A Comparison of Computer-Assisted and Manual Wound Size Measurement

   The ability to accurately and precisely measure the size of a wound is critical in documenting the progress to healing and in assessing the effectiveness of interventions on the healing process in clinical and research settings.^1-3 Several techniques are available for documenting wound size, including measuring the dimensions of a wound using a disposable ruler.

Ruler-based assessments of wound size have good reliability.⁴ However, deciding which dimensions to measure if the wound is irregular can be difficult.⁵ Furthermore, this ruler-based technique tends to overestimate the actual size of the wound, and the reliability of this technique decreases as wound size increases.⁶

   Tracing the wound edges onto a transparency is also a popular method of assessing wound size. The tracings can be placed onto metric grid paper and the number of square millimeters counted to document the surface area of the wound.⁷ This process is time consuming, especially if the wound is large. The tracing also can be cut out and weighed on a precision scale.⁸ This method may be faster than the square counting method, but the second transfer and cutting out the shape of the wound reduces the accuracy of the technique.^8,9 Tracing the edge of a wound onto a transparency and using an electronic or computerized device such as a planimeter to quantify the surface area of the wound appears to be a popular and practical method for assessing wound surface area and has been utilized and studied extensively.^8,10,11 Excellent interrater and intrarater reliability has been reported with this manual technique.^7,12 Inexpensive and convenient to use, it requires minimal training and provides readily available results. Potential disadvantages include: difficulty identifying the edge of a wound,^8,13 inaccurately tracing wounds in the presence of a skin fold, or, when conforming the transparency over the wound, causing a deformation of wound shape and size.¹⁴ The reliability of this manual technique lessens with decreasing wound size.⁶

   Stereophotogrammetry (SPG) refers to computer-assisted measurement of digital and photographic images, including images of wounds. Stereophotogrammetry utilizes two cameras to reconstruct a three-dimensional image from which the surface area and volume of a wound can be determined.¹⁴ This three-dimensional technique is time consuming, expensive, and requires specialized training, making it less than ideal for routine clinical use. Three studies have examined the reliability and precision of SPG for measurement of wound dimensions. Bulstrode et al,¹ using artificial leg ulcers, found less than 1% difference between the known surface area of the wound and surface area measurements obtained using SPG. Frantz and Johnson¹⁵ reported Pearson correlation coefficients ranging from 0.96 to 0.99, indicating excellent interrater reliability for determining surface area, circumference, depth, and volume of pressure ulcers using SPG.

   These results are similar to those reported by Eriksson et al¹⁶ who used SPG to determine the dimensions of venous stasis ulcers.

   A variation of this method, referred to as the computerized technique, utilizes a single digital camera and customized computer software. This new computerized technique is noninvasive and provides documentation of serial images of a wound for both determining wound dimensions and visual characteristics. The technique requires less training and is considerably less expensive and time consuming than SPG. In contrast to most manual wound assessment techniques, the computerized technique does not involve wound bed contact. The computerized technique used in this study utilizes a digital video camera, a frame grabber, and a computer that houses wound assessment software (VeV Measurement Documentation, Vista Medical, Winnipeg, Manitoba, Canada). Using this system, a digital image of the wound is obtained with a target plate placed in the field of view of the camera. The computer software uses photogrammetry to analyze the target plate and calibrate the system, correcting for any distortion imparted to the digital image of the wound from the curvature of the lens. Langemo et al,¹⁷ using models of wounds constructed from plaster of Paris, found that determining the surface area of the models using the computerized system was more accurate and less biased than using the manual technique, ruler-based measurement, or computerized determination of wound length and width.

   Even though this computerized technique has been validated using wound models, its validity and reliability in a more realistic, clinical and pre-clinical setting is unknown. Therefore, the overall objective of this study was to assess the validity and reliability of this new computerized technique when used with chronic human wounds in a clinical setting and with full- thickness excisional wounds in laboratory animals. Four questions were addressed in the study:
  1. What is the intrarater and interrater reliability of the new computerized technique when assessing the surface area of human wounds?
  2. Is this new computerized technique reliable and valid when measuring small wounds such as those found on animals in laboratory experiments?
  3. Is the intrarater and interrater reliability of this new computerized technique comparable to the reliability of the well-established manual technique?
  4. Can the precision of each technique improve when the average of three repeated measurements of surface area is used compared with only single measurements of surface area?

