CAD/CAM dentistry: Trapped by what you know?

Jan. 1, 2012
“I will never have one of those machines in my office.” I turned to face the source of that comment and smiled at a participant in our Facially ...

Gary M. DeWood, DDS, MS

“I will never have one of those machines in my office.” I turned to face the source of that comment and smiled at a participant in our Facially Generated Treatment Planning (FGTP) workshop. He was pointing at one of the many CEREC machines we have in residence at our Scottsdale Center facility. He clearly had a strong opinion about its usefulness that he wanted to talk about.

Now I am no stranger to unbridled contempt for CAD/CAM dentistry. In the early days of this technology, I had an experience with CEREC 1 that soured me on the idea for a long time. So hearing an unsolicited outburst like this made me wonder what experiences had created such emotion for this individual.

As it turned out, he had purchased a CEREC 2, a machine that in his words cut essential corners that made the restoration mediocre at best. He had returned it, angry and disgusted, refusing to even look at the technology since. Wow! It’s déjà-vu all over again.

Like my friend, after my venture into CAD/CAM, I also remained staunchly opposed to this technology. I refused to even listen to those who sang the praises of the newest versions. Obviously they were easily swayed by the sales hype. I, on the other hand, knew the truth. I might still be actively dismissing this part of dentistry if not for my experience in Seattle when Frank, Gregg, and I were introduced to the CEREC AC machine and the Bluecam.

I was surprised and impressed. This was most definitely not what I had experienced years ago. It was easy, precise, and most importantly, it was accurate. I realized in that moment that what I thought I knew had held me back from seeing the evidence and adopting an excellent technology for my patients and my practice. My daughter is a second-year dental student at Midwestern University in Phoenix, and she is already working with E4D. This technology will be normal for her practice. CAD/CAM is not only a part of dentistry, it is also an important part of dental education. I am working on catching up.

The two restorations seen in Figures 1-3 were placed in 90 minutes for a friend visiting from the UK who broke a tooth. They are milled from a lithium disilicate (e-Max) blue block that I conditioned and glazed after milling. The oven even told me what program to run. Figure 2 shows the use of Gluma PowerGel to wet and desensitize prior to bonding the restoration to place.

Fig. 1

Fig. 2

Fig. 3



Gordon J. Christensen, DDS, MSD, PhD

One of my least desirable and successful clinical procedures is removable partial dentures. Many patients admit that they do not wear their partial dentures, except for esthetic reasons. It seems that I am always tightening clasps, and too often breaking clasps. Is there any way to make this undesirable treatment more acceptable for patients and for me?

You are definitely not alone in expressing your dissatisfaction with removable partial denture (RPD) treatment. Most patients do not like their RPDs for the reasons you expressed. You can make RPDs much more acceptable for them by following some of these suggestions:

Use of implants

One of the most important suggestions I can make concerning RPD fabrication is placing implants in strategic locations under the RPD. Placement of implants under removable partial dentures is well known to increase patient satisfaction by improving retention and stability. The implants are placed primarily for additional support and to provide some or all of the RPD retention. Although most dentists do not often accomplish this simple technique, I feel that implants should be used whenever patients can afford to have them placed. Where should the implants be placed?

There is a triangle of remaining bone mesial or distal to almost every remaining natural tooth that does not have a tooth directly adjacent to it (Fig. 1). This small amount of bone is often adequate for placement of an implant of conventional diameter, such as 3.5 to 3.75 mm. When there is not as much bone present, a small-diameter implant can be placed, ranging from 1.8 to 2.9 mm (Fig. 2).

The implants should be placed as parallel as possible with the planned path of insertion and removal of the RPD. Depending on the attachment used, the parallelism necessary with the path of insertion varies. Spheres used as the abutments on the implants, with “O” ring rubber washers, allow the most variance from parallelism with the planned path of RPD insertion (Fig. 3).

Housings located to exactly coincide with the implant abutments are placed in the denture base resin of the RPD. There must be at least 4 mm of denture base material present from the tissue side of the denture to the artificial tooth to allow space for a housing to be placed.

