CHAPTER 5 Major and Minor Connectors
Figure 5-1 A, Framework for mandibular removable partial denture with the following components: 1, lingual bar major connector; 2a, minor connector by which the resin denture base will be attached; 2b, minor connector, proximal plate, which is part of clasp assembly; 2c, minor connector used to connect rests to major connectors; 3, occlusal rests; 4, direct retainer arm, which is part of the total clasp assembly; 5, stabilizing or reciprocal components of clasp assembly (includes minor connectors); and 6, an indirect retainer consisting of a minor connector and an occlusal rest. B, Maxillary removable partial denture with resin denture bases supporting artificial posterior teeth. Bases are attached to metal framework by ladderlike minor connectors similar to those seen in 2a. C, Mandibular bilateral distal extension removable partial denture with resin denture bases supporting artificial posterior teeth.
When a prosthesis that can be removed from the mouth is used, the prosthesis must extend to both sides of the arch. This enables transfer of functional forces of occlusion from the denture base to all supporting teeth and tissues within an arch for optimum stability. It is through this cross-arch tooth contact, which occurs at some distance from the functional force, that optimum resistance can be achieved. This is most effectively accomplished when a rigid major connector joins the portion of the prosthesis receiving the function to selected regions throughout the arch.
The chief functions of a major connector include unification of the major parts of the prosthesis, distribution of the applied force throughout the arch to selected teeth and tissue, and minimization of torque to the teeth. A properly designed rigid major connector effectively distributes forces throughout the arch and acts to reduce the load to any one area while effectively controlling prosthesis movement.
The principle of leverage is connected with this component part. A rigid major connector will limit movement possibilities by acting as a counteracting lever. This phenomenon is referred to as cross-arch stability. Cross-arch stability becomes more important in situations associated with high potential for greater prosthesis movement (e.g., distal extensions).
A major connector is the component of the partial denture that connects the parts of the prosthesis located on one side of the arch with those on the opposite side. It is that unit of the partial denture to which all other parts are directly or indirectly attached. This component also provides cross-arch stability to help resist displacement by functional stresses.
The major connector may be compared with the frame of an automobile or with the foundation of a building. It is through the major connector that other components of the partial denture become unified and effective. If the major connector is flexible, the ineffectiveness of connected components jeopardizes the supporting oral structures and can be a detriment to the comfort of the patient. Failure of the major connector to provide rigidity may be manifest by traumatic damage to periodontal support of the abutment teeth, injury to residual ridges, or impingement of underlying tissue. It is the dentist’s responsibility to ensure that appropriate design and fabrication of the major connector are accomplished.
Appropriate relief beneath the major connector avoids the need for its adjustment after tissue damage has occurred. In addition to being time consuming, grinding to provide relief from impingement may seriously weaken the major connector, which can result in flexibility or possibly fracture. Major connectors should be carefully designed for proper shape, thickness, and location. Alteration of these dimensions by grinding can only be detrimental. Relief is covered at the end of this chapter and is expanded in Chapter 11.
Margins of major connectors adjacent to gingival tissue should be located far enough from the tissue to avoid any possible impingement. To accomplish this, it is recommended that the superior border of a lingual bar connector be located a minimum of 4 mm below the gingival margin(s) (Figure 5-2). At the inferior border of the lingual bar connector, the limiting factor is the height of the moving tissue in the floor of the mouth. Because the connector must have sufficient width and bulk to provide rigidity, a linguoplate is commonly used when space is insufficient for a lingual bar.
Figure 5-2 A, Lingual bar major connector should be located at least 4 mm inferior to gingival margins and farther if possible. The vertical height of a finished lingual bar should be at least 4 mm for strength and rigidity. If less than 8 mm exists between gingival margins and the movable floor of the mouth, a linguoplate (B), a sublingual bar (C), or a continuous bar (D) is preferred as a major connector. Relief is provided for soft tissue under all portions of the mandibular major connector and at any location where the framework crosses the marginal gingiva. The inferior border of mandibular major connectors should be gently rounded after being cast to eliminate a sharp edge.
