CHARACTERISTICS OF A SADDLE JOINT – AN ALTERNATIVE GEOMETRY FOR BEARING SURFACES
*Valdevit, A; ** Kambic, H; # Errico, JP; #Zubok, R, ##Salveson, B
*Lutheran Medical Center, Brooklyn, NY, **The Cleveland Clinic Foundation, Cleveland, OH, #SpineCore Inc, Summit, NJ, ##Datum Industries,
Patterson, NJ
[email protected]
Introduction: The use of conforming surfaces such as a ball and
retrieved wear debris was analyzed using scanning electron microscopy
socket is widespread in the orthopaedic implant industry. Geometric
and chemical composition was determined using mass spectroscopy.
constraints have impeded the development of such a configuration to
Results: All devices survived axial fatigue testing with no observable
include some of the smaller articulating joints comprising the
or mechanical compromise to the performance of the device. No
musculoskeletal system. The authors present a saddle joint
evidence of wear was observed on the mating surface of the devices. In
configuration that may provide an alternative to the ball and socket
order to elucidate an overall evaluation of the device deflection, and
geometry for use in these smaller articulating geometries. High
hence integrity, a non-linear regression of the mean deflection data was
frequency fatigue testing was conducted in compression in order to
performed and resulted in an exponential curve with a time constant
endure mechanical endurance of the saddle configuration. In addition a
equal to 852,400 cycles.(Figure 4)
motion simulator was developed in order to investigate the wear
characteristics of the saddle configuration.
Figure 4. The deflection decay curve for the saddle joint over 10 million
Materials and Methods: The saddle joint geometry was fabricated
cycles.
from Co-Cr-Mo and is shown in Figure 1. The distal component
possesses different radii in the X and Y directions respectively.(Figure
An overall decrease in deflection from (0.068±0.001)mm to
1B) In order for the proximal saddle to articulate with the distal saddle,
(0.05564±0.0002)mm was computed by setting the initial cycle value to
MEAN DEFLECTION: AXIAL FATIGUE - NON-LINEAR FIT
the respective X and Y radii are similar but not identical in the
zero in the regression
respective directions.(Figure 1C) With these particular radii, the saddle
equation and achieving
O
joint is capable of ±7.5 of rotation in both the X and Y axes while
the decay to the
O
permitting ±3 of axial rotation prior to separation. Both components
computed plateau..
are equipped with a mounting flange in order to facilitate mounting
From the recorded data,
within test fixtures.
No significant changes
Figure 1. A) The
in deflection were noted
Z
saddle configuration.
beyond 1.5 million
B) Distal saddle
cycles (P>0.05) while no statistically significant difference in stiffness
Y
component. C)
was detected beyond 3 million cycles (P>0.05)
X
Proximal saddle
With respect to the wear material, regression analyses for the total
A
B
C
component.
volume of debris generated for cobalt (Figure 5A) and chromium
(Figure 5B) were computed as 0.000002cm3 /million cycles and
High frequency fatigue testing was accomplished using a dedicated
0.000216cm3 /million cycles respectively
materials testing machine (Endurtec, ELF 3300, Minnetoka,
MN).(Figure 2A) Six saddle joints were mounted on a polyethylene
block with 4mm screws and subsequently secured to a stainless steel
fixture. The fixture was mounted in the testing machine and subjected to
ten million cycles of cyclic compressive loading from –30N to –300N at
a frequency of 60Hz.(Figure 2B) Data was collected at 15,000 cycles
and at intervals of 250,000 cycles thereafter.
A
B
Figure 5. The total volume of wear material generated over 10 million
Figure 2. A) High
cycles of motion. A) Cobalt. B) Chromium.
frequency testing machine.
B) Saddle component
While evidence of wear is seen throughout the 10 million cycles of
mounted in testing fixture.
simulated motion, no evidence of cavitation or pitting was evident.
Lines of wear manifest the degree to which the contact wear between the
A
B
upper and lower components was seen. In addition, it appeared that the
upper components displayed increased evidence of wear relative to the
In order to evaluate the durability of the saddle
distal components.(Figure 6)
2 million
6 million
10 million
configuration under repeated motion a custom
joint simulator was designed and fabricated.
(Figure 3).
DEFLECTION (mm)
0.10
NON-LINEAR FIT FOR
MEAN DEFLECTION
2
R = 0.867
0.08
PLATEAU OF
0.05564 mm
0.06
0.04
0
2000000
4000000
6000000
8000000
10000000
12000000
CYCLES
K1 H A L F L I F E
852,400 CYCLES
Volume Co (cm3 )
Volume Cr (cm 3 )
2.3×10 -3
1.8×10 -5
2.0×10 -3
1.6×10 -5
1.8×10
MEAN
1.0×10 -5
TOTAL
3
1.2×10 -5
Volume (cm)
3
Volume (cm)
1.4×10 -5
8.0×10 -6
6.0×10
-6
4.0×10
-6
0
1.5×10 -3
MEAN
1.3×10 -3
TOTAL
1.0×10 -3
7.5×10 -4
5.0×10
2.0×10 -6
-3
-4
2.5×10 -4
0
1
2
3
4
5
6
7
8
9
10
0
0
1
CYCLES (Millions)
Figure 3. The simulator designed to apply
repeated motion under compressive loading to the
saddle joint.
Upper
Lower
2
3
4
5
6
7
8
9
10
CYCLES (Millions)
Upper
Lower
Upper
Lower
Figure 6. Although patterns of wear were observed surface pitting or
cavitation was not evident over 10 million cycles.
The simulator permits both undulation (±7.5O per
side) and oscillation (±3 O per side). Undulation is
accomplished through an offset base at a preset
distance from a rotating can while oscillation is
performed through the motion of a secondary arm
mounted off-axis to a second cam. The test information is input on a
single touch screen for each of 6 stations and includes applied load
(70N), unique frequency input for each of undulation (1.5Hz) and
oscillation (1.5Hz), total number of cycles required (10 million) and the
number of cycles between intervals (1 million). Each of the six stations
contained a reservoir that was filled with a 1:1 ratio of bovine serum and
phosphate buffered saline with 0.2% sodium azide as preservative. Six
saddle configurations were subjected to 10 million cycles with wear
debris retrieval and photographic images taken of the bearing surfaces
taken at intervals of 1 million and 2 million cycles respectively. The
Discussion: While wear markings were evident on the articulating
surfaces of the prosthesis components, no component failed to perform
to the designed range of motion. With respect to the wear debris, it has
been reported that approximately 10mg of material is lost by 5 million
cycles in simulator tests involving metal-on-metal Co-Cr hip
components. Using the methods in this experiment lead to a mass loss
of less than 8mg at 5 million cycles. The fatigue response of this design
has shown that such a geometric configuration can sustain loads on the
order of 70N.
Conclusions: These mechanical and material characteristics have
demonstrated that a saddle design may be applicable in cases where
large loads are rare and geometric constraints make application of the
traditional ball and socket design difficult. Applications may include
such joints as the elbow, wrist and cervical spine.
51st Annual Meeting of the Orthopaedic Research Society
Poster No: 0841