Planimetry is best way to measure wounds

By Liza G. Ovington, PhD, CWS
Ovington & Associates
Ft. Lauderdale, FL

Wound healing often is expressed as a change in area over time. Ongoing measurement of the dimensions of a wound is critical in order to gauge healing progress in response to treatment or to compare healing effects of two different treatments. Payers and regulators also are interested in wound measurements as indicators of treatment efficacy.

Wound measurement also is inherently difficult. Wounds are three-dimensional; they possess area and volume. The most accurate way to assess these dimensions often is not the most convenient for clinical usage. Many of the measurement methods commonly used are not entirely accurate and may not capture the full extent of the healing response.

For example, it is common to measure a wound’s length, its width, and its depth. However, wounds — especially chronic wounds — very rarely have uniform length, width and depth; they have irregular borders and may be shallow in one area and deep in another. They may even possess shelves and tunnels. So where should the measurement be taken for each of three irregular dimensions? General practice is to measure the longest length, widest width, and deepest depth. The location of these sites is rarely marked in any way to allow subsequent measurements to be taken in the same locations.

Tracing wound margins

Even if the measurements were taken in the same place each time, multiplying length and width to determine wound area overestimates the size of the wound unless it is a square or rectangle. A mathematical correction factor can be applied to the area to better approximate wounds that are oval or round.

A more accurate method of determining wound area is tracing of the wound margins onto a clear plastic film. The tracing can then be measured by planimetry to derive the area. If the tracing is made onto a material of uniform density, it also may be cut out and weighed to give an indication of wound area. Finally, the tracing can be superimposed over a grid or graph paper and the squares encompassed by the tracing counted to reflect the area. All of these steps subsequent to the actual tracing of the wound edges require time. Frequently, a tracing is simply compared to its predecessor for a qualitative indication of healing.

Performing and retaining the wound tracing present sterility issues. There are several tracing devices that consist of a double layer of clear, sterile material. Once the material has been placed on the wound for tracing, the layer that contacted the wound can be removed and disposed of, while the top layer containing the tracing is retained for subsequent analysis or placed in the chart. A nonsterile, low-tech version of a tracing device is a plastic sandwich bag, where the side of the bag that touched the wound is simply cut away after the tracing.

Photographs also may be used to determine wound area if the focal distance is uniform from one photo to the next or if a standard area reference unit is included in the photograph. After the wound is photographed, the image can be digitized to reflect the wound area. This method has an advantage in that it requires no wound contact and may show promise as digital cameras and software become more affordable.

Making wound impressions

Wound area is only part of the story in wound measurement. Full-thickness wounds have appreciable depth, and that dimension also must be followed. Also, wounds occur on sites that have surface curvature, such as lower extremities, and this affects volume measurements.

Multiplying length times width times depth to estimate wound volume is problematic. Wounds are more often bowl-shaped as opposed to having walls that are perpendicular to their base. It is possible to get a more exact assessment of wound volume by filling the wound with a material that hardens and can be removed — such as dental impression material (vinyl polysiloxane, alginate gels). Once the mold of the wound is removed, it can then be weighed or used to displace a measured amount of water in a calibrated container to indicate an accurate wound volume. However, such molds are time-consuming to make and may be considered to be invasive. Alginate volume molds are a popular wound measurement method for use in clinical trials, but rarely are used in daily practice.

Wound volumes also can be measured accurately with devices that utilize laser light and cameras to image the wound. Such devices are not yet in mainstream use, but show promise for the future in terms of their potential accuracy and the fact that they require no wound contact.

None of the methods of wound measurement mentioned here adequately address wound undermining or tunneling. Such features must be assessed using gentle, cautious probing of the wound and a linear measuring device. Whatever method of wound measurement is utilized, it is important to maintain consistency from one assessment period to the next.

Commercially Available Products for Wound Measurement

Product Company Information Line
Carrington Rule Carrington Labs (800) 358-5205
Dermassist Wound Measuring Guide Assistec Medical (800) 224-4488
Disposable Measuring Guide Trademark Medical (800) 325-9044
E-Z Graph Wound Assessment System EZGraph of Victoria (800) 975-9528
Measure-it Dumex Medical (800) 463-0106
MediRule, MediRule II Briggs Corp. (800) 247-2343