Altered Cast Impression Technique to Correct over-trimmed master cast

A case report

Abstract

Bilateral edentulous posterior areas in dental arch posterior to natural teeth (Kennedy Class I) is one of the most common cases in dental practice. The Removable Partial Denture (RPD) that used to replace the missing posterior teeth is mainly tissue borne denture. Altered cast impression technique is used in dental practice for recording distal extension bases. It precisely records the edentulous area distal to the last tooth in order to distribute the forces applied on the teeth and soft tissue. In this case a concept of correcting the laboratory error of over trimmed mandibular master cast can be done using altered cast technique without the need of remaking final impression was reported.

INTRODUCTION

Bilateral edentulous posterior areas in dental arch posterior to natural teeth (Kennedy Class I) is one of the most common cases in dental practice. The Removable Partial Denture (RPD) that used to replace the missing posterior teeth is mainly tissue borne denture (Carr AB, Brown DT, 2011), (Jayachandran D, 2017). Altered cast impression technique is used in dental practice for recording distal extension bases (Sayed M, Jain S, 2019). It precisely records the edentulous area distal to the last tooth in order to distribute the forces applied on the teeth and soft tissue (Feit DB, 1999). The soft tissue is recorded as it functions and then the distal edentulous area of the cast is replaced with a new impression and the cast is poured (Baloch HR, Vohra F, 2014) (Phoenix, DR, et al., 2008) (Chandrashekar Sajjan, 2010). However, there is some controversy as to the impression technique for management of the distal-extension RPD (Prasad S, Monaco EA, 2010).

This article reports a concept of correcting over trimmed mandibular master cast using altered cast technique without the need of remaking final impression.

CASE PRESENTATION

A 45-year-old female presented to the undergraduate dental clinics, College of Dentistry at University of Ha’il, 9 October 2017. She was complaining of lost mandibular molars in both sides and and old defective mandibular denture. Her chief complain was pain while using the denture. She was medically fit. Dental history revealed periodontal disease and need for multi-division dental treatments. Clinical and radiographic examination showed gingival recession, with average size alveolar ridge. Teeth no. 18, 15, 14, 12, 23, 25, 28, 38-36, 44-48 were missing. Metal ceramic bridges was present on teeth 16 to 13 and 24 to 26. Caries was present on 35, 34, 33 and 43.

The treatment plane considering available equipment and patient preference was include periodontal treatment, resin restoration of carious teeth, RCT and fixed surveyed crown on tooth 34. Definitive RPD was planned for the mandibular arch to replace missing posterior teeth.

After several appointments, the mandibular arch was prepared to receive RPD. The elastomeric master impression was done (Figure 1) to be used for metallic framework construction and as was planned for the final processing of the RPD. The design of metallic framework was consisting of lingual plate as major connector, combination clasp for direct retention, on 35 and 43, lingual plate with cingulum rests were considered as indirect retainer and finally meshwork minor connectors were used for the acrylic denture bases.

Figure 1: The master elastomeric impression for framework construction

The elastomeric master impression was sent to the laboratory, private laboratory, for master cast pouring and metallic framework construction. When the master cast poured and metal framework fabricated, the posterior ridge areas in the master cast was removed by cast over trimming as laboratory error. Metal framework was checked on the cast then tried in the patient mouth (Figure 2).

Figure 2: metallic framework for class I RPD on the cast that by the laboratory (A), Metallic framework try in intraorally (B).

The metal framework was accurately seated in the patient mouth. To overcome the problem of over trimmed master cast, the altered cast impression technique was

To overcome the problem of over trimmed master cast, the altered cast impression technique was planned. The procedure for altered cast impression technique involve:

1. An acrylic resin custom trays were added to the meshwork extensions. (Figure 3, A). Before adding the impression tray material, the master cast was carefully examined. Undercuts that would interfere with removal of the tray were blocked out using baseplate wax.
2. Border refining was done using Low fusing modelling plastic sticks (impression compound, KERR, USA) that was applied to the border of the tray, tempered in a water bath, and seated in the patient’s mouth and guiding the placement of the cheek and tongue. This procedure was repeated until an accurate border moulding has been accomplished. To provide space for the impression material at the border, 1 mm of modelling plastic is removed from the fitting surface. (Figure 3, B, C&D).
3. An altered cast impression was made with a light-bodied poly vinyl siloxane impression, material (PERFECT- F LIGHT- Premium Fast, HAN DAE CHEMICAL CO., LTD, Korea) (Figure 3, E).
4. The master cast was altered to accommodate the secondary impression, and corrected cast was generated. (Figure 3, F, G&H).
5. The generated altered cast with accurate posterior extension was used for all the subsequent procedure for RPD construction including record block construction and mounting for setting of artificial teeth (Figure 3, H&I). Also, it was used for processing of definitive RPD (Figure 4, A).
6. Removable partial denture was inserted intraorally and pressure indicating past was used to check for any pressure area (Figure 4 B, C).

