New Horizons in Arthroplasty - Hydroxylapatite Coatings
Degenerative arthritis of the hip is a disabling affliction affecting
millions of Americans. Surgical solutions to this important problem
have undergone a rapid evolution over the past several years driven by
a synthesis of new technologies. Indications for joint replacement, which
were quite limited initially, have been expanded. New technologies applied
to orthopedic surgery have provided more durable, biocompatible materials
such as the building blocks of artificial joints.
As the result of these advances, prosthetic longevity of total hip replacement
continues to increase. In addition, reconstructive problems which were
previously thought to be unmanageable because of severe bone destruction
(such as might be seen in revising a failed total hip replacement)
or because of a patient's relative youth are now handled on a routine
basis.
In the early 1960's, Sir John Charnley brought about a revolution in the
treatment of arthritis with the introduction of the first successful total
hip replacement. This represented a high-technology solution incorporating
new developments in plastics, polymer chemistry and metallurgy. Continued
achievements in total joint replacement are driven, in large part, by
advances in technology.
Increased understanding of joint biomechanics, and advances in imaging
technology and material sciences are contributing to the dramatic improvements
in the practice of total joint replacement. Hip Joints take a tremendous
amount of punishment. Under the gentlest of conditions they are routinely
exposed to stresses that are three times body weight.
A person engaging in moderate activity may repeat this stress cycle more
than one million times each year. Despite these demands, artificial
hips implanted in the elderly using the conventional acrylic cement techniques
pioneered by Charnley have a 90%+ rate of continued success after 15 years.
However, when these same techniques are attempted in high risk patients
under 40 to 50 years of age, failure rates of 40% after only five years
have been reported.
In those cases where a conventional joint replacement has already failed,
revision surgery using cemented techniques has even worse results. In
both situations, failure is often due to breakdown of the acrylic cement
anchoring the prosthesis to the host bone.
To address this problem with cement failure, a new type of prosthesis
was developed that obtained fixation directly to the patient's bone without
the use of cement. The first generation of these new devices had porous
metal surfaces into which bone could grow and anchor the prosthesis. This
approach was successful for many patients, but still resulted in occasional
failures.
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And We're Off
Judging from the wonderful response to our letter, the future of this
informative newsletter appears to be good. In addition to some humorous
tales from several people, there were 29 suggestions for a title,
of which three of the names were submitted twice . The title, Joint Effort
suggested by Adelaide Peterson, came out the winner.
A very positive note: Friends of Orthopaedic Research (F.O.R) will be
merged with Sutter Institute for Medical Research (SIMR).
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Patient Concerns
Q: Why do I need so many X-rays?
A: Pre-operative films are required for diagnosing a joint problem and also
as a tool for planning the most appropriate procedure if indicated,
The best methods for the physician to evaluate post-operative results are
talking with the patient, physical examinations and studying the X-ray films.
It is necessary to compare the most recent X-rays with previous films. X-rays
must be taken with the patient positioned in a similar manner each time
in order to obtain a meaningful comparison. Having an X-ray unit in
the office and a technician there is a consistency in producing quality
films.
Q: What should I do if I am having problems with my artificial hip or knee?
A: Please call the office at, 733-5066, and ask to speak to the physician
who performed your surgery. Frequently your concerns can be handled with
a telephone call. If you call when the office is closed, the answering
service will relay the message to the office and your call will be
returned by either your doctor or the nurse who will advise as to the best
course to take to alleviate the problem.
Bioactive Ceramic Coatings - Hydroxylapatite
To resolve these persistent failures, researchers turned their attention
towards a bioactive ceramic called hydroxylapatite. This ceramic promised
to provide a more hospitable environment for biological bony fixation
of prosthetic implants.
Bone is a composite material made up of 60 to 70% inorganic mineral crystal
and 30 to 40% organic matrix consisting mostly of collagen protein fibers.
The major mineral constituent of bone is hydroxylapatite in the form of
tiny elongated crystals. These crystals are combined with the collagen
fiber organic matrix of bone in a highly organized fashion and stiffen
the bone structure in much the way that glass fibers stiffen softer plastics
in the synthetic composite fiberglass.
When the body encounters exposed hydroxylapatite surfaces adjacent to
healing bone, the natural response is to form new bone along those
exposed surfaces until the healing bone is anchored there. Synthetically
prepared hydroxylapatite ceramic has the same chemical composition as
this major mineral constituent of bone.
When implanted in sites adjacent to healing bone, it incites this same
natural healing response with an actual physical chemical bond forming
between the patient's healing bone and the implanted bioceramic. This
bond is so strong that once bone has become attached to the surface of
this material and samples are stressed to failure, fracture occurs through
the bone some distance away from the actual bone ceramic interface.
When it was discovered that synthetic ceramic hydroxylapatite provided
such a hospitable environment for bone attachment and biologic fixation,
researchers began to study its direct application in clinical settings.
After more than 10 years of experience in the dental field with successful
utilization of synthetic hydroxylapatite as an aid to bone reconstruction,
and as a mechanism for anchoring metallic prostheses, orthopedic researchers
began seriously considering its use in total hip replacement. To be successful
in this especially demanding environment, the hydroxylapatite must be
applied in an ultra thin uniform layer to the desired surfaces of the
prosthesis.
