Critical Evaluation Of Material And Procedures Used For The Functional Peripheral Moulding

Critical Evaluation Of Material And Procedures Used For The Functional Peripheral Moulding
Irfan Qureshi*, Saqib Rashid**, Sameer Qureshi***, Amin Ur Rehman****

Assistant Professor of Deptf Prosthodontics Altamash Institute of Dental Medicine. Karachi
Professor, Dept of Conservative Dentistry Fatima Jinnah Dental College Hospital Karachi.
Assistant Professor Dept of Conservative Dentistry Fatima Jinnah Dental College Hospital Karachi.
Adjunct Associate Professor, Dept of Periodontology, University of North Carolina, Chapel Hill. U.S.A
Correspondence: “Dr. Irfan Qureshi”

How to CITE:

J Pak Dent Assoc 2010;19(2):129-132.



Abstract

Border molding is done to record the peripheral margins of the denture and to ensure their extension to the sulcus reflection and in the post dam area in the maxilla. With the use of green stick compound for border moulding it is possible to record the border details accurately but it tends to be messy and it is difficult to determine the consistency and proper temperature. Therefore, the use of thermoplastic compound has been examined and recommended to be kept to a minimum due to the availability of simpler techniques and easy to handle and accurate materials. Students may now be introduced to the various techniques and materials for border moulding along with the concept behind the procedure. This will enable them to decide on their own which technique to employ and master. In this paper, a critical evaluation of the various materials and techniques used for border moulding of the special tray is given. It is not the authors` intention to criticize the current methods of teaching in our dental schools but to encourage the readers to broaden their vision of the often complex but fascinating craft of complete denture construction.

KEY WORDS:

Peripheral Moulding, Special denture, Complete denture, Thermoplastic compound.

Introduction

Achieving retention in removable complete dentures has been a traditional problem, particularly in lower complete dentures. The traditional technique being taught in most Pakistani dental institutes of using thermoplastic green stick modeling compound for border molding to achieve peripheral border seal remains a demanding and in most instances “messy” job. It is difficult to manipulate and handle this material.

Complete denture prosthodontics remains one of the most demanding endeavors for any dental practitioner. It is not uncommon to notice a practitioner or a dental student struggling to achieve retention in complete removable dentures. The original material used for border molding was modeling compound which was introduced in 1907 by the Green Brothers.1 The conventional technique of border molding using green stick modeling compound taught at the University of Washington before 1976 required a minimum of “24” insertions of the trays, eight for the maxillary and sixteen for the mandibular, provided the proper extensions were secured on the first insertion for each section.2 Woelfel et al determined that seven dentists required an average of seventeen insertions to secure a final maxillary impression on the same patient when using modeling compound for border molding.3

It is important to first determine the objective behind recording the periphery of the custom tray or the denture base, with border molding using green stick modeling compound, which is to create a peripheral seal, around the denture margins to prevent ingress of air, thus providing retention. One of the most time consuming steps in complete denture construction is border-molding that establishes the impression borders to assure optimal extensions for the final prosthesis. The objective of border the maximal extension and accuracy of the peripheral borderswith no functional impingement of the tissues.4

The other factors that aid in retention are the wetabilty of the denture bearing mucosa, the consistency of the saliva, surface tension, forces of cohesion and adhesion, and close mucosal adaptation of the denture base to the underlying denture bearing tissue.5

A mucocompressive impression is taken using impression compound or an addition silicone followed by a wash impression with less pressure on the tissues using a less viscous impression material like alginate or light body silicone, so that the recoil of the soft tissue doesn’t displace the denture base. If the impression is accurate and the permanent processed denture base is made precisely according to the impression, it will ensure close mucosal contact. If the margins of the base are short of the tissue reflection in the sulcus, it will not form a peripheral seal and if they are over extended beyond the tissue reflection the elastic recoil of the muscle attachments will exert pressure on the denture base. Both instances would lead denture displacement from the residual alveolar ridges with loss of retention. 6

Border molding is done to record the peripheral margins of the denture, and to ensure their extension upto the sulcus reflection and in the post dam area in the maxilla. However, performing border molding using green stick modeling compound can record the border details accurately but it tends to be messy and it is difficult to determine the consistency and proper temperature.7 The question thus arises, whether an alternative material or technique exists which can achieve the above mentioned objectives.

Alternative materials and approaches

Several authors have suggested various ways of performing border molding accurately without the use of thermoplastic green stick modeling compound. Chaffee et al suggested the use of vinyl polysiloxane for recording the periphery of the denture base margins, using a tray adhesive. An advantage of this technique is the ability to accurately reposition the partially border-molded tray during each application. The setting time of the material is 4minutes.4 Kirk and Holt successfully used a perio pack to perform border molding. They reported accurate periphery recording along with ample working time of the material. Furthermore, use of tray adhesive is not necessary with this technique.8

Thermoplastic materials can also be avoided by performing border molding with high molecular weight polymethylmeythacrylate (PEMA) based self cure acrylic resin with no methylmethacrylate (MMA) monomer. This material is easy to use and can accurately record peripheral border details as well as giving sufficient working time, without having to use a water bath or a naked flame.9

