For an ideal restoration, a casting must be made to fit the
prepared tooth intimately.
However cast crowns do not seat completely when cemented.
The more accurately the casting fits the prepared tooth, the
more difficult it is for cement trapped between the crown and
the occlusal surface of the tooth to escape.
Thus, a properly constructed fixed prosthesis may fail as a
result of incomplete seating during cementation.
Most research on cementation of crowns relates seating
failure to the thickness of the cement film.
FACTORS AFFECTING FILM
THICKNESS OF CEMENT
• Jorgensen investigated factors influencing the
thickness of cement between the restoration and the
• The optimum cementation force required to reduce the
film thickness of cement was 5 kg for 1 minute.
• Film thickness increase with an increase in
Smaller the tapper ; <10degrees (when occlusal
surface is not perforated)
• According to Windler
Occlusal displacement =
film thickness at axial wall
sine(1/2 tapper angle)
Windeler AS. Powder enrichment effects on film thickness of zinc
phosphate cement. J PROSTHET DENT 1979;42:299-303.
FILM THICKNESS MEASUREMENTS
• Dies with cemented crowns sectioned and measured directly with a
• Indirectly by recording the distance between the margin of the casting
and the finishing line of the preparation before and after cementation, or
between reference marks on the casting and die.
• The ADA specification for film thickness of type I zinc phosphate
cement is 25 um and for type II 40 um.
• Most investigators found film thickness ranging from 60um to
• The wide range may relate to different methods of measurement and
experimental design of the models used.
Filtration process results in much higher film thickness
than ADA specification No. 8 for zinc phosphate cemenet
During cementation cement
accumulates on the occlusal surface
Cement can escape through cervical
As the crown approaches its final
position this space becomes smaller
The flow of the non compressible
liquid cement is inhibited and
seating of the crown is resisted
Hydrodynamic pressure causes the
cement to separate in solid and
The solid particles form a mass that only
allows passageof the thinner liquid,
causing further separation and filtration of
the cement (FILTRATION PROCESS)
According to study by Hoard et al
• Intra coronal pressures during crown cementation were measured
with zinc phosphate cement.
• Maximum pressure was recorded at the occlusal surface of a die
after 2 seconds; it reduced to zero after 1 minute.
• Less pressure reducing to zero was recorded at the cusp tip and at
the axial walls, reducing to 25% after 1 minute.
• In a comparison of different cements, the more viscous the
cement, the greater was the hydraulic pressure that did not reduce
• Thus, it was concluded that internal back pressure plays a
limited role in preventing the complete seating of a crown
• suggested that the filtration process of Jorgensen is responsible for
increased film thickness of cement (accumulation of cement
DISCREPANCIES OF MARGINAL FIT
Complications caused by incomplete seating of
Creation of premature contacts
Alteration of contact areas with adjacent teeth
A reduction in crown retention by 19% to 32%
Discrepancies of marginal fit of the crown.
• Crowns may fail because of cement wash-out at the margins.
• Ill-fitting margins expose large amounts of cement to the oral
fluids, &-easing the rate of deposition of plaque.
• Dissolution of cement in oral fluids results in marginal
leakage and a rough surface that accumulates debris and
• After setting, zinc phosphate cement has been shown to elute
zinc, magnesium, sodium, and phosphate ions into an aqueous
• Swartz et al. found that early exposure of zinc phosphate
cement to water after setting increased its solubility and
suggested coating the exposed surface with cavity varnish.
• The configuration of the finishing line of the
preparation affects the width of the band of
cement exposed to oral fluids. regardless of
the discrepancy of seating.
• Gavelis et al. confirmed that feathered edges
and shoulders had the smallest marginal
• Clinically, there is no clear correlation
between the thickness of the exposed band of
cement at the crown margin and the amount
of cement dissolved by the oral fluids.
• Cement manipulation or environmental
factors localized to the tooth should be
Techniques to improve marginal fit
• Internal relief of the castings &
• modified cementation techniques
• A small hole prepared in the occlusal or axial
surface of the restoration allows exit of excess
cement during cementation.
STUDY Improved vertical seating
Cooper et al 45 um
Van Nortwick and Cettleman 290um
Kaufman et al 175um
Retention increased by 19% to
Dimashkieh & Davies
Venting of crowns is less critical
when the taper angle is increased.
