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Posted: Jun 8th, 2018 - 4:16 am In Reply to: pick arthroplasty (glenohumeral joint) - PatH

+*Müller, D; *Marti, A; *Herzig, P
+*Robert Mathys Foundation, Bischmattstrasse 12, CH-2544 Bettlach, Switzerland. Bischmattstrasse 12, P.O. Box, CH-2544 Bettlach, Switzerland, +41/32 644 14 55,
Fax: +41/32 644 11 76, muellerd@rms-foundation.ch
The Mathys shoulder hemiprosthesis made of POM-C (Polyoxymethylene)
have been successfully implanted for years. This biocompatible thermoplastic
polymer exhibits high strength, rigidity and in addition a high resistance
against solvents and disinfectants. In the last years, the polymer PEEK
(Polyetheretherketone) has been more and more used for manufacturing
synthetic medical implants. Its characteristics are the very good mechanical
properties, the high resistance to chemicals, wear and γ− irradiation.
The aim of this study was to compare the wear behavior of the two polymers
when articulating with bovine cartilage in view of a possible replacement of
POM-C by PEEK in shoulder prostheses. Therefore, two in vitro tests were
performed, a pin-on-disc test and a more physiological investigation.
Materials and Methods
For screening a pin-on-disc test was carried out on the OrthoPOD wear testing
machine (AMTI Watertown, USA). POM-C (n=6) and PEEK (n=6) pins of a
diameter of 9.5mm, a length of 20mm and a tip radius of 22mm were moved
over cartilage specimens. Therefore, bovine discs with a diameter of 40mm
retrieved from the patella and immersed in a 30% serum solution were taken.
Thanks to pneumatic regulation, the pins were able to follow the established
rectangular path (5mmx3mm) on predefined load conditions even if the
surface of the cartilage discs was not perfectly flat. The applied load was
changed from 0.2MPa to 0.4MPa in each single cycle (Fig.1).
Figure 1: Wear path and applied load pattern for pin-on-disc test.
The number of cycles was 200’000 with a frequency of 1Hz. The weight
difference measured after conditioning before and after the tests was used to
establish the volumetric wear of POM-C and PEEK pins and the weight loss
of the bovine discs.
The second wear test consisted in a physiological test of the Mathys 44mm
shoulder hemiprosthesis made of POM-C (n=2) and PEEK (n=3). Bovine
cartilage discs taken from the acetabulum were used as gliding partners.
During testing, the discs (20mm dia.) were fully immersed in a 0.9% NaCl
solution, whereas the shoulder prosthesis was immersed only partially.
Figure 2: Left) Pin-on-disc test set-up Right) Physiological shoulder test setup.

The testing machine simulated a swinging phase of 90° and axial rotation of
30° forward and backward during one single cycle (Fig.2). The number of
cycles was again 200'000 at a frequency of 1Hz. A constant load of 200N in
the axis of the shoulder prosthesis was applied to the prosthesis-disc system.
In order to somewhat relieve the cartilage, the testing machine was only
functioning 12 hours a day and staying in unloaded rest for the other 12 hours.
The difference in weight before and after the test allowed to establish the
volumetric wear of the hemiprosthesis.
In order to establish the absolute loss of weight, the swelling amount of POMC
and PEEK pins and hemiprostheses was measured and taken into
consideration in both tests.
The two following diagrams show the volumetric wear of the tested implants:
Figure 3: Volumetric wear of PEEK and POM-C on bovine cartilage. Left)
Pin-on-disc test Right) Physiological test set-up.
A different behavior between the two materials could be observed with regard
to the wear rate of the tested samples.
In the pin-on-disc test, the mean wear volume of POM-C and PEEK pins after
200’000 cycles was 1.85±0.52mm3
and 0.32±0.42mm3
respectively. The
PEEK material presented a wear factor amounting to 17% of that of POM-C.
In the physiologic test, the mean wear volume of POM-C and PEEK shoulder
hemiprostheses after 200’000 cycles was 10.02±2.14mm3
and 1.64±5.94mm3
respectively. The PEEK material presented a wear factor amounting to 16% of
that of POM-C.
The swelling factors of both materials play a very important role when
determining the absolute wear behavior of the tested components. These
factors, provided by the weight difference of the reference bodies fully
immersed in serum (pin-on-disc test) and partially immersed in Ringer
solution (physiological test) for a defined testing time, depended on the way
of weighing these bodies requiring a strict protocol. Before the test, the pins
and shoulder prostheses were washed with alcohol and placed in an desiccator
for at least 4 hours in a relative atmospheric humidity of 50%. After the tests,
the following operations were carried out prior weighing in order to eliminate
salt crystals and proteins from the different components: ultrasonic bathing in
deionized water for 10 minutes, re-washing the surface with deionized water,
replacing them in the desiccator for 1 hour. The precision of the balance was
of 10-2mg for the pins and 10-1mg for the shoulder prostheses. The swelling
factors of POM-C and PEEK were respectively: 0.4% vs. 0.1% (pin-on-disc
test) for pins and 0.058% vs. 0.024% (physiological test) for shoulder
prostheses respectively.
The tests showed the higher wear resistance of PEEK on bovine cartilage in
comparison with POM-C. As PEEK has also good mechanical properties, high
chemical resistance and favorable irradiation resistance, it is probable that this
material will be used for manufacture of future shoulder hemiprostheses.
Further studies are in progress to evaluate optimal slide pairing for total
shoulder arthroplasty.


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