Our aim is to investigate the true level of precision of ordinary routine-procedures in the dental labor, based on literature data, on our personal experience and on simple reasoning.
The marginal opening of a direct or indirect restoration allows plaque accumulation resulting in microleakage, caries, periodontal disease and decrease of the longevity of the restoration itself.
Definition of microleakage:
"leakage of minute amounts of fluids, debris, and microorganisms through the microscopic space between a dental restoration or its cement and the adjacent surface of the cavity preparation; it may progress through the dentin into the pulp" (Miller-Keane Encyclopedia and Dictionary of Medicine, Nursing, and Allied Health, Seventh Edition).
Definition of fit:
"the particular way in which something, especially a garment or a component, fits" (Oxford Dictionary).
In Prosthodontics the particular way is the quality, condition, or fact of achieving the zero-gap:
gap = 0 micron.
1 micron = 1/1000 millimeter
This condition is considered crucial for the long term survival of our restorations.
We as dentists strive to optimize the marginal fit
to prevent the microleakage
Micro-leakage on E-Max crown - shoulder prep - adhesive cementation - check up after 10 years
Literature says in vitro we can't have less than 50 micron of un-fit i.e. gap.
That means in vivo, in the oral enviroment, the basic line is starting from 50+x micron, that is: we have at least 50+1 micron of gap.
That is, the gap is unavoidable.
The findings of literature are in agreement with the clinical acceptance of a marginal openings range of 120 - 150 µm.
Nonetheless a marginal gap ranging from 25 to 40 µm for cemented restorations has been suggested as a clinical goal.
(In vitro evaluation of hte marginal and internal discrepancies of different esthetic restorations - Amal Abdelsamad SAKRANA - J. Appl. Oral Sci. Vol.21 no°6 Bauru Nov./Dec. 2013).
The unavoidavle gap should be sealed with the most thin film of cement.
Cement is a viscous fluid.
A viscous fluid (cement) is made of liquid.
A liquid is a nearly incompressible fluid that conforms to the shape of its container (crown) but retains a (nearly) constant volume independent of pressure.
The smallest constant volume i.e. thickness in vitro for cement is
Film thickness of different cement
technicians are striving for the minimum possible opening at margins that is 25 to 40 micron (which is unreal in vivo);
for the reason above mentioned they are used to put a die-spacing=0 micron in the first 2 mm at margin's level, that is, they like to have a high level of friction right there;
dentists are using cement (a viscous fluid) which has an in vitro minimum thickness of 20 micron. In vivo is > than 20 micron.
we want to force under pressure (cementation of a crown) a viscous fluid (cement)
of 20 µm to fill into a micro-space of
exactly where we have the maximum level of friction
to get the perfect seal.
The above mentioned conditions we have only in the ideal in vitro scenario. Even in vitro we believe it hardly to happen to get leakage-proof margins.
In vivo reality is of course different:
gaps on average are ~ 120 µm
cement thickness is ~ 40 µm or even more
crown elevation after cementation scores from 116 to 650 µm with huge marginal openings of more than 200 µm
literature states gaps of 200 micron filled with cement are working fine for many years.
Friction aka retention fit has been since always obsessively considered the key feature of a good-quality indirect restoration (crown).
a high friction's fitting is mandatory for a proper indexing of position of stump on the plaster die-cast. Technician can manage occlusion, interdental contact points and finishing line fit;
a high friction's rate of restoration (crown) is beneficial to the dentist during the cementation procedure: the lodging of the crown during try-in and final cementation is unambiguous (just one way);
however the retention of a crown has been claimed to be improved by a ‘frictional fit’ between the casting and the tooth, this is a false believe. Many authors have investigated the relationship between post-cementation retention and friction: the more the friction – the lower the retention.
Kaufman et al. and Merker et al. found that restorations made on unspaced castings show up significantly lower frictional retention values after cemented compared with those made on die-spaced castings.
Eames et al. (1978) using crowns manufactured on extracted human teeth and die-castings abutments coated (spaced) with four layers of die-spacer, found out crowns being 25% more retentive than crowns made on unspaced die-cast abutments.
