Fixed Prosthodontics
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PRE-PROSTHODONTIC TREATMENT
Pre-prosthodontic treatment includes endodontic, orthodontic, and periodontal treatment as required. These treatments may include:
- Crown lengthening – in order to establish a ferrule some compromised teeth may require periodontal surgery to expose more tooth structure.
- Orthodontic extrusion – a large force will “pull” the tooth coronally. A change in alveolar ridge crest height may also be seen, and an additional crown lengthening procedure may be required.
- Alveoplasty – alveolar ridge augmentation to remove high spots or eliminate undercuts. Sharp prominent ridges may need augmentation.
- Vestibuloplasty – “increasing” the height of the alveolar ridge by shifting the vestibule apically. The mandibular insertion of the buccinator, mentalis, and mylohyoid can be surgically shifted.
- Soft tissue surgery – often implemented to remove excessive, flabby tissue to improve denture support. An ill fitting denture may have caused inflammatory hyperplasia (Epulis fissuratum) that requires removal. Palatal papillary hyperplasia (denture papillomatosis) is a common lesion that develops on the hard palate in response to chronic denture irritation.
- Frenectomy – a pronounced frenum may dislodge a denture or interfere with the peripheral seal. Z-plasty is a plastic surgery technique usually used to improve the cosmetic appearance of scars, but can also be used in the case of obtrusive frenum attachments.
It is important to understand the term biological width and how it relates to fixed prosthodontics. Biological width refers to the combined measurement of the junctional epithelium (~1mm) and connective tissue attachment (~1mm) to the tooth above the alveolar crest. A crown margin that impinges on the biological width can lead to chronic periodontal problems. Surgical crown lengthening should be considered in cases with a short clinical crown or when margins will impinge on the biological width. If an equigingival finish margin is planned, another ~1mm is needed to accommodate the depth of a “normal” gingival sulcus. Recall a ferrule is 1.5-2mm of tooth structure above the finish line.
Palatal tori (singular: torus) are benign bony enlargements seen in 20-25% of patients (mostly females). They usually reach maximum size around 30-40 years of age. Tori are often covered by a thin mucosa which is prone to trauma and doesn’t heal well due to inadequate blood supply. Palatal tori are not commonly removed before denture fabrication, but removal may be considered if:
- a torus impinges on soft tissues.
- it is so large that the primary support of the denture is compromised.
- it has an undercut.
- it extends far enough posteriorly to interfere with the palatal seal.
- it is psychologically disturbing to the patient (fear of cancer).
Mandibular tori may interfere with the lingual bar or a lower appliance and are removed surgically prior to denture fabrication. Thin, easily traumatized mucosa can also be found in the mylohyoid area, and on the lingual surface of the mandibular alveolar ridge.
Radiographic evidence of occlusal trauma includes periapical hypercementosis, root resorption, alterations in the lamina dura and widening of the periodontal ligament space. Periodontal pocketing is not a radiographic sign of occlusal trauma. Endodontically treated teeth with cast-post cores or large, tapered preparations are prone to fracture. The main symptom of a root fracture is pain when biting. Initially the radiograph will appear normal, however chronic inflammation associated with a vertical root fracture will often be visible on a periapical film as a J-shaped radiolucency around the affected root. A deep localized periodontal pocket can confirm the presence of a root fracture.
TOOTH PREPARATION
The goal of tooth reduction is to conservatively remove only enough tooth structure to accommodate the fixed prosthesis. Tooth structure removal allows a technician enough volume to design a prosthesis with optimum mechanical properties. Tooth preparation must be carried out in a manner that prevents pulpal injury (know the morphology of the pulp), maintains long term periodontal health, and conserves tooth structure. If possible, partial coverage is preferred over complete coverage to conserve tooth structure. Some components of an acceptable tooth preparation are:
- Retention form – features incorporated in the preparation that resist dislodgement of the crown in a vertical direction parallel to the path of insertion.
