18 Curt Coffman1, Chris Visser1, Jason Soukup2, and Michael Peak3 1 Arizona Veterinary Dental Specialists, Scottdale, AZ, USA 2 School of Veterinary Medicine, University of Wisconsin‐Madison, Madison, WI, USA 3 Tampa Bay Veterinary Specialists, Largo, FL, USA Prosthodontics is the dental specialty pertaining to the diagnosis, treatment planning, rehabilitation and maintenance of oral function, comfort, appearance, and health of patients with clinical conditions associated with missing or deficient teeth and/or oral and maxillofacial issues (American Dental Association, Specialty Dentistry Definitions: http://www.ada.org/en/education‐careers/careers‐in‐dentistry/dental‐specialties/specialty‐definitions; accessed 5 November 2017). According to the American College of Prosthodontists, a Prosthodontist is a dentist who specializes in the esthetic (cosmetic) restoration and replacement of teeth, including the treatment planning and restoration of implants, temporomandibular joint disorder, and rehabilitation of occlusion with prostheses. Prosthodontic treatment in veterinary dentistry involves restoration of tooth structure lost to trauma, wear, or disease. Prosthodontic treatment is used most frequently in veterinary patients to restore fractured, worn, or decayed teeth and following endodontic treatment. Less common prosthodontic treatments for veterinary patients include inlays, onlays, partial crowns or veneers, bridges, and dental implants. The majority of the topics contained in this chapter pertain to treatments and materials commonly used in veterinary patients. While some of the information discussed may be more pertinent to human dentistry, these items are presented to expand the general knowledge base for veterinary dentists. Many of the elemental principles as well as materials and equipment used in prosthodontics are also integral to operative dentistry. Describing defects in tooth structure using modifications of human cavity classification systems (see below) can be useful in treatment planning. However, use of American Veterinary Dental College (ADVC) nomenclature found at www.avdc.org is advised (AVDC Nomenclature Committee: www.avdc.org/nomenclature; accessed 5 November 2017). Additional discussion of operative dentistry techniques is included later in this chapter and also in Chapter 17 – Restorative Dentistry. When used in dental charting and AVDC case log entries, the tooth fracture abbreviations noted above are stated as T/FX/(specific abbreviation) (e.g., T/FX/CCF). This tooth fracture classification can be applied for brachydont and hypsodont teeth, which covers domesticated species and many wild species. Fractures of teeth in some wild species may not fit into this classification because of differences in the tissues present in the teeth. Other abbreviations used for restorative terms can also be found on the AVDC website (https://www.avdc.org/traineeinfo.html) (see Table 18.1). The G.V.Black cavity preparation classification system can be found in Chapter 17 – Restorative Dentistry [1]. Table 18.1 AVDC nomenclature abbreviations (https://www.avdc.org/traineeinfo.html; accessed 5 November 2017). A working understanding of dental anatomy is essential when considering extensive prosthodontic restorative work such as crown restoration. While most teeth requiring prosthodontic restoration in veterinary patients are non‐vital and have received standard endodontic therapy, some vital teeth require attention as well. Crown preparation on vital teeth commonly leaves many open dentin tubules, resulting in dental pain, and possible pulpitis. A temporary crown may be placed to protect the tubules and pulp, knowing that these can sometimes be difficult to maintain on the animal. Alternatively, the tubules should be sealed with a bonding agent temporarily, until the permanent restoration is placed. When restoratives are placed upon vital teeth, it is extremely important to perform follow‐up examinations to reassess and confirm continued tooth vitality. Restorative dentistry not only concerns itself with the crowns of teeth but also with the teeth as anchorage units with the alveolar bone. In a simplified sense, teeth with a large amount of root inside the bone are considered to have favorable crown‐to‐root ratio [2]. When alveolar bone support is lost the ratio becomes less favorable. This basic concept of anchorage should be considered in treatment planning for restorations, but also underscores the importance of maintaining proper periodontal health and attachment in all restored teeth. Because periodontal disease is a common cause of tooth loss in veterinary patients, the clinician must be aware of each patient’s periodontal health status when planning prosthodontic treatment. The patient’s current periodontal health as well as the pet owner’s ability and willingness to perform necessary future oral hygiene are vital