In near parallel-sided prepared teeth,
venting was necessary to avoid an
excessive occlusal cement thickness.
Time of final seating of 1 minute
in non vented crowns was reduced to
15 seconds in vented crowns.
• An extra visit is necessary to fill the vent hole
• The material used to fill the vent hole may wear
• Marginal leakage &
• Occlusal venting of ceramic or ceramometal
crowns may weaken the porcelain.
• Vent holes in crowns may be closed with
amalgam or composite.
• Bassett described a technique of casting a pin
that was sized to fill a hole made by a No. 699
• Seberg prepared a hole in the wax pattern and
cast a matching Williams pin. The pin was
seated in the vent hole immediat.ely after the
escape of the cement.
• A space is, created between the casting and the
prepared tooth to accommodate excess cement
after closure of the marginal pathway of escape.
• Pascoe demonstrated that the margins of
oversized castings fit better than undersized
castings after cementation.
• The same effect can be obtained by relieving the
internal surface of the crown.
A number of methods exist to achieve
Grinding the inside of the casting
Internal carving of the wax pattern
Etching the internal surface of the wax
restoration with aqua regia.
Electrochemical milling of the inside of the
• These methods have been rejected as
inaccurate and inconsistent. It is impossible
to obtain a uniform space for the cement.
• These methods cannot be used for
Internal relief can also be achieved by cutting internal
channels in prepared abutment teeth and in the internal
surface of the crowns before cementation.
Channels are 0.5 mm deep extending from the occlusal
surface to within 1 mm of the cervical margin.
No significant difference was found after modification
of the occlusal surface of the preparation.
This technique causes loss of adaptation
of tooth & casting only in discreet areas
where channels are prepared.
• An effective method of obtaining internal
relief is die spacing.
• Die spacer is an agent applied to a die to
provide space for the luting agent in the
finished casting. GPT 8
• A clinically acceptable relief is considered to
be 20 to 40 um/ axial wall
THE CEMENTATION TECHNIQUE
Three variations of the cementation technique
have been used in an effort to obtain improved
Site of application of cement, and the
Magnitude of cementation pressure.
• Oliveira et al found that vibration improved the
fitting of MOD inlays by 16 um and crowns by 27
• Van Nortwick and Gettleman indicated that
vibration applied horizontally while the cement was
setting did not affect crown seating.
• Koyano et al. reported that the cement film was
thinner when vibrating pressure was applied.
• Vertical vibration was slightly more effective than
horizontal vibration but less comfortable to the
Site of application of cement
Ishikiriama et al. found that cement painted
on the inner walls of the crown promoted a
better fit than when the crown was
completely filled with cement.
The flow pathway of cement and the amount of
incomplete marginal seating of the restoration
has been studied.
Cement was applied at various locations on the
internal surface of the crown and on the
Rimmer Y. The flow of zinc phosphate cement under a full
coverage restoration according to the location of its
application and its effect on the marginal adaptation.
Location of Cementation Marginal
Margin of crown 54um
Apical half of axial wall of crown 106um
Margin of prepared tooth 10um
Apical half of axial wall of preparation 40um
Magnitude of cementation pressure
• Jorgensen noted that an increase in load
above 5 kg had little effect on the result.
• Fusayama et al reported that excessive
amounts of pressure (15 to 50 kg) applied
during cementation produced no significant
difference in the thickness of cement on the
shoulders of preparations.
• Grajower et al reported that a pressure increase of 2 to
10 kg applied at the first try-on stage of cast full
crowns caused an average apical movement by the
crown of 63 um.
• This is attributed to protuberances on the internal
surface of the casting arresting the complete seating
of the crown.
• On application of pressure, these protrusions created
furrows in the axial surface of the prepared tooth
• The average elevation of non precious metal
crowns caused by cementation was found to be
• This elevation corresponds to an effective
minimum cement thickness of 4.7 um at the
• On application of pressure, protrusions on the
casting surface could cause high localized
pressure on the cement and might therefore
penetrate the cement layer.
