Outcomes of Subatmospheric Pressure Dressing Therapy on Wounds of the Diabetic Foot

   Diabetic foot wounds commonly proceed to lower extremity amputation. They have been identified as the key component in nearly nine in 10 of these ablative procedures.¹ Clearly, healing these wounds in as rapidly a manner as possible may result in a substantial reduction in morbidity.

   One recently introduced method used to heal defects of the skin involves topical subatmospheric pressure dressing (SPD) therapy. This modality is most commonly delivered to the wound by way of an open-cell foam dressing covered with a clear membrane over the wound. The dressing is attached to a pump that provides either intermittent or continuous suction with a standard setting of 125 mm Hg.^2-8 Several studies have reported varying levels of success in different categories of wounds in both human and animal models.^9-22 Although one randomized trial of 10 patients with diabetic foot wound exists,⁵ the authors are unaware of any other studies in the peer-reviewed medical literature that have specifically assessed outcomes (by way of primary data) associated with this type of therapy in this very high risk population. Therefore, this preliminary study was conducted to evaluate outcomes associated with application of SPD therapy on diabetic foot wounds.

Materials and Methods

   Data were collected from 31 consecutive patients with diabetes (see Table 1), 77.4% male, aged 56.1 ± 11.7 years presenting for care at two large, multidisciplinary, referral-based wound care centers. All patients had a previous diagnosis of diabetes mellitus as assessed by their attending physician. All patients received surgical debridement for indolent diabetic foot wounds and were subsequently placed on SPD therapy delivered using VAC devices (Vacuum Assisted Closure, KCI, Inc., San Antonio, Texas, USA). Wounds tended to be large (27.9 ± 19.5 cm²), as they were evaluated after the aforementioned surgical debridement. Data were collected from patients over a 1-year period. Retrospective collection of these data was determined to be exempt from consent policy according to the governing institutional review board.

   All wounds were described and classified using the University of Texas diabetic foot wound classification system.²³ The prevalence of University of Texas grade 1, 2, and 3 lesions in this population was 3.2%, 45.2%, and 51.6%, respectively. The foot wounds were located in the digit/ray, transmetatarsal, midfoot, and heel (see Table 2). Although all wounds were initially infected, all were converted to stage "A" (noninfected, nonischemic) before device application, using a combination of surgical debridement and culture-directed systemic antibiotic therapy. Outcomes evaluated included time to complete wound closure, proportion of patients achieving wound healing at the level of initial debridement, and complications associated with use of the device. The mean duration of wounds before therapy was 25.4 ± 23.8 weeks. Following sharp debridement, patients were treated with SPD therapy at 125 mm Hg using a protocol that called for consideration for cessation of therapy when the consistency of the wound bed approached 100% granulation tissue with no overtly exposed tendon, joint capsule, or bone. Subatmospheric pressure dressing therapy dressings were changed every 48 hours.

   All descriptive data are described as mean ± standard deviation. The authors used Pearson's test to evaluate correlation between continuous variables such as time to healing based on wound duration. To compare differences in time to healing by gender, a Student's t-test for independent samples was used. For all analyses, a significance (alpha) level of 0.05 was used.²⁴

Results

   Subatmospheric pressure dressing therapy was used for a mean 4.7 ± 4.2 weeks (mode = 2 weeks) until the wound bed approached 100% granular tissue (see Figure 1). No significant difference in age, gender, duration of wounds, or time to healing between centers was noted. In total, 90.3% (n = 28) of wounds healed at the level of debridement without the need for further bony resection in a mean 8.1 ± 5.5 weeks. The remaining 9.7% (n = 3) went on to higher level amputation (below knee amputation = 3.2% [n = 1] and transmetatarsal amputation = 6.5% [n =2]). Complications included periwound maceration (19.4% [n = 6]), periwound cellulitis (3.2% [n = 1]), and deep space infection (3.2% [n = 1]). In this small sample, no significant difference in healing time (P = 0.2) or proportion of patients healing (P = 0.6) based on gender was noted. Likewise, no significant association was reported between age (P = 0.4) or duration of the wound (P = 0.9) and time to wound healing (P=0.4). Similarly, the authors could detect no association between wound size and time to healing (P = 0.5).

Discussion

   The use of SPD therapy has been proposed as a novel method of manipulating the chronic wound environment in a way that potentially reduces bacterial burden and chronic interstitial wound fluid, increases vascularity and cytokine expression, and, to an extent, mechanically exploits the viscoelasticity of periwound tissues.^2,4,25-28 Although SPD therapy has appeared anecdotally promising, only one small randomized trial evaluating it in treatment of diabetic foot wounds can be found in the literature. McCallon et al⁵ enrolled 10 patients with diabetes and concomitant foot ulceration. Patients in this study were evenly randomized (by way of an alternating randomization technique) to either SPD therapy or to a control group (twice daily dressing changes with saline-moistened gauze). At the conclusion of the trial, the authors reported a difference in time to healing in patients receiving SPD therapy compared with standard therapy (23 ± 17 vs. 43 ± 32 days). No statistical analysis was performed on this admittedly underpowered sample. Most of the patients in the SPD group received definitive closure by way of delayed primary closure (4 out of 5 SPD versus 2 out of 5 control). Although this study may have had identifiable issues related to selection and performance bias between groups (as no readily identifiable method of randomization occurred), it did suggest a guardedly optimistic assessment of this treatment regime. Further works in this area may help to substantiate the potential of SPD as noted in the McCallon study.

   In the authors' study, which intended to provide pilot data for future randomized controlled trials, valuable experience was gained in determining how this technology might be best used in the diabetic foot. It has become the authors' general protocol to treat patients with SPD therapy until they reach coverage of vital periarticular, tendinous, or osseous structures and/or a 100% granular base. The clinician may then choose to employ other wound healing modalities to move the wound toward complete epithelialization. This is reflected in the relatively short amount of time the device was used in this study. The protocol used most frequently called for SPD therapy for 1 to 3 weeks (see Figure 1) to achieve the aforementioned goals. It also should be noted that the patients in this study had proportionately much larger wounds than those in the McCallon trial, largely because the centers participating in the study utilized SPD therapy following a protocol of aggressive surgical debridement of chronic or infected diabetic foot wounds.

Limitations

   This was a retrospective study and, as such, could not control for the myriad variables that could confound many conclusions. For example, the authors were unable to control for vascular status, degree of glucose control, nutritional status, activity level, or numerous other potential variables that could be important in assessing overall results. However, the objective of this study was to report outcomes and experiences using this treatment at busy diabetic foot clinics, not to provide evidence to show SPD is more (or less) effective than other modalities.

Conclusion

   This study provides additional pilot data to support, in a guarded sense, the judicious use of subatmospheric pressure dressing therapy in the diabetic foot. The results of this study suggest that appropriate use of subatmospheric pressure dressing therapy to achieve a rapid granular base in diabetic foot wounds may show promise in treating this population of patients who are at high risk for lower extremity amputation. To the authors' knowledge, this is the largest study to date to evaluate vacuum therapy in this specific wound category. The study will provide pilot data for future large, multicenter, randomized controlled trials of this wound healing modality to further clarify its utility in treating indolent diabetic foot wounds. Ultimately, this may yield improved results in therapy and a reduction in the unnecessarily high prevalence of lower extremity amputations in the developed and developing world.

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