In the maxillary arch, because no moving tissue is present in the palate as in the floor of the mouth, the borders of the major connector may be placed well away from gingival tissue. Structurally, the tissue covering the palate is well suited for placement of the connector because of the presence of firm submucosal connective tissue and an adequate, deep blood supply. However, when soft tissue covering the midline of the palate is less displaceable than the tissue covering the residual ridge, varying amounts of relief under the connectors must be provided to avoid impingement of tissue. The amount of relief required is directly proportional to the difference in displaceability of the tissue covering the midline of the palate and the tissue covering the residual ridges. The gingival tissue, on the other hand, must have an unrestricted superficial blood supply to remain healthy. To accomplish this, it is recommended that the borders of the palatal connector be placed a minimum of 6 mm away from and parallel to the gingival margins. Minor connectors that must cross gingival tissue should do so abruptly, joining the major connector at nearly a right angle (Figure 5-3). In this way, maximum freedom is ensured for gingival tissue.
Figure 5-3 Palatal major connector should be located at least 6 mm away from gingival margins and parallel to their mean curvature. All adjoining minor connectors should cross gingival tissues abruptly and should join major connectors at nearly a right angle.
Except for a palatal torus or a prominent median palatal suture area, palatal connectors ordinarily require no relief. Intimate contact between the connector and the supporting tissue adds much to the support, stability, and retention of the denture. Except for gingival areas, intimacy of contact elsewhere in the palate is not detrimental to the health of the tissue if rests are provided on abutment teeth to prevent tissue-ward movement.
An anterior palatal strap or the anterior border of a palatal plate also should be located as far as possible posteriorly to avoid interference with the tongue in the area of the rugae. It should be uniformly thin and its anterior border should be located to follow the contours between the crests of the rugae. The anterior borders of such palatal major connectors therefore will be irregular in outline as they follow the contours between the rugae. The tongue may then pass from one ruga prominence to another without encountering the border of the connector. When the connector border must cross a ruga crest, this should be done abruptly, while avoiding the crest as much as possible. The posterior limitation of a maxillary major connector should be just anterior to the vibrating line. A useful rule applied to major connectors and throughout partial denture design is to try to avoid adding any part of the denture framework to an already convex surface.
The basic form of a mandibular major connector is a half-pear shape, located above moving tissue but as far below the gingival tissue as possible. It is usually made of reinforced, 6-gauge, half-pear–shaped wax or a similar plastic pattern (Figure 5-5).
Figure 5-5 Sagittal section showing half-pear shape of lingual bar. A taper of the superior border of the bar to the soft tissues above will minimize interference with the tongue and will be more acceptable to the patient than would a dissimilar contour. Tissue relief is necessary to protect the soft tissue of the floor of the mouth.
The major connector must be contoured so that it does not present sharp margins to the tongue and cause irritation or annoyance by an angular form. The superior border of a lingual bar connector should be tapered toward the gingival tissue superiorly, with its greatest bulk at the inferior border, resulting in a contour that has a half-pear shape. Lingual bar patterns, both wax and plastic, are made in this conventional shape. However, the inferior border of the lingual bar should be slightly rounded when the framework is polished. A rounded border will not impinge on the lingual tissue when the denture bases rotate inferiorly under occlusal loads. Frequently, additional bulk is necessary to provide rigidity, particularly when the bar is long or when a less rigid alloy is used. This is accomplished by lining the ready-made form underneath with a sheet of 24-gauge casting wax rather than altering the original half-pear shape.
The inferior border of a lingual mandibular major connector must be located so that it does not impinge on the tissue in the floor of the mouth because it changes elevations during the normal activities of mastication, swallowing, speaking, licking the lips, and so forth. Yet at the same time, it seems logical to locate the inferior border of these connectors as far inferiorly as possible to avoid interference with the resting tongue and trapping of food substances when they are introduced into the mouth. In addition, the more inferiorly a lingual bar can be located, the farther the superior border of the bar can be placed from the lingual gingival crevices of adjacent teeth, thereby avoiding impingement on the gingival tissue.
At least two clinically acceptable methods may be used to determine the relative height of the floor of the mouth and locate the inferior border of a lingual mandibular major connector. The first method is to measure the height of the floor of the mouth in relation to the lingual gingival margins of adjacent teeth with a periodontal probe (Figure 5-6). When these measurements are taken, the tip of the patient’s tongue should just lightly touch the vermilion border of the upper lip. Recording of these measurements permits their transfer to both diagnostic and master casts, thus ensuring a rather advantageous location of the inferior border of the major connector. The second method is to use an individualized impression tray for which lingual borders are 3 mm short of the elevated floor of the mouth, and then to use an impression material that will permit the impression to be accurately molded as the patient licks the lips. The inferior border of the planned major connector can then be located at the height of the lingual sulcus of the cast resulting from such an impression. Of the two methods, we have found measuring the height of the floor of the mouth to be less variable and more clinically acceptable.
