Interview: Dr. John P. Collier
Director, R & D
Cook Engineering Design Center Dartmouth College
Hanover, New Hampshire
The Cook Engineering Design Center at Dartmouth is well-
respected by the medical community for its many advances in the field
of orthopaedic implant design for both the knee and hip. To study
the metallurgy of implants and how to improve those implants, the
Implant Retrieval Lab utilizes state-of-the-art equipment for both
video image transmission and photomicroscopy.
Q. How would you outline your activities in the Implant Retrieval
Lab?
We perform analytic and experimental orthopaedic implant design
of both the hip and knee prostheses. We work at the Medical Center,
the Medical School, and the Thayer School of Engineering, all within
walking distance of one another.
One of the key areas where we gather to share our findings
is the Implant Retrieval Lab. There we examine retrieved prostheses
and the histologies made from them. The Lab is open to all
orthopaedic surgeons, residents, and medical and engineering school
students. The focus of our examinations is the interface between
host tissue and porous-coated prostheses.
Q. What are the methods you use for examining histologies?
More than one hundred surgeons have made prostheses available to
us. They are sent to us in a ten percent formalon solution.
Photographs are taken of the numbered and labelled specimens.
Prostheses sent to us intact in cadaver specimens are X-rayed and
then bi-valved. Specimens retrieved through revision surgery are
examined and mapped for areas of bone and fibrous tissues adherence.
This permits us to visualize the fit of the prosthesis within the
bone, and estimate the amount of bone and fibrous tissue ingrowth as
a percentage of total available porous surface.
The photographed specimens are sectioned and fully dried in a
series of alcohol and acetone solutions over a two-week period.
Specimens are embedded in ethylmethacrylate and cured under
pressure to prevent void formation. Thin slices of each specimen are
cut with a Bronwill saw using a carbide blade. The 1mm-thick slides
are hand polished on one side, and that side is glued with epoxy to a
standard microscope slide. The sections are subsequently hand-ground
using a Buehler 8001 hand-held, petrographic slide holder until a
thickness of between 40 and 60um has been produced. Final polishing
is done with a .3um aluminum oxide polishing powder. The finished
slides are then stained with H&E and cover-glassed. All histological
photography is performed on a Zeiss Photomicroscope III.
Q. How do you integrate microscopy with video and photography?
With the fully automatic, integrated design of the Zeiss
Photomicroscope III, we have the ability to switch between video and
photography, depending on what we want to do. The transition needs
to be smooth and not time consuming.
Q. How often do you use video as compared to photography and
direct viewing through the eyepiece?
Basically, we're using video most of the time, and we do it
ourselves, without a medical video production crew. That's one of
the reasons why we use the camera we do, the JVC KY-M280. You don't
have to be a video technician to operate or maintain it properly.
That allows us to concentrate on examining histologies without
costly interruptions or downtime.
Q. Why do you use video, as compared to other mediums?
We use video primarily so that students, orthopaedic residents,
and orthopaedic surgeons may simultaneously observe and discuss the
details our research unfolds. For us, video is basically a mech-
anism for teaching. It works beautifully because first we look at
photographs of these devices taken through the Zeiss, then we go to
video to discuss the details with one another.
There are five of us working in the Lab, and color video
monitors let us simultaneously observe the specimens to understand
better what's going on at the interface between the host and the
implant.
Q. I understand that early successes with porous-coated prostheses evaluated
in this Lab led the orthopaedic industry to a number of important
advances?
Yes, the early successful results of Drs. Lunceford, Mayor, Engh
and others in the early to mid-1970s led the industry to produce a
variety of sizes of femoral hip prostheses so that each medullary
canal can be precisely filled. In fact, at the recent American
Association of Orthopaedic Surgeons in San Francisco in January,
1987, we presented a poster which provides information about our work
over the years.
The-state-of-the-art, as of say the mid 60s early 70s, was that
if you had an artificial joint implanted in you, bone cement
(methylmethacrylate), which is very similar to plexiglas, was
put in a doughy form into the reamed medullary canal of the femur.
