Perspective article

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EOS Ease of closureLDI Laser Doppler imaging

Science of measurements in wound healing

RAJ MANI, PhD, FACA

Wounds that are slow to heal are poorly understood. Clinicians and researchers have attempted to predict treatmentoutcomes from simple physical measurements without, as yet, understanding the pathogenesis or the role of compli-cations on chronic wounds. Laboratory studies on tissues from biopsies and wound fluids are essential. These must beassociated with carefully conducted physiological measurements before, the significance of measurements in woundhealing is established. (WOUND REP REG 1999;7:330–334)

To paraphrase a well-known saying, “to measure adifference, there must be a difference.” More than inmany other clinical subspecialties, wound care pro-viders are constantly frustrated by the chronicity ofleg ulcers, pressure ulcers, and/or de novo ulcerationof the foot of a diabetic. Are objective measurementsbeneficial for treatment, and if so, why and whatshould we measure? And when we do, what realisticexpectations may we have of scientific measurements?

Most wounds are caused by trauma, and a largepercentage of these wounds heal without complica-tions. However, when there are underlying patholo-gies such as circulatory dysfunction, diabetes, or otherserious conditions, the repair process is frequentlyimpaired, i.e., frustratingly slow or complicated byinfection, edema, pain or a combination of such prob-lems. Therefore, it is vital to identify the etiology ofa wound before initiating treatment.1

Infection, edema and pain are complications thatimpede wound healing. In order to measure param-

eters that are either direct or surrogate markers2 ofthese complications, it is important to understandhow these factors complicate healing, as well as meth-ods of controlling them. The type and level of woundinfection may be assessed from wound swabs or bi-opsies, and treated. But is it realistic to measure thelevel of bio-burden in every wound?

For some, pain is reported to be the worst aspectof suffering caused by a wound.3 Pain is subjectiveand problematic to measure, although visual analogscores are helpful. How pain delays healing is difficultto explain and a relevant measurement is, healthrelated quality of life assessment.4

Edema delays leg ulcer healing2 arguably by im-peding the diffusion of oxygen,5 although there couldbe other reasons. Edematous leg ulcers appear to ex-ude wound fluids more than normal leg wounds, withthe levels of exudate decreasing as healing occurs.What are we to measure in order to treat more effec-tively: the volume or the contents of exudate?

In wound healing, measurements have been tra-ditionally conducted in physiology or biochemistrylaboratories. Biochemical measurements have beenstimulated by the need to discover the contents ofwound fluids, and their effects on tissue growth andrepair. It has long been recognized that cells influencehealing by the nature of their cellular products, sur-

From the Peripheral Vascular Group, Medical Physicsand Medical Engineering, Southampton Univer-sity Hospitals Trust, Southampton, United King-dom.

Reprint requests: Raj Mani, PhD, Peripheral VascularGroup, Medical Physics and Medical Engineer-ing, Southampton University Hospitals Trust,Southampton S016 6YD, United Kingdom. Email:rm1@soton.ac.uk.

Copyright © 1999 by the Wound Healing Society.1067-1927 $5.00 + 0

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faces and the milieu in which they exist.6 Scarring,its causes, its preventive mechanisms, and the poten-tial for gene therapy are excluded from this paper.This article briefly reviews the basis of some mea-surements, and examines what the future may offerto wound healers.

BEGINNING AT THE ENDSuccessful healing is evident by wound closure on thesurface, a satisfactory end point so long as the struc-ture and function of the ulcerated area are restored.For most cutaneous lesions, measuring wound areasfrom contour traces reliably estimates healing despitethe errors introduced by flattening a curved surface.Legs taper and this complicates the precision withwhich ulcer areas may be measured from contours.7

Solomon8 proposed an elegant technique of measure-ment that also accounted for the taper in legs. Allthese methods are practical and satisfactory.

Ulcer photographs may be quantitated, simply byplacing a linear scale within the field of view of thelens. Stereo-photography can be used to measurewound volumes as well as areas.9 In recent years, anovel method of measuring wound volumes using astructured light and a video camera attached to a com-puter was developed.10 Is it necessary to measure vol-ume routinely? In cavity wounds, yes. For cavitylesions, inert dental prosthetic material is best suitedto make “negative” casts of wounds.11 In diabetic feet,a sterile probe is recommended to assess depth, andto document tunneling.12 Wound parameters are use-ful for a continuous record of treatment efficacy. Mea-surements of the square root of the diameter permitsforecasting with a degree of precision as proposed byLawrence.13

Physiological measurementsVascular measurements to identify wound etiologyare well known and widely available.14,15 An excitingnew development, Laser Doppler Imaging (LDI), of-fers vast potential to assess dermal damage, nonin-vasively. LDI an advance on the laser Doppler flow-meter, uses a low powered helium-neon source (1 mW)to illuminate tissues (Figure 1). In principle, laserlight is directed on tissues by a mirror that is movedin a controlled fashion using a stepper motor. Back-scattered light from tissues is detected by photo-diodes, and processed to produce images and valuesof perfusion using the Doppler principle. This devicepermits noninvasive, repetitive imaging of dermal

perfusion that is of immense value to burn woundmanagement (Figure 2).16,17 It could benefit traumamanagement in general.