to the long‐term success for any restored tooth. The clinician planning a prosthodontic restoration must identify and institute effective periodontal treatment before completing definitive restorative treatment. It is a major disappointment to lose a properly restored tooth as a result of uncontrolled periodontal disease. If the cause of tooth trauma or disease can be determined, steps should be taken to resolve the problem when possible. Any traumatic injury to the head or face can cause accompanying tooth fractures in dogs and cats. A common cause of tooth fracture in pet dogs is chewing on hard bones, objects, or toys, but any traumatic injury to the head or face may potentially cause accompanying tooth fractures in dogs and cats [3]. Many products marketed as chew toys are hard enough to cause tooth fracture and abrasion from a dog’s normal chewing forces. Working dogs may injure teeth during training, patrol, or protection work. Cage biting, tug of war games, and uncontrolled dermatologic problems are also common causes of tooth wear and injury. The long‐term success of any planned restorative treatment can be compromised if the initial problem cannot be identified and resolved. The selection of the material and design of the restoration should be based on several factors [4]: If the degree of tooth destruction from the original injury requires the restoration to add strength and protection, a cast metal or ceramic crown may be indicated over composite resin restoration. Esthetic considerations may not be as important for pets as in people. However, many pet owners consider their pets to be a family member and may request a tooth colored restoration over cast metal. Recent advances in ceramic technology allow some tooth colored restorations to approach the strength of cast metal restorations, but cast metal must be considered the “gold standard” for veterinary restorations until solid research proves otherwise. Plaque control and the health of the periodontal tissues are critical concerns when planning a restoration. If the supporting tissues of the tooth are diseased, the periodontal disease must be treated and controlled before placement of definitive restorations. The pet owner must be willing to commit to routine oral hygiene at home and be willing to present the pet for proper follow‐up in the veterinary office. The clinician must consider the source of the original trauma to the tooth and the goal for protection weighed with preservation of a tooth. In some dogs that are “cage biters,” reduction of less coronal structure with a ¾ crown onlay may help prevent further abrasion of the distal aspects of the canine teeth [5]. However, full coverage crowns are generally accepted to be the most retentive restorations when compared to ¾ crowns and other types of partial coverage crowns [4]. Designing a restoration for a tooth that has been previously treated endodontically is dependent primarily on the amount of remaining tooth structure. Often in veterinary patients, the amount of tooth structure lost to the original injury precludes restoration of teeth to their original height and contour. The remaining tooth structure must be evaluated for its ability to sustain normal occlusal load and stress, its retentive qualities, and the esthetic requirements of the restoration. Because of the size variability of veterinary patients, there is no easy rule of how much remaining crown structure is required to support a successful crown restoration. In general, the greater the amount of remaining sound tooth structure, the more retentive and resistant the final crown restoration will be. If the amount of remaining tooth structure is deemed insufficient, a crown lengthening procedure can be considered. The encirclement of the axial tooth structure within the walls of a full crown creates a ferrule effect that will protect the tooth from fracture. In humans approximately 2 mm of a sound vertical tooth structure must be present to achieve this effect [4]. When contemplating crown restorations, some considerations for the crown and preparation design include: material, preparation type (for full crown coverage or partial crown coverage either inlay or onlay), the supragingival or subgingival margin, the need for additional retentive designs and tooth preservation. The material used for the crown should be based on the needs of the patient and the desires of the owner. If strength and maximum protection is needed, then a full metal crown is the best choice. If esthetics are the primary concern, then other crown restorative materials such as ceramic, PFM, or composite may be more appropriate. Each of these materials has its advantages and disadvantages as well as preferred methods for preparation of the tooth. The clinician should be familiar with all these crown substrates and their requirements. While the general focus of the clinical techniques