The Impact of Noncontact, Nonthermal, Low-Frequency Ultrasound on Bacterial Counts in Experimental and Chronic Wounds
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Pre- and post-treatment tissue biopsies were obtained by following a specified protocol and sent to an independent central laboratory (LabCorp, Burlington, NC) for quantitative analysis.
Data collection and analysis. Data collection and documentation were performed by investigative site personnel. The study sponsor’s clinical personnel monitored data abstraction, completion of study documentation, and correctness of case report forms. Descriptive statistics were performed to summarize and compare baseline data with outcomes following 2 weeks of noncontact ultrasound therapy. Bacteria quantities were summarized in CFU/g of tissue.
Depth of penetration. Nile red dye penetrated into the intact porcine skin samples 2.0 mm to 2.5 mm with noncontact ultrasound therapy compared with 0.05 mm to 0.07 mm with the sham therapy. The majority of the dye was concentrated in the epidermis and stratum corneum for both treatments. In wounded skin samples, Nile red dye penetrated 3 mm to 3.5 mm with noncontact ultrasound therapy and 0.35 mm to 0.50 mm with sham therapy. With noncontact ultrasound therapy, the dye penetrated into the reticular dermis but not into the underlying subcutaneous fat (see Table 1).
In vitro bacteria reduction. Following the noncontact ultrasound treatment, the percentage of dead bacteria was 33%, 40%, and 27% for P. aeruginosa, E. coli, and E. faecalis, respectively (see Figures 2, 3a,b). The sham treatment resulted in no dead organisms (0%). Using this treatment protocol, noncontact ultrasound had little or no effect on MRSA (1% increase of live bacteria) and S. aureus (0% change).
Scanning electron microscopic images of E. faecalis showed structural changes to the round shape of the bacteria with distinct cell wall punctures or wall destruction after noncontact ultrasound treatment compared with intact cell walls in sham-treated controls (see Figure 4).
In vivo bacteria reduction. Histologically, no differences were noted between the noncontact ultrasound and sham treatment groups with regard to edema, granulation tissue formation, or the presence of eschar. All animals remained healthy and gained weight during the 7-day study. Both the noncontact ultrasound therapy and silver antimicrobial dressing resulted in an overall reduction of bacterial counts (see Table 2). Overall bacterial colony counts increased consistently over time in the moist control group; whereas, bacterial counts in sham therapy-treated wounds decreased or increased at various treatment times. However, the patterns for P. aeruginosa and S. aureus were different. Although noncontact ultrasound therapy resulted in reduced bacterial counts of both organisms during days 3 to 7, during that time, bacterial counts of the Gram-negative bacteria P. aeruginosa changed from 8 ± 0.73 to 5.8 ± 0.74 in the ultrasound, and from 5.7 ± 2. to 4.7 ± 0.85 in the silver antimicrobial dressing group. Given the small sample sizes (as low as n = 2), statistical comparisons between treatment groups were not performed.
Clinical bacteria reduction study. Of the 18 patients with Stage III pressure wounds enrolled between November 2006 and April 2007, 11 completed baseline and post-treatment biopsies and were considered evaluable for the effectiveness analysis.
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