- Resistance form – features incorporated in the preparation that resist dislodgement in any direction other than the path of insertion. Though both are important, the resistance form is more crucial than the retention form.
The most important features to accomplish the above are
- Taper – the convergence of the walls of the preparations, ideally at 3-6°. The more parallel, the better the retention and resistance. The convergent angles of the opposing walls should not exceed 10°. Axial contours should correspond to the emergence profile of the tooth.
- Preparation height – taller preparations have better resistance and retention.
- Increasing the surface area and roughness of the preparations or including boxes and/or grooves will aid in retention.
Reduction is dictated by the material chosen and the expected forces in the area. The functional cusp bevel allows for extra thickness of metal and/or porcelain to withstand heavy occlusal contact.
In the case of anterior teeth:
- All ceramic crowns require 2.0mm incisal reduction, 1.0mm lingual reduction.
- Metal-ceramic crowns require 2.0mm incisal reduction, 0.5-1.0mm lingual reduction.
- Porcelain-fused-to-zirconia crowns require 2.0mm incisal reduction, 0.5-1.0mm lingual reduction.
In the case of posterior teeth:
- All ceramic crowns require 2.0mm reduction for non-functional cusps, 2.5mm reduction for functional cusps.
- Metal-ceramic crowns require 2.0mm reduction for non-functional cusps, 2.5mm reduction for functional cusps.
- Porcelain-fused-to-zirconia crowns require 2.0mm reduction for non-functional cusps, 2.5mm reduction for functional cusps.
- Metal crowns require 1mm reduction for non-functional cusps, 1.5mm reduction for functional cusps.
- Zirconia crowns require 1mm reduction for non-functional cusps, 1.5mm reduction for functional cusps.
A minimum of 0.3-0.5mm space is needed at a metal margin. In non-load bearing areas the absolute minimum thickness of porcelain is 0.7mm, but 1mm is more commonly used to ensure adequate volume for strength. 2mm of porcelain is required for optimal esthetics. A porcelain to metal junction should be positioned at least 1.5mm away from occlusal contacts. A metal-ceramic restoration requires 0.5mm for the metal and at least 1mm for the porcelain, 1.5mm combined.
Finish line depth and margin configurations are largely selected based on the materials used.
A bevel (feather-edge) margin is a tapered margin or chamfer used when a thin metal margin is desired. A bevel allows the margin to be burnished, which is really only useful for gold restorations. Theoretically this is the best margin for gold, but practically a beveled margin can be difficult to identify on the impression and the die cast and may lead to margin overextension or distortion of the wax pattern. A beveled margin has the least marginal strength. A minimum of 0.5mm space is needed at a metal margin to prevent distortion during function and to assist in proper casting.
A chamfer margin is the preferred finish line for gold restorations. When compared to a beveled margin, the chamfer has increased marginal strength, and a well prepared chamfer is easily identifiable on the die cast.
A shoulder margin (butt joint) is the finish of choice for porcelain jacket and all ceramic crowns, due to the lack of edge strength of porcelain. It provides resistance to occlusal forces and minimizes stress concentration in the porcelain. Butt joints are very easy to identify on a cast. This margin is the least favorable when using a cast metal restoration.
A shoulder with a bevel is a modification of a regular butt joint. It allows a sliding fit to occur at the margin to minimize the gap between the restoration and the tooth, reducing the thickness of cement required. A shoulder with a bevel can be used for metal-ceramic restorations with a metal collar.
In the case of posterior and anterior teeth:
- All-ceramic or porcelain jacket crowns – 1.0mm shoulder or heavy chamfer.
- Metal-ceramic with porcelain to the marginal edge – 1.5mm shoulder or heavy chamfer.
- Full gold crown – bevel, but more practically a 0.5mm chamfer.
- Metal-ceramic, with a metal collar to the marginal edge – 0.5mm chamfer or shoulder with bevel.
