A crown or “dental cap” is a dental restoration that fully caps or covers a tooth or dental implant.
Crowns may be required when a large cavity endangers the health of a tooth. It is generally bonded to the tooth by dental cement. Several materials, such as zirconia, can be utilized to create them. Crowns can enhance the stability or esthetics of teeth and halt tooth degeneration. A crown preparation is the design of the tooth after “reducing it” to permit room for a crown. Dr. Hawryluk’s preparation design depends on the material, prior fillings, and whether the tooth has fractured or has had a root canal.
For a crown to be stable, there must be enough tooth structure for the crown to adhere. In addition, it must have adequate resistance and retention form. Resistance form is the feature of the tooth that enhances stability on the tooth. Patients have protrusive and excursive moments of their jaw. The resistance form is the feature that could help dislodgement in the mouth by forces it encounters. Retention forms are the features of the tooth that resist the dislodgement of a crown in a vertical direction. It is the “up and down” forces or those or along the path of its placement. The shorter the tooth, the more Dr. Hawryluk needs to consider aspects of crown engineering to meet these criteria.
When designing your crown, Dr. Hawryluk strives to preserve tooth structure as much as possible. We need to give ceramists room for the crown material, but there is a fine line. We don’t want to over-prepare a tooth and weaken the tooth or compromise the dental pulp. The marginal integrity of the crown must provide an air-tight seal with a tooth. Finally, Dr. Hawryluk designs crown with proper gum health in mind. An appropriate depth of a crown prevents over contouring which aids in hygiene. When we do tooth preparation, we want to keep things conservative. When preparing zirconia crowns, Dr. Hawryluk’s crown margins tend to be about three-tenths of a millimeter with a taper of about 10-20 degrees. If possible, the preparations are kept tall and parallel. We prepare crowns with electric handpieces because we get a ton of torque- which means Dr. Hawryluk does not have to push as hard on your tooth to do the work needed. For your comfort and tooth health, our electric handpieces are the famed NSK Ti-Max Z95L. We usually prepare the anterior teeth first when doing full mouth rehabilitations. Very rarely will we prepare 14 teeth at a time. After the anterior, we will do one posterior quadrant at a time.
The minimum preparation height for the anterior (front) tooth is 3mm. However, for a molar, it is 4mm. Molars are wider, so the crown preparation must be taller to retain the crown as a tooth gets wider. If there is a situation where a crown preparation is less than these dimensions, the tooth will need a crown lengthening procedure before placing the cap. Crown lengthening surgery involves the removal of gum tissue, bone, or both to expose more of a tooth so it can be fixed or restored. The tooth which will most commonly need crown lengthening is the mandibular molars.
In visible areas, Dr. Hawryluk will try “hide” the margin of the crown below the gumline to maximize esthetics. It is prudent to review a few definitions first to understand how he accomplishes this in different situations. The “biologic width” is the biological seal that extends around the tooth, shielding the alveolar bone from infection and disease. The space right below the gum line is named the “gingival sulcus”. Initially, Dr. Hawryluk will prepare teeth “equi-gingival” (at the gumline and measure the sulcus depth with a periodontal probe. If the sulcus measures .5mm or less, we cannot extend the preparation below the gum line, or else we would get a biologic width invasion. We must stay equi-gingival. However, if the sulcus is greater than 1mm, we can take the crown preparation to half the sulcus depth. The findings from these sulcus measurements may dictate the type of material used to make a crown or whether the dentist should consider the modification of the gum before placing a crown.
Matching the shade of teeth is sometimes tricky. Luckily, there are a lot of time-tested methods to support Dr. Hawryluk when making aesthetic decisions. A “shade” considers hue, chroma, and value. Hue refers to the color, whereas chroma is the intensity or saturation of the hue. Value is how dark or light the shade is. A simple tool for color matching is a shade guide. It is a plastic card containing various shades of teeth. Dr. Hawryluk will hold the guide by your teeth and approximate the natural shade to the examples until he finds a matching shade. The dentist usually starts by choosing the value of the tooth and then assesses the hue and chroma. While selecting a shade is relatively subjective, some science can help. First, the ceramist should make color determinations under natural light conditions (like near a window) or fluorescent lights that imitate natural light. Traditional incandescent lights tend to be too yellow and make colors seem different. The ceramist periodically asks patients to remove any bright lipstick or other makeup, affecting the perception of the tooth shade. Dr. Hawryluk also uses the Trios system for digital shade selection.
When a crown preparation is into the dentin, sometimes we must wait at least a day to take the crown shade. However, if the preparation is only into the enamel, we can sometimes take the shade right away. There is substantial tooth dehydration when we prepare a crown into dentin. Enamel has less than half of water content compared to dentin, has 25% water content, and its percent water loss is 6% during tooth preparation. Enamel has 12% water content, and the percent water loss is 1%.
Sometimes teeth are “internally bleached” before placing crowns. Internal bleaching is the method of bleaching or whitening teeth from the inside out. If internal bleaching is to be done before preparing a crown, we need to wait at least two weeks to prepare the final crown as there will be free oxygen radicals escaping from the tooth, which will inhibit the bonding.
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