THE JOURNAL OF PROSTHETIC DENTISTRY
BINON
Evaluation of the effectiveness of a technique to prevent screw loosening
Paul P. Binon, DDS, MSDa
Roseville, Calif.
Statement of problem. Abutment screw loosening of single and multiple fixed partial dentures attached
to external hexagonal implants is commonly encountered.
Purpose. A technique that has been reported to prevent abutment screw loosening was tested with cyclic
loading to determine its effectiveness.
Material and methods. A sample using identical components and test protocol, with the abutment
screw tightened to 20 Ncm, was used as control.
Results. On the basis of the data obtained, the recommended technique does not increase resistance to
screw loosening. The control sample sustained a mean number of cycles to screw loosening of more than
twice that of the technique evaluated. (J Prosthet Dent 1998;79:430-2.)
CLINICAL IMPLICATIONS
The procedure evaluated in this article cannot be relied on to keep abutment screws
from loosening. Long-term screw joint stability can only be obtained when all critical
controlling factors are optimized.
A
butment screw loosening of single and multiple
fixed partial prostheses attached to external hexagonal
implants is a commonly encountered occurrence.1-9 To
mitigate this problem, Cavazos and Bell10 advocated an
abutment screw tightening protocol that involves the
application of hand torque, the addition of undercuts in
the internal surface of the screw access chamber, the injection of impression material, and the condensation of
a cotton pellet into the material before set. The
rationale is that once the impression material sets and
engages the head of the screw and the undercuts in the
chamber, it prevents screw “back out” due to any functional micromovements of the abutment. Anecdotal reports of apparently useful clinical techniques frequently
have substance and merit. However, techniques that involve the critical parameters that affect implant prosthesis stability require more prudent and thorough clinical and laboratory evaluation before acceptance.
The purpose of this study was to evaluate the effectiveness of the screw stabilizing protocol when subjected
to cyclic loading.
MATERIAL AND METHODS
Standard 3.75 × 10 mm external hexagonal implants
(ASTM Grade 3 CPT, Implant Support Systems, Irvine,
Calif.) were selected from a pack of implants, which have
a mean hexagonal flat to net dimension of 2.684 mm
(STD 0.007). Each implant was embedded in a sample
a
Private Practice; Assistant Research Scientist, Department of Restorative Dentistry, School of Dentistry, University of California San
Francisco, San Francisco, Calif.; and Adjunct Professor of Prosthodontics, Indiana University, Indianapolis, Ind.
430
THE JOURNAL OF PROSTHETIC DENTISTRY
Fig. 1. Control group implant–abutment test assembly.
Nonsegmental (UCLA) type abutment has 0.1085-inch hexagonal recess.
cup with Sample-kwick resin (Bushier, Lake Bluff, Ill.),
as described in a previous study.11
Custom conical nonsegmented (UCLA type) abutments, 8 mm long with a major diameter of 8 mm and a
loading table of 3 mm, made of ASTM grade 3 titanium
were used to replicate the prosthetic element (Fig. 1).
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BINON
THE JOURNAL OF PROSTHETIC DENTISTRY
Table I. Cyclic loading test sample
Sample
1
2
3
4
5
Mean
SD
Cycles
1,826,430
1,184,500
2,067,355
492,430
136,645
1,141,472
830,544
Table II. Cyclic loading reference sample
Sample
Cycles
1
2
3
4
5
Mean
SD
2,038,950
2,530,920
1,716,099
2,177,180
3,772,920
2,447,214
796,654
The abutment had a midrange size hexagonal receptacle
of 0.1085 inch, with a rotational tolerance (misfit) of 7
degrees. Abutments were connected to the implants with
standard ASTM titanium grade 23 abutment screws (Implant Support Systems). For the experimental group, the
abutment was attached to the implant in accordance with
the following technique.10
PROCEDURE
The abutment was attached to the implant and a ½
round bur was used to place four dimples inside the abutment cylinder immediately above the screw head at 90degree intervals. The implant abutment stack was then
disassembled and the parts were debrided with an air
syringe. The stack was reassembled and the screw was
tightened to maximum finger torque. After 10 minutes,
the screw was retightened.
A 30-gauge needle was placed into the screw hexagonal to act as an air vent. With a free flow disposable
syringe, the chamber was filled with Impregum impression material (Espe America, Norristown, Pa.) while
aspirating with the 30-gauge needle syringe. Before set,
a small cotton pellet was condensed on top of and into
the impression material with an amalgam condenser.