Methods

   Human subjects. Study participants included outpatients (N = 45) of the Parkwood Hospital Wound Management Clinic (St. Joseph's Health Care, London, Ontario, Canada). Study approval was obtained from the Review Board for Health Sciences Research Involving Human Subjects at the University of Western Ontario and from the Research Committee at Parkwood Hospital (St. Joseph's Health Care, London, Ontario, Canada). All wounds were located in the lower extremities and varied in size (0.99 cm² to 18.80 cm²) and etiology (arterial, n = 3; decubitus, n = 14; neuropathic, n = 14; venous, n = 14). Wounds that could not be captured in their entirety with a single digital image (eg, those involving or wrapping around more than one surface of the lower extremity, ankle, or foot) were excluded from the study. Assessment of wound surface area was completed after receiving written consent from each patient and with the understanding that their identities would be concealed at all times.

   Animal. The wounds (0.10 cm² to 2.20 cm²) of 38 CD-1 male mice were randomly chosen from an on-going, unrelated laboratory experiment. Assessment of wound size using the manual and computerized techniques was completed as required by the study protocol of the experiment. Only the assessment data obtained was used for this study. The mice were administered general anesthesia (acepromazine, 0.05 mL/10 g of body weight; ketamine, 0.75 mL/10 g of body weight) before surgical placement of full-thickness excisional wounds dorsally between the shoulder blades using previously established techniques.^18,19 Assessment of the surface area of each animal wound was completed immediately after surgery (n = 19) and 10 days following surgery when all the animals were sacrificed (n = 19). Over the 10-day experimental period, animals received morphine (2.5 mg/kg of body weight) to assist with pain management. All experimental procedures were in accordance with guidelines of the Canadian Council for Animal Care and were approved by the University Council on Animal Care - Animal Use Subcommittee at the University of Western Ontario.

   Manual wound measurement technique. Three tracings of each human and animal wound were made during the assessment session. A transparency (EZ GRAPH; Victoria, Tex.) was placed directly over the wound, and the edge of the wound was traced with an indelible fine-tipped marker. The orientation of the wound and the position of the patient at the time of the tracing were recorded. Care was taken to avoid movement of the transparency and exertion of pressure over the wounds in order to avoid distortion. All wounds were debrided regularly to aid healing; it was observed that regular debridement assisted in identifying the margins of the wound. Because the greatest source of error in tracing wounds may be in the tracing itself,⁸ a single assessor completed all tracings.

   A Planix 7 Tamaya Digital Planimeter (Koizuni Sokki Manufacturing Company, Ltd., Nagaoka-Shi, Japan) was used to calculate the surface area (cm²) from the three tracings of each wound by tracing the outlined wound edge with the tip of the digital planimeter. According to the manufacturer, the planimeter has an accuracy of ± 0.2% and a resolution of 0.1cm². Two assessors digitized tracings in random order.

   Computerized wound measurement technique. Digital images of both chronic human and postsurgical animal wounds were obtained by one assessor during a single assessment session using a SHARP View Cam (Model VL-H850U, Sharp Corporation, Osaka, Japan.) analog video recording device. The video camera permits recordings of a still image and was equipped with a quarter-inch CCD image sensor (with approximately 410,000 pixels), a 16X zoom lens (F1.4, f = 4.0 to 64.0 mm), and a full-color LCD (TFT active matrix) monitor. A target plate (VeV Measurement Documentation. Vista Medical, Winnipeg, Manitoba, Canada) of appropriate size to the wound was positioned adjacent to, and in the plane of, the wound before capturing the image. Lighting conditions were optimized to reduce glare and shadows and increase image contrast. The distance between the camera and the wound was varied in order to provide an image that captured the entire wound, a sample of the periulcer skin, and the target plate. Captured digital images were transferred in the Transfer Image File Format (TIFF) to retain maximum image quality using a frame grabber (MRT 801 PCMCIA Type III, MRT, Norway) onto a laptop (DELL, Inspiron, Austin, Tex.) that housed the software (VeV Measurement Documentation Vista Medical, Winnipeg, Manitoba, Canada) used to assess the surface area of the wounds. To be able to use the methodology described in the present study, the laptop must meet the following minimal hardware requirements: a Pentium I Processor, 32 MB RAM, and 2 MB hard drive.

   Each digital image (39 pixels/cm; 24-bit color) was visualized on the screen (14 inch; 800 X 600 pixels) of the laptop housing the assessment software and a computer-pointing device (Microsoft USM IntelliMouse Inspiron, Woodland Hill, Calif.) was used to identify successive points located on the edge of the wound. The assessors who used the manual technique also traced each wound three times in a random order. The software applies photogrammetric techniques, allowing interpretation of the digitized image; thereby, producing measurements of the dimensions of the wound such as surface area, perimeter, length, and width. Because accuracy is affected by the relative size of the target plate to the wound, for maximal accuracy the manufacturer recommends using a target plate that approximates the size of the wound. Accuracy also is increased by choosing successive points located close to each other along the edge of the wound.

