CEREC doctors.com - Q3 2016 - page 38

36 | CERECDOCTORS.COM | QUARTER 3 | 2016
Oneofthefirststudiestopromoteasingleretainerdesignconcept
was a short term clinical study of 38 glass ceramic RBFPDs.5 The
first 26 RBFPDs were fabricated from Empress 2 (Ivoclar), and
the last 12 fabricated from e.max (Ivoclar). A conventional two-
retainer deign was used for 17 of the RBFPDs, and 21 had a canti-
levered single-retainer design. The retainers had minimal lingual
veneer preparations, a groove on the cingulum and shallow prox-
imal boxes adjacent to the pontic. The mean observation periods
were 21.2 months for the two-retainer RBFPDs, and 15.1 months
for the single retainer RBFPDs. One fracture was reported for
the single retainer RBFPDs and six fractures for the two-retainer
RBFPDs. The Kaplan-Meier survival rates were calculated to be
60.3 percent for the two-retainer RBFPDs, and 90.9 percent for
the single-retainer RBFPDs.
Another clinical study compared the longevityof 12 two-retainer
RBFPDs and 13 single-retainer cantilevered RBFPDs for replace-
ment of maxillary incisors over a period of 45 months.6 They
reported loss of retention in only one of the two-retainer RBFPDs
that was successfully rebonded. No failures were reported for any
of the single-retainer RBFPDs. There is no significant difference
in the longevity for this small clinical trial, but it does support the
premise that single-retainer RBFPDs are clinically successful.
One long-term clinical study compared the clinical outcome
of maxillary anterior RBFPDs fabricated from glass-infiltrated
alumina ceramic (In-Ceram/Vita).7
A total of 37 anterior RBFPDs were divided between 16 two-
retainer designs and 21 cantilever single-retainer designs. All
RBFPDs were bonded using Panavia or Panavia 21 after the glass
ceramic retainers were silica-coated and silanated. RBFPDs were
evaluated on a yearly recall schedule with a mean observation
time of 75.8 months for the two-retainer group, and 51.7 months
for the single-retainer group. No RBFPDs debonded during the
course of the study. One of the two-retainer design RBFPD frac-
tured through the connectors at three months. Four additional
two-retainer design RBFPDs fractured within 15 months at one
connector with the pontic, and the remaining retainer main-
tained the RBFPD in service. One single-retainer design RBFPD
fractured and was lost 48 months after insertion. The five-year
survival rates reported were 73.9 percent for the two-retainer
group, and 92.3 percent for the single-retainer group. The author
concluded that a single-retainer cantilever designwas a promising
treatment alternative for RBFPDs.
One consistently reported technique for improving adhesive
retention is to include abutment preparation features that protect
the adhesive interface from shear forces.8,9 This would include
| | | FA S B I N D E R
Fig. 1: Pre-operative view of congenitally missing teeth #7 and #10
Fig. 2: Post-operative view of transitional RBFPDs. Note wide
appearance to pontics #7 and #10 with a no preparation,
two-plane design
proximal grooves, proximal guide planes, chamfer margins and
proximal boxes. These preparation features serve to limit the path
of insertion of the RBFPD and provide a measure of resistance
form to minimize shear stress on the adhesive bond. Although
these features may be beneficial for metal-based retainers, there
is no clinical evidence that these mechanical preparation features
improve clinical longevity of all-ceramic RBFPDs. There is
actually a strong desire to avoid any tooth preparation when
provisional RBFPDs are used for periods of healing following
implant placement. In this application, attempting to create a
two-plane design in the retainer using the lingual surface and
proximal surface to the missing tooth can enhance the resistance
and surface area for bonding (Fig. 1) However, this design may
have a tendency to increase the visual appearance of the pontic