The average cost of treating an infection at the site of a total
joint arthroplasty is approximately $60,000, with an average net loss to the
hospital of $20,000 per patient based on the typical reimbursement that a
hospital receives1.
Many factors, such as the expense of antibiotics, contribute to the high cost
of treating such infections.
The use of antibiotics in acrylic bone cement spacers for the treatment of
infection following total joint arthroplasty has been well documented in
numerous clinical and in vitro
studies2-11.
Early studies by Buchholz et al. showed the efficacy of gentamicin
sulfate-impregnated cement in the treatment of infection at the site of total
hip
arthroplasty12,13.
Premixed powdered gentamicin with cement has been recently introduced in the
United States, but it is very expensive. Tobramycin, which offers nearly
identical antimicrobial coverage, is used almost exclusively in cement
spacers, and its potency has been well
established14-17.
Tobramycin is expensive (with a cost to our hospital of approximately $310
for a 1.2-g vial), and its availability is problematic at this time. Premixed
powdered gentamicin has a similar cost. Gentamicin liquid, on the other hand,
is readily available and is much less expensive (with a cost of $4 for a
480-mg dose) than the more commonly used tobramycin. Liquid antibiotics have
not been used in cement in the past because earlier studies have demonstrated
that the mechanical properties of the cement diminish by approximately 30% to
50% when liquid antibiotics are
added6,18.
However, cement spacers are temporary, typically being needed for only six to
twelve weeks. Therefore, if liquid gentamicin could be proven to be potent and
clinically effective for the treatment of infection at the site of a total
joint arthroplasty, substantial health-care dollars could be saved.
The purpose of the present study was to test the mechanical properties,
elution characteristics, and antibacterial activity of liquid gentamicin in an
acrylic bone cement spacer.
Compression Tests
Palacos R bone cement (Biomet Orthopaedics, Warsaw, Indiana) was
mixed according to the manufacturer's directions with and without the addition
of tobramycin or gentamicin. When tobramycin was used, 1.2 g of powdered
tobramycin (Eli Lilly and Company, Indianapolis, Indiana) was hand-mixed with
40 g of Palacos R bone cement powder before the liquid monomer was added. When
gentamicin was used, six 80 mg/2 mL vials (480 mg total) of liquid gentamicin
(Abbott Laboratories, North Chicago, Illinois) were added to 20 mL of liquid
monomer before the powder was added. The additional liquid antibiotic was
miscible with the monomer and polymer and did not appreciably alter the
consistency or texture of the cement. Palacos cement without any antibiotic
served as the control. After the cement had reached a doughy phase, it was
manually pressed into a cylindrical mold (6 mm in diameter by 12 mm long) and
was allowed to cure for one hour at room temperature. Ten specimens were
formed for each of the three test groups. Typically, both ends of the
specimens were concave after curing. To ensure that the end surfaces of the
specimens were flat and parallel, both ends of the specimens were polished
with 600-grit sand paper. The lengths of the specimens were then measured
once, and the diameter was measured in three different locations with
calipers.
The ten specimens from each group were tested quasistatically in
compression in accordance with American Society for Testing and Materials
(ASTM) specification F451-99a (standard specification for acrylic bone cement)
on a servohydraulic testing machine with use of polished, self-leveling
compression plates that were coated with mineral oil to reduce friction during
testing. The tests were run under displacement control at a rate of 1
mm/minute according to the ASTM standard. Load and displacement data were
collected at 2 Hz on a personal computer equipped with an analog-to-digital
(A/D) board and data-acquisition software. The tests were run until the load
reached a maximum value in compression.
The load and the actuator displacement were converted into stress and
strain. Stress was calculated by dividing the load by the initial
cross-sectional area of the cylinder. Strain was calculated by dividing the
actuator displacement by the initial length of the cylinder. Ultimate
compression strength was defined as the maximum stress achieved in compression
during the test. Compression modulus was calculated as the slope of the
stress-strain curve between 20% and 80% of the ultimate compression
strength.