The most hygienic margins are supragingival. Equigingival and subgingival margins are more aesthetic but subgingival margins are rarely indicated due to the potential for long term periodontal problems. Supragingival and equigingival margins will be easier to prepare, easier to take an impression of, and better for patient care. Crown lengthening surgery should be considered over subgingival margin placement.
The term “ferrule” describes the circumferential remaining tooth structure above the margin. 1.5 – 2.0 mm is the minimum necessary amount to minimize the risk of failure. Occlusal point contacts are preferred over broad area contacts, to prevent unnecessary wear.
CROWNS
An indirect restoration may rely on a post and/or core buildup. A post is used to retain the core material, but is not needed if the remaining tooth structure is able to retain the core material itself. A post does not reinforce the tooth. Most molars are able to provide enough retention due to their large pulp chamber, but if a post is needed the largest root is selected. For maxillary teeth this is the palatal root, for mandibular molars it is the distal root. The post should be at least as long as the crown to be placed, taking up ⅔ to ¾ of the root length. A minimum of 4-5mm of apical gutta-percha should be left inside the canal to allow for an adequate apical seal. Post width selection is a balancing act. The post should be wide enough to be sufficiently strong, but not not too wide as to compromise tooth root integrity. Most dental materials can be used as a core material in an indirect method.
A cast post and core may be used to restore endodontically treated teeth which have lost a significant amount of coronal structure. The cast post and core does not strengthen the tooth, it only helps to retain the restoration by replacing missing tooth structure. In a separate procedure, a crown is fabricated to sit over the cast post core to restore form, function and esthetics.
A partial crown is a restoration that covers more than half but not all of the clinical crown. The primary reason for using a ¾ or ⅞ crown over a full cast crown in the preservation of tooth structure, and improved esthetics. More of the restoration margin is easily accessible for finishing and cleaning once placed. Partial crowns are not very popular today for anterior teeth (display of gold/metal) but can be used on posterior teeth. A reverse ¾ crown is commonly used for mandibular molars, where the design preserves a relatively intact lingual surface if the buccal side is more damaged. A ⅞ crown can be esthetically acceptable on a posterior tooth since the relatively unrestored mesio-buccal cusp obscures the disto-buccal one. There is more coverage compared to a ¾ crown which provides more resistance, and a ⅞ crown can function as an abutment for a bridge. The path of insertion for an anterior ¾ crown parallels the incisal ½ or ⅔ of the labial surface, not the tooth’s long axis. The occlusion of a gold restoration is best checked with a silver plastic shim stock.
BRIDGES
A bridge is a fixed prosthesis that spans a gap left by one or more missing teeth. The quality of bone surrounding the roots and root configuration will be the most important factors when assessing an abutment’s suitability from a periodontal standpoint. Ante’s law states the surface area of the roots of abutment teeth should equal or exceed the surface area of the roots of the teeth being replaced. Multirooted teeth, long or widely separated roots, and roots that are not round provide the best periodontal support, compared to roots that converge, fuse, or are conically shaped. A tooth with a crown to root ratio of 1:2 is ideal for a bridge abutment. 1:1 is the minimum acceptable ratio.
Parallelism is an important feature of abutment preparation. Preparations on multiple abutment teeth need to be parallel to allow for a common path of insertion. The longer the span, the more likely failure will result. Replacing more than three teeth with a single bridge would be considered high risk.
The pontic is the component that replaces the missing tooth. In a posterior fixed bridge, the pontic should be in contact in centric relation, may contact during working side excursive movements, but does not contact during non-working side excursive movements. Pontic designs include:
Ridge lap pontic (saddle pontic) – concave base straddles the alveolar ridge. It is an esthetic option but the apical concavity is very difficult to clean. This design is not recommended.
Sanitary pontic (hygienic pontic) – convex base that sits 2-4mm away from the alveolar ridge. It is very easy to clean but unaesthetic and can be a food trap.