Data Analysis

   Several investigators have utilized the Pearson product-moment correlation coefficient to assess concurrent validity between different wound size assessment techniques.^1,10,20 However, several problems exist with this approach. First, correlation does not reflect agreement, only consistency.²¹ Clinically, the assessment of concurrent validity seeks to delineate the agreement between, rather than the consistency of, two measurement techniques, such as the computerized and manual techniques. Further, the correlation coefficient is not a true reliability coefficient because it is unable to separate the variance due to error and true variance, the latter being the variance shared by the measurements obtained using the two techniques.²¹ Because measurement error can adversely affect the analysis and interpretation of data, having a tool that readily quantifies the measurement error is important. Lastly, high correlation coefficients may actually mask poor agreement between two measurement techniques because these correlation coefficients reflect covariance or rank order characteristics within the data, which does not equate to a high degree of agreement between repeated measurements.²¹ The intraclass correlation coefficient (ICC) is a reliability coefficient that is calculated using variance estimates obtained through an analysis of variance (ANOVA).²¹ Therefore, the ICC reflects both the degree of correspondence and agreement among two or more measurement techniques, assessors, or ratings,²¹ as well as the extent to which two measurement techniques, assessors, or ratings are interchangeable.²² Shrout and Fleiss²³ describe three models of the ICC distinguished according to how the raters are chosen and assigned to the subjects. Each of the three ICC models can be expressed in two forms, producing six types of ICCs, depending on whether the scores are single or mean ratings.²¹ The six types of ICCs are classified using two numbers in parentheses. The first number designates the model (1, 2, or 3), and the second number signifies the form, using either a single measurement (1) or the mean of several measurements (k).

   Reliability. The ICC was used to determine the degree to which 1) a single rater agreed on measurements of surface area obtained from the same wounds on two occasions (intrarater reliability), and 2) two different raters agreed on measurements of surface area obtained on the same wounds on a single occasion (interrater reliability). For the present study, ICCs (3,1) and (3,2) were used to assess the intrarater reliability of each measurement technique when single and the average of three repeated measurements of surface area were used respectively.²³ The intraclass correlation coefficients (2,2) and (2,1) were used to assess the interrater reliability of each measurement technique when a single or the average of three repeated measurements of surface area was used respectively.

   Precision. Standard errors of measurement (SEMs) also were determined to quantify the error with which surface area could be measured using either technique²⁴; thereby, providing an indication of the imprecision of each measurement technique. Smaller values of the SEM reflect more precise measurements.

   Concurrent validity. To assess the concurrent validity between measurements of wound size obtained using the manual and computerized techniques, or to demonstrate whether the computerized technique is interchangeable with the well-established manual technique when used by equally trained assessors, ICCs (2,1) and (2,2) were used when single or the average of three repeated measurements from each technique was used respectively.^21,23 A repeated measures one-way ANOVA was used to obtain the variance components for calculating each ICC.

   The ICCs were interpreted as follows: excellent (0.75 to 1.00), modest (0.40 to 0.74), or poor (0.00 to 0.39).²¹ All statistical analyses were completed using SPSS for Windows (Release 10.0.7; SPSS Inc. Headquarters. Chicago, Ill.).

Results

   The mean, range, and standard deviation of the surface area of both human and animal wounds assessed using both techniques are presented in Table 1. In both human and animal wounds, the measurements of surface area obtained using the manual technique were significantly greater than those obtained by using the computerized technique (P < 0.05).

   Single surface area measurement of both human and animal wounds using either technique produced excellent intrarater reliability (ICC (3,1) > 0.75) (see Table 2). Although both techniques demonstrated greater precision, indicated by lower SEMs, when measuring the surface area of animal wounds, the computerized technique was more precise than the manual technique.
Interrater reliability was excellent (ICC (2,1) > 0.75) for surface area measurement of both human and animal wounds, using either technique. However, the best interrater reliability and most precise measurements were observed when measuring the smaller animal wounds using the computerized technique (see Table 3). Interrater reliability of each measurement technique for both human and animal wounds improved when the average of three repeated measurements of surface area was considered (see Table 3). The precision of each technique with human wounds and the precision of the manual technique with animal wounds also improved with the average of three repeated measurements of surface area, demonstrated by smaller values of the SEM (see Table 3).

   Concurrent validity between the manual and computerized techniques using both single and the average of three repeated measurements was excellent when assessing the surface area of human wounds (ICC (2,1) and ICC (2,2) > 0.75) (see Table 4). Poor (ICC < 0.39) concurrent validity was obtained for the smaller animal wounds using either the single or average of repeated measurement results.

