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dentistry Dominic Thorncroft

Acrylic Resin Crowns Part 4

Acrylic Resin Crowns

Prefabricated forms are ready for use as temporary crowns, and  made of polycarbonates or cellulose acetate – for anterior teeth. They should be always modified by a process of relining (rebasing) with materials of the same chemical composition, and further contouring and shaping.

Temporary forms made of aluminium, silver-tin alloy or nickel-chrome alloy, can be used for posterior teeth. Ni-Cr ESF are intended (designed) for use in cases of severely damaged primary posterior teeth (molars) in children. They are not relined  or  rebased,  but  need  careful  adjustment  and  fixing,  cementation  with  a permanent cement.

The basic procedures for fabrication of temporary crowns are: indirect, direct and combined – indirect/direct method. Indirect method means that the crown or bridge is fabricated on a master cast.

The first step is taking an impression from the prepared abutment teeth and soft tissues, pouring a stone model, master cast, wax modeling of the restoration. Modeling can be performed not in wax, but with special acrylic resin material and then a heat-curing process is accomplished.

The indirect method has a number of advantages and is widely used in practice. All adverse effects of the liquid component of the acrylic resin material (monomer, or methyl-methacrylate) can be avoided – allergy, sensitivity, overheating the pulp due to the exothermic reaction during polymerization, greater accuracy and precise fit of the crown margin to the finish line and gingival margin – the direct procedure is always associated with shrinkage and deformities of the resin material.

The rigid stone model prevents the material from such volumetric changes and deformities.

Due to all these reasons the indirect or combined procedures are the preferred approach in temporary resin crown fabrication.

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dentistry Dominic Thorncroft

Acrylic Resin Crowns Part 3

Acrylic Resin Crowns

Aesthetic Requirements:

1. The temporary crown should be easily shaped, contoured.

2. To demonstrate stability of colour, and its matching to the colour and translucency of the adjacent natural teeth.

The perfect temporary restoration should meet all these biologic, mechanical and esthetic requirements. Although the ideal dental material for temporary crowns and bridges is not discovered yet, contemporary dental medicine can use a number of materials for their fabrication that  demonstrate optimum clinical and technological quality.

The temporary crowns can be fabricated using various materials and methods. A common feature of all technological procedures is the use of 2 components:

  • ESF (External Surface Form), usually a silicone impression, or a pre-fabricated prototype.
  • ITF (Internal Tissue Form) – the abutment or the abutments and mucosa for the bridge restorations.

The Plastic Material Is Pressed Or Syringed Between The Two Forms

There are two basic types of ESF: prefabricated and custom. The custom made forms are more popular and frequently used – heavy body silicones, thermoplastic foil (polypropylene or cellulose acetate) – fabricated in special devices under pressure or in vacuum. The custom-made forms need a full-arch impression before tooth preparation. The impression is the external form, by which the plastic material is pressed to the prepared tooth or teeth on the model.

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dentistry Dominic Thorncroft

Acrylic Resin Crowns Part 2

Acrylic Resin Crowns

The importance of maintaining occlusion: Inadequate occlusal contacts allow overeruption. This is (vertical movement). Missing proximal contacts can cause migration – horizontal movement – of the abutment under the influence of mastication forces.

This migration can make the adjustment of the final restoration quite difficult. Orthodontic movement is sometimes indicated to shift the abutment to its initial position. The temporary crown protects the abutment from fracture, because tooth mechanical stability has been reduced as a result of its preparation.

Mechanical Requirements:

1. The first one is to withstand functional loading.

2. The temporary crown should not be easily separated from the abutment. This means to have good retention to the abutment tooth. Important factors for good retention of the crown to the abutment are: height of the abutment, surface area, inclination of axial walls, thickness of cement layer, type of cement and mix.

3. Another requirement is to prevent the abutment from migration, including the maintenance of the abutment positions in fixed bridge restoration.

