Conservative Tooth Preparation, Minimal Intervention, and/or Narrow-parallel Preparations: A Narrative

INTRODUCTION

During my senior year in dental school (1963), I heard a lecture by Dr Miles Markley and a few months later another by Dr Wilmer Eames. Both touched on the subject of smaller posterior preparations that did not use GV Black's standard one-third intercuspal width for posterior teeth. I had done a little reading in GV Black's Operative Dentistry during the preclinical operative course. However, I learned early that in the first year of dental school one does not ask questions. Fortunately, I was able to connect with Dr Eames and we had a discussion on cavity preparation.

Black's original ideas on class I and II cavity preparations were to have the occlusal preparation at least one-third isthmus width and the depth at the dentin–enamel junction.¹ In the class II preparation, the margin of the proximal box was carried out just past the adjacent tooth.² This was called “extension for prevention."³ As a fourth-year student, I was aware that patient factors, areas of decalcification of the enamel, and interception of the caries process changed where margin could be placed.

After graduation, I was in the postgraduate program at Indiana. I was enrolled in this program with majors in operative dentistry and dental materials. I also maintained a part-time private practice. A few months into the graduate program, I found an article by Vale⁴ that indicated one-quarter isthmus prepared teeth were almost twice as strong as teeth with a one-third isthmus width.

I was HOOKED!

I HAD TO FIND A WAY TO HAVE STRONGER TEETH AFTER CAVITY PREPARATION

I began preparing extracted teeth with different burs looking for a better way to achieve narrower preparations than the one-third isthmus I had been taught. I had discussions with faculty and read the literature constantly. My research on narrower preparations had many start/stop periods in those two years. I prepared teeth, both typodont and extracted, multiple times. I sectioned teeth and photographed the results.

I concentrated on the use of divergent burs in the second year. I was able to have a manufacturer make special divergent burs. I had a difficult time with preparations using divergent burs. If I lifted the bur out of the preparation, I had a parallel preparation. Following the undulating fissures of a molar enamel surface with the neck of the bur as a guide was exceedingly difficult. Even with experience, this did not get easier. I tried to start at the deepest portion of the tooth normally used in cavity preparation and go straight out the grooves. That was not a good solution with different widths of the preparation at the occlusal surface. I found I needed smaller and less divergent burs for lower premolars, especially lower first premolars. Also, if the opposing wall was touched with a bur you had a flared wall that was almost impossible to correct. After a couple months, I finally abandoned divergent burs.

Then, I tried cylindrical burs. How easy they were. The #56 (1.0 mm diameter) seemed to be a perfect fit. Go to your depth just above the dentin–enamel junction and cut out the grooves and fissures. Where caries were found, remove the decay and widen the preparation as necessary. This narrow-parallel preparation was so easy that I almost could not believe it. If I could do this with so little difficulty, I knew a student could learn it readily. I conducted limited trials on strength and was satisfied these narrow preparations did not substantially weaken the tooth.

However, I recognized the problem of having an acute angle of restorative material at the cavosurface margin. Our metallurgical professor had discussed the problem of greater fracture rates of brittle materials like amalgam when they were at an acute angle. By using the narrow-parallel walled preparation, I was in a bind. I finally gave up on resolving this acute angle dilemma and, when indicated, continued preparing patients' teeth with the narrow-parallel preparation.

In 1966, I had been on faculty at SUNY Buffalo for one year and was asked to be the director of the preclinical operative course. A new preclinical laboratory was to be finished in the fall and my only request was to install high-speed units because these had not been in the plans.

Several faculty members and I prepared a detailed manual and made plaster and typodont models for the students. We began teaching the narrow-parallel width preparations described above. The first three typodont preparations the students learned involved class I and II variations of the narrow-parallel preparations to open the fissures. We continued that philosophy through to the students' graduation. Interestingly, at year 4, I was asked by the New York State Dental Board to explain our narrow-parallel preparation. After my presentation, their question to me was why don't other schools teach this? I had and continued to share this technique with faculty from other dental schools. I ran the operative program at Buffalo until I left in 1973.

My next position was at my old school, Indiana. I was in the dental materials department in charge of clinical research. I was asked to take over a large portion of the department's lectures to dental, hygiene, and graduate students. It was an exciting time in dental materials. Formulas for dental amalgam had radically changed, and clinical research was needed to find the best amalgam alloys. Composite resins were under accelerated development, and glass ionomers had just been introduced. The department was able to publish many papers in the next years.

AS FATE WILL HAVE IT

A coauthor of our clinical trials wanted to do some additional work on the amalgam clinical studies. Dr Elliot Gale, PhD psychologist, felt other factors could skew the data. He designed our clinical studies and calculated the statistics. He also told us how many restorations were needed to be placed in a study. Therefore, when he spoke, we listened. He chose a clinical trial where I had placed amalgams during 1974-1975. I had used standard preparations and the narrow-parallel walled preparation. We evaluated 12 amalgam alloys, tooth position, and width of preparations. Three different preparations were 1) one-quarter or less, 2) one-third width, and 3) greater than one-third. We evaluated the amalgam restorations at two different times. At two years, we evaluated 429 amalgam restorations.^5,6 The data indicated that the narrow-parallel prepared teeth with an amalgam exhibited less marginal fracture than amalgams in wider preparations. At the 13- to 14-year evaluation, we examined 193 amalgam restorations.^5,7 Again, results indicated the narrower amalgams exhibited significantly less marginal fracture than in wider preparations. We noted that gamma-2–containing amalgams, which usually show high rates of marginal breakdown in the narrow-parallel preparation, were rated the same as the best high copper amalgams. We also found that the difference between width in upper bicuspids showed a highly significant difference. Both studies were published in the Journal of Dental Research.^6,7

That “feared” acute angle at the cavosurface margin of tooth played an insignificant role in the fracture of amalgam margins. Narrowness seemed to be the key. We found no recurrent decay in any narrow-parallel preparations. On reviewing the data, Dr Gale said, “See, that narrow prep of yours skewed the data. All the amalgams looked the same.”

I became a faculty member at SUNY Stony Brook in 1978 and continued the narrow-parallel preparation saga for nine years.

I have two theories why these narrow-parallel preparations have such superior results. First, the teeth prepared in this manner do not flex as much as when undercut or with wider preparations. Second, in teeth with narrow-parallel preparations, no restoration is touched by the opposing dentition.

One thing still fascinates me about these amalgam restorations in the narrow-parallel preparations. I have never seen one replaced. Dentists love to replace fillings. Of the hundreds I have placed and the years I have observed them, you would think I would have seen one replaced. But none, that is astonishing.

SUMMARY

During my postgraduate years after dental school, I wanted to have a better way of preparing teeth than the 60+-year-old preparation I was taught in preclinical labs. I developed a procedure to prepare teeth using small diameter cylindrical burs with the expressed desire to have stronger teeth. I started using this technique of opening fissures with narrow burs in my patients. The early caries situation was the best time to use this procedure. In 1966, I began teaching a narrow-parallel preparation to dental students at SUNY Buffalo. This philosophy was carried through until their graduation. The faculty started using these preparations and an attitude of prevention was more prevalent. Clinical trials demonstrated the significant positive affect of narrow-parallel preparation for amalgam restorations. A prepared cavity for posterior teeth using a small diameter cylindrical bur for the narrow-parallel preparation exhibited significantly less fracture at the margins of amalgams. The positive effect of this narrow-parallel preparation to open fissures was not only longer lasting restorations, but a new excitement that we could provide a better service.

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