Figure 3: Acrylic custom tray attached to the metallic framework(A). Border refinement using green compound(B). Scalpel was used to trim the excess (C). Border refinement intraorally E. Elastomeric impression material was used to record the altered cast impression(D). The metallic framework with the impression was seated on the sectioned over trimmed cast (F). Accurate seating of the metallic framework was preserved by green sticks (G). The altered cast was poured after boxing the impression and is used for record block construction for jaw relation record (H). The altered cast was used for mounting and setting of artificial teeth (I).

Figure 4: The definitive RPD. Extraorally (A), during check of pressure area at insertion appointment (B) and intraorally (C).

DISCUSSION

Kennedy’s class I RPD is a challenge in prosthodontics. Maximum tissue coverage in mandibular Kennedy’s class I RPD is important to provide maximum support (Carr AB, Brown DT, 2011). Altered cast or corrected cast impression technique provide advantages over anatomical impression. It provides maximum support from the residual ridges (Phoenix, DR, et al., 2008) (Chandrashekar Sajjan, 2010) (Baloch HR, Vohra F, 2014). Master cast must include the important posterior anatomical landmarks, retro molar pad area. (Carr AB, Brown DT, 2011).

The success of the dentures depends largely on the relation of the dentures to the anatomic landmarks, including, supporting, and limiting structures. The removable partial denture must extended posteriorly to cover the retromolar pads because it form the posterior seal of the mandibular denture (Phoenix, et al, 2008). Accurate extension of denture periphery is important for denture success. Errors of master cast pouring, and trimming must be corrected. The correction can be done through remake the impression. In this case report, the metallic framework was accurate intraorally but there was error in the master cast that will be used in processing of acrylic part of denture base. If this error was not solved the denture periphery will not be accurate. If the cast error was corrected by remaking the impression, the metallic framework cannot be used. The lingual plate as the major cannot be seated in new cast of new impression because it is rested on soft tissue. The soft tissue contour is differ from one impression to the other because it is resilient and contain fluid. M. M. Devan, 1952 mentioned that the oral cavity is lined with resilient tissue that differs in amount of displace ability, according to its type, thickness, its rigidity, level, and direction of the forces applied to it during the impression. Hary L. Page in 1946, he specified that all soft tissues were mainly fluid and 80% or more of the tissues are formed of water. The planning in this case report was to preserve the metallic framework and to correct the master cast. This was done through the altered (corrected) cast impression technique.

Although altered cast impression technique has specific use, where it is used primarily to record the ridge in a functional or physiologic form, in this case study it is used to correct the laboratory error of master cast. This error was discovered at metallic framework try in appointment. The correction of the master cast error, over trimmed posteriorly, may be considered as another advantage of altered cast impression technique.

This altered cast impression technique had given good results and the RPD was accurately extended. The class I RPD was delivered to the patient successfully.

Accurately extended denture base will provide stimulation to the underlying tissue, bone and allows masticatory force distribution uniformly. The altered cast technique allows the ridge, recorded in functional form, to be accurately related to the remaining teeth with accurate tissue coverage (Chandrashekar Sajjan, 2010). RPD constructed with altered cast impression technique exhibited less vertical movement of distal extension denture bases compared to those constructed using conventional-single impression techniques (Sayed M, Jain S, 2019).

CONCLUSION

Accurately extended distal extension denture base is important from the biomechanical point of view. Altered cast impression technique not only allow the physiological/ functional record of the edentulous ridge but can be used to correct a laboratory errors of over trimmed master cast.

CONFLICTS OF INTEREST

There is no funding agency / research support / conflict of interest for this study.

ACKNOWLEDGEMENT

Many thanks to the Dental technicians of the dental college, Ha’il University, especially Razaz, who perform the laboratory procedure of denture construction.