Optimum Fits for Outsourcing PFM Copings in the Digital Workflow

Technicians are often challenged with knowing how to modify parameters to create proper fit of digital workflow units versus analog workflow units. Without proper training and an understanding of the process, the journey can be frustrating and many technicians revert back to their comfort zone—conventional analog technique. With the right training and time investment, proper fit in a digital workflow and an increase in your bottom line can be quickly achieved.

There are many benefits of outsourcing PFM manufacturing. First, labor savings by avoiding waxing, spruing, investing, burnout, casting, divesting, and removal of the sprues. Next, lower cost of process since there's no need for casting rings/formers, vacuum investing equipment, burnout ovens, torches, casting machines, divesting equipment, maintenance, etc. Material cost savings, such as for casting wax, wax sprues, investment, alloy, etc, cannot be overlooked; nor can the issue of maintaining inventory. And last but not least: avoiding all of the mess.

Evaluating the case's geometry must include recognition of retention, lack of retention, undercuts, line angles, rough surfaces, and, on bridges, line of draw (Figure 1). An experienced technician picks up a die and their eyes, brain, and hands effortlessly coordinate, instantly identifying what needs to be done to make the coping work. After marking the margin, evaluation of retention begins. If the die has lack of retention, the technician may opt to place no die paint to achieve optimum fit with no die spacer. If another die has very parallel walls, the technician might use two to three coats of die paint to give extra relief for ease of seating. Undercuts are blocked out with utility wax, die lube is painted on the die, and the coping is waxed. The coping is removed from the die and fit is analyzed. This is done with tactile feel and dexterity from years of experience. If the fit is too snug, a light scrape of the intaglio surface might suffice. In some cases, if the fit is inadequate, the die might require re-waxing. Seal the margin, sprue, and invest. Be sure to invest with the correct expansion ratio for the type of alloy, prep geometry, and type of restoration. In the analog world, technicians use different expansion ratios for copings (external fit) and inlays (internal fit). Different techniques and parameters are a must in the analog world to achieve proper fit.

Block-out is required if undercuts are present or axial walls are divergent (Figure 2). It is paramount in rough areas, sharp line angles, and undercuts. If block-outs are not done, the copings will either distort or crack, requiring a re-wax either before or after casting. Incisors have a unique geometry different from most canines and posteriors. In many cases, the preparation style creates a sharp line angle where the lingual surface meets the proximal wall (Figure 3 and Figure 4).  

Light block-out of the lingual surface over this line angle creates copings that literally drop on the die. This light block-out has to be done in the analog world or the castings will not fit. If done correctly, it saves labor, time, and frustration during metal finishing.

For many cases, parameters will need to be altered for optimum fit of the individual die. Altering parameters requires evaluation of prep geometry at the die interface stage. One size does not fit all. Learning how to alter parameters to achieve the desired fit takes time, patience, and perseverance, but the payoff is very rewarding. Tweaking and testing parameter settings will likely result in a control panel change (creating customized default settings) within 10 to 15 units, sometimes fewer. Those tested and proven settings will likely apply to 90% of the work in any given laboratory.

In a digital workflow, the die is scanned by a scan technician. In many cases, that is the last time the die is seen until the coping reaches the metal finisher. Next, the design technician designs the case with no tactile feel or sense of the die. Prep geometry is discarded because the die has been converted into a 3D image that captivates the mind and distracts the technician from reality. Within the software, there are multiple fields, drop down menus, and boxes to click, creating an illusion of technical superiority.

Some of the most misunderstood steps in the digital workflow are removing undercuts, sculpting on die, and default library parameter settings. Many technicians think if they just check the remove undercuts box, all undercuts will be removed. This may or may not be true depending on the manufacturing process. For instance, print path, an additive manufacturing process, is very different from mill path, a reductive manufacturing process. Undercut removal can vary between these two processes and for selective laser melting and conventional casting techniques using a printed wax pattern.

Caution is recommended when using a sculpt on die. This tool allows the technician to do virtual block out on the die using the wax knife. Using the remove or smooth option of this tool can actually take away the surface of the die, inhibiting the coping to seat on the die.

The misunderstanding of default settings could be the second biggest reason for misfit in the digital workflow, the lack of block-out being the biggest. Manufacturers' default settings are a middle-of-the-road guideline and a baseline for different skill levels. Default settings for prep geometry are based on general preps, not the most or least retentive.

The fit of a coping can be altered simply by increasing or decreasing the distance to the margin line. Increasing the distance to the margin line gives you a wider area of intimate fit above the margin, creating a tighter fit. Decreasing the distance to the margin line will narrow the area of intimate fit, creating a looser fit (Figure 5). Cement gap is the setting of intimacy below distance to margin line. Cement gap can be altered, but in most cases the default setting will be fine (Figure 6). Extra cement gap is the setting of intimacy above distance to margin line (Figure 7). This can be increased to create a looser fit on a very parallel and retentive prep, decreased for a non-retentive prep. Note: In some cases of non-retentive preps, the extra cement gap can be set equal to the cement gap to minimize spacer. Drill radius is applied where the axial wall meets the occlusal surface of the prep, creating a sharp line angle (Figure 8 and Figure 9). This is a slight over-mill of the intaglio surface at that line angle to allow for ease of fitting.

Technicians should not be afraid to alter parameters on a case-per-case basis. Testing and experimentation will increase their knowledge and confidence using the system. Fit is subjective. Remember, in the analog world, technicians alter the fit of the coping with tactile feel during the wax-up phase. Investment ratios are altered to achieve the proper fit based on prep geometry and type of restoration. In the digital world, a wax spatula is exchanged for a mouse. Without making corrective moves and altering of parameters, success can be limited. Adjust the parameters by pre-designing the die interface: evaluate retention, the distance to margin line adjustment, the cement gap, the extra cement gap, and the drill radius.

Pre-scan block-out is the most important, most under-used technique in the digital workflow. One constant that should always be taken into consideration during pre-evaluation of each case is prep geometry, such as recognizing retention, lack of retention, undercuts, rough surfaces, and line angles. Block-out and slight changes of parameters will go a long way in achieving proper fit of the units.  Prior to scanning, use scan wax to block out undercuts, rough areas, divergent axial walls, and sharp line angles where drill radius can vary depending on path of insertion (Figure 10).

Close communication between the design and metal finishing departments is vital. When all team members make the commitment to digital workflow, you will achieve success.

Rob White, CDT, has been a technician for 39 years and is a Technical Support Specialist for The Argen Corporation.

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