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dentistry Dominic Thorncroft

Dental Wax Modeling Techniques

During central closure in the normal dentition the lingual cusps of the maxillary posterior teeth and the buccal cusps of the mandibular posterior teeth make contact with the occlusal fossa or the marginal ridges of the opposing teeth.

They grind food like a molar during mastication and are called functional (supporting) cusps.

On the other hand, the buccal cusps of the maxillary molars and premolars and the lingual cusps of the mandibular posterior teeth do not contact the opposing teeth.

These cusps prevent food from overflowing, confine food within the sulcus, and protect the buccal mucosa and the tongue by keeping them away from the functional cusps. Since these cusps do not make direct contact with opposing teeth, they are called non-functional cusps.

The type, number and distribution of occlusal contacts is called an occlusal scheme. The occlusal scheme can be classified by the location of the occlusal contact made by the functional cusp on the opposing tooth in centric relation. There are two types of occlusal schemes: cusp-fossa and cusp-marginal ridge.

The – cusp-marginal ridge – the cusp-marginal ridge relation is the type of occlusal scheme in which the functional cusp contacts the opposing occlusal surfaces on the marginal ridges of the opposing pair of teeth, or in a fossa. Therefore, a cusp-marginal ridge occlusion is basically a one-tooth-to-two-teeth arrangement.

This is the most natural type of occlusion and is found in 95% of all adults. Since the majority of adults exhibit the cusp-marginal ridge type of occlusion, it is an occlusal pattern widely utilized in daily practice. It can be used for single restorations.

The waxing technique used for cusp-marginal ridge occlusion was originally devised by E.V. Payne was the first wax-added (addition) technique for functional waxing. The same technique, modified by the use of color-coded waxes, has become a widely used method for teaching functional waxing.

The – cusp-fossa    – the cusp-fossa relation is an occlusal pattern in which each functional cusp is positioned (contacts) into the occlusal fossa of the opposing tooth. It is a tooth-to-tooth arrangement. Although considered to be an ideal occlusal pattern, it is rarely found in its pure form in natural teeth. 

Each centric cusp should make contact with the occlusal fossa of the opposing tooth at three points. The contact points are on the mesial and distal ridge (incline) and the inner facing incline of the cusp, producing a tripod contact. Since the cusp tip itself never comes in contact with the opposing tooth, the cusp tip can be maintained for a long time with a minimum of wear.

The technique used for producing wax patterns with an exclusively cusp-fossa occlusion was developed by P.K.

Thomas. It is important to keep in mind, however, that the same technique, utilizing the same sequence of morphologic development, can be used with excellent results for developing a cusp-marginal ridge occlusal relationship.When the cusp-marginal ridge arrangement is the desired end result, cusp placement is altered slightly.

Occlusal forces are directed parallel to the long axis of the tooth. These forces are near the center of the tooth – placing very little lateral stress on the tooth.

Since this type of occlusion is rarely found in natural teeth, it usually can be used only when restoring several contacting teeth and the teeth opposing them, as well as for full mouth reconstruction.

The basic purpose of wax modeling and developing optimal occlusal contacts between the maxillary and mandibular dental arch is to reconstruct the lost anatomic shape and function without trauma to the supporting structures and with a uniform distribution of forces during mastication. All this depends on the type of occlusal scheme and the proper distribution of the occlusal contacts.

A good restoration adapts perfectly to the prepared tooth and the gingival margin,restores the proximal contacts, the continuity of the dental arch, the interocclusal relationship and the lost harmony of the dental arch. With the wax modeling we restore (reconstruct) the shape, size, proportion and all characteristics of the destroyed or fully completely lost tooth crown.

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dentistry Dominic Thorncroft

Case Study – Brushing Once a Day

Case Study

Dominic Thorncroft went through a patient’s treatment that he had examined in the previous week. This 13 year old patient in the mixed dentition phase presented with a 6 monthly check up and had no issues since last time. The patient’s personal details and medical history were checked. A dental history was taken which revealed that the patient had been tooth brushing just once a day.