Carter and Wilson (1966) investigated the effect of die spacing on crown retention using natural teeth prepared to a standardized full crown preparation. They applied paint-on die spacing in either 0, 2, 4, 6 or 8 layers to the dies with full occlusal and axial coverage and used a 25 N seating force. Mean post-cementation retention, in this study, increased from 250 N (zero coats of spacer) to 375 N (eight coats of spacer).
Several studies have investigated the relationship between die-spacing and crown seating i.e. fit: frictional fit causes an elevation of the crown. Die-spacing is a technique commonly used to provide the physical space for cement. A material is layered (coating) on the abutment producing an over-sized die. In analogic labor-work is supposed to be used either a paint-on material (varnish) or a foil applied to the die. In the digitalized CAD-CAM way the software allows the technician to manage the space.
Eames et al. (1978) noted that the use of four layers of die-spacer resulted in a decreased elevation of full crowns from 143 μ to 45 μ after cementation with zinc phosphate cement to 10° tapered prepared natural teeth.
Van Nortwick and Gettleman (1981) found three layers of die-spacer resulted in a mean change in vertical seating from 333 μ to 52 μ when alloy crowns were cemented to chromium-nickel-cobalt dies with zinc phosphate cement.
Campagni et al. (1986) found a significant decrease in crown elevation following cementation of full gold casted crowns onto prepared teeth, with and without axial grooves, after the application of six coats of aircraft dope (plasticised lacquer) to the dies on which the crowns were manufactured.
Oliva and Lowe (1987) in a study investigating the effect of die-spacer on the seating of cast restorations onto composite core preparations stated that the marginal opening of castings made on dies without relief (no-spaced) was substantially greater than that for castings made on relieved (spaced) dies.
Grajower et al. (1989) used a casting technique designed to produce undersized, full crown castings. They noted a decrease in crown elevation from 649 μ for crowns without spacer to 29 μ for crowns with four layers of new (i.e. unopened) spacer.
Wang et al. (1992) found the application of four coats of die-spacer to dies prior to wax pattern construction resulted in cast full crowns with decreased seating discrepancies from 116 μ to 12 μ when a light seating force of 5 lb (22 N) was applied and 73 μ to 2 μ when a heavier seating force of 30 lb (133 N) was applied.
We may conclude, from all the above mentioned studies, that:
- frictional retention decreases the overall retention
- frictional retention causes an elevation of the crown from 116 micron to 650 micron, compared to a die-spaced crown whose elevation ranges from 12 μ to 52 μ.
That is :
cervical friction is preventing the full seating of the crown
cervical friction on margin's level is preventing cement to flow into the the marginal gap and to seal
cervical friction - preventing the full seating of the crown - is lowering retention.
If we have right at the margins level the highest point of friction - applying a force onto the crown - the cement will be stressed i.e. pressed into the gap. Liquids have tensile strengths, which when exceeded causes the onset of cavitation. Cavitation is the formation of vapour cavities in a liquid – i.e. small liquid-free zones ("bubbles" or "voids") – that are the consequence of forces acting upon the liquid. It usually occurs when a liquid is subjected to rapid changes of pressure. When subjected to higher pressure, the voids implode and can generate the break-down of the liquid (cement) in his basic components.
If we have at the margins level a tight closure aka friction we prevent the venting-effect during cementation. Cement excesses can't escape. They bottle-up inside the crown. As a consequence we have crown elevation.
If the amount of cement is not enough to fill-in the occlusal void produced from the common die-spacing procedure, we may have an over-seating of the crown in case of vertical preparation or champfer.
Venting - the little hole is working like an escape valve
Dr. Roberto Magallanes Ramos
we can't avoid gaps of ~ 120 micron in vivo
Cement is the only way to contrast the micro-leakage
cement can't be reduced by compression to less than 20 micron in vitro
the less friction in the cervical area the better cement can seal this area: cement can escape through the marginal gap (venting) without being stressed
the less friction the better can seat the crown (no elevation)
if we have a big die-space on the occlusal surface of stump – overseating of the crown occurs in case of vertical preparations or champfer.
Technicians routinely in die-casting are used to create a No-Die-Spacer-Area on the first 2 mm of the cervical part of the abutment
Fitcheck try-in - iuxta-cervical friction and no-cement zone
Dr. Attilio Bedendo - AIOP official website
Fitchecker try-in - iuxta-cervical friction and no-cement zone
Dr. Vasile Cirimpei
At margin level there is the no-die-spacing-area i.e. the cervical friction area: NO CEMENT!
