Investigation of the Optimal Pressure for Skin Grafting Using Negative Pressure Wound Therapy

Abstract

Background:

Recent studies suggest that negative pressure wound therapy (NPWT) is more effective than tie-over bolsters for skin graft fixation, improving graft take rates and reducing operation times. However, the relationship between negative pressure and compression pressure remains unclear. This study aims to explore that relationship.

Methods:

Study 1 investigated compression pressure at the thigh, anterior tibia, and posterior lower leg under varying negative pressure settings. Study 2 measured compression pressure at the anterior ankle joint and knee joint during flexion. Study 3 compared compression pressure between sheet skin grafts and grafts with drain holes.

Results:

Compression pressure did not strongly correlate with increasing negative pressure at body sites. At the dorsiflexed ankle joint, compression pressure decreased despite increased negative pressure, whereas at the knee joint, pressure increased with the flexion angle. In the sheet skin graft model with continuous sutures, compression pressure rose in response to negative pressure. Additionally, when a drain hole was created, a compression pressure of 20 mm Hg was achieved at a negative pressure of −25 mm Hg.

Conclusions:

The effectiveness of NPWT in skin grafting seems to result from adherence rather than direct compression because air is released between the graft and wound surface. The optimal negative pressure setting varied by graft type: 125 mm Hg for sheet grafts and 25 mm Hg for grafts with drain holes. These findings underscore the importance of adjusting negative pressure based on graft type and anatomical location to maximize the effectiveness of NPWT in skin grafting.

Takeaways

Question: What is the relationship between the negative pressure wound therapy setting and the compression pressure on the skin?

Findings: There is no correlation between increased negative pressure and compression pressure on the skin. When the sheet skin graft model with continuous sutures and a drain hole was created, a compression pressure of 20 mm Hg was obtained at a negative pressure of −25 mm Hg.

Meaning: Crimping can be obtained by creating drain holes and densely fixing the skin grafts, enabling effective compression fixation when using negative pressure wound therapy.

INTRODUCTION

Recently, the use of negative pressure wound therapy (NPWT) in skin grafting has been found to be more effective than conventional tie-over bolsters in terms of the graft take rate, reoperation rates, operation time, and postoperative pain.^1–4 In addition, when NPWT is used to fixate skin grafts in joints, the graft can take without the need for a splint.⁵

Although the optimal pressure for a conventional tie-over bolster is 20 mm Hg,⁶ the negative pressure used for NPWT has been reported to range from 50 to 125 mm Hg.^7–10 However, the relationship between negative pressure and compression pressure—as well as between compression pressure and the type of skin graft, such as sheet grafts or grafts with drain holes—has not been clarified. In this study, we investigated the relationship between the negative pressure of NPWT and compression pressure and examined the effect of drain holes on compression pressure.

METHODS

This study was approved by the ethical committee of the Medical Research Specialty Group of Osaka Medical and Pharmaceutical University, Osaka, Japan (approval number: 2024-041).

Study 1

To clarify the relationship between the negative pressure setting in NPWT and compression pressure, a compression pressure measuring device (Pico Press; Microlab Elettronica, Veneto, Italy) was placed on the body surface of 5 healthy participants (3 men and 2 women; average age, 36 y; age range, 31–47 y). A small-sized V.A.C. NPWT device (Solventum, Maplewood, MN) was then applied over the Pico Press. The set negative pressure of the V.A.C. and the resulting compression pressure were measured (Fig. 1). The measurement sites were the anterior thigh, anterior tibia, and posterior lower leg, with negative pressure settings ranging from 0 to 200 mm Hg.

Fig. 1.

Study schema. In studies 1 and 2, a Pico Press was placed on the body surface, followed by the application of a V.A.C. form over the Pico Press. The setup was then sealed with a clear occlusive drape.

Study 2

To clarify the relationship between the joint flexion angle and joint compression pressure, the Pico Press and V.A.C. were applied to the dorsiflexed ankle joint and extended knee joint in the same manner as in study 1. Changes in compression pressure were then observed (Fig. 2).

Fig. 2.

In study 2, the V.A.C. and Pico Press were fixed on the dorsiflexed ankle joint and the extended knee joint (arrows), and compression pressure changes due to joint motion were measured.

Study 3

To clarify the relationship between the shape of the skin graft and compression pressure, a 2-mm-thick artificial skin sheet (Marui & Co., Ltd., Osaka, Japan) made of a soft elastomer with the same elastic modulus as human skin was grafted onto the Pico Press, which was placed on an artificial ulcer model. We created drain holes for the skin graft model, each 5 mm long and spaced 5 mm apart, at a density of approximately 3.5 holes per cm². The Pico Press was placed in the center of the ulcer to avoid affecting the shape of the defect. First, a full sheet of the artificial skin model was sutured to the ulcer model using continuous sutures, and the compression pressure was measured. Next, a drain hole was created in the skin graft model, and the compression pressure was measured again (Figs. 3–7).