The increase in patient satisfaction provided by implant placement under RPDs is due to the following improvements: clasps can be reduced in number or eliminated; and the retention provided by denture attachments such as ERAs and Locators or spheres and rubber washers is usually significantly more than that present with conventional clasps. When retention is reduced during service, it can be improved quickly and simply by placing a tighter plastic element in the attachment or replacing the rubber washer. Usually, implants can be placed in numerous sites depending on the locations of the remaining teeth.

Keeping metal frameworks directly in

contact with the gingival tissues

Many dental technicians place a spacer on the soft tissue represented on the working cast to allow space under the framework. This technique allows the framework to easily go into place without pressure on the soft tissue, but it also creates a food trap that collects food debris unless the partial is cleaned frequently. Also, air often escapes from under the denture, which is disagreeable to the patient. I suggest the framework should rest directly on the soft tissue, and that there should be NO relief of the stone cast (Fig. 4). When using this concept, a sore spot will occasionally occur after seating the RPD. The sore spot can be eliminated quickly and easily with various pressure evaluation materials, such as Fit Checker from GC, to help find the spot and eliminate the pressure. This is a small concession for the elimination of food entrapment and air escape.

Using thin frameworks

Typical RPD frameworks are often made thick and bulky, apparently in an attempt to reduce breakage. The resulting bulky and large RPD is objectionable to the patient. The clasps are rigid and cannot flex. When the clasps of such frameworks are tightened, the partial frequently will not fit back easily into the mouth. I suggest that RPD frameworks should be made thin to allow flexibility (Fig. 5). A thin RPD framework can be slightly flexed when held in your hands and force is applied. Of course, the negative aspect of thin frameworks is breakage, but this is so infrequent that it should not be a major deterrent. Patients appreciate thin and flexible frameworks because the RPD does not have the peculiar, objectionable, large, foreign object feeling of a thick framework.

Making clasps thin and flexible

Thick, nonflexible clasps are unsightly, cannot be easily tightened, and are bulky and uncomfortable for patients, thus contributing to patients’ reluctance to wear an RPD. I suggest making thin, flexible clasps that easily spring into undercuts (Fig. 7). Most laboratory technicians need instruction to make thin, flexible clasps, since they usually make them thick in an attempt to avoid breakage. In my experience, thin, flexible clasps do not require significant tightening, they retain RPDs well, and they satisfy patients far more often.

Smooth RPD frameworks

Some labs make the external surfaces of frameworks rough, apparently in an attempt to make them similar to the irregularities on natural gingival tissue. In my experience, patients prefer smooth external RPD surfaces that can be polished to a high shine with extremely smooth surfaces (Fig. 6).

Lingual clasps

Historically, most retentive and supportive clasps for RPDs have wrapped around teeth, and they show metal on the facial surfaces of the teeth. Some clasps, such as “I” bars, locate a vertical bar of metal on the facial surfaces of the supportive teeth. Placing clasps in any area where the patient can see them negatively influences patient acceptance. Additionally, observers can easily see the clasps, which immediately says that the patient is wearing an RPD. Reducing or eliminating this metal display can be accomplished simply by using the following procedure:

• Identify the locations for the RPD design where retention is necessary.

• Prepare occlusal rests in the teeth involved where retention is necessary. The rests should be at least 1 mm deep and should eliminate the possibility that the framework can move out of the rest seat toward the facial or lingual.

• If the rests must extend into dentin, a resin-based composite restoration should be placed in the deepest portion of the rest, and the patient should place fluoride gel onto the rest seat metal at least once a day. (An example is Colgate PreviDent 5000 Plus fluoride gel.) Potential retentive undercut areas should be identified on the lingual tooth surfaces of the affected teeth.

• If gingival tissue blocks the entry of a clasp into the undercut, a small piece of soft-tissue should be removed with an electrosurgery unit or a laser, allowing the tip of the clasp to enter the undercut.

The framework is designed with minimal or no facial clasp display (Fig. 7).