4 years old PFM with iuxta-cervical friction and no-cement zone
On the basis of the former considerations about friction, retention, cement-thickness and cement-rheology and on behalf of the many above cited studies about the topic, Tomorrow Tooth has moved on to a NEW approach in die-casting:
the frictional grip-area - chief for getting the index of the restoration has to be RELOCATED from the OLD cervical area
(as taught by all horizontalist schools, as from verticalist schools like BOPT school of Ignazio Loi or Mascarella’s school of Di Febo and others)
to the NEW close-to-the-occlusal-surface area,
as shown in the following pictures: the two different friction-approaches
Cervical Frictional Retention Occlusal Frictional Retention
onto the cervical area onto the occlusal area
(Ignazio Loi BOPT, Di Febo, etc.) (Tomorrow Tooth Approach)
(Martignoni, Wirz-Hoffmann, Schillingburg, S.D. Tylman, Massironi, Fradeani, Ignazio Loi, Di Febo, etc)
- high risk of chipping and/or cracks of the thinnest and weakest part of the crown (the cervical part), especially with ceramic-layered crowns (PFM or Zirconia, E-Max would just fracture!)
- impossibility to manage undercuts in the cervical area in dental labor: the clinician is forced to multiple, messy, fit-checker trial and error attempts to get the crown seated
- bad flow (rheology) of cement, with filtration (the above mentioned cavitation effect) and elevation of crown
- cement is missing in the most critical area: on the cervical part i.e. on the margins of restoration (like in pictures) there is NO cement
- placing the spacer on the head of the abutment, we get in many cases the paradox-effect of an over-seating of the crown: if the empty space (created from occlusal die-spacer) is not filled with cement – the crown can seat at a deeper level, causing:
chipping of the border (margins are trying to seat in a no-prep zone with a high probability to meet undercuts!)
a big horizontal over-contour i.e. a big gap
- is creating an homogeneous thickness of the cement-film in the most critical area (the cervical region)
- is avoiding chipping and cracks in the thinnest and weakest part of the crown (the cervical part) : stops over-seating
- is moving the crown friction to the more robust part of the crown, avoiding the more thin cervical part (more prone to crack)
- we have an easier management of the cervical undercuts
- we get an improved flow of the cement venting through the marginal gap instead to bottle-up on the axial walls and onto the occlusal surface with big elevation of crown.
A big misunderstanding (aka hoax) about the perfect fit was perpetrated for many years. Many of us were truly frustrated in pursuing the zero-gap. We honestly believed to papers and books and lecturers trumpeting of marginal precision. Working with low magnification it's impossible to judge a gap: you'll never feel the difference between 10 and 50 micron.
Technicians more realistically were trying to solve the everyday's problems on their own way: incorrect prepared abutment (too little, too conical, with dirty finishing lines) needed crowns. Whatever kind of it. They could not be at odds to lose their customer, the well educated dentist.
Therefore they tailored the die-spacing job: the worse the tooth was prepared - the more die-spaced had to be the stump.
Note at least 500 micron of cement on the occlusal intaglio surface
Even on bad-prepared abutments coming from poor skilled doctors they naively believed in achieving the best possible marginal fit. That is they always located the free-die-spacer-zone on the cervical: the first 2 mm.
Doing so they dramatically increse friction on the cervical area. The freshly casted or milled framework does hardly seat on the stone model. The good-willed technician makes the crown seat in the right way. A hell of a job. Drilling the intaglio surface of the crown and creating more space to overcome little undercuts and the damn friction. Working for hours on 2-3-4 duplicated die-castings. Otherwise plaster is wearing out faking a good seating. And they had even to guess how much occlusal space for cement: the more the better. With horizontal preparation it was easy because the shoulder is the bottom limit: no problem of over-seating.
Shoulder is a finishing line, therefore the crown has a stop!
With the "extended champfer technique" or with vertical prepping there is no bottom limit: if you have a free space on the occlusal surface of the abutment and cement is not enough to fill it, over-seating may occur, causing a big horizontal over-contour or cracks of the cervical margins.
The cone without a stop can just slip more deep into the sulcus!
massimo mazza & pasquale venuti