Fig. 3.

Study schema. In study 3, a Pico Press was placed under artificial skin, then the V.A.C. form was applied over the artificial skin and sealed with a clear occlusive drape.

Fig. 7.

Artificial sheet skin graft with a drain hole.

Fig. 5.

Wound model created on artificial skin made of a soft elastomer with the same elastic modulus as human skin.

Fig. 6.

Artificial sheet skin graft without a drain hole sutured onto the wound model.

RESULTS

Study 1

The average compression pressure and its range at each measurement site due to negative pressure were as follows: thigh, 3.13 (range, 3–6) mm Hg; anterior tibia, 6.73 (range, 5–7) mm Hg; and posterior lower leg, 3.37 (range, 3–7) mm Hg (Fig. 8). The correlation between negative pressure and compression pressure was statistically weak at all sites (Pearson correlation coefficient: thigh = −0.41; anterior tibia = −0.50; posterior lower leg = −0.45).

Fig. 8.

Results of study 1. Line graphs showing the relationship between NPWT settings and compression pressure measured by Pico Press on the thigh (A), anterior tibia (B), and posterior lower leg (C).

Study 2

The maximum pressure at the extensor side of the knee joint was 46.8 mm Hg, observed at 120 degrees of flexion and 0 mm Hg of negative pressure. The minimum pressure was 2 mm Hg, recorded at 0 degrees of flexion and 125 mm Hg of negative pressure. At the dorsiflexed ankle joint, the maximum pressure was 11.8 mm Hg, observed at 45 degrees of flexion and 0 mm Hg of negative pressure. The minimum pressure was −7.6 mm Hg, recorded at −20 degrees of flexion and 125 mm Hg of negative pressure. In the knee joint, there was a correlation between the flexion angle and compression pressure. By contrast, at the dorsiflexed ankle joint, negative pressure was sometimes not transmitted effectively because of the joint flexion angle (Fig. 9).

Fig. 9.

Results of study 2. Line graphs showing the relationship between the joint flexion angle and compression pressure under various NPWT settings for the knee (A) and dorsiflexed ankle joints (B).

Study 3

The average compression pressure was 17.22 (range, 8–24) mm Hg in the sheet implantation model and 34.89 (range, 12–47) mm Hg in the implantation model with drain holes (Fig. 10). Both models showed a statistically strong positive correlation between negative pressure and compression pressure (Pearson correlation coefficient: sheeted = 0.95, with drain holes = 0.95).

Fig. 10.

Results of study 3. Graph showing the relationship between negative pressure and compression pressure in sheet skin graft models with and without drain holes.

DISCUSSION

In previous reports, the optimal NPWT setting for skin graft fixation ranged from 50 to 125 mm Hg. In our study, however, skin compression did not correlate with increased negative pressure in NPWT. Areas with a hard wound surface, such as the anterior tibia, were more susceptible to negative pressure. Furthermore, study 2 revealed that NPWT applied to the dorsiflexion side of the ankle joint did not generate any compression pressure, likely because of the concave shape of the fixation site causing the dressing to lift under suction. These findings suggest that NPWT does not exert compressive force such as a vacuum pack but instead applies continuous suction that creates traction on the skin surface. In study 3, compression pressure increased when a drain hole was created in the skin graft, indicating that graft fixation is not achieved through direct pressure, but through adherence—a process driven by the evacuation of air between the graft and the wound surface. The presence of a drain hole facilitates exudate removal¹¹ and allows for even suction across the wound surface, promoting better adherence and fixation. This is supported by the observation that in sheet grafts without drain holes, compression pressure did not increase despite higher negative pressure settings. The appropriate compression pressure for tie-over dressings has been reported to be 20 mm Hg.⁶ Our findings indicate that to achieve a comparable compression pressure using NPWT, the optimal negative pressure is 25 mm Hg for sheet skin grafts with a drain hole and 125 mm Hg for grafts without a drain hole (Fig. 4). Because the optimal negative pressure setting varies depending on the graft type, it is essential to adjust NPWT settings accordingly.

Fig. 4.

Actual setup of the Pico Press and V.A.C. applied to artificial skin.

LIMITATIONS

The effect of joint movement was not directly evaluated in study 3 because the measurements were conducted using a skin graft model rather than on actual patients. We performed measurements using artificial skin of a fixed thickness, but it would be desirable to measure using actual skin grafts or to measure with different thicknesses of skin grafts.

CONCLUSIONS

NPWT is effective for graft fixation. This study revealed the actual compression pressure at each negative pressure and also demonstrated that adherence between the wound bed and the skin graft is important for fixation. The findings will also be useful for determining negative pressure settings and selecting types of skin grafts. The data that support the findings of this study are available from the corresponding author upon reasonable request.

DISCLOSURES

The authors have no financial interest to declare in relation to the content of this article. The article processing charge was paid for by the authors.