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Author: Dr. Devendra Chopra,

Abstract
In a conventional 3 unit FPD the occlusal forces are transmitted through the connectors to the terminal retainers. In the case of a pier being used as an abutment requires special consideration as the middle abutment acts as a fulcrum which causes failure of the weaker retainer.
The remedy to this problem is to neutralize the forces which are concentrate on the pier abutment. This can be achieved by the incorporation of a non rigid connector which can minimize the effect of these deleterious forces with the help of these stress breakers.
This article describes a technique, how to place a non rigid connector in a pier abutment to accommodate the female component of the nonrigid connector.

Introduction
A pier abutment is that tooth in the arch where an edentulous space is present both anterior and posterior to it. This pier abutment acts as a fulcrum because of its strategic position when it is subjected to occlusal forces acting on the ends of the prostheses that will tend to lift the other end like a class I lever causing stress on the terminal abutments and ultimately failure of the F.P.D and trauma to the periodontium.1

In order to prevent the above, a stress breaker should be provided near the pier abutment. The stress breaker is a nonrigid connector with a key in a keyway. The keyway is usually placed on the distal surface of the pier abutment. The male component or the key is attached to the mesial surface of the mesial pontic of the distal edentulous space.2

During mastication, molars tend to tip mesially and hence excessive stress is transferred through the connector to the pier abutment. Forces acting on the mesial end of the distal edentulous space will disengage the connector and prevent stress to the pier abutment. The potential stress on the pier abutment needs to be dissipated. When the edentulous space is only one tooth long, one can incorporate a non rigid connector on the distal portion of the pier abutment to act as a stress relief point.

Markley (1951) suggested that non rigid connector should be placed at one of the terminal retainers.3 Gill (1952) recommended placing non rigid connector at one side or both sides of pier abutment. Schillinburg et al (1973) suggested that patrix of nonrigid connector should be placed distal to the pier retainer & matrix should be in distal pontic.4

Two types of connectors are mainly used in fixed partial denture prostheses, rigid and non - rigid. The use of rigid connectors between pontics and retainers is common for conventional bridges. Rigid connections in metal can be made by casting, soldering, or welding. Cast connectors are shaped in the wax pattern itself.

In an FPD requiring the restoration of two missing teeth and where an intermediate pier abutment is present with a single casting (rigid connectors) is not an ideal treatment.

Nonrigid connectors are indicated -

  1. When it is not possible to prepare two abutments for a fixed partial denture (FPD) with a common path of placement.
  2. When there is an uncertainity about an abutment`s prognosis and if the abutment fails, only a portion of a fixed partial denture may need to be remade.
  3. In the mandibular arch, nonrigid connectors are indicated when a complex FPD consists of anterior and posterior segments.
  4. Disparity in retentive capacity of the abutments.

Case Report
A 46 year old female patient was referred to the Post Graduate department of prosthodontics Babu Banarasi Das College of Dental Sciences with a complaint of difficulty to masticate on the right side of the mouth. No relevant medical history reported. Examination showed missing 45 & 47. (fig.1)

Radiographs showed root canal treated 44 and 46. Treatment plan was to restore the area with a fixed partial denture with the use of non – rigid connector (Tenon & Mortise) in a design. The non rigid connector used consists of a male component (also called as tenon or key) and a female component (also called as mortise or keyway) (fig.2). It was noted that there was lack of adequate space left for the pontic of the distal segment/part of the FPD. Due to this space problem we planned to prepare a ‘Proximal box’ on the distal aspect of the pier abutment i.e. 46 to accommodate the female component (keyway) for the mesial segment of the FPD.