Flexible denture base materials for some RPDs

This type of RPD is not the restoration of choice for most patients, because the RPD may sink into soft tissue, break, or cause occlusion challenges by collapsing the occlusion and causing opposing tooth extrusion. Although condemned by some dentists, in my opinion, flexible, non-metal-containing RPDs serve some purposes well.

Their use is acceptable for treated periodontal cases with somewhat mobile teeth, for those patients not suitable for implants, or for those patients who have difficulty wearing conventional metal framework containing RPDs. In my opinion, they are a satisfactory clinical solution if they’re accompanied with adequate informed consent and patient education.

In summary, current excellent materials, placement of implants, and improved patient-friendly techniques can make RPDs acceptable treatment. I suggest that you look into the various suggestions I have made and implement those that fit your patient population.

Our newest DVD, “Predictable Removable Partial Dentures” (Item# V2551), illustrates many of the concepts we have identified in this article, and shows me, Dr. Christensen, in live, close-up video accomplishing the procedures. Visit our website or call Practical Clinical Courses at 800-223-6569 for additional details.

Dr. Christensen is a practicing prosthodontist in Provo, Utah. He is the founder and director of Practical Clinical Courses, an international continuing-education organization initiated in 1981 for dental professionals. Dr. Christensen is a cofounder (with his wife, Rella) and senior consultant of CLINICIANS REPORT (formerly Clinical Research Associates).

Fig. 1 — One implant has been placed in the almost-always present triangle of bone directly adjacent to remaining teeth.

Fig. 2 — Small-diameter implants can be placed in minimal remaining bone.

Fig. 3 — Rubber washers in housings, used as RPD retention, allow significant lack of parallelism of implants and still function well.

Fig. 4 — The stone cast has not been relieved for this RPD, and the metal framework rests directly on the soft tissue. This technique reduces metal bulk and reduces food impaction around the RPD.

Fig. 5 — This metal framework has purposely been made thin and flexible, reducing rigidity and bulk and improving patient acceptance.

Fig. 6 — Note the smooth external surface on this RPD framework. Most patients prefer this type of surface to the rough surface often placed on RPDs by technicians.

Fig. 7 — Note the thin and flexible clasps on this RPD. Such clasps allow easy insertion and removal of the RPD, spring easily in and out of undercuts without distortion, and are less objectionable esthetically for patients.

Frankly Speaking: Insight from Spear Education

Ask Dr. Christensen

Figures 4-6 show a milled composite overlay used to test an altered VDO and occlusal scheme for a TMD patient. Mounted casts were used to scan my “preps,” the existing surfaces of the mandibular teeth, and then design a new occlusion using a buccal bite technique to create a precise occlusion against the maxillary arch. Spot etching on cusp tips and seating with flowable composite provides a much better test than an appliance because the patient functions normally with the new occlusion at all times. The exception is the need to thread floss under the overlay.

As I have become more familiar with CAD/CAM, I have learned that there are some significant differences between what I was taught back in the 1970s and what works best for this technology. Preparation design is infinitely more important than it is with other restorative techniques. Square and sharp have been replaced by round and flowing. The margin must be as perfectly smooth as possible so it can be milled. Magnification becomes not just suggested but mandatory. The diameter of the milling diamonds must be known and the prep must conform to ensure that the restoration will not be overmilled, creating areas to accommodate the milling diamond size.

Better preparation is almost always the first thing people mention when they talk about how CAD/CAM has changed their practice. I also learned this. Unless you are retiring as you read this, CAD/CAM dentistry will change your practice, too. Don’t be trapped by what you know. This technology is not coming; it is here. I turned to the gentleman mentioned earlier who had made the comment at the FGTP workshop and said, “We need to talk.”

Gary M. DeWood, DDS, MS, earned a DDS from Case Western Reserve University in Cleveland, Ohio, and an MS in biomedical sciences from the University of Toledo College of Medicine. He serves as executive vice president for curriculum for Spear Education, teaching and practicing in Scottsdale, Ariz. Contact him at [email protected].

Fig. 4

Fig. 5

Fig. 6

Speaking at the Hinman 100th Dental Meeting March 22-24, 2012

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