In a more recent study, Olivieri et al used visible light polymerized resin on a custom tray for border molding. The resin used for border molding was of a contrasting colour to the custom tray which was fabricated 24 hours in advance to the final impression and border molding appointment to account for dimensional changes. The material has several advantages, it requires less armamentarium than thermoplastic impression compound, it may be border molded in a single step, can be corrected in single areas, and may be used in areas where the tray may be under extended.10

Smith et al suggested the use of polyether impression material for a one-step border molding technique,2 however, Schwarz WD argued that using polyether carried a risk of hypersensitivity to both the patient as well as the person handling the material.11

Wax and self cure acrylic resin has also been used for border molding and final impression taking. Wax, unlike modeling compound is not brittle and can easily be trimmed with a knife. It reduces the chair side time, but this method is technique sensitive.12

Modeling compound is also not the first choice material for border molding at the first author`s alma mater at Guy`s Hospital, King`s College London. At the Dental Institute at Guy`s Hospital, water baths and naked flames have been disallowed in clinics due to risk of cross infection and injury. A technique using silicone with tray adhesive for border molding is routinely employed with success.

Since chair side border molding tends to be time consuming and tedious, another technique involves performing border molding on the primary model in the laboratory prior to the chair side appointment. This technique may reduce chair side time, but the problem of difficult manipulation of Kerr`s compound (thermoplastic green stick modeling compound) still persists.13

Firtell and Koumijan suggested the use of a modified glue gun for ease of use with thermoplastic impression compound14, however this equipment is not available in Pakistan and is expensive

A rather interesting and innovative technique was suggested by Allen P and Worollo S15, where they suggested heating the green stick compound in a microwave. Small pieces of green stick modeling compound were placed in a syringe and the syringe was then placed in a microwave for 4 minutes. The technique allowed use of the green stick compound in dry state. Another technique was described by Knap F involving a secondary impression using zinc oxide eugenol paste prior to the border molding. This would improve retention of the lower custom tray and the border molding may be performed more accurately.16

Drago CJ in a recent retrospective study compared the post insertion adjustments between two groups of patients, the first group had dentures made from border molded definitive impressions using modeling plastic impressin compound (traditional technique) and the second group had their dentures made from border molded custom trays using heavy-body vinyl-polysiloxane impression material. Each group comprised 78 patients, and post insertion visits were quantified for one year following denture insertion. He concluded that within the limitations of the study, both the materials provided similar results in terms of post insertion visits for one year.17

It is also important to note here, that border molding is increasingly declining in dental institutes and some leading dental schools of the world no longer teach border molding to their students. According to a survey conducted in 2001 in the prosthodontic/restorative departments of 54 U.S. dental schools, 17% reported that they do not teach border molding to their students.18 This may be due to the improved clarity of concepts of denture retention and the fact that modern impression materials sufficiently fulfill the criteria for an efficient border seal. In the same survey 31% dental schools reported using materials other than modeling compound. These materials included polysulphide, polyvinylsiloxane and wax.19 In clinical practice, an increasing number of dentists have abandoned a lot of techniques and steps taught in undergraduate degrees, including border molding, some would use it only when it is required.20

Modeling compound was also not the first choice material for border molding at the first author`s alma mater at Guy`s Hospital, King`s College London during his MSc Prosthetic Dentistry course. At the Dental Institute at Guy`s Hospital, water baths and naked flames have been disallowed in clinics due to risk of cross infection and injury. A technique using silicone with tray adhesive for border molding is routinely employed with a great degree of success


CONCLUSION

Most dental students and practitioners face difficulties in making retentive dentures. They, in particular, struggle with border molding due to poor understanding of the mechanism through which retention maybe achieved in complete dentures. The authors have observed that upon questioning students why they are asked to border mold complete dentures custom trays, most did not have a clear concept and just seemed to be going through the motions trying to get “suction” with green stick modeling compound without realizing that the objectives of border molding can be achieved using simpler techniques and materials. Clinicians should realize that peripheral borders of the denture base can be recorded accurately using various materials which are not thermoplastic. Also the fact that the traditional technique of border molding of the tray can be modified with encouraging results, and in some cases border molding may not even be required. Many dental students get discouraged and grow averse to removable complete denture prosthetics because they are asked to employ complicated and complex techniques with materials such as modeling compound which are difficult to handle even for experienced clinicians. Often, clinical instructors in dental schools in Pakistan seem reluctant to try alternative techniques and materials for border molding, or even to recognize certain instances where this step is not required at all. Making complete dentures is a difficult craft to learn, let alone master and one would be well served to strive to make it simpler and more effective.

The authors are of the opinion that the use of thermoplastic compound should be kept to a minimum due to the availability of simpler techniques and easy to handle and accurate modern materials. In our institutes the students should be introduced to various techniques and materials for border molding along with the concept behind the procedure and should decide on their own which technique to employ. This would encourage the students to think laterally and out of the box, and keep their vision from being limited. The students would then not be disheartened or frustrated with complete denture construction as is often observed.

It is not the authors` intention to criticize the current methods of teaching in our dental schools but to encourage the readers to broaden their vision of the often complex but fascinating craft of complete denture construction


ACKNOWLEDGMENT

This paper is dedicated to Drs Andrzej Juszczyk and David Radford, and Prof. R.K.F. Clark, who were the first author`s instructors at King`s College London, Guy`s Hospital, Dental Institute, London (UK).


REFERENCES

 

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