described in this chapter pertains to metal crown restorations, veterinary dentists may occasionally choose materials other than metal at the pet owner’s request. Metal, in general, can be classified as either ferrous or non‐ferrous based on the content of iron. Ferrous metals include metals such as iron or steel. Non‐ferrous metals can be further broken down into noble metals, base metals, and light metals [6]. Prosthodontic crowns are usually made from non‐ferrous metals. Noble metals contain gold and the platinum group of metals: platinum, palladium, ruthium, rhodium, iridium, and osmium. Gold has long been used for metal crown restorations. It has the advantage of being relatively non‐reactive and resists corrosion in the oral cavity. It is very malleable which helps in finishing the margins and allows for a good marginal fit. The disadvantages of gold include its hardness (gold is relatively soft), its cost, and its color when considering esthetics. Noble metals are relatively non‐reactive and resistant to corrosion and tarnishing. Silver is not considered a noble metal in dentistry since it does corrode in the oral cavity. All of the noble metals plus silver can be grouped together as precious metals. Other metals are considered “non‐precious” or “base” metals. A combination of several of metals is known as an alloy. The vast number of alloy systems requires greater reliance upon laboratory guidance. However, some of the new non‐precious alloys often provide very good results more economically than the costlier precious alloys. Casting alloys are typically classified into two broad groups based upon their fusion temperatures [7]. The normal fusing alloys are formulated for all metal restoration, while high fusing alloys are designed for PFM restorations [8]. However, high fusing metals are sometimes used for all metal restorations. A high fusing metal must be used for the metal layer of PFM restorations, so that they do not melt or creep during the repeated heating required to bake on the porcelain layers [9]. When cost is not a concern, gold alloy is a good overall selection. However, there are many newer precious metal alloy combinations that provide good results more economically than gold. The base metals, although highly durable and inexpensive, have disadvantages that must be taken into consideration [10–12]. The gold–palladium alloys have many good characteristics for PFM use and are considered a good choice by many human clinicians. High palladium alloys are a combination primarily of palladium, with a moderate amount of base metal and a small amount of gold and silver. They are very hard and compatible with most porcelain systems, but are more technique‐sensitive and more difficult to cast well [9]. Base alloys contain less than 25% noble metal and have a place in veterinary dentistry due to their lower cost, hardness, and strength. They are used for full‐cast restorations as well as PFM restorations. As a group they are much harder, stronger, and have twice as high an elastic modulus as do the high noble and noble metal alloys. This latter property is advantageous because casting can be made thinner and still retain rigidity. The ADA Council revised the classification system in 2003 with regard to the use of titanium and titanium alloys in dentistry (http://www.ada.org/2190.aspx). Titanium and titanium alloys have been added between high noble and noble alloys because of their excellent biocompatibility in the revised classification system (Table 18.2). Table 18.2 Revised classification system for alloys for fixed prosthodontics (ADA) (http://www.ada.org/en/about‐the‐ada/ada‐positions‐policies‐and‐statements/revised‐classification‐system‐for‐alloys‐for‐fixed‐prosthodontics). a Metals of the platinum group are platinum, palladium, rhodium, iridium, osmium. and ruthenium. The indirect composite resin technique requires the composite restoration to be fabricated on a model rather than directly upon the tooth. Indirect techniques do have some advantages over direct applications. Composite resins, both direct and indirect, when cured have shrinkage in the resin matrix during polymerization. With directly applied composites, this can result in gaps in the marginal seal and possible marginal leakage. With indirect technique the shrinkage occurs prior to placement, which is chiefly offset with the luting or bonding agent at the time of placement. This results in less marginal gapping and a decreased risk of marginal leakage [9]. In addition, laboratories can use ultraviolet light (direct composites normally use visible light curing composites), heat, and vacuum to provide a superiorly cured composite resin restorative [13]. This results in a composite resin that may be harder and have greater tensile strength, which would then be a stronger, longer lasting restoration [14]. Composite resins are softer than porcelain, do