Discussion

   In this study, both the computerized and manual techniques were reliable measurement techniques for assessing the surface area of human and animal wounds. These results are in agreement with previous studies, which report excellent intrarater and interrater reliability for the manual technique^7,12 for assessing human wounds and excellent interrater reliability of the computerized technique when assessing constructed wound models.¹⁷ Previous results indicate that, compared with manual technique, the computerized technique is more accurate and less biased when measuring rigid plaster of Paris wound models.¹⁷ Such models are not representative of clinical practice where the influence of patient positioning and resultant distortions to the wound would affect measurements.¹⁷ In this study, both techniques were found to be equally reliable and precise when only single measurements of the surface area of human wounds were considered, indicated by the similar ICCs and SEMs. The computerized technique was found to be more reliable and precise than the manual technique for the measurement of the surface area of the smaller animal wounds. The precision of both techniques was improved by using the average of three repeated measurements of surface area compared to a single measurement of surface area of both human and animal wounds.

    Excellent concurrent validity, indicated by high ICC values, was observed between measurements of the surface area of human wounds obtained using the two techniques. This suggests that the computerized technique is as good as the well-established manual technique for assessing the surface area of human wounds. However, concurrent validity between measurements of the surface area of the smaller animal wounds obtained using the two technique was poor, as indicated by low ICC values.

   Two plausible reasons for obtaining low ICCs must be considered before concluding that the low ICC value obtained between the two techniques when measuring the surface area of smaller animal wounds truly indicates a lack of agreement between measurements. An "interaction" may exist between the techniques used by the assessors and the subjects (wounds).²¹ For example, the results indicate the manual technique consistently rendered measurements of the surface area of the smaller animal wounds that were greater compared to those obtained using the computerized technique; this difference was statistically significant. This tendency of the manual technique to overestimate surface area of the smaller animal wounds was not evident when the surface area of human wounds was determined with either technique and was likely due to the inherent difficulty associated with assessing tracings obtained from the smaller animal wounds with a planimeter.^8,25,26

   The second reason a low ICC may be obtained is when samples with limited variability are used.²¹ Looking for significance of the between-subjects variance in the one-way ANOVA can check for this effect.²¹ If the measurements are similar, this source of variance will not be significant. In this case, with both single measurements and the average of three repeated measurements, the between-subjects variance was not significantly different. The low ICC value has been attributed to the small range of surface area measurements obtained from the smaller animal wounds and to interrater interactions where measurements of surface area from the manual technique were consistently greater than those obtained using the computerized technique.

   Clinical relevance. The results of this study suggest that a trained assessor can use either the manual or computerized technique and that a single measurement of surface area using the protocol adopted for the present study is sufficient. Multiple assessors commonly document the surface area of wounds in a clinical setting, but reliability is improved if the same assessor completes all assessments over time using either technique. In the case of less experienced assessors, reliability and precision may be improved by using the average of three repeated measurements. The computerized technique may be indicated for use with smaller wounds, such as those typically encountered in laboratory experiments, and in situations where the results do not have to be readily available. Ideally, a single user should complete all assessments using the computerized system - a scenario that is most likely to occur in a research setting where one investigator is specifically trained to complete assessments according to a predetermined protocol.

   Implications for future research. Studies to examine the effects of different types of wounds on the reliability and concurrent validity between clinical methods used to assess the size of wounds are needed. Also, comparisons between the new computerized technique and commonly used measurements of wound length and width using a ruler and standard metric scale should be made. The effect of inexperienced assessors on the reliability and concurrent validity of the two techniques also has not been determined.

Conclusion

   Evaluation of the surface area of a wound is a predictor of the healing process and provides a baseline measurement for comparison following the administration of an intervention. The results herein indicate that the computerized technique studied is as reliable and precise as the manual technique for measuring the surface area of human wounds and is more reliable and precise for assessing the surface area of smaller animal wounds. Although the computerized technique is more costly and the results may not be readily available, the technique does not require contact between the digital camera and the wound, a desirable feature when pain, contamination, and damage to the wound bed are a concern. This computerized technique eliminates the difficult task of trying to replicate each time the distance and angle at which a digital image of the wound is obtained.¹⁷ The manual technique is considerably cheaper, and if a planimeter is available in the clinical setting, measurements can quickly be incorporated into the patient's records. However, the manual technique requires the transparency to come into contact with the wound, fostering a risk of contamination, pain, and damage to the wound bed. Ultimately, the decision to use a manual or computerized technique in a clinical or a research setting should be based on patient, practicality, and economic considerations.

Acknowledgments

   The authors wish to thank the support personnel at the Outpatient Wound Management Clinic (Parkwood Hospital, St. Joseph's Health Care, London, Ontario, Canada) for their assistance with the recruitment of subjects. The primary author acknowledges the financial support received from the Ministry of Training and Education in the form of a Province of Ontario Graduate Scholarship and from the Physiotherapy Foundation of Canada in the form of an Ann Collins Whitmore Memorial Award.

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