Figure 5-6 A, Height of floor of the mouth (tongue elevated) in relation to lingual gingival sulci measured with a periodontal probe. B, Recorded measurements are transferred to a diagnostic cast and then to a master cast after mouth preparations are completed. The line connecting marks indicates the location of the inferior border of the major connector. If periodontal surgery is performed, the line on the cast can be related to incisal edges of teeth and the measurements recorded for subsequent use. C, Impression made with functional movement of the tongue to demonstrate maximum shortening of the floor of the mouth. This allows visualization of the anatomic feature that establishes the inferior extent of a major connector. If a stock tray causes impingement on this functional position, an individualized or custom tray may be used for the same purpose.
Figure 5-7 View of mandibular Class I design with contoured linguoplate. Linguoplate is made as thin as possible and should follow the lingual contours of the teeth contacted. Doing so will often result in a scalloped superior margin. In this example, the straight superior margin can be bulky at the cingulum region, causing tongue discomfort.
A linguoplate should be made as thin as is technically feasible and should be contoured to follow the contours of the teeth and the embrasures (Figure 5-8). The patient should be aware of as little added bulk and as few altered contours as possible. The upper border should follow the natural curvature of the supracingular surfaces of the teeth and should not be located above the middle third of the lingual surface, except to cover interproximal spaces to the contact points. The half-pear shape of a lingual bar should still form the inferior border that provides the greatest bulk and rigidity. All gingival crevices and deep embrasures must be blocked out parallel to the path of placement to avoid gingival irritation and any wedging effect between the teeth. In many instances, judicious recontouring of the lingual proximal surfaces of overlapped anterior teeth permits closer adaptation of the linguoplate major connector, eliminating otherwise deep interproximal embrasures to be covered (Figure 5-9).
Figure 5-8 Apron of linguoplate (tissue side) is closely adapted to the teeth extending into nonundercut interproximal embrasures, resulting in a scalloped form. When well adapted, this form will benefit from some anterior teeth acting together to help resist horizontal rotational tendencies of the prosthesis, especially if the posterior ridge form does not resist such movement.
Figure 5-9 If a linguoplate major connector was indicated for this patient with overlapped anterior teeth, judicious recontouring of the lingual proximal surfaces of right lateral, right central, and left lateral incisors would eliminate excessive undercuts and permit closer adaptation of the lingual apron of the major connector.
The linguoplate does not in itself serve as an indirect retainer. When indirect retention is required, definite rests must be provided for this purpose. Both the linguoplate and the cingulum bar ideally should have a terminal rest at each end, regardless of the need for indirect retention. However, when indirect retainers are necessary, these rests may also serve as terminal rests for the linguoplate or continuous bar.
Because no component part of a removable partial denture should be added arbitrarily, each component should be added to serve a definite purpose. Indications for the use of a linguoplate may be listed as follows:
The same reasons for use of a linguoplate anteriorly apply to its use elsewhere in the mandibular arch. If a lingual bar alone is to be used anteriorly, there is no reason to add an apron elsewhere. However, when auxiliary splinting is used for stabilization of the remaining teeth or for horizontal stabilization of the prosthesis, or for both, small rectangular spaces sometimes remain. Tissue response to such small spaces is better when they are bridged with an apron than when they are left open. Generally, the apron is used to avoid gingival irritation or entrapment of food debris or to cover generously relieved areas that would be irritating to the tongue (Figure 5-10).
Figure 5-10 Sagittal section through the linguoplate demonstrating a basic half-pear–shaped inferior border with the metallic apron extending superiorly. Extension of linguoplate to the height of contour on the premolar was accomplished to enclose a rather large triangular interproximal space inferior to the contact point between the canine and premolar. Such spaces may often be bridged to eliminate obvious food traps. Relief is provided for soft tissue under all portions of the mandibular major connector and at any location where the framework crosses the marginal gingiva.
Sometimes a dentist is faced with a clinical situation wherein a linguoplate is indicated as the major connector of choice even though the anterior teeth are quite spaced and the patient strenuously objects to metal showing through the spaces. The linguoplate can then be constructed so that the metal will not appreciably show through the spaced anterior teeth (Figure 5-11). The rigidity of the major connector is not greatly altered. However, such a design may be as much of a food trap as the continuous bar type of major connector.