image002 image004 image006 image008

Figure 1: Missing teeth 45 & 47

Figure 2: Tenon, Mortise, Surveying tool

Figure 3: Wax pattern for ant. segment

Figure 4: Anterior segment


image010 image012 image014 image016

Figure 5: Wax pattern for post. segment

Figure 6: checking the fit of the FPD inside the patient`s mouth

Figure 7: Finished ant. & post. segments

Figure 8: Finished ant. & post. segments


Technique:
  1. Facebow transfer done and the diagnostic models are mounted on the whipmix articulator.
  2. After evaluating the diagnostic models tooth preparartion was done on abutments(44, 46 & 48) followed by the check impression and later the final impression.
  3. Bite registrations are done with aluwax (bite registration wax) and the final cast models are transferred to the articulator.
  4. After that patterns were fabricated. The anterior segment of the FPD with a female component or mortise on the distal aspect or inside of the pier abutment retainer was attached in the wax pattern (fig.3).
  5. Then pattern for 48 was made and the male component (key) was attached with the help of the surveying tool provided by the manufacturers CEKA PRECLINE ATTACHMENTS LTD. placed inside the female component (keyway).
  6. The anterior segment of the FPD is invested and casted (fig.4).
  7. After seating of the anterior segment of the FPD on the model, then fabricate the wax pattern for a posterior segment of the FPD and the male component (key) was attached to the distal aspect of the pontic and adjusted accordingly (fig.5).
  8. The fit was rechecked by disengaging and placing the posterior segment of the FPD on the anterior segment.
  9. Now the distal segment is invested and casted.Then the necessary adjustments were made in the laboratory and the seating was checked on the articulated models.
  10. Ensuring the fit of both the segments of FPD (mesial & distal) on the articulator the FPD was adjusted inside the patient`s mouth (fig.6).
  11. After all the occlusal adjustments have been made and the fit is confirmed polishing of the prostheses is being done (fig.7).
  12. Finally both the segments of the FPD was luted with the glass ionomer cement. Firstly the anterior segment of the prostheses is luted then the posterior segment of the prostheses was luted (fig.8).

Discussion
Non rigid connector transfers shear stresses to supporting bone rather than concentrating them in connectors. It minimizes mesiodistal torquing of abutments & permits them to move independently 5. According to a study conducted by Selcuk Oruc & Arzu Atay, the stress distribution & values of an FPD and pier abutment are affected by the presence & location of a non rigid connector. The area of minimum stress concentration occurs in pier abutments when a non rigid connector is located at the distal region of the pier abutment for a 5 unit fixed partial denture with a pier abutment.6

Russell D et al studied the stress transfer patterns with variable implant support and simulated natural teeth through rigid and nonrigid connection under simulated functional loads. They concluded that rigid connector in particular situations caused only slightly higher stresses in the supporting structure and demonstrated more widespread stress transfer. Recommendations for selection of connector design should be based on sound clinical periodontal health of a tooth and the support provided by implants.7
There are many designs of the nonrigid connectors like key & keyway, cross – pin & wing connectors etc. The most common design of Non – rigid connector that is used in such situations consists of a mortise (female component) placed within the contours of the retainers and a Tenon (male component) attached to the pontic. The mortise is placed on the distal aspect of the anterior segment. Accurate alignment of the dovetail or cylindrically shaped mortise is critical, it must parallel the path of withdrawl of a distal retainer. Paralleling is accomplished with a dental surveyor. The path of placement of the retainer & that of the tenon and mortise is identified on the cast. The mortise in the other retainer is then shaped so its path of insertion permits concurrent seating of the tenon and its corresponding retainer. As it was noted that there was a space constraint to place a keyway on the distal aspect of the pier abutment so we had planned to create a proximal box on the distal surface of the pier abutment.

This accommodated the keyway and also directed the occlusal load more towards the long axis of the tooth thus minimizing the cantilever effect on the anterior segment of the FPD. This was only possible as the pier was a molar tooth.

Conclusion
The size, shape and type of connectors plays an important role in the success of a fixed partial denture. Abnormal stress concentration in a FPD is induced by various forces like torquing and flexure. Stress concentration is found in the connectors of the FPD. These stress concentrations result in a failure of the long span fixed partial dentures. To combate these forces the use of non – rigid connectors is advocated with which the affect of the harmful forces has been negated.

REFERENCES
  1. Rosenstiel, Land , Fujimoto – Contemporary fixed prosthodontics 3rd edition.
  2. Shillinburg , Hobo, Whitsett – Fundamentals of fixed prosthodontics 3rd edition.
  3. Markley K Broken – Stress Principle & design in fixed prosthesis. J Prosthet Dent 1951; 1: 416 – 23.
  4. Shillinburg HT, Fisher DW. Nonrigid connectors for fixed partial dentures. J Am Dent Assoc 1973; 87 : 1195 – 99.
  5. Sutherland JK, Holland GA, Sluder TB, Whie JT. A photoelastic stress analysis of stress distribution in bone supporting fixed partial dentures of rigid & nonrigid designs. J Prosthet Dent 2008 ; 44 : 616 -23.
  6. Seluk oruc, Oguz Eraslan , Aiper Tukay , Arzu Atay, Stress analysis of effects on nonrigid connectors on fixed partial dentures with pier abutments, J Prosthet Dent 2008, 99, 185 – 192.
  7. Nishimura RD, Ochiai KT, Caputo AA, Jeong CM, Photoelastic stress analysis of load transfer to implants and natural teeth comparing rigid and semirigid connectors. J Prosthet Dent 1999 ; 81, 696 – 703.

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