As a ceramic material, hydroxylapatite is quite brittle, and if the coating
on the implant surface is too thick, then failure of fixation between
the bone and the prosthesis may occur through cracks which develop within
the substance of the coating. The ideal thickness is approximately 70
microns: thinner than the width of a human hair.
Results, after two years of experimental clinical orthopedic experience,
have shown a striking potential to promote bone growth directly onto the
surface of the implant providing excellent biologic fixation. This bioactive
coating can be applied over whatever area is deemed necessary by the surgeon
and is now being utilized in selected high-risk patients.
It has already demonstrated some qualities which are superior to the previously
used porous metal ingrowth surfaces. It appears to be "more hospitable"
to the attachment of bone to the surface of the prosthesis. In addition,
it can be applied over larger areas than was previously possible
with some of the porous surface metal techniques. This is especially critical
in the setting of revision total hip replacement where obtaining adequate
host bone contact with biological fixation surfaces on the implant is
a crucial issue.
Several centers in the United States and Europe are experimenting with
this application of hydroxylapatite in total hip implants and have reported
excellent short-term results. We must wait some time before we can get
a picture of the long term success of this material.
Several concerns have been raised which are currently being studied. The
first is that the coating may crack off, leading to loosening of the component
and ultimate failure of fixation. This was a problem that was seen in
early dental implants which depended upon hydroxylapatite for biologic
fixation.
In retrospect, it can be seen that the coating on these failed dental
implants was of a non-uniform character: sometimes being applied too thick
and in an impure form. In addition, many of the implants were designed
with smooth sides in which forces through the implant were transmitted
to the bone via the hydroxylapatite coating in a shearing fashion. Both
bone and hydroxylapatite ceramic are weakest when subjected to shear and
tensile forces, and strongest in compression.
Newer designs of dental implants, which transfer load to the bone through
the hydroxylapatite coating under compression, have shown excellent longevity.
These design features have been incorporated into total hip implants and
allow the hydroxylapatite coating to be loaded in compression rather than
shear.
Another potential problem is that hip implants coated with hydroxylapatite
may become so well anchored in the host bone that their removal, should
it ever become necessary, would be exceedingly difficult.
Hydroxylapatite coatings promise to become a new and powerful tool in
the surgical equipment of orthopedics. As a bridging link between high-tech
metallurgy and bone biology, it shows promise as the best method yet developed
to obtain secure biologic fixation of total Joint implants.
We are continuing to conduct research with this important new material
as we make it available to selected high-risk patients in total hip replacement.
Jeffery K. Taylor, M.D., is an orthopedic surgeon specializing In joint
reconstructive surgery He performed his medical training and orthopedic
residency at the Johns Hopkins Medical Institutions and subsequently completed
a fellowship In lower extremity reconstructive surgery. He practices
with Dr. William L. Bargar,where both physicians are actively engaged
in ongoing research at the Sutter Institute for Medical Research and at
the University of California, Davis, Orthopedic Research Laboratories.
X-rays And Your Health By Mary J. Minix, E.R.T.
Being the X-ray technologist for Dr's. Bargar and Taylor, I hear this question
quite often. I would like to address this concern and, I hope, alleviate
any fears regarding the use of diagnostic X-rays.
According to Troy Brannan, nuclear physicist for Sutter Hospitals, "There
are no measurable effects from diagnostic X-ray". The average series
of hip films delivers .689 millirads or 1/1000 of a rad (Radiation Absorbed
Dose). He compared this to a plane ride from coast to coast, which delivers
25 millirads. "To see any negative effects from X-rays you must exceed
10 rads, at which time a change could be noted in the blood system, namely
a temporary decrease in the leukocyte count", Brannan said.
Therefore, it would take over 10,000 hip X-rays to cause a temporary reduction
in leukocytes. After receiving 200-300 rads "serious radiation sickness";
bone marrow syndrome/hemorrhaging becomes a concern. This would equate to
20,000 to 30,000 hip X-rays.
Though the possibility of damage is minimal, well-trained technicians are
taught that the safety of the patient is of the utmost importance. Shielding
is used on patients that are still in child-bearlng years, repeat
exams due to improper positioning are avoided when possible, and the minimum
amount of exposure is used.
Also, the equipment used is designed to limit the radiation to the clinical
area of interest and to prohibit exposure of radiation to other parts of
the body. After considering all the health effects of diagnostic X-rays,
it is apparent the risk is far outweighed by the benefits.
Dr's. Bargar and Taylor would be unable to help any of their patients without
the information provided to them by the X-rays.
Mary J. Minix, E.R.T, has been with Dr. Bargar since January, 1987.
Return to top Dr. Bargar Accepts New Fellow
We said our farewell to Dr. Mark A. Newman, who completed a year as a fellow
in joint reconstruction under the auspices of Dr. William Bargar. Dr. Newman
plans to enter private practice in Fountain Valley. Being a southern Californian,
he will be returning to home after an absence of 10 years. During this time
he attended medical school in Chicago, completed a five year orthopedic
residency at UC Davis Medical Center—this last year as a fellow. We
all wish him success in his new practice and new location. Dr. William J.
Murzic from Duke University, Durham, North Carolina will be starting August
1 as Dr. Bargar's new fellow.
Hydroxylapatite