Introduction

Pressure ulcers are a common complication of spinal cord injury (SCI), with up to 95% of people experiencing at least one pressure ulcer during their lives.1, 2 Pressure ulcers affect quality of life and participation. They delay rehabilitation and can lead to contractures, scarring, deformities, osteomyelitis, loss of limb and sepsis.3 In addition, they affect a person’s family life and are costly and difficult to manage. Pressure ulcers can also be life-threatening, particularly in low-income and middle-income countries.4

Clinical assessment is an important aspect of effective pressure ulcer management. Most guidelines5, 6, 7 recommend weekly assessment using a standardised assessment scale that captures many different features of a pressure ulcer. One feature is the extent to which a pressure ulcer is undermined. Undermining is the region underneath the overlying loose skin around a pressure ulcer (see Figure 1). It is an important feature of a pressure ulcer because it reflects the real size of a pressure ulcer. For example, a pressure ulcer with a small surface area may have extensive undermining. Therefore, sole reliance on the surface area may underestimate the severity of a pressure ulcer. The presence of undermining is also important to capture because undermining can mask the extent of exudate and the type of tissue at the floor of the pressure ulcer.

Figure 1
figure 1

Pressure ulcer with undermining (two-dimensional view).

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Measurements of wound undermining are commonly used in clinical practice and form part of the Sussman Wound Healing Tool8 and the Bates–Jensen Wound Assessment Tool (previously known as Pressure Sore Status Tool).9 There are different techniques to measure undermining but the most widely used is that described by Sussman et al. in 1991. It involves measuring wound undermining at four cardinal points of the clock, namely, 12 O'clock, 3 O'clock, 6 O'clock and 9 O'clock.8 It is appealing because it is simple and has face validity. In addition, it can be used for research purposes because the sum of the four measures provides a single continuous value. However, surprisingly, there are no data on its reliability. Therefore, the purpose of this study was to determine the reliability of measuring wound undermining, using the four cardinal points in people with SCI.

Materials and methods

The inter-rater reliability and intra-rater reliability of the wound undermining measure were determined.

Participants

Thirty inpatients and outpatients were recruited from the Indian Spinal Injuries Centre, New Delhi, India. Participants were included if they had a complete or incomplete traumatic or non-traumatic SCI, and a pressure ulcer with wound undermining. Participants were excluded if they had a clinically significant or unstable medical condition including psychiatric, behavioural or terminal illness. We certify that all applicable institutional and governmental regulations concerning the ethical use of human volunteers were followed during the course of this research. All participants gave consent to participate.

Procedure and data analysis

Demographic and clinical data to describe the population were collected. This included the American Spinal Injury Association Impairment Scale and neurological level of injury according to the International Standards for Neurological Classification of Spinal Cord Injury.10 In addition, each pressure ulcer was scored on the Pressure Ulcer Scale for Healing (PUSH) version 3.0.11 The PUSH tool version 3.0 rates pressure ulcers according to the surface area of the pressure ulcer, amount of exudate and type of tissue damage. The total scores range from 0 to 17 points, with higher scores indicating a severe pressure ulcer and a score of zero representing no pressure ulcer.12 The original plan was to correlate the undermining scores with the PUSH to provide some evidence about the validity of the undermining measurements. However, this aspect of the study was subsequently abandoned when it became clear that the PUSH was not a reasonable ‘gold standard’ for undermining. We, however, report the PUSH data and its correlation with the undermining scores.

Measuring wound undermining

Wound undermining was measured at four cardinal points of the clock, namely, 12 O'clock, 3 O'clock, 6 O'clock and 9 O'clock, where 12 O'clock was defined as towards the head. A wet cotton-tipped scaled probe with normal saline was inserted gently into the undermined region. The size of wound undermining in each direction was measured to the nearest millimetre and recorded in centimetres from the probe.8 All four scores were then summed to derive one overall score in centimetres. Scores were continuous where a score of zero represented no undermining.

Wound undermining was measured three times to determine inter- and intra-rater reliability (Figure 2):

  1. a

    Inter-rater reliability. This was determined by comparing the scores of wound undermining taken on two different occasions by two different assessors. The order in which the two assessors tested participants was randomised. The inter-rater reliability of the measures of wound undermining was determined using an intraclass correlation coefficient (ICC) and percent close agreements.

  2. b

    Intra-rater reliability. This was determined by comparing the scores of wound undermining by the same assessor taken on 2 different days. The assessments were separated by 3–5 days to avoid recall. The intra-rater reliability of the measures of wound undermining was determined using ICC and percent close agreements.

Figure 2
figure 2

Schematic diagram for the study.

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Data were managed and transcribed into electronic format using Research Electronic Data Capture (REDCap) tools hosted at the University of Sydney. REDCap is a secure, web-based application designed to support data capture for research studies (Nashville, TN, USA).12 STATA 13 for Windows was used for all analyses. All assessments were taken blinded to the results of all previous assessments whether taken by the same assessor or a second assessor.

The ICC values were interpreted according to the rating system suggested by Shrout and Fleiss13 (>0.75 excellent, 0.40–0.75 fair to good reliability and <0.40 poor reliability).

Results

Demographic characteristics of the 30 participants and their PUSH results are presented in Table 1. The median (interquartile range) extent of wound undermining was 3.2 cm (1.0–7.1), and the median (interquartile range) PUSH score was 13 (10–13). The Pearson Correlation Coefficient describing the relationship between wound undermining and PUSH scores was imprecise and poor with a correlation coefficient (95% confidence interval) of 0.42 (0.08–0.72).

Table 1 Characteristics of the participants
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Inter-rater reliability

The ICC (95% confidence interval) reflecting the agreement of the two wound undermining scores by two different assessors was 0.996 (0.992–0.999). The percent close agreements are shown in Table 2. In summary, the two wound undermining scores were within 0.2, 0.4 and 0.6 cm, 67%, 90% and 93% of the time, respectively.

Table 2 Percent close agreement (cumulative percentages) of the repeat assessments of the undermining measure
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Intra-rater reliability

The ICC (95% confidence interval) reflecting the agreement of the two wound undermining scores by the same assessor was 0.998 (0.996–0.999). The percent close agreements are shown in Table 2. In summary, the two wound undermining scores were within 0.2, 0.4 and 0.6 cm, 67%, 83% and 100% of the time, respectively.

Discussion

Clinical guidelines5, 6, 7 are increasingly recommending the routine measurement of wound undermining. However, little attention has been directed at determining the psychometric properties of the most widely used method for measuring wound undermining, which is the method that uses the four cardinal points of a clock. Our study shows that this method has excellent inter- and intra-rater reliability, and hence could readily be used in the clinical setting.

Measurements of wound undermining have excellent face validity. Future studies could consider using ultrasound or computed tomography scan to further validate measurements of wound undermining, although this is probably not necessary. The poor correlation between wound undermining and PUSH scores indicates that a less severe pressure ulcer can have extensive undermining and vice versa. Our study only included participants with wound undermining. If we had included people with severe pressure ulcers (that is, high PUSH scores) and no undermining then probably the correlation between the PUSH and wound undermining would have been extremely poor. This has been previously noted.14 The poor correlation between PUSH scores and undermining does, however, lend weight to the argument that PUSH scores alone are not sufficient to capture all aspects of a pressure ulcer, and that improvements in PUSH scores may not necessarily be mirrored by reductions in undermining.

Wound undermining captures an important aspect of a wound that is not reflected in the PUSH. It is relatively easy to measure wound undermining, and these measurements have excellent inter-rater and intra-rater reliability.