• The results of Grajower et al indicate that the
effective minimum cement thickness at the
axial walls can be lower than 4 um and that the
possibility for metal-dentin contact in certain
locations cannot be excluded.
• This raises a question as to the relevance of the
ADA Specification No. 8. Jorgensen and
Esbenser believe that the ADA Specification No.
8 relates more to the viscosity of the cement
than to the film thickness.
• Grajower et al suggest that studies regarding
cementation and crown elevation be done on
teeth instead of on dies because mechanical
properties of the dentin may affect the
• The amount of incomplete seating of a cemented
crown is equal to the film thickness of the
cement at the occlusal surface.
• It is related to the cement thickness at the axial
walls of the tooth preparation by the equation:
Film thickness at occlusal surface =
film thickness at axial wall
Sine 1/2 the taper angle
• The Jorgensen filtration process explaining
why the thickness of cement exceeds the ADA
Specification No. 8 is based on two
1. On placing the crown containing the
cement on the prepared tooth, cement
accumulates on the occlusal surface, and
2. Hydrodynamic pressure develops within
the cement as the marginal opening
prevents cement escaping.
In well controlled circumstances, these two
assumptions can be contradicted:
1. The amount of cement reaching the occlusal surface
can be controlled and even totally prevented by
applying cement only to specific areas of the crown
or preparation. It is not yet clear whether complete
elimination of cement at the occlusal surface is
desirable inasmuch as an empty space may
encourage microbial growth.
2. At the try-on stage of the crown, a space exists
between it and the tooth preparation. If the volume of
this space could be calculated and the correct
amount of cement introduced into the space,
hydrodynamic pressure would not develop.
Controlling the amount of cement may well
eliminate the filtration process of Jorgensen.
Windelers claims that the occlusal discrepancy
is caused by excess cement that exceeds the space
available for it and suggests the equation:
Occlusal discrepancy =
film thickness on axial walls minus space available for cement
sine 1/2 taper angle of the preparation
• As shown by Grajower et al.a film thickness as
small as 4 pm can be reached at the axial walls.
Fresh cement painted with a camel brush in
the part of the crown to be cemented promotes
a better fit than when the crown is completely
filled with cement.
Influence of some factors on the fit of cemented crowns.
JPD 1981 Apr;45(4):400-4.
• Brushed on cement on the inner surface of
restoration produces a seating discrepancy
one third less than that resulting from
filling the crown half full and more than
2/3rd less than that resulting from filling
the crown full.
Tan K, ibbetson RJ: The effect of cement
volume on crown seating . J Dent Res
Seating force must be adequate to ensure
complete seating of the crown but sudden
excessive force may result in the elastic strain of
the dentin, creating a rebound effect which
results in the crown being partly dislodged due
to elastic recoil.
JPD 1992; 68(3): 476-81
Orange wood blocks (wooden sticks or
plastic wafer) used for seating crowns after
cementation produced far less seating
discrepancy as compared to cotton rolls that
showed highest seating discrepancy.
Influence of pressure and vibration during
cementation. JPD 1979 41(2) 173-177
A delay in applying the seating force reduces
the quality of cementation. A 20 second
delay caused an increase in the seating gap
of 0.02mm and 2um marginal gap.
JPD 1986; 55(1): 13-18
• The crown is inserted slowly to about half a
distance, it is then withdrawn by a few
millimeters and reinserted to almost the full
extent of its length. The process is then repeated.
Use a slight up and down movement along this
path to assist the layering of the cement. When
the operator no longer feels any resistance , the
crown is pushed to the finish line for final
Int J Prosthodont Restor Dent 2012: 2(2):77-81
The art and cementation is to choose cement with an
inherently low film thickness and use technique
which allow it to escape while the crown is being
Cement flow can be hindered by preparation features
which cause a build up of hydrostatic pressure.
The problem can be overcome by die spacing and
controlled cement application or by venting of the
• Research should continue to determine the correct
amount of cement to place in the crown, the site of
application, and the amount of cementation pressure
so that space between the axial wall and the crown will
accommodate the available cement.
• The emphasis in the literature about incomplete
seating of crowns during cementation has been
placed on eliminating marginal discrepancy by
venting and internal relief.
• More stress should be placed on the cementation
technique as a means of solving the problem of