Remakes are inevitable in dentistry due to the multiple variables in play during the crown fabrication process. However, remakes can be reduced greatly by applying a consistent prep technique, controlling moisture with haemostatic agents and developing an impression technique ideal for your chosen impression material.

The most common problem addressed in our laboratory is inadequate reduction. Every practitioner should develop a prep technique that can be refined through feedback from your dental laboratory.  The prep technique should be developed to accomplish four main goals.

• Adequate overall reduction
• Well defined margin
• Smoothness of prep
• Reduced prep time

A well defined chamfer margin is necessary for the dental technician to match the shade requested without over contouring the crown. A crown result that is opaque in appearance at the gingival 1/3 is typical of lack of reduction in this area. This will minimize porcelain thickness, raising the value of the crown resulting in the opaquey look.  A well defined margin also helps the dental laboratory identify the margin and provide a more accurate marginal trim.

A smooth prep may seem less important than some of the other topics I have discussed, but the single most common aspect of prep design amongst the dentists with the lowest remake percentage is a smooth prep. Jagged edges, spikes or sharp corners can break off when the model is pulled from the impression and often are very subtle differences that are not noticed by the model technician. The result is a crown that rocks on the often elusive high spot.

The second most common problem addressed in laboratories is washed out margins.

A good moisture control technique is vital to an accurate impression and in the end an accurate crown. Improvement in impression accuracy can be made by following a few recommendations.

• Rinse and dry prep area before taking the impression
• Stop bleeding with haemostatic agent
• Leave cord in sulcus until no blood or saliva are present before syringing the light body impression material
• Consider two cord retraction to displace tissue and control fluids
  
  
  

Third most common problem addressed in our dental laboratory is pulls in the impression.

A consistent impression technique through the understanding of your chosen impression materials is the final step to a very accurate impression. Facial and lingual pulls are common problems noticed in our lab and can be reduced through these steps.

• Seat impression early in the recommended seat time so as not to exceed the working time
• Leave in longer than recommended set up time to ensure complete set up
• Do not move tray after seating
• Use more material to create back flow
• Use an impression tray with proper support for material

This article was written to help inform dentists of the most common problems with impressions seen in our laboratory and there resulting effects on the final restoration. Taking the time to improve the techniques for prep design, control of moisture and the final impression will result in reduced time in seating of restorations and greatly increase patient satisfaction and confidence in the doctor’s abilities. In every group of dentists that a laboratory supports there is a wide range of abilities and the dentists that are on the upper end of this range tend to also be better communicators. As a dentist this is where you want to be. The benefits in profitability, reduced stress, patient satisfaction and the high regard from your laboratory will make your life in dentistry much more enjoyable.

The info in this article is an accumulation of my own experience as a laboratory owner coupled with images and information gathered from 3M MAKING BETTER IMPRESSIONS GUIDE, which can be accessed through the PDF library at our web site RVDALAB.com

There are a multitude of technical problems involved with the prep/impression process that can be addressed through our library.