Tooth Crowns

In dentistry, crown refers to the anatomical area of teeth, usually covered by enamel. The crown is usually visible in the mouth after developing below the gingiva and then erupting into place.

In dentistry a crown can also refer to a metal or porcelain device placed over the tooth to prevent further tooth decay and its structures underneath as well as to provide functionality for that tooth.

Tooth Bridges

A dental bridge, otherwise known as a fixed partial denture, is a prosthesis used to replace missing teeth and is not removable by the patient. A prosthesis that is removable by the patient is called a removable partial denture.

A dental bridge is fabricated by reducing the teeth on either side of the missing tooth or teeth by a preparation pattern determined by the location of the teeth and by the material from which the bridge is fabricated. In other words the abutment teeth are reduced in size to accommodate the material to be used to restore the size and shape of the original teeth in a correct alignment and contact with the opposing teeth. The dimensions of the bridge are defined by Ante's Law: "The root surface area of the abutment teeth has to equal or surpass that of the teeth being replaced with pontics" [1].

The materials used for the bridge include gold, porcelain fused to metal, or in the correct situation porcelain alone. The amount and type of reduction done to the abutment teeth varies slightly with the different materials used. The recipient of such a bridge must be careful to clean well under this prosthesis.

When restoring an edentulous space with a fixed partial denture that will crown the teeth adjacent to the space and bridge the gap with a pontic, or "dummy tooth", the restoration is referred to as a bridge. Besides all of the preceding information that concerns single-unit crowns, bridges possess a few additional considerations when it comes to case selection and treatment planning, tooth preparation and restoration fabrication.

Tooth preparation

As with preparations for single-unit crowns, the preparations for multiple-unit bridges must also possess proper taper to facilitate the insertion of the prosthesis onto the teeth. However, there is an added dimension when it comes to bridges, because the bridge must be able to fit onto the abutment teeth simultaneously. Thus, the taper of the abutment teeth must match in order to properly seat the bridge; this is known as requiring parallelism among the abutments. When this is not possible, due to severe tipping of one of more of the abutments, for example, an attachment may be useful, as in the photo at right, so that one of the abutments may be cemented first, and the other abutment, attached to the pontic, can then be inserted, with an arm on the pontic slipping into a groove on the cemented crown to achieve a span across the edentulous area.

Restoration fabrication

As with single-unit crowns, bridges may be fabricated using the lost-wax technique if the restoration is to be either a multiple-unit FGC or PFM. As mentioned in the paragraph above, there are special considerations when preparing for a multiple-unit restoration in that the relationship between the two or more abutments must be maintained in the restoration; that is, there must be proper parallelism in order for the bridge to be able to seat properly on the margins. Sometimes, the bridge does not seat, but the dentist is unsure whether or not it is only because the spacial relationship of the two or more abutments is incorrect, or whether the abutments do not actually fit the preparations. The only way to determine this would be to section the bridge and try in each abutment by itself. If they all fit individually, it must have simply been that the spacial relationship was incorrect, and the abutment that was sectioned from the pontic must now be reattached to the pontic according to the newly confirmed spacial relationship. This is accomplished with a solder index.

The proximal surfaces of the sectioned units (that is, the adjacent surfaces of the metal at the cut) are roughened and the relationship is preserved with a material that will hold onto both sides, such as GC pattern resin. With the two bridge abutments individually seated on their prepared abutment teeth, the resin is applied to the location of the sectioning to reestablish a proper spacial relationship between the two pieces. This can then be sent to the lab where the two pieces will be soldered and returned for another try-in or final cementation.
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Venners

In dentistry, a veneer is a thin layer of restorative material placed over a tooth surface, either to improve the aesthetics of a tooth, or to protect a damaged tooth surface. There are two types of material used in a veneer, composite and porcelain. A composite veneer may be directly placed (built-up in the mouth), or indirectly fabricated by a dental technician in a dental laboratory, and later bonded to the tooth, typically using a resin cement such as Panavia. In contrast, a porcelain veneer may only be indirectly fabricated.

The advantages of using a veneer to restore a tooth are many. Very good aesthetics can be obtained, with minimal tooth preparation (i.e. drilling). Normally a reduction of around 0.5 mm is required for a porcelain veneer on a labial tooth surface. Composite veneers are becoming more popular as they are easy to repair, whereas porcelain veneers have a tendency to fracture. It can be very difficult to match the shade of an individual veneer to the remaining teeth, hence the tendency to place several veneers.

Veneers and laminates are another conservative approach to enhancing the look of your teeth. They are a thin shell made of porcelain or composite resin that is cemented to the front surface of the tooth. Like bonding, veneers are used to cover up discoloration, cracks and chipping, and to change the shape or size of your teeth. However, porcelain veneers outperform bonding with greater resilience and a greater resistance to stains and dulling. Also, they tend to look more natural than a bonded tooth.

Veneers may be used cosmetically to resurface teeth such as to make them appear straighter and possess a more aesthetically pleasing alignment. This may be a quick way to improve the appearance of malposed teeth without need to use orthodontics. However, the amount of malposition of teeth may be such that veneers alone may not be enough to correct the aesthetic imbalance. Instead, orthodontics would need to be used, or orthodontics combined with veneers. The dentist who places veneers must be careful since veneers could increase the thickness of the front face of the teeth. If the teeth are too thick on the face they may appear to stand out and push out the lips. The effect may be enough to give the patient a full or chipmunk appearance when the lips are closed. Veneers must also be created such that the patient bites into them with minimal force. Otherwise, they may chip off. So, patients whose lower jaw protrudes out farther than their upper jaw, otherwise known as a class III bite, may not be good candidates for veneers because the teeth of the lower jaw may bite into the teeth of the upper jaw such as to dislodge the veneers.

Indications treatable by veneers include

stained/ defective restorations
gap between front teeth
fracture lines
wearing of teeth
discolored teeth
malformed teeth
slight malposition
gum recession exposing the roots
Erosion/ abrasion of teeth
in case of children teeth (with large pulp}

Comparison of veneer systems

Composite resin veneers
Composite resins ( same as used for tooth -colored fillings) can be used in a direct chairside technique to form a veneer. Advantages of these direct composite veneers are:

Only one appointment is required
cost to the patient is less
composite resin veneers are reparable
color and form can be controlled by the dentist

Porcelain veneers: These veneers promise" the highest esthetic potential to date for restoration of anterior tooth defects." Porcelain is the optimum material for its color stability, esthetics, wear resistance and tissue compatibility. But unlike composite resin veneers these are constructed in labs from dies made from patient's impression.

Porcelain veneers

Frequently do not require anesthetic and are less stressful to the patient.
does not usually cause sensitivity
maintains natural contacts between teeth
eliminates display of metal at the gum margin
does not usually require temporization

But these veneers are expensive. These are extremely difficult to repair if fractured and the technique involved in placement of veneers is extremely delicate.
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