Decrease Crown Seating Time with Better Bite Records
4 principles to increase accuracy and ensure proper fit
When speaking with dentists and dental laboratories across the country, one of the most common discussions is the need for and usefulness of interocclusal bite records. Dentists typically send a bite record of some type with their impressions, even if it is a dual-arch tray, to help the lab verify the bite. These are many times not used by the laboratory because of a concern that they may introduce more inaccuracies into the restoration. Often the injectable material used covers the preparations and opposing teeth, obscuring the view of intercuspation as the patient closes.
When placing crowns, better marginal and interproximal fit and less occlusal adjustment lead to decreased seating time and better overall result for our patients. The quality of the product we get back from the laboratory is determined by the quality of the information we as restorative dentists give them. Proper technique and use of the right materials will ensure fewer problems. Whether for full-arch cases or single units, the accuracy and stability of the bite registration is critical to success. Although there are many different techniques and materials available, there are a few key principles to remember when selecting the right one.1
Principle 1: Stable Material
It is best to use a rigid polyvinyl siloxane (PVS) material because of its dimensional stability2 and ease of trimming with a scalpel (eg, Futar® D, Kettenbach LP, www.kettenbach.us). Some PVS materials can be thin and flexible, making them difficult to trim without breaking. Hard waxes that have a high melting point can also be used if handled carefully to avoid breaking. Wax wafers also have the advantage of providing cross-arch stability (DeLar Bite Registration Wax, DeLar Corporation, www.delar.com). Soft wax is easily distorted by changes in temperature or damaged in shipping and handling and is therefore not advised.3
Principle 2: Accurate Material
Following the same thought process as principle 1, rigid PVS or very hard wax materials are also the most accurate.4 Remember that the purpose of bite records is to fit the prep model and the opposing model with equal accuracy. If care is not taken in the final impression, or especially the opposing model, it is likely that the bite record will be more accurate than the model and will not fit properly. Accurate working models come from accurate impressions.
Principle 3: Use an Index to Verify the Correct Bite
Bite discrepancies are often introduced when the patient unintentionally closes into a non-functional position while the material is setting. Patients’ proprioception is decreased by local anesthetic, causing them to bite down into the bite record material in a different position than their normal maximal intercuspal postion. A simple way to alleviate this problem is to make a PVS index on the contra-lateral side before the patient is anesthetized (Figure 1). Once this index has set, remove it from the mouth and complete the preparation and impression. Just before taking the bite registration, place the index back in (Figure 2), inject the bite record material over the prepared teeth, and have the patient close down into the index (Figure 3). The index assures that the mandible returns to the same bite position that was verified at the beginning of the appointment (Figure 4).
This approach is effective whether you are using a quadrant tray, a triple tray, or a full-arch tray. It is especially helpful when preparing multiple anterior teeth. In that case, the preparations eliminate the preoperative anterior stop, creating the opportunity for the patient’s mandible to drift forward into the bite registration, altering the relation of the lower incisal edge to the upper lingual surface—a critical functional area.
Principle 4: Verify the Fit of the Bite Record to the Prepared Teeth and to the Dental Casts
Because of the hardness of the material, it should be easy to trim the excess with a scalpel in order to visualize and verify complete seating in the mouth. When the case comes back from the dental laboratory, the same bite record can be used to verify that it fits exactly the same on the articulated model as it did in the mouth. This can be used as a quality control check before the patient’s appointment. Any discrepancy can be discussed with the lab and corrected before wasting an appointment trying in a crown that will not fit or will need a large amount of occlusal adjustment.
Attention to these four principles will greatly decrease the time it takes to seat single or multiple units by decreasing the adjustments necessary. It will also improve esthetics by not grinding away the characterization built in by the laboratory technician.
1. Dawson PE. Functional Occlusion: From TMJ to Smile Design. St. Louis, Mo: Mosby; 2007.
2. Ghazal M, Albashaireh ZS, Kern M. The ability of different materials to reproduce accurate records of interocclusal relationships in the vertical dimension. J Oral Rehab. 2008;35(11):816-820.
3. Millstein PL, Clark RE. Determination of the accuracy of laminated wax interocclusal wafers. J Prosthet Dent. 1983;50(3):327-331.
4. Vassilis KV, Aris-Petros DT. Evaluation of vertical accuracy of interocclusal records. Int J Prosthodont. 2003;16(4):365-368.
About the Authors
Jeff Scott, DMD
St Petersburg, FL
St. Petersburg, FL
American Academy of Restorative Dentistry