Then, the implant was pressed into to canal. That way, the bone cement itself
acted as a grout between the end of the bone and the
implant. The trouble with this bone cement technique is that bone
cement is relatively brittle and weak, tending to break down over
time.
So in the early 70s research here was initiated with a goal of
eliminating bone cement. The most viable solution to date apperas to
be the application of a porous surface to the surfaces of the implant
in contact with bone. For example, hip prostheses are stabilized by
a stem which goes down inside the medullary canal of the bone, which
is hollow. The stem has a porous-coating made of beads of the same
metal that the implants are made of. This provides a three-
dimensional porous network that looks something like a sponge, and
the pores that are generated are on the order of 300 to 500 microns
in diameter which is sufficient to permit bone to grow into them.
When implanted in the body, bone grows into the coating and over
time the bond between the host and the implant gets stronger rather
than weaker. Occassionally these devices are retrieved if it is
necessary to change implant position, obtain a better match between
components, or eliminate an infection. When these devices are
retrieved, we examine them. That's the focus of the histology lab.
Q. Does the inherent magnification through the video monitor
provide greater recognition of detailed anatomical landmarks?
Yes, to some extent. We are examining very thin, transverse
cross-sections of bone, fibrous tissues fats, and metal. When we put
our Zeiss images up on a monitor to share what's going on as a group,
there is indeed a certain degree of image enhancement. It's provided
by the video system, especially under polarized light because it
brightens up the field. You can often get a better picture this way
on the video monitor, rather than by simply looking through the
eyepieces.
Q. Do you ever use the video monitor as a screen for helping to focus the
camera lens? That is, do you put an image up on the monitor and then
focus, instead of first looking through the eyepiece?
All the time. We never need to go back to the eyepiece because
there's no guarantee that the focal length of the two is absolutely
the same. So when we're using the monitor we do all our focusing on
the monitor itself.
Q. How was it that you came to specify a three-tube color video
camera from JVC?
Our video dealer, AZI (Avon, Massachusetts), was responsible
for showing up with the right equipment for our particular needs.
Basically, all we did was ask AZI for a video camera that would give
us an image that was as good as, or better than what we could see
through our microscope eyepieces. We simply had to decide whether or
not the camera could do this.
AZI brought over the KY-M280 from JVC, and showed how easily the
head fits onto our Zeiss Photomicroscope III which we've been using
for years. AZI's Erwin Deutsch said so far for most video
microscopy applications, his firm had been accustomed to selling
single-tube black and white cameras. That was because single tubes
are small, have excellent resolution, are easy to maintain, and
lightweight enough to be mounted on a microscope.
Deutsch also said that with the new KY-M280 three-tube camera
from JVC, we can meet the criteria for use with a microscope. Under
three pounds, its camera head is lightweight and compact, plus it
gives us the benefits of high-resolution color. The camera outputs
RGB signals to two RGB monitors for maximum resolution, and also
composite signals for standard monitors and recorders. We use
a very high resolution 12-inch monitor, and the other is a
larger 21-inch monitor with less resolution.
Deutsch believes that with our three-tube and its high
resolution, we can satisfy whatever biomedical work we get involved
with here at the Lab. The availability of separate RGB outputs was a
major factor in going with the KY-M280. AZI told us it would assure
better color in the monitor, and that's what we want.
Deutsch points out that when hooked up to a microscope, a good
video camera makes the big difference at the lower magnifications,
not the higher ones. He says that when you go to low magnification
there are many more pixels of information. For example, we do much
of our work with a 100X oil immersion lens. Deutsch says a less
sophisticted video camera could handle 100X oil, but when we get down
to 2.5X he says the KY-M280 is needed to avoid color blur. He says
that's where a three-tube is so much better than a single-tube, and
now of course with the KY-M280 three-tubes have come way down in size
and weight.
Deutsch says that before the KY-M280 was available, three-tube
cameras for lab applications suffered from lack of flexibility in
that they were too sensitive to adjust. He told us that changing the
magnification of the microscope used to mean rather large changes in
the color of the produced images, as if the color temperature of the
light source had changed. Three-tubes also made it more difficult
to compensate for this change. But now, with the KY-M280, the
controls are so good that if the colors are off-balance, we can
quickly get them back.