not generally accelerate wear of the opposing natural tooth structure, and are easy to re‐polish following adjustment. These advantages, along with continued improvement in composite technology, have resulted in direct composite resin becoming one of the most commonly used types of restoration in both human and veterinary dentistry. Dental ceramics are man‐made, inorganic, non‐metallic materials produced by the heating of raw minerals at high temperatures [15]. Ceramics are widely used in prosthetic dentistry because they are aesthetically pleasing in color, shade, and luster, and they are chemically stable. The main constituents of dental ceramic are silicon‐based inorganic materials, such as feldspar, quartz, and silica [16] (Table 18.3). Traditional feldspar‐based ceramics are also referred to as “porcelain.” Ceramics are generally considered brittle, which means that they display a high compressive strength but low tensile strength and may be fractured under very low strain (0.1%, 0.2%). As restorative materials, traditional dental ceramics have disadvantages mostly due to their inability to withstand functional forces that are present in the oral cavity. Hence, initially, they found limited application in the premolar and molar areas, although further development in these materials has enabled their use as posterior prosthetic restorations and structures over dental implants in people [17]. All dental ceramics display relatively lower fracture toughness when compared with dental alloys [18]. The main difference between a regular ceramic and a dental ceramic is the proportion of feldspar, quartz, and silica contained in the ceramic. A dental ceramic is a multiphase system, containing a dispersed crystalline phase surrounded by a continuous amorphous phase (a glassy phase). Modern dental ceramics contain a higher proportion of the crystalline phase that significantly improves the biomechanical properties of these ceramics. Table 18.3 Types of dental ceramics. Source: Adapted from reference . The so‐called cast‐glass ceramics are different from traditional feldspar‐based (porcelain) ceramics because of the larger crystalline phase that helps stop crack growth. Examples of these high crystalline ceramics include lithium disilicate and zirconia. These infiltrated ceramics such as In–Ceram (Vita) are made through a process called slip‐casting, which involves the condensation of an aqueous porcelain slip on a refractory die [19]. This fired porous core is later glass infiltrated, a process by which molten glass is drawn into the pores by capillary action at high temperatures. Materials processed in this way exhibit less porosity, fewer defects from processing, greater strength, and higher toughness than conventional feldspathic porcelains [20]. These are made of a glass‐infiltrated core that is later veneered with a feldspathic ceramic for final esthetics. The Vita In‐Ceram slip‐casting system makes use of three different materials to gain a good compromise between strength and esthetics. Three variations of In‐Ceram are spinell, alumina, and zirconia. Solid‐sintered ceramics have the highest potential for strength and toughness but, because of high firing temperatures and sintering shrinkage techniques, they were not available to use as high‐strength frameworks for crowns and fixed partial dentures until recently. Solid‐sintered monophase ceramics are materials that are formed by directly sintering crystals together without any intervening matrix to form a dense, air‐free, glass‐free, polycrystalline structure [21]. There are several different processing techniques that allow the fabrication of either solid‐sintered aluminous oxide or zirconia oxide frameworks. Examples of these systems include Procera (Nobel Biocare), Lava (3M ESPE), and Cercon (Dentsply). Cast glass ceramics or solid sintered ceramics can be considered for esthetic repairs of canine teeth of non‐working dogs. However, even these newest ceramics may still be vulnerable to the complex shearing forces of animal carnassial teeth, where metal restorations are generally better suited. Traditional feldspar porcelain is infrequently used as a singular restorative material in veterinary patients because of its tendency to fracture under strain. Porcelains with a higher coefficient of thermal expansion were developed in the early 1960s [9]. This allowed the compatible fusing of porcelains to that of casted metal dental alloys, and the acceptable fabrication of the PFM restoration. While the outer porcelain coating of PFM restorations remain relatively brittle compared to ceramic and full metal restorations, the metal interior of PFM restorations provide a strong inner core to protect the tooth structure. Porcelain fuses to the metal by both a chemical and mechanical bonding. The chemical bond occurs as the porcelain forms a crystalline attachment to oxides on the