Bio Mechanical Considerations in the use of Hinges in Partial Denture RestorationPREAT Corporation
Distal Stress Eliminator–DSE Hinge
The necessity of using a design and construction capable of relieving or eliminating torque on the abutment teeth of a partial prosthesis has long been recognized. No single answer exists in how best to achieve the desired end result.
Part of the problem is the infinite number of variations encountered in the condition of the oral cavity which influences the design and construction of a partial prosthesis.
A major factor to be considered in the designing of a partial with distal extension saddles is the number of posterior teeth missing. The larger the edentulous area, the larger is the occlusal load to be shared with the remaining teeth.
To be considered also is the status of the opposing dentition. If a lower partial is opposed by a full denture, the resulting occlusal force generated is decidedly less than the forces generated with all or part of the natural teeth intact. Often one observes where normal healing of the edentulous area after tooth extraction has not taken place. The patient may have been informed to come back within a certain time frame for a relining of the saddles. Frequently, the patient will ignore the set appointment. If this occurs, rapid resorption is nevitable with the resulting damage to the abutment tooth. Since approximately 75% of the partial lower and 65% of partial upper restorations involve distal extension saddles, solving the problem of relieving the abutment teeth of excessive stress is of prime importance, particularly if the abutments show loss of bone support.
The typical removable partial denture is a one-piece casting with clasps, a lingual bar, or in the case of an upper restoration, either a horseshoe type or a palatal bar which is rigidly connected with the saddle retention necessary for securing the acrylic saddles. It is generally recognized that as long as the denture saddles remain in contact with the tissue, moderate occlusal forces can be absorbed without causing damage to the abutment teeth.
The damage occurs when the ridge resorption takes place. On a partial prosthesis where all or most of the posterior teeth are missing, the restoration, in effect, becomes a Class I lever with the occlusal rests as fulcrum points. Furthermore, ridge resorption, due to the patient’s chewing habits of favoring one side of the arch over the other, is very seldom equal, causing a further strain on the abutment teeth. This condition can easily be verified. When the saddles are relined, the thickness of the reline material from one saddle to the other can vary greatly. A solution favored by some dentists is the use of a buccal wrought wire arm of approximately 18 gauge. It is true that the strain on the abutment is reduced; again, however, if ridge resorption takes place, tipping action of the abutment teeth will occur.
Orthodontic wire of even thinner gauge is used to move teeth. Another factor to be considered in the use of wrought wire arms is the loss of retentive power when undercut conditions are often insufficient for a wrought wire clasp, but more than adequate for a more rigid properly tapered chrome cobalt cast clasp.
Another approach in relieving stress on the abutments is the use of stress-reducing attachments. Sixty years ago, Dr. Kennedy, who made the use of the Kennedy bar popular, had this to say about the use of stress breaker:
“Since the advent of the cast clasp and the removable bridge, a great number have advocated the use of ‘stress breakers’ between their saddles and the clasps. These have been shown to be absolutely essential by dentists who had used cast clasps for partial dentures. They found that in a short time the teeth to which such clasps were attached loosened, and that this was due mainly the rigidity of the clasp.”
To add to this quote, with the popularity of precision attachments increasing, the same reason for using a stress reducer is equally applicable to rigid precision attachments.
Sixty years ago, the stress-equalizing attachments available at that time were made from plastic patterns, and after casting allowed excessive lateral play, which frequently caused soreness in the edentulous areas. Today, a number of attachments precision prefabricated in metal are available, and if properly applied, will assure lateral stability.
The most universally used attachment is the DSE Hinge. The Distal Stress Release Trunnion attachment, which came on the market in 1953, has recently been improved. The new design is much smaller with a width of three millimeters and a vertical height of one and a half millimeters.
The simplicity of the laboratory phase, and the durability and economy of this attachment have contributed to the popularity and acceptance by the dental profession of this stress-equalizing attachment. The attachment can be used on both conventional clasp cases and in conjunction with a variety of precision attachments.
There is a fundamental difference in the use and function of the Distal Stress Equalizer when compared with other stress-breaking devices. The illustrations will explain the difference in function of the DSE and other attachments.
Illustrates a hypothetical cut through the master cast and the acrylic saddles posterior to the first molar replacement tooth.
In this drawing, the acrylic saddles are in firm contact with the patient’s saddle tissue.
The fulcrum points are the occlusal rests on both sides of the mandible.
Acrylic saddles in contact with ridge tissue
Shows the space between the acrylic saddles and the patient’s saddle tissue.
Acrylic saddles lost in contact with ridge tissue
Schematic Drawing #2b
Illustrates the Class I Lever effect on the abutment teeth.
Class I Lever affect on abutment teeth
Illustrates a further complication when uneven resorption occurs.
In this case, the rest of the right abutment becomes the fulcrum point.
If resorption under one saddle is sever enough, the tendency would be for the right abutment to drift posteriorly with a corresponding drift of the left bicuspid abutment.
Right saddle out of contact with ridge tissue
Demonstrates a different situation. It is assumed that the right side of the arch is tooth bearing with clasps on the left second molar and first bicuspid, and a clasp on the right first bicuspid.
With resorption of the tissue bearing right saddle, the fulcrum points will be the occlusal rests on the right first bicuspid and the left second molar.
Provided the abutments have adequate bone support and a normal ridge contour, a stress equalizer would be counter indicated.
Left saddle tooth bearing
Regardless of the type of retentive mechanism used–clasps, intracoronal, or extracoronal precision attachments–the object of a rational case design should first be to direct occlusal loads so that the load as nearly as possible is applied along the vertical axis of the abutment tooth. If a stress relieving attachment is used on distal extension saddles, there are two different approaches.
Illustrates the use of a Trunnion-type stress equalizer. To better demonstrate the action of the stress-relieving device, the replacement teeth are omitted.
Drawing #5 also shows interconnected saddles. Occlusal forces generated on one side of the partial prosthesis are transmitted to the opposite side.
Again, as long as the acrylic saddles stay in intimate contact with the patient’s saddle tissue, no adverse affects will occur.
Ridge resorption, particularly if it is uneven, will adversely affect the abutment teeth.
Use of Trunnion stress equalizers with interconnected saddles
Illustrates the unilateral approach, which is preferred by most dentists.
In the unilateral application, occlusal forces are absorbed by the individual saddle.
If more ridge resorption occurs on one side of the mandible, the other side will not be affected and undesirable stress on the abutments will be avoided.
Preferred use of the Trunnion stress equalizer on unilateral acting saddles
A most desirable by-product of this type of construction is overall function of the unilateral approach. Under occlusal loads, the clasps or precision attachments and the lingual bar remain stationary and passive. Movement is confined to the saddle area only, with the occlusal rests taking the major share of the occlusal load. Compensation for uneven tissue resorption is automatic with each saddle finding its own level.
Supporting the rationale of inserting a Distal Stress Release (DSE) Trunnion between the saddle and supporting structure of a partial prosthesis is the report from a number of dentists who have kept the Trunnion attachment immobilized in the acrylic saddle for a number of weeks. After freeing the Trunnion, the patients reported a lessening of stress on the abutment teeth.
Since loss of vertical will result in loss of chewing check-ups and possible saddle relines.