Bullet pontic (conical pontic) – tapered bullet-shaped base with the tip sitting on the alveolar ridge. This design is not very aesthetic, and is most commonly used in molar areas where aesthetics is not of significant concern.
Modified ridge lap pontic (Steinpontic) – essentially a ridge lap on the buccal and a bullet on the lingual, this pontic design has a convex base with pinpoint alveolar ridge contact on the buccal side. It can be used in most areas with aesthetic concern.
Ovate pontic – the most esthetic, resembling a tooth emerging from the alveolar socket. It is the most difficult to achieve because the design may require surgical ridge augmentation prior to fabrication. Patients may be unwilling to sign up for additional surgeries.
VENEERS
Veneers can be a highly aesthetic treatment option to treat a range of cases, including:
- Covering surface defects, for example hypoplasia.
- Masking discolored teeth (tetracycline staining, traumatic dental injuries etc.).
- Repairing structural damage.
- Improvement in tooth shape and color (e.g. peg laterals).
But veneers are contraindicated in the following cases:
- Traumatic occlusal contacts.
- Unfavorable tooth morphology.
- The severe imbrication of teeth.
- Insufficient tooth structure.
- Insufficient enamel.
A pinledge restoration is a thin, modified lingual inlay used on anterior teeth that usually has 3 parallel pins (1.5-2mm long) that penetrate the lingual dentine for retention.
COLOR IN DENTISTRY
Color is produced by light reflecting from an object. Pigment color reproduction is determined by selective absorption and selective radiating (scattering). Tooth color is influenced in part by optical reflection, refraction and dispersion. The three dimensions of color:
- Hue – the actual color (e.g., red, yellow, green, etc.). On a Vita shade guide this would mean selecting A (reddish-brownish), B (reddish-yellowish) , C (greyish shades) or D (reddish-grey). Slight changes in hue may be accomplished within a restoration.
- Chroma – the amount of saturation/intensity/strength of the color or shade. Once the hue is selected (say B shade) the saturation can be determined (B1, B2, B3 etc). Chroma can be successfully increased by external colorants/stains.
- Value – the relative lightness/brightness or darkness. Value and chroma are inversely proportional (higher value, lower chroma). External colorants can be used to lower the value, but it is almost impossible to increase the value of an existing ceramic restoration. Generally, older patients will have teeth with lower value and higher chroma, compared to younger patients. Intensity is included in the term value.
Only two modifications to hue are commonly accomplished. The addition of pink-purple will move yellow towards yellow-red. The addition of yellow will decrease the red content of a yellow-red shade.
Characterization is the art of producing natural defects to enhance the realism of a restoration.
Metamerism is the optical phenomenon when an object appears to change color under different lighting conditions. A crown color matched in a dental chair using the dental light may be a poor color match when the patient steps out into direct sunlight. It is important to select a shade in different lighting conditions.
Fluorescence describes the reflection of (longer wavelength) visible light when an object is exposed to ultraviolet (UV) light. Dentine will “glow” blue-white hues (400-450nm) when exposed to UV light. Fluorescence in porcelain is important to minimize metamerism in differing light conditions.
Blue fatigue increases yellow sensitivity. If a dentist stares at a blue object (blue curtains for example) it can cause blue fatigue which accentuates the ability to discriminate yellow shades while selecting the tooth color.
Opalescence describes the optical effect of a translucent material. An opalescent object will appear to be one color when light is reflected from it and another color when light is transmitted through it. For example, teeth appear more blue in reflected light and more red-orange in transmitted light. This effect can be created in porcelain restorations by incorporating submicron particles of porcelain into the enamel layers.
When selecting the shade of a tooth, the surrounding colors caused by bright lipstick, heavy make-up, dark glasses, or articles of clothing can affect the perception of color. Make sure the teeth are clean before shade matching. Staring too long will cause fatigue, decreasing the ability to discriminate colors and shades. Half closing your eyes can increase the sensitivity of retinal rods to better select value.