The following systematic approach to the design of a mandibular lingual bar and linguoplate major connectors can be readily used with diagnostic casts after the diagnostic data are considered and related to the basic principles of major connector design:
Figure 5-12 Sequence of design considerations for a mandibular major connector. A, Diagnostic cast with basal seat areas outlined. B, Inferior border of the major connector is outlined. Location of the inferior border was determined as suggested in Figure 5-6 and extends to the mesial of the mandibular right molar. C, Superior border of the major connector is outlined. Limited space for the lingual bar requires use of the linguoplate major connector. Linguoplate requires that rest seats be used on canines and the first premolar for positive support. D, Rest seat areas on the posterior teeth are outlined, and minor connectors for retention of resin denture bases are sketched.
A modification of the lingual bar that has been demonstrated to be useful when the height of the floor of the mouth does not allow placement of the superior border of the bar at least 4 mm below the free gingival margin is the sublingual bar. The bar shape remains essentially the same as that of a lingual bar, but placement is inferior and posterior to the usual placement of a lingual bar, lying over and parallel to the anterior floor of the mouth. It is generally accepted that a sublingual bar can be used in lieu of a lingual plate if the lingual frenum does not interfere, or in the presence of an anterior lingual undercut that would require considerable blockout for a conventional lingual bar. Contraindications include interfering lingual tori, high attachment of a lingual frenum, and interference with elevation of the floor of the mouth during functional movements.
When a linguoplate is the major connector of choice, but the axial alignment of the anterior teeth is such that excessive blockout of interproximal undercuts must be made, a cingulum bar may be considered. A cingulum bar located on or slightly above the cingula of the anterior teeth may be added to the lingual bar or can be used independently (Figure 5-13). In addition, when wide diastemata exist between the lower anterior teeth, a continuous bar retainer may be more esthetically acceptable than a linguoplate.
Figure 5-13 A, Lingual bar and cingulum bar (continuous bar) major connector. Upper portion of this major connector is located on the cingula of anterior teeth. The requirement of positive support by rest seats, at least as far anteriorly as the canines, is critical. Note that the superior border of the lingual bar portion is often placed objectionably close to the gingival margins if sufficient bulk for rigidity is to be obtained. This type of major connector easily traps food and is often more objectionable to patients than a linguoplate. B, Cingulum bar (continuous bar) major connector. Although this design may reduce the possibility of food entrapment, it may not provide adequate rigidity.
Fortunately, in only a few situations does extreme lingual inclination of the remaining lower premolar and incisor teeth prevent the use of a lingual bar major connector. With the use of conservative mouth preparations in the form of recontouring and blockout, a lingual major connector can almost always be used. Lingually inclined teeth sometimes may have to be reshaped by means of crowns. Although the use of a labial major connector may be necessary in rare instances, this should be avoided by resorting to necessary mouth preparations rather than by accepting a condition that is otherwise correctable (Figure 5-14). The same applies to the use of a labial bar when a mandibular torus interferes with placement of a lingual bar. Unless surgery is definitely contraindicated, interfering mandibular tori should be removed so that the use of a labial bar connector may be avoided.
Figure 5-14 A, Lingual inclination of patient’s canines and premolars precludes use of the lingual bar. B, Labial bar major connector was used in treatment. Retention was obtained on terminal abutments. Support and stabilization were gained by using rests, minor connectors arising from the labial bar, and well-fitting denture bases.
A modification to the linguoplate is the hinged continuous labial bar. This concept is incorporated in the Swing-Lock* design, which consists of a labial or buccal bar that is connected to the major connector by a hinge at one end and a latch at the other end (Figure 5-15).
Figure 5-15 The hinge for this continuous labial bar connector is located buccal and distal to the remaining dentition (area of tooth #21). The latching mechanism is opposite to the hinge, adjacent to tooth #28. In this location, it will be housed within the buccal flange of the denture.
Support is provided by multiple rests on the remaining natural teeth. Stabilization and reciprocation are provided by a linguoplate that contacts the remaining teeth and are supplemented by the labial bar with its retentive struts. Retention is provided by a bar type of retentive clasp with arms projecting from the labial or buccal bar and contacting the infrabulge areas on the labial surfaces of the teeth.