Date Published: March 5, 2018
Publisher: Public Library of Science
Author(s): Piotr Foltynski, Yih-Kuen Jan.
Wound surface area measurement is important as therapeutic decisions may depend on the change of wound surface area over time. Digital planimetry is a popular technique in wound area measurement. It is accurate and repeatable when calibration is performed with 2 rulers placed at opposite sides of a wound. The aim of the current study was improving accuracy and precision of wound area measurement using capabilities of smart devices.
The correction of area measurement based on calculated camera tilt angle and the calculation of calibration coefficient of linear dimensions as weighted average were proposed. These and other improvements were applied in the Planimator app for Android, which was then used in the study. Accuracy and precision of the Planimator app were compared to the Visitrak device, the SilhouetteMobile device, the AreaMe software, and to the digital planimetry based on 2-ruler calibration with pictures taken by the smartphone, compact, and D-SLR cameras. Areas of 40 wound shapes of area ranged from 0.14 to 31.72 cm2 were measured with each device. Medians of relative errors (REs) were compared in the accuracy tests and standard deviations (SDs) of relative differences (RDs) were compared in the tests of precision.
The median of REs for the Planimator app was not significantly different from the medians of REs for the digital planimetry based on pictures from the compact or D-SLR cameras, but it was significantly lower than the medians of REs for the Visitrak and SilhouetteMobile devices, the AreaMe software and the digital planimetry based on pictures from a smartphone camera. The SD of RDs for the Planimator app was not significantly different from the SDs of RDs for the digital planimetry based on pictures from the compact or D-SLR cameras, but it was significantly lower than the SDs of RDs for the Visitrak and SilhouetteMobile devices, the AreaMe software and the digital planimetry based on pictures from a smartphone camera. The Planimator app installed at a smartphone revealed to be 2-fold more accurate and 1.5-fold more precise than the measurements with using ImageJ software based on pictures taken with the same smartphone.
The Planimator app occurred to have the same accuracy and precision as measurements with digital planimetry with 2-ruler calibration and based on pictures from a compact camera or a D-SLR camera. This app showed better accuracy and precision than the Visitrak and SilhouetteMobile devices, the AreaMe software and the digital planimetry based on pictures from a smartphone camera.
Using a measurement tool in medicine is very important. Ring  states that Carl Wunderlich made in second half of 19th century the greatest progress in medicine by his development of the clinical thermometer and the progression of body temperature could be monitored. Simple measurement of body temperature and creating its graphs gave an opportunity to improve the treatment. To measure a progress of treatment one must have a measurement tool, a start point, and an endpoint. In the case of wound closure the lack of wound, e.g. wound with zero surface area, would be the good endpoint and the start point would be the current surface area of wound. Measurement tool must be reliable and giving similar results under consistent conditions. It is obvious that a more reliable method will enable to make a better diagnosis, which is important for the patient. Therefore it is necessary to use and develop more and more reliable measurement methods. As precision is a synonym for reliability, good measurement method should be precise, but not only. Accuracy is also important, and ideal measurement device is both accurate and precise.
There were many attempts for more accurate and precise wound area measurement using digital planimetry. In some apps one two-dimensional (2D) marker is used for area calibration [14, 15]. In one patent application  the use of 2 or more 2D markers was proposed for area calibration, but an app was not presented. In the case of more than one marker an average area from all markers at the picture was proposed to be used for area calibration. The use of multiple 2D markers may give good accuracy in the case of evenly distributed wounds and when markers are at even distances from wound edges. Moreover, flat 2D markers must lie on the same plane as the measured wound. This is easy to assure on large and flat areas of skin, but it may be problematic on a curved area, where the use of narrow and flexible rulers may bring better measurement accuracy. The use of dedicated calibration markers to an app is of course convenient, because they can be automatically detected in the picture, but the user needs to have a continuous supply of such markers to perform measurements. Thus, the use of standard paper rulers for calibration as in the Planimator app may be a more versatile solution than the use of dedicated markers. Some researchers may try to use the Adobe Photoshop software for wound area measurement with a convenient tool called “magnetic lasso”, which enables fast pixels counting in scanned image of wound outline .