you explain the principle of indirect retention in RPD design?

Answer:

An indirect retainer is part of an extension RPD which inhibits the extension base from lifting off the ridge tissue. This indirect retention occurs because the indirect retainer resists the dislodging forces exerted against the denture on the opposite side of the axis of rotation (fulcrum line).

When an extension base is part of an RPD case design, the requirement for supplemental indirect retention should immediately come to mind. A movement of a toothborne RPD in an occlusal direction is prevented by clasps on abutment teeth located at each end of the edentulous space. This condition is not completely true for an extension RPD. The direct retainer on the abutment tooth closest to the edentulous space is only minimally effective in preventing rotational movement of the extension base away from the ridge tissue.

When rotational movement happens, it occurs around an axis. For example, in bilateral distal extension cases, the axis of rotation passes through the most posterior abutment on both sides of the dental arch. If something like sticky food is acting to pull an extension base away from ridge tissue, the part of the RPD on the other side of the axis of rotation moves in the opposite direction, if that portion is unsupported. When adequate support is present on the other side of the rotation axis, the extension base would have less tendency to lose contact with ridge tissue. Support to resist occlusal displacement of the extension base usually comes from one or more strategically located rests. This might be auxiliary rests or part of a clasp assembly. The effectiveness of the rest as an indirect retainer is directly proportional to its distance away from the axis of rotation; effectiveness increases with distance.

Thus to achieve adequate indirect retention on a bilateral distal extension case you may opt to include a mesial occlusal rest on a 1st premolar, a cingulum rest on a canine, and/or select a major connector that has a lingual plate resting on the cingulums of the anterior incisors. Inversely, on an anterior extension case, the indirect retention would be placed as far posteriorly as possible.