Examination

An extra oral examination, soft tissue check, charting, occlusal examination, periodontal assessment including BPE scores, and oral hygiene assessment were recorded. It was noted from the examination that there was an occlusal tooth wear lesion on LL7.

A diagnosis was recorded and a suitable treatment plan made to have this tooth restored with flowable composite without tooth preparation. This was agreed with the patient. We discussed other possible materials including glass ionomer, posterior composite and amalgam.

The use of this technique was appropriate for this case. Dominic Thorncroft included a discussion of all options including no treatment and advantages and disadvantages of possible solutions. The patient consented for the placement of a flowable composite with etching/bonding. The treatment was carried out on the same visit and included a detailed description of the filling with the necessary post operative instructions.

Treatment

Also he carried out additional preventative advice and a suitable recall period was made for the patient. With regards to the recall period, it could be argued that in this patient who needs motivation, increased dental care and awareness of the damage that fizzy drinks can cause, a 3 monthly recall period would be also entirely justified. The treatment for the LL6 was also discussed.

The lower left first molar is heavily restored with an MOD amalgam restoration. In addition, there is a small distal overhang with a larger mesial overhang. The clinical examination showed that the buccal cusps were weak and the buccal wall of the tooth was very thin.

This tooth has a reduced prognosis and a crown was suggested. However this is not so straightforward because this patient is young and on average a crown that will last 10 years. This means that this patient will have several replacement cycles and each placement cycle reduces the prognosis of the tooth which will ultimately result in a large gap for this patient.

We also saw that teeth with crowns have a 20% chance that they will require root canal treatment at some point in the future. With this particular patient, these chances even further increased as he is young and there hasn’t been much opportunity for a significant amount of tertiary dentin to form.

A further discussion was planned regarding this tooth and the options.

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dentistry Dominic Thorncroft

Acrylic Resin Crowns Part 1

Acrylic Resin Crowns

Indications

As a temporary crown. Short term or Immediate term.

Why

Pulpal protection and also Protects the finishing line of crown margins. It maintains occlusion and the Aesthetics. Ensures Against breaking of the crown prep.

Nowadays some other contemporary materials can also be also used for this purpose, like
self-curing and light-curing composites such as Protemp.

Acrylic Resin Crowns can be chair side OR lab made.

The conventional technology for the fabrication of an acrylic resin crown is the heat-curing processing of PMMA. Wax prototype is invested using dental stone in a small flask and at the end of the curing process the wax is replaced by acrylic resin material.

PMMA can demonstrate good esthetics, yet this material has a number of disadvantages – insufficient mechanical strength (poor mechanical properties), imbibition of liquids (absorbs liquids, saliva), aging with colour alteration, etc.

All these disadvantages determine the indications for treatment with acrylic resin crowns – for temporary, provisory, preliminary artificial crowns and bridge restorations, and probably the most accurate term – interim restorations. 


Temporary restorations are of extreme importance to the quality of final restoration and success of the prosthetic treatment.

The interim restoration should meet a number of closely interrelated requirements – biologic, mechanical and aesthetic. It protects the pulp of the tooth from bacterial, chemical and physical damage. The preparation process, particularly the complete crown preparation can cause trauma to the pulp – between 40 000 and 70 000 dentin tubules are cut and widely opened during this process. They contain the odontoblast processes and the nuclei of the cells are positioned into the pulp.

If the prepared tooth surface remained unprotected from the harmful external factors, the damage to the pulp may be permanent and irretrievable, accompanied by pulp irritation, contamination and inflammation, and needing root canal treatment – that means a loss of its vitality.

The temporary restoration protects the marginal periodontium – this is achieved with an optimum shaping and perfect fit of the crown margins to the finish line, shoulder or chamfer. Any inaccuracy in this area, the area of the crown margin, may cause not only trauma of the periodontium, but injury of the pulp due to uncovered zones, sections, not protected by the temporary crown.

The provisional restoration should establish or maintain proper contacts with adjacent and opposing teeth – i.e. to maintain proximal contacts and occlusion.