surface of the metal alloy. The mechanical interlock occurs both on a gross level from the design and on a micromechanical level from the abrasive treatment of the metal, which forms a surface texture for bonding [22]. The metals for the substructure must be able to take repeated firing for the application of the various layers of porcelain without deformation or creep. This means that the substructure metal of a PFM should be a high fusing alloy (see Section 18.6.1 – Metals, above) [23]. The porcelain must be able to resist slumping and devitrification. Slumping results in slow deformation of porcelain from repeated heating. Devitrification is the crystallization that occurs from the repeated firings, resulting in a clouding of the porcelain and an opaque, non‐vital appearance to the restoration [22]. The axial reduction with PFM needs to be in the range of 1.5 mm to allow for a 0.5 mm metal substructure and the 1 mm of porcelain needed to cover the metal esthetically. The incisal reduction should be in the 2.0 mm and occlusal 1.5 mm range [9]. Failure to provide sufficient reduction will result in poor esthetics, a weak restoration, or an oversized restoration [24]. In addition, the porcelain layer should not generally exceed 2 mm in thickness, as it is not required for esthetics and the porcelain is the weak link in the restoration. Therefore, the metal substructure should be designed to prevent overlayering of the porcelain [25]. The esthetic outer structure of PFM crowns is certainly susceptible to chipping in veterinary patients, but the inner metal core will provide circumferential protection for the tooth identical to a full metal veneer, so PFMs can be considered if the pet owner understands the risk of damage to the porcelain. Preparation of teeth for crown restoration does not require an extensive armamentarium. Most preparations require a selection burs and a high‐speed air turbine handpiece. Small pointed diamond burs used with a water–air spray will allow precise removal of tooth structure. Gross reduction of the tooth is accomplished with coarse diamond burs. In order to leave a smooth finish line at the margin, a fine diamond or other finishing bur should be used to smooth the preparation margin [25]. This will facilitate fabrication of the restoration and allow for a smooth, continuous finish line and a well‐adapted margin. Cementation of the restoration requires simple restorative materials for use with adhesive cements. A basic list of equipment and materials for prosthodontics includes: When a defect occurs in the hard tissues of the tooth (enamel, dentin), optimally it is best to preserve the function and structure of the object by restorative means. It is essential to understand the basic components of restorative dentistry, including cavity preparation, as well as skills involved in shaping the final preparation. The clinician should be familiar with the basics concepts of restorations including conservation, esthetics, contours and contacts, extension for prevention, cavity preparation, and identification and resolution of the cause of the tooth defect (see Chapter 17 – Restorative Dentistry). Various walls, lines, and angles are created during cavity preparation (see Chapter 17 – Restorative Dentistry) In crown and bridge preparation the primary walls of concern are the pulpal, axial, and gingival. Additionally, there are a few subdivisions of the walls, such as the enamel and dentinal walls. The enamel wall is that portion of the preparation wall that consists of enamel. The dentinal wall is that portion of the wall that consists of dentin. The dentin–enamel junction is that juncture in the wall where the dentinal and enamel walls meet. Where two walls meet, a line angle is formed. At the point where three walls meet a point angle is produced. The cavosurface angle is the line angle formed between a wall of the prepared surface and the unprepared tooth surface. The cavosurface angle is also sometimes termed the preparation margin, especially once the preparation is restored. The combined peripheral extent of all of the cavosurface or preparation margins is termed the cavity or preparation outline. In dealing with restoratives, the restorative margin is the restorative surface that abuts the cavosurface angle or preparation margin. Design of the cavosurface angle requires special consideration in its preparation. The preparation margin greatly affects retentive qualities of the restoration, resistance to marginal leakage, physiologic contour reactions, gingival health, and resistance to attrition, abrasion, and fracture of the restoration and the restored tooth. Selection of the specific cavosurface angle for the margin is dependent upon the type of restoration selected, restorative materials to be used, and the degree of anticipated stress demand upon the restoration. The preparation margin comprises two