Tension Tests
Five specimens from each group were prepared in a similar manner as
described for the specimens used for the compression tests, although they were
molded into a standard "dog-bone" type rectangular
tension-specimen geometry with a gauge length of 50 mm, a width of 6.35 mm,
and a thickness of 5 mm. The thickness and width of the gauge region of the
specimens were measured at three different locations with calipers.
The five specimens from each group were tested quasistatically in tension
in accordance with ASTM test method D638-99 (standard test method for tensile
properties of plastics) on a servohydraulic testing machine with use of
tension grips. A 25-N preload was placed on the specimen, and the specimen was
loaded to 225 N under load control at a rate of 25 N/second according to the
ASTM standard. An extensometer was attached to the gauge region of the test
specimen to directly measure strain (for the purpose of determining the
tension elastic modulus). The specimen was then unloaded, the extensometer was
removed, and the specimen was then reloaded to failure at a rate of 25
N/second. Load and extensometer data were collected at 25 Hz on a personal
computer equipped with an A/D board and data-acquisition software.
The load and extensometer data were converted into stress and strain.
Stress was calculated by dividing the load by the initial cross-sectional area
of the gauge region of the specimen. Tension modulus was calculated as the
slope of the stress-strain curve between 50 and 150 N of load during the
tension modulus test. For the tests to failure, strain was calculated by
dividing the displacement of the extensometer by the initial length of the
extensometer. Ultimate tension strength was recorded as the maximum stress
achieved during the test to failure.
Elution Testing
Ten additional specimens of Palacos R bone cement with and without
gentamicin were prepared as described for the compression test and were used
for the elution and antibacterial testing. Fluorescence polarization
immunoassay (FPIA) (Axsym System, Abbott Laboratories, Abbott Park, Illinois)
was used to confirm the bioassay results and to measure the quantity of
gentamicin being eluted from the
samples19. The
results were recorded in mcg/mL. The low-end sensitivity of this technique was
0.30 mcg/mL.
Briefly, fluorescence polarization immunoassay is based on the principles
of competitive antibody binding and fluorescence polarization. In this case,
competitive antibody binding involves the competition between gentamicin and
fluorescein-labeled gentamicin for antibody binding sites. Fluorescence
polarization is based on the notion that larger molecules rotate more slowly
(Brownian motion) and have higher fluorescence emissions than smaller
molecules do. Therefore, the lower the concentration of gentamicin present in
solution, the less it will be bound to the antibody and the more the labeled
gentamicin will be bound. The antibody-bound labeled gentamicin is a larger
molecule and, as such, rotates more slowly, resulting in more fluorescence
emission when excited with polarized light.
Antibacterial Testing
To determine the bioactivity of the gentamicin in the tested specimens,
aliquots of culture medium from the specimens used in the elution tests were
analyzed with use of a standard bioassay. Samples were taken on Days 1, 2, 3,
4, 7, 14, 21, 28, 35, and 42. The tube dilution bioassay was employed with
serial twofold fluid dilutions with use of a selected Escherichia
coli strain (minimum inhibitory concentration, 0.8 to 1.6 mcg/mL)
(National Committee for Clinical Laboratory Standards
1997)20. The
accuracy of the bioassay is ±1 tube dilution. Although Escherichia
coli is not necessarily a common organism isolated from infections at the
sites of total joint arthroplasties, it is sensitive to gentamicin and for
this reason was used to test the potency of the eluted antibiotic.
Statistics
Compression modulus, ultimate compression strength, tension modulus, and
ultimate tension strength in the three groups were compared with use of
Duncan's multiple range test, with the level of significance set at p =
0.01.
The addition of 1.2 g of powdered tobramycin to Palacos R cement had
no significant effect on the measured biomechanical properties of the cement
(Table I). The addition of 480
mg of liquid gentamicin, however, had a detrimental effect on the
biomechanical properties of the cement. The mean compression modulus was
significantly lower for the specimens containing liquid gentamicin than it was
for the control specimens and the specimens containing tobramycin (1.31
compared with 2.23 and 2.24 Gpa, respectively), representing decreases of 41%
and 42%, respectively (p < 0.01). The mean tension modulus also was
significantly lower for the specimens containing gentamicin than it was for
the control specimens and the specimens containing tobramycin (2.00 compared
with 3.06 and 3.09 Gpa, respectively), representing a 35% decrease in both
cases (p < 0.01). The ultimate compression strength and the ultimate
tension strength were similarly significantly decreased for the specimens
containing liquid gentamicin compared with the control specimens and the
specimens containing tobramycin (p < 0.01).
