By Dr.Eshan Verma Page 1 Stress Breaker Introduction In RPDs, the broad distribution of stresses to the supporting structures of the arch is generally achieved by using rigid major and minor connectors 1 . However in distal extension cases (Keneddy’s class I &II) this might be injurious to the abutment tooth. 1, 2 This is because of less vertical displaceability of natural teeth compared to the soft tissues covering the edentulous ridge, transferring most of the stresses to the abutment tooth. 2 Therefore, use of non rigid connectors or stress breaker is necessary to protect the abutment tooth. 1, 2 Defining Stress breaker Stress breaker is defined as a device or system that relieves specific dental structures of part or all of the occlusal forces and redirects those forces to other bearing structures or regions. 3 Stress-breakers are those elements of partial dentures which are interposed in a connector system in order to introduce a controlled and intentional degree of flexibility into the structure. 4 In simple words, it is a device, which allows movement between the denture base and the retainer to reduce lateral and tipping forces on abutment teeth 5 . It is also known as Stress director 2 , stress equalizer 1 , broken stress partial denture 6 , articulated prosthesis 6 , and semi rigid connectors 6 . Principle concerning their application 4 Stress breakers are based on the principle of selective load distribution between the supporting tissues. When load is applied to a free-end saddle which is tooth-and- mucosa-supported, part of the load will be applied to the abutment tooth and part will be applied to the mucosa underlying the free-end saddle. The distribution of the applied load between these two supporting tissues can be altered by varying the rigidity of the connector used to join By Dr. Eshan Verma Page 2 the support/retention unit on the abutment tooth to the saddle. If the rigidity of this connector is high, the proportion of the load applied to the abutment tooth will be at a maximum. Whereas when a flexible/movable connector is used, the proportion of load applied to the abutment tooth will decrease and that applied to the mucous membrane under the saddle will increase. The more flexible/mobile is the connector which is used, the greater will be the proportion of the load falling on the mucous membrane. Flexible/ movable connectors used for the purpose of achieving such selective load distribution are termed stress- breakers. Classification Based on the location, stress breaker can be classified as 6 Intracoronal Extracoronal Based on the type of movement, stress breaker can be classified as 4 Type I - Those utilising a hinge or moveable joint. Type 2 - Those utilising flexible connection. Type 1 Stress-breakers: This joint may be in the form of hinges (e.g. D-E hinge type stress breaker, trummion type stress breaker), sleeves (e.g. Baca design), sleeves with springs (e.g. Dalbo retainer and Crismani retainer) and cylinders or ball and socket Devices (e.g. ASC-52 attachment). 1 These are mostly Dalbo Retainer By Dr. Eshan Verma Page 3 used in association with precision attachments (e.g. intracoronal Crismani combined unit –and Dalbo extracoronal retainer) however can also be used with clasp units (e.g. the Wipla Unit). 4 The joint allows vertical and hinge movement of the base to prevent direct transmission of tipping forces to the abutment. 5 Type 2 Stress-breakers These are normally used in association with clasp units as direct retainers. Various forms are possible such as- 1. Wrought wire connector 4,6 - e.g. torsion bar Torsion bar may be used in the design of a lower partial denture carrying bi-lateral free- end saddles. Bars extend anteriorly from the clasp units on each side to join a lingual bar near the mid line .Flexibility can be controlled by varying the cross-section of the ASC-52 attachment By Dr. Eshan Verma Page 4 torsion bars, the method of construction (cast or wrought) and the material of construction (normally gold alloys or cobalt chromium alloys).Disadvantages are associated with the use of the torsion bar structure in that the double bar system is liable to trap food and cause irritation to the tongue. 2. Partial division of the connectors/ split bar major connector 4 - e.g. Split lingual bar, split palatal plate, Ticonium “Hidden lock” design The principle can be applied in both upper and lower dentures. The connecter is spitted by an anterio-posterior slot into upper and lower portion. Upper portion is attached to the retainer unit on the abutment while lower portion is connected to the saddle, thus creating a degree of flexibility between the two. 3. Mesial placement of occlusal rests 4 This offers the simplest available approach to stress- breaking. The degree of stress- breaking achieved is, though, much less than that available where more complex devices are employed. It may be used in the design of either upper or lower dentures. By positioning the rest of the clasp unit on the mesial instead of on the distal fossa of the By Dr. Eshan Verma Page 5 abutment tooth and by using a minor connector to link the rest to a major connector (for example, a lingual bar) some flexibility may be introduced into the clasp unit/saddle link. 4. Clasps having stress breaking action 5 1) Gingivally approaching- resilient I-bar clasp. 2) Occlusally approaching clasp having resilient retentive wrought gold wire arm (combination clasp). 3) Back-action clasp. 4) Reverse back-action clasp. 5) Extended-arm clasp. 6) Ring clasp. 7) Wrought wire clasp. 8) RPI clasp. 9) RPA clasp. Indications The decision as to whether or not a stress breaker is to be used and, where indicated, the type of stress-breaker to be chosen for use is based on a clinical assessment of the load-bearing potential of the two supporting tissues. Where it is judged that the abutment tooth can safely bear the brunt of the applied load, the use of rigid connection between the saddle and the support & retention unit on the abutment tooth is indicated. At the opposite end of the scale, where the periodontal status of the abutment tooth is suspect and the trabecular structure of the alveolar bone underlying the free-end saddle indicates that a favourable response to load-bearing by the bone is likely, the use of a flexible type of stress-breaker would seem to be indicated. Intermediate clinical findings may indicate the use of a stress-breaker of moderate flexibility. It should, however, be pointed out that the period for which flexible stress-breakers remain functionally active in service can be disappointingly short. Doubts have also been expressed as to whether their theoretical benefits are fully achieved in the clinical situation. 4 The main indications for the use of stress breaker include 5 1- When internal attachments are used (because of the unyielding nature of intracoronal retentive elements) 2 2- In distal extension removable partial dentures to distribute the load between the abutment teeth and the ridge. 3- In cases exhibiting weak abutment teeth and well formed ridges. By Dr. Eshan Verma Page 6 Advantages and disadvantages 2, 5 Advantages: Decrease horizontal forces (tipping forces) acting on the abutment teeth thus it preserves alveolar support of these teeth. Distribute the stress between the abutment teeth and the residual ridge. Prevent the quick damage of abutment teeth if relining is needed but not done. Providing physiological stimulation of bone which prevent bone resorption. Call for minimal direct retention because the denture bases operate more independently than do those used in conventional removable partial denture applications. Disadvantages: Difficult to construct and expensive. Less tolerated by the patient. Flexible connectors may be bent and distorted. Some split connectors pinch the underlying soft tissue or tongue as they open and close under function. The effectiveness of indirect retainers is reduced or eliminated. Repair and maintenance of any stress breaker is difficult. All mechanical devices that are free to move in the mouth may collect debris and become unclean. By Dr. Eshan Verma Page 7 References 1. McCracken's Removable Partial Prosthodontics 11th Ed 2. Stewart's Clinical Removable Partial Prosthodontics 3 rd ed 3. The Glossary of Prosthodontic Terms 8 th ed 4. An Introduction to Removable Denture Prosthodontics 1983 - Grant,Johnson 5. Partial denture theory and practice 2010- Fayad M 6. Review of Removable partial denture- Lovely M DOSTAFA FAYAD