components: the recess and the finish line. The recess is the depth of tooth structure removed and must provide enough room for the intended restoration. The finish line delineates the most apical border of the preparation. An optimal finish line should be well‐defined and crisp [26]. Historically many configurations and variations of finish lines have been used for margins of cemented crown restorations and onlays. The basic preparation margins commonly used with current restorative materials in veterinary patients (chamfer, shoulder, and heavy (deep) chamfer) are discussed further below. Shoulders and heavy chamfers may be provided with a bevel, which is an oblique cut into the external angle of the recess. The potential benefit to adding a bevel is that it will decrease the thickness of the cement interface at the restoration margin [1]. The use of occult finish lines or very steep bevels may leave a stronger tooth substructure to support the crown, but also results in an oversized restoration. Theoretically, an occult finish line may benefit cases where additional foundation strength is required (i.e., military working dogs), but there are no published studies to support this idea. Additionally, these finish lines are clinically more difficult to prepare and reproduce in the dental laboratory. In order to place a cast restoration on a tooth, some tooth structure must be removed. This process is termed tooth preparation. The guiding principles of tooth preparation in humans, although only studied to a small degree in dogs, are likely to be applicable in companion animals. However, continued research in this field of veterinary dentistry is highly encouraged in order to promote evidence‐based tooth preparations. There are five governing principles for the design of a tooth preparation: preservation of tooth structure, retention and resistance, structural durability, marginal integrity, and preservation of the periodontium. Tooth preparation is a balance between removing the appropriate quantity of tooth material to place and maintain a restoration and maintaining the appropriate quantity of tooth material to preserve tooth strength and pulpal health. Efforts should be made to maintain as much normal tooth structure as possible. If a partial veneer crown achieves the clinical goal while respecting the other governing principles, especially retention and resistance, it may be chosen to minimize removal of the tooth structure. Partial veneer crowns (¾ crowns) have been advocated for the management of distal abrasion of canine teeth in dogs [4]. However, studies have shown that full veneer crowns are generally more retentive than partial veneer designs [26–28] (Figure 18.1). Therefore, the operator must weigh the pros and cons of each design for their patient’s unique clinical presentation. Figure 18.1 Full metal crown. While subgingival finish lines may increase clinical crown height, there are many disadvantages to their use in companion animals in regard to periodontal health. When not dictated by a client’s esthetic preferences, the use of subgingival finish lines should be avoided in order to maintain as much tooth structure as possible.
Crowns and Prosthodontics
18.1 Introduction
18.2 Definitions of Prosthodontic Terms
18.3 Classifications of Lesions in Teeth
18.3.1 The AVDC Tooth Fracture Classifications
CR
Crown
CR/M
Crown metal
CR/PFM
Crown porcelain fused to metal
CR/P
Crown preparation
CRA
Crown amputation
CRL
Crown lengthening
CRR
Crown reduction
CBU
Core buildup
IM
Impression and model
IMP
Implant
R
Restoration of tooth
R/A
Restoration with amalgam
R/C
Restoration with composite
R/CP
Restoration with compomer
R/I
Restoration with glass ionomer
18.4 Dental and Periodontal Anatomy Concerns
18.5 Treatment Planning
18.5.1 Material and Design
18.6 Materials Used for Crown Restorations
18.6.1 Metals
Classification
Requirement
High noble alloys
Noble metal content ≥60% (gold + platinum groupa) and gold ≥40%
Titanium and titanium alloys
Titanium ≥85%
Noble alloys
Noble metal content ≥25% (gold + platinum groupa)
Predominantly base alloys
Noble metal content ≤25% (gold + platinum groupa)
18.6.2 Composites
18.6.3 Ceramics
Restoration type
All ceramic
Traditional dental ceramics
Silica‐based ceramics
Glass ceramics
Leucite‐feldspar ceramics
Fluormica ceramics
Lithium disilicate ceramics
Porcelain fused metal (PFM)–high‐strength core ceramics
Glass‐infiltrated ceramics
Metal oxide ceramics
Alumina‐based ceramics
Zirconia‐based ceramics
18.6.4 Porcelain – Porcelain Fused to Metal (PFM)
18.7 Operative Equipment Required
18.8 Tooth Preparation
18.8.1 General Concepts for Restorative Procedures
18.8.2 Components of Prepared Cavities
18.8.3 General Concepts of Prosthodontic Preparations
18.9 Principles of Tooth Preparation for Cast Restorations
18.9.1 Preservation of Tooth Structure
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