Extract
External fixation is a versatile method of fracture treatment. However,
pin-track infections remain a major complication. Despite the routine use of
antibiotics, pin-track infections occur in 57% to 75% of
cases1,2.
One factor contributing to the high prevalence of infection is the access that
the pin track provides to bacteria and the ability of bacteria to colonize the
metal surface of the pin or wire. The OrthoGuard AB antimicrobial sleeve
(Smith and Nephew, Memphis, Tennessee) inhibits bacterial colonization on
external fixation pins and wires by providing sustained local delivery of
gentamicin to the pin or wire site.
External fixation is a versatile method of fracture treatment. However,
pin-track infections remain a major complication. Despite the routine use of
antibiotics, pin-track infections occur in 57% to 75% of
cases1,2.
One factor contributing to the high prevalence of infection is the access that
the pin track provides to bacteria and the ability of bacteria to colonize the
metal surface of the pin or wire. The OrthoGuard AB antimicrobial sleeve
(Smith and Nephew, Memphis, Tennessee) inhibits bacterial colonization on
external fixation pins and wires by providing sustained local delivery of
gentamicin to the pin or wire site.
In order for local antibiotic delivery to be bactericidal, (1) the
antibiotic must be effective against pathogens common to pin-track infections
and (2) the sleeve must deliver an effective dose into the pin track in a
sustained manner.
The purpose of the present study was to evaluate the results of in vitro
testing in order to establish the long-term bactericidal properties of the
OrthoGuard AB antimicrobial sleeve.
The OrthoGuard AB sleeve consists of polyurethane tubing that has a thin
gentamicin-loaded coating on its inner and outer surfaces. After insertion of
the fixation pin or wire, the sleeve is placed over the pin or wire and is
manually pushed through the subcutaneous tissue up to the point of contact
with the bone (Fig. 1). The
sleeve is measured and cut before placement so that at least 1 cm of the
sleeve is protruding from the skin surface in order to allow for swelling and
movement of the skin during the healing process. OrthoGuard AB sleeves are
designed to fit over common pin and wire sizes used for external fixation
(Fig. 2).
The efficacy of gentamicin against pathogens commonly associated with
pin-track infections was determined on the basis of data collected for the
SENTRY Antimicrobial Surveillance Program
(1997-2002)3.
Surveillance data were collated for clinical isolates of Escherichia coli,
Pseudomonas aeruginosa, Staphylococcus aureus, and Staphylococcus
epidermidis from infected surgical wounds. These four pathogens were
chosen because they have been shown to represent 87% of the bacteria that
cause pin-track
infections4,5.
The level of susceptibility of these clinical isolates to gentamicin and their
respective minimum inhibitory concentrations (MIC) are reported in
Table I. The MIC breakpoint has
been established by the National Committee for Clinical Laboratory Standards
(NCCLS) to be 4 µg/mL for gentamicin against common pin-track pathogens
(that is, gentamicin concentrations of =4 µg/mL will inhibit susceptible
pathogens). The overall susceptibility level for the 1456 clinical isolates
evaluated against gentamicin was 83.1%. Therefore, local gentamicin
concentrations of =4 µg/mL would have inhibited at least 83.1% of these
pathogens isolated from surgical wound infections. It is important to note
that the SENTRY data were based on confirmed surgical wound infections and as
such represent a worst case susceptibility population of bacteria. The
surveillance data therefore demonstrate that gentamicin has a high level of
clinical efficacy against pathogens commonly associated with pin-track
infections.
Zone of inhibition testing is a standard method used to determine the
sensitivity of bacteria to an antibiotic. Agar plates are first seeded with
bacteria. The test antibiotic on a filter disc is then added to the plate.
During an incubation period of twenty-four hours, the antibiotic diffuses into
the bacteria-seeded agar. Inhibition of bacterial growth is evident as a clear
region surrounding the disc.
Zone of inhibition testing of twelve OrthoGuard AB sleeves was performed
after the sleeves had been soaked in saline solution at 37°C for two hours
and for two, six, and fifteen weeks. At each time-point, a 5-mm length of
sleeve was placed over a stainless steel pin and this construct was then set
directly on a plate seeded with Staphylococcus epidermidis. The zone
produced by each sample construct was compared with the zone produced by a
control disc containing 10 µg of gentamicin.
Figure 3 plots the size of the
zone produced by the sleeve as a percentage of the zone produced by the
control disc at each time-point. Examples of the zone of inhibition around a
test pin-sleeve construct and around a control disc are shown in
Figure 4. These data
demonstrated bactericidal activity throughout the fifteen weeks of simulated
use and helped to establish the long-term efficacy of the OrthoGuard AB
sleeve.
Elution testing of six OrthoGuard AB antimicrobial sleeves was performed by
measuring gentamicin release at various time-points while the sleeves were
agitated in saline solution at 37°C. The solution was removed and replaced
at two, twenty-four, forty-eight, and seventy-two hours and once a week
thereafter for twenty-six weeks. The amount of gentamicin in each solution was
then measured with use of a standard microbiological assay. Gentamicin
concentrations that would be present in the pin track were then estimated with
use of theoretical pin-track volumes.
Figure 5 shows the gentamicin
concentration (expressed in µg/mL) at the time-points between one week and
twenty-six weeks. Note that, after ten weeks, the solution was removed and
replaced every week but the gentamicin content was only measured at the
fifteen, twenty, and twenty-six-week time-points.
Gentamicin concentrations remained above 4 µg/mL through twenty weeks of
elution, a level that would have inhibited at least 83.1% of the pathogens
collected from infected surgical wounds (as demonstrated by the surveillance
data).
Surveillance data demonstrated that gentamicin will provide high-level
clinical efficacy (83.1% susceptibility) against pathogens commonly associated
with pin-track infections.
Zone of inhibition testing demonstrated the bactericidal activity of
OrthoGuard AB sleeves throughout a period of fifteen weeks. Elution studies
established that gentamicin release from the sleeves was >4 µg/mL (the
susceptibility threshold) throughout twenty weeks.
The mean pin-track concentration at one week was 43.3 ± 3.36
µg/mL. Such a high concentration of gentamicin delivered directly into the
pin track can only be achieved by local release from an OrthoGuard AB sleeve.
Oral and intravenous administration of gentamicin is limited by the maximum
safe-serum concentration (10 to 12 µg/mL). In situ, the OrthoGuard AB
sleeve will infuse the surrounding tissue with gentamicin, creating an
effective barrier against bacterial colonization. The initial burst and
subsequent long-term sustained release of gentamicin from the OrthoGuard AB
sleeve should inhibit bacterial colonization.
The present study establishes the long-term antibacterial properties of the
OrthoGuard AB sleeve. OrthoGuard AB sleeves can be used to inhibit bacterial
colonization on external fixation pins and wires.
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