Excess impression material was removed with the condenser head. Immediately after set, any flash remaining
on the screw access chamber wall was removed with an
explorer. Fermit composite (Ivoclar Vivadent Inc.,
Amherst, N.Y.) was placed and light cured to achieve an
occlusal seal (Fig. 2).
The control reference sample was made up of identical components, without modification to the inside of
the screw access chamber. The abutment screw was tightAPRIL 1998
Fig. 2. Schematic of implant–abutment test assembly with four
dimples (D) immediately above screw, injected impression
material (A), condensed cotton pellet (B), and Fermit occlusal
seal (C) as advocated by Cavazos and Bell.10 Nonsegmental
(UCLA) type abutment has 0.1085-inch hexagonal recess.
ened to 20 Ncm of torque with a torque wrench (Lifecore
Biomedical, Chaska, Minn.) as recommended by the
manufacturer. No impression material/cotton plug was
introduced into the screw access chamber. The test
samples were allowed to bench set overnight. They were
inserted into split collect sample holders and placed into
the cyclic loading stations. The samples were cyclically
loaded using the instrumentation and protocol previously published.11,12 The instruments loading head delivers a 30 pound (133.3 N) load to each abutment
through a 0.090-inch diameter stylus.
The samples were rotated counterclockwise, 360 degrees off-axis under the loading styli. A minute counter
tracked the number of loading cycles and an automatic
cut-off system monitored screw joint failure. Screw joint
failure is defined as abutment mobility resulting from
screw loosening, screw fracture, or implant fracture.
During the test, when the cut-off system shorted, abutment mobility was verified manually. If mobility was
questionable, the cut-off pointer was recalibrated and
the instrument restarted. This was repeated until mobility was manually verified. The number of cycles to
failure (screw loosening) was reported under these identical loading conditions. Five samples of each group were
tested at different time intervals. Instrumentation was
recalibrated and standardized in an identical manner for
both groups at 1 million cycles interval, for all samples.
The only variable in the study was the screw stabilizing
technique advocated by Cavazos and Bell,10 which con431
THE JOURNAL OF PROSTHETIC DENTISTRY
sists of maximum hand tightening the abutment screw
and the addition of an impression material plug over the
screw.
RESULTS
The test results are presented in Table I. Screw joint
failure ranged from 136,645 to 2,067,355 cycles with a
sample mean of 1,141,472 (SD 830,544). Failure constitutes screw back out that was verified manually with
abutment mobility. None of the implants or screws fractured during the test. The control group recorded a mean
of 2,447,214 (SD 796,654) cycles (Table II).
BINON
statistical analysis of the data is not relevant. However,
it established a clear trend and demonstrated that the
proposed screw stabilizing technique did not appear to
offer superior performance.
CONCLUSIONS
Test results in this study indicated the following.
1. The hand tightening, impression material/cotton
pellet screw access sealing technique does not inhibit
screw back out.
2. The impression material/cotton pellet technique
is a viable method of sealing the screw access hole.
DISCUSSION
REFERENCES
Results obtained with identical components, assembled per manufacturer’s recommendations, and
tested in an identical manner, were significantly better
than in the test sample that used the impression material/cotton pellet sealing technique (Table II).11 The
control test, recorded a mean of 2.4 million cycles to
screw loosening compared with 1.1 million cycles for
the impression material/cotton pellet technique. This
may be attributed to several factors. Foremost is that
the experimental technique recommended and relied on
finger tightening of the abutment screw. In the reference sample, the abutment screw was tightened to 20
Ncm as recommended by the manufacturer. Standardized torque application with a calibrated torque gauge
has been documented to be more consistent and predictable in obtaining optimal torque and preload in a
screw.13
The second consideration is that regardless of its inherent rigidity, impression materials will not inhibit vibration or lateral movement within the implant–
abutment complex. On the basis of the results of this
test, the advocated screw stabilizing protocol does not
improve screw resistance to screw loosening when compared with the conventional technique that calls for the
application of 20 Ncm of tightening torque with a torque
wrench. Sakaguchi and Borgersen14 have demonstrated
that off axis loading results in micromovement within
the screw joint complex that ultimately leads to screw
loosening.
The impression material/cotton plug technique did
not appear to inhibit micromovement within the screw
joint complex. The technique may be a viable method
of obturating the screw access channel. Clinical experience also indicated that it permits easy access for reentry; however, improved screw joint stability was not demonstrated. Because the number of samples tested is small,
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1158 CIRBY WAY
ROSEVILLE, CA 95661
Copyright © 1998 by The Editorial Council of The Journal of Prosthetic Dentistry.
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