Other advances in physiological measurementsinclude surface pH, durometry and extensiometry.Surface pH may be easily and reliably measured.18

Reports suggest that an alkaline ulcer surface milieuchanges to acidic in consonance with healing.19,20

These observations endorse long-held prejudices re-lating acidic milieu favorably with healing. It mustbe remembered that values reported are of surfacepH and correlation with tissue pH must be examined.It may also be necessary to study the effects of tissueinfection on pH.

Durometry is a simple reliable means of measur-ing tissue hardness and is useful to classify lipoder-matosclerosis, which increases the risk of venousulcers.21 Extensiometry, also a noninvasive method ofmeasuring extensibility and relaxation properties of-fers promise to study visco-elastic properties of tissuesat risk of ulceration.22 Given that tissue pressures areraised in an edematous limb which would cause tissuetensions to increase, how would this affect ulcer heal-ing? Unrelieved edema is associated with raised totaltissue pressure and its effect may be far-reaching, re-quiring that the mechanical as well the biochemicaleffects be understood. Advances in ultrasound imag-ing technology permit location of edema,23 which isessential to understand its pathophysiology.

Biochemical measurements In recent years, several attempts to study woundfluids have been reported from different researchcenters.24,25 These workers have used differentmethods to extract wound fluids that they analyzedfor collagen precursors and or matrix metallopro-teinases. Different wound hormones have also beeninvestigated.26 The paradigm common to these stud-ies seems to be that an imbalance exists in extracel-lular matrix dynamics, which may affect collagensynthesis. Wound fluid studies show a divergent ex-pression in collagen based on measurements of suchcollagen precursors as type I collagen C-propeptideand type III collagen N-propeptide.27 It is importantto correlate these observations with infection in tis-sue before their significance is determined. Staceyand colleagues28 devised a protocol to aspiratewound fluid from beneath leg ulcers covered by Op-site® (Smith & Nephew, London, U.K.) in patientswho had fasted overnight. Patients were given a literof water to drink 1 hour before sampling with their

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Figure 2. Example of an LDI display of a burn injury. Varying levels of perfusion (A) are noted on the upper trunk of this male burnpatient (B). Courtesy of Moor Instruments, Devon, U.K.

Figure 1. Schematic representation ofthe LDI method to measure dermal per-fusion.

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legs dependent. These patients were due to havesurgery and bed rest for the night prior to tests.Stacey has analyzed wound fluids and developed aprofile that is different for chronic wounds in somerespects than the profile for acute wounds. This mayopen up new vistas. Other wound investigators usedsponges or filter paper to absorb fluid from woundbeds and then recover fluid by crushing the absorb-ing medium. Do the different techniques equate andare the data comparable? These studies are in theirinfancy; methodological problems need to be re-solved.

Are wound biopsies likely to represent tissue sta-tus with greater fidelity? It does appear that explantcultures reveal reliable data about the host bed.29

Hussain et al30 have also shown that fibroblast cul-tures derived from foot ulcer tissue show more brokenDNA chains compared with a similar examination ofcells from from periodontal tissues.

FORECASTING FROM MEASUREMENTSAn important value of measurements is the abilityto forecast outcomes of treatment. Robson31 proposeda scoring system to assess the problems of assistedwound closure, which they term Ease of Closure(EOS). EOS was applied to a randomized, double-blind controlled study of pressure ulcers in whichpatients were assigned to 3 dosages of treatment withplatelet-derived growth factor, compared with vehiclecontrol. Outcome measures included area measure-ments. Analysis of the data showed EOS scores cor-related well with fractional changes in area (r2 = 0.52overall including all dosages and control) and a rea-sonably good fit exists over the range tested. Therespective rate of change in both EOS and area mea-surements, are similar.

EOS is a bold initiative. Has this any value toforecast healing? Possibly, but this rather assumesthat consensus exists on the treatment of choice forthat lesion which it clearly does in the case chosenby Robson. It is interesting to note in this trial thatall 4 treatment options had similar rates of healingfor the first 8 weeks after which all slowed down.Others have also observed a reduction in rate of heal-ing after an initial period following altered therapy.Standardizing treatment is anathema to cliniciansbut certain procedures offer a basis for protocols tobe developed. Predicting outcomes is helpful clinicallyas well as from a resource management perspective,but is fraught with complications.

THE FUTUREVenous ulcers represent the vast majority of chronicwounds. Evidence has recently appeared that associ-ates raised levels of homocysteine with recurrentvenous thrombosis.32–34 Hyperhomocysteinemia maybe treated with a combination of multivitamins andfolates. Speculative though it is, these data that im-proved nutrition may reduce the incidence of venousthrombosis and therefore, of venous ulcers.

The aim of this article was to briefly review thescience of measurements in wound healing. From theevidence, predicting outcomes based on physical mea-surements is still not developed. Modeling chronicwounds is difficult, and is further complicated by sig-nificant comorbid conditions. We need to understandthe role of common complications in delayed healing,such as infection and edema, which makes imperativeboth biochemical and cellular measurements onwound fluids and biopsies. Only then will the linksbetween physiological, biochemical and cellular mea-surements become apparent. For now, we must con-tinue to “measure to make the difference.”

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