Q. How do you use the two different types of video monitors you
have?
It depends on what we're looking for. We're set up to take
Polaroid shots from the small, high-resolution monitor. Ninety
percent of what we do with video involves the larger monitor.
Q. Is there any particular reason, why you selected the
Zeiss Photomicroscope over others? Was this recommended as part of the
package?
Well, we had the Zeiss Photomicroscope III for a long time
before we bought the JVC, and so there was a straightforward match.
It certainly is a reliable model that works beautifully. The
question is whether we can see all of the things we're looking for,
and the answer is yes. Our criterion is the limit of resolution for
the thickness of the samples we're able to grind. In other words,
typically we're looking through cross-sections that are forty to
sixty microns thick. The metal is so much harder than tissue, so
that even with the embedding techniques we use, if we try to go much
thinner than that, we lose both the soft and hard tissue that we
wanted to see.
Q. Do you use reflected or transmitted light?
Virtually all transmitted light. That's because what we're
looking at are relatively thin cross-sections. When we look at the
metallurgy of the prostheses themselves, we use reflected light, and
we do that from time to time.
Q. What kind of contrasting techniques do you use?
We typically don't use phase contrast, differential interference
contrast, or darkfield, but instead bi-linear or circular polarized
light. That's because both the bone and fibrous tissue collagen
materials we're looking at are birefringent. Polarized light lets us
examine the orientation of these collagen fibers which we often see
holding bone in close, direct apposition to smooth regions of the
substrate below the porous-coated devices. They're not always
in-grown by bone. Specifically, tibial knee components and
acetabular components often have bone adhering to the porous-coating
through a layer of fibrous tissue.
Q. Have you modified the microscope system in any way to suit your
needs?
No, there's no need to. It comes with a full set of accessories
for doing just about anything we want. We prepare slides that
include fat cells as well as a very tough metal embedded in them, and
dealing with that difficulty is where we have the opportunity to be
innovators. The hardness of the materials varies by orders of
magnitude, therefore getting them both in the same field of vision is
where we get innovative. It's not a particularly simple process.
Q. What are some of the Zeiss accessories you use?
For example, there are light filters you can typically adjust
so that lighting is well within the range that the KY-M280 camera is
comfortable with. If the light intensity is a little bit too low,
you can pick it up with the image enhancement capability of the
camera. If it's a little bit too high, you can cut down on the light
either by using the iris aperture (which also helps your contrast),
or simply by cutting down on the light from the quartz iodine light
source which we use almost entirely. We almost never use a mercury
light source. And simply by turning that quartz iodine light down a
bit, we can reduce light intensity without radically changing the
color. We're really happy with the way the equipment performs.
Q. What would you say are the most common problems you encounter?
The one you just described? The thickness problems?
Absolutely, preparing the samples is the tough part. Each
specimen takes about 10 hours to prepare. Photographing them is
easy. The microscope system works fine. The hassle is trying to get
samples that are thin enough, so that you can get the resolution that
you need. It's really quite tricky. We've produced and examined
around 300 of them now.
The embedding process is also very difficult, and now finally we
have that down to a reasonable science. If we don't do it right, we
end up with a lot of air bubbles in the sample. Then we have
something that looks terrible when we section it.
Q. Who does the photography in the Lab?
The vast majority of the photography that's now done is by
technicians who have picked it up over the years. When they do
photography, it's almost always 35mm slides. As I said before, we're
set up to take Polaroid shots off the small high-resolution video
monitor, although its not something we do frequently.
Q. What would you describe as your department's strong points?
We work together as a team encompassing a variety of different
disciplines. We can also move around from assignment to assignment,
sharing different responsibilities.
We have two technicians, Leo Dauphinais and Helene Surprenant, who
do all of the embedding and sectioning, and they end up doing much of
the photography as well. Our team includes two professionals,
Michael Mayer, an orthopedic surgeon at the Hitchcock Clinic, and
myself. My background is really in biomaterials. I'm an engineer by
profession. We also have a research engineer who works with us,
Victor Surprenant. Together, with the students and residents the
focus here at the Lab is examining these porous-coated prostheses and
improving the bonding of metal to tissues.
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