THE CHARACTERIZATION OF PARTICULATE DEBRIS OBTAINED FROM FAILED
ORTHOPEDIC IMPLANTS:
Chapters 6-8
6Scope
of This Research
This research project is concerned primarily with total hip joint
prostheses that have produced particulate debris within the joint
cavity of the patient. Thus, all implants considered by this research
can be considered as having failed according to the constraints
given in the previous section.
Samples of tissue containing the particulate debris were removed
from the patient's joint cavity at the time of revision arthroplasty
(surgical repair or removal of a joint prosthesis). Many authors
have investigated the biological aspects of this metal and polymer
contaminated tissue. Severe contamination of the joint cavity
with metal particles was first reported in the early 1970s by
Meachim and Williams. The condition was then commonly dubbed 'metallosis'.
The adverse effects of metallosis vary from none to severe infection-like
reactions, depending on the metal particle composition. The morphology
of the retrieved metal particles are reported to vary from sub-micron
sized particles to particles as large as 100µm.
7Project
Hypothesis
Although many aspects of implant failure are addressed by this research, the primary goal of this project is to verify the following hypothesis:
The metallic debris that has been collected from the tissue
surrounding non-cemented Ti6Al4V-CoCrMo alloy modular implant
systems is generated primarily by fretting wear.
8Experimental
Design
Rationale and Hypothesis
The objective of this project is to determine the mechanism of
the generation of microscopic metallic debris particles collected
from the tissue surrounding failed hip joint prostheses. The implant
system under consideration is of a modular design (CoCr alloy
head, ultrahigh molecular weight polyethylene (UHMWPE) socket
liner, porous coated Ti6Al4V acetabular socket, Ti6Al4V stem
with sintered CP Ti mesh). This particular implant system is frequently
used by many surgeons around the world. Unfortunately we were
unable to obtain actual implants that seen actual service conditions
within the body. As stated above, the hypothesis of this research
is that the metallic debris particles found in tissues around
the implant at the time of the revision operation are primarily
products of wear processes. The procedure for confirming or denying
this hypothesis will be based on experimental analysis and on
the theory of wear and fretting mechanisms.
The experimental procedure consists of many approaches: 1) isolate
the metallic particles from the tissue specimens for examination,
2) classify the various particulate types found among the different
tissue specimens, 3) examine each class of particle by scanning
electron microscopy to determine the characteristic size and morphology;
and couple this with electron diffraction data, where possible
to determine crystallography and verify composition, and finally,
4) perform conventional metallographic examination of the various
metallurgical aspects of an unimplanted hip joint prosthesis stem,
and to relate the findings of this examination with the particle
characterization data to determine the likely source of origin
of the various particle types.
Standard metallographic preparation techniques combined with optical
and electron microscopy were used to examine the internal and
external microstructure of the stem and mesh. Transmission electron
microscopy (TEM) was used to characterize the crystallographic
structure and composition of individual debris particles. With
its high resolution and depth of field, scanning electron microscopy
is an ideal technique for examining the size distribution and
morphology of metallic particles of this size distribution.
Materials and Procedures
Preparation of Particulate Specimens
A total of twelve debris specimens generated in vivo were
examined. Each of these twelve specimens was extracted chemically
from tissue removed from the region surrounding the implant by
the Stanford Orthopedic Research team. Biological tissues were
enzymatically digested with a papain solution (an enzyme derived
from the papaya fruit) and centrifuged to separate the metallic
and polymeric components from the liquid media. The liquid medium
was extracted and replaced with bovine serum. The specimens were
kept at 4°C prior to initial examination. Each specimen consisted
of approximately 0.1 ml of particulate debris (metallic and polymeric)
suspended in approximately 1.5 ml of fluid similar to the fluid
that surrounded the implant.
Two types of 'control' debris specimens were obtained. One consisted
of titanium alloy particles and the other consisted of a nickel
alloy. The titanium control wear debris particles were generated
by mechanically loading two pieces of Ti6Al4V in sliding contact
while immersed in Ringer's solution (a body fluid substitute).
This control wear debris specimen was provided by Zimmer Inc.
(Warsaw, IN). The exact mechanical parameters under which the
debris was generated (e.g., frequency) were not reported.
The nickel alloy powder specimen was chosen because it exhibited
a purely ductile mode of formation, and the particles were of
roughly equivalent size and shape as the titanium particles obtained
from the extracted tissue. These nickel powders (supplied by Novamet
Specialty Products Corp., Wyckoff, NJ) were ball milled and sieved
to the particle size distribution shown in Figure 20.
Preparation of Unimplanted Stem Component
An unimplanted Ti6Al4V stem (obtained from a major implant manufacturer)
with sintered CP Ti mesh was obtained in the 'as received' (though
non-sterile) condition. This stem component was sectioned as shown
in Figure 21 for metallurgical analysis. These sections
were then mounted and examined with both optical and electron
microscopy to determine their microstructure. Microhardness measurements
were also performed on the mounted specimens.
Analysis Methods
Optical Microscopy
The synovial and factory generated particulate specimens were
mounted in epoxy and polished using standard metallographic techniques
and examined using a Nikon Epiphot inverted microscope. The unimplanted
stem was sectioned as shown in Figure 21 above with a SiC
cutoff wheel or, in the case of small cross sections, a diamond
saw, and then mounted into glass fiber reinforced thermosetting
mounts for polishing. Kroll's etchant was used to reveal the microconstituents
in the various regions of the implant. Knoop microhardness measurements
were taken at the mesh-stem interface.
Scanning Electron Microscopy
A Hitachi S-520 scanning electron microscope (SEM) was used to
examine and analyze the particulate debris. Suspended particles
were transferred via sterile syringe onto an aluminum SEM specimen
mount covered with copper tape. The copper taped substrate prevented
the aluminum mount from influencing the energy dispersive spectrograph
(EDS) This is important because the titanium alloy used in the
implant contains aluminum. A drop of methanol was added to each
mount to enhance dehydration and evaporation. Once the fluid had
completely evaporated, the particles were ion etched and sputtered
with gold to a final film thickness of approximately 150Å.
Specimens were then examined at 20keV and at a working
distance of 15 cm. Metallic particles were analyzed by energy
dispersive x-ray analysis (EDX). This determined the relative
chemical content of individual particles. Representative particles
from each specimen were photographed at various magnifications
to document particle morphology.
Sectioned portions of the unimplanted stem were polished by standard
metallographic procedures and they too were then examined and
photographed. Both their sectioned and external surfaces were
examined.
Transmission Electron Microscopy
A scanning transmission electron microscope (STEM) exhibits much
higher resolution than an SEM and can provide information about
the crystallinity and structure (via electron diffraction) of
materials. After many attempts, suitably thin particles were isolated
and examined on a Philips TEM at 400kV. Spot patterns, ring patterns,
and STEM photos were taken. Electron diffraction patterns for
gold were used to calibrate the diffraction data obtained from
the debris particle.
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