Coefficient of friction and preload
in implant abutment screw connections
S.
S. WENTASCHEK,
WENTASCHEK, S.
S. HARTMANN,
HARTMANN,
K.
K. LEHMANN,
LEHMANN, H.
H. SCHELLER
SCHELLER
Objectives
Objectives
Results
Results
The clamping force needed to keep implant
parts tightly together is called preload.
Preload is generated by a screw when a
torquing force is applied to the screw. The
ratio of preload is predominately affected by
the applied torque and coefficient of friction
between the screw thread and implant
internal thread and screw head and
abutment. The coefficient of friction has to
be known for the calculation of preload. The
application of optimal preload is the most
important aspect to prevent screw
loosening.
Mean preload value for dry components: 224 ±
3.2 N at first tightening (Fig. 3, left red box), and
191 ± 14.5 N at second tightening (Fig. 3, left
green box). Thread friction component: 5.8 ± 0.8
Ncm at first tightening (Fig. 4, left red box), and
6.2 ± 1.3 Ncm at second tightening (Fig. 4, left
green box).
The aim of this in vitro study was to develop
an experimental approach to determine the
torque-tension relationship for a typically
used implant abutment complex.
Mean preload value for lubricated components:
185 ± 12.6 N at first tightening (Fig. 3, right red
box)), and 169 ± 17.7 N at second tightening
(Fig. 3, right green box). Thread friction
component: 6.7 ± 0.3 Ncm at first (Fig. 4, right
red box), and 6.8 ± 0.3 Ncm at second tightening
(Fig. 4, right green box).
Calculated coefficient of friction is 0.54 at first
tightening of the dry components (Fig. 5).
Methods
Methods
240
5
Thread friction component [Ncm]
160
6,0
5,5
5,0
140
4,5
Thread dry
Fig. 3
Thread lubricated
First tightening
Thread dry
Thread lubricated
Second tightening
Fig. 4
C
Fig. 1
B
A
Converted formula to coefficient of friction
Nomenclature used in the VDI 2230 procedure:
Applied make-up torque MA = 25 Ncm = 250 Nmm
Preload force FVM = 225 N
Pitch diameter d2 =1.575 mm
The half angle of thread δ/2 = 30°
Outer head friction diameter dW= 2.2 mm
Inner head friction diameter dh = 1.85 mm
Helix angle of the screw thread φ = 4.06°
E
3
B
4
A
4 D
3
B2
5
180
6,5
Fig. 5
2
1
200
7,0
Formula to calculate the tightening torque
4
D
3
7,5
220
Preload [N]
A custom load frame was constructed (Fig.
1+2). In a pilot test eight titanium abutment
screws were tested in an unused stack of
components. In four screws the component
stack was dry, and four screws were
lubricated with a material based on a
silicone matrix (GapSeal®, Hager &
Werken, Duisburg, Germany). All screws
were torqued twice to 25 Ncm.
8,0
1
A
C
2
Fig. 2
Coefficient of friction
μGes = 0,54
1
Fig. 1 + 2: Method to determine the preload:
1. Lower metal plate to hold the abutment (A).
2. Free rotating assembly of a lower implant holding base (B) and an upper base
which is countered by a deflecting torque wrench (C) against the upper metal
plate (3).
3. Upper metal plate with planar beam load cell (PB-37,5kg-C3; Flintec,
Meckesheim, Germany) (D).
4. Clamp to transmit the load from the rotating assembly (2) to the load cell (D).
5. Disc to introduce the tightening torque in a steady and repeatable manner by a
weight which turns the disc, by sliding in a water filled tube (E).
Conclusions
Conclusions
The experimental approach seems to be
adequate to measure preload and the thread
friction component. Lower preload was achieved
through repeated use of titanium screws. The
results suggest that the application of a used
screw (e.g. from the try-in appointment) may be
unfavorable for obtaining optimal preload.
Acknowledgements
The authors would like to thank bredent medical (Senden, Germany) for providing the implant components.
Department of Prosthodontics, University Medical Center of the Johannes
Gutenberg-University, Mainz, Germany
correspondence:
[email protected]