The majority of gentamicin was released from the specimens quickly. Within
the first twenty-four hours, the concentration of gentamicin averaged 26.4
mcg/mL. By three weeks the average concentration had decreased to 4.15 mcg/mL,
and by six weeks it had decreased to 0.65 mcg/mL. Based on the minimum
inhibitory concentration of the Escherichia coli strain tested (0.8
to 1.6 mcg/mL), bactericidal concentrations of gentamicin were eluted from the
specimens for at least three weeks. However, by four weeks, the concentrations
of gentamicin being eluted approached the lower limit of the minimum
inhibitory concentration of the Escherichia coli strain being
tested.
The bioassay results showed that the gentamicin being released from the
specimens retained its bactericidal activity against the Escherichia
coli strain being tested (Fig.
1). The concentrations of active gentamicin in the sampled
aliquots based on fluorescence polarization immunoassay testing generally fell
within the expected ranges based on the bioassay results
(Fig. 1).
It has been estimated that >400,000 total joint arthroplasties
will be performed this year in the United States, and that number is expected
to as much as double over the next twenty to thirty
years21. It follows
that we will see a commensurate increase in the number of infections at the
sites of total joint arthroplasties, with the number of such infections
reaching as high as 8000 to 10,000 per year.
Presently, two-stage exchange arthroplasty is the standard of care in the
United States for the treatment of infection following total joint
arthroplasty22-28.
The acrylic bone cement spacer is a temporary device that primarily serves to
maintain spatial relationships among the adjacent tissues after removal of the
prosthesis in anticipation of later reimplantation. Because the spacer
represents a foreign body, its presence in an infected site can be associated
with the persistence of pathogens. Conceptually, antibiotics leaching out from
the spacer eradicate susceptible microorganisms on the polymer surface.
Aminoglycoside antibiotics (gentamicin and tobramycin) have functioned well in
this
role12,13.
Bactericidal levels of these antimicrobial agents against most staphylococcal,
streptococcal, and gram-negative aerobic bacilli are released into local
tissues. The placement of an antibiotic-impregnated acrylic bone cement spacer
combined with the administration of a course of intravenous antibiotics has
allowed for successful reimplantation in approximately 85% to 90% of
cases22,25-28.
In the United States, tobramycin is available in powdered form and is
incorporated with acrylic bone cement to produce spacers for the treatment of
infection following total joint arthroplasty. However, powdered tobramycin is
extraordinarily expensive and recently has been in short supply. Gentamicin is
only available as a liquid in the United States. Premixed powder gentamicin
with cement has recently been introduced in the United States but is very
expensive as well.
The cost of revision total joint arthroplasty can be as much as three times
the cost of primary total joint
arthroplasty1, and
the treatment of infection following arthroplasty is associated with
substantial health-care costs. Eighty infections around total joint implants
were treated with cement spacers at our institution in the year 2000. If
liquid gentamicin had been used instead of tobramycin, approximately $148,000
could have been saved by the institution for the year. On a national scale,
approximately 4000 infections at the sites of total joint arthroplasties are
treated each year. Assuming an average annual infection rate of
1%1, this could
translate into a cost savings of $7,400,000 per year.
The present study demonstrates that liquid gentamicin in bone cement, at
the quantity used in our experiments (480 mg), is eluted effectively and
maintains its bactericidal activity. The lower mechanical properties of cement
containing liquid gentamicin would preclude it from being used for the
implantation of a prosthesis. However, given the temporary nature of the
cement spacer, the use of 480 mg of liquid gentamicin in cement may be
worthwhile given the substantial cost savings available. Additional studies
are needed to determine its clinical efficacy and safety. ?