Original Article

Validation of the Spanish Version of the WOMAC Questionnaire forPatients with Hip or Knee Osteoarthritis

A. Escobar1, J. M. Quintana2, A. Bilbao2, J. Azkarate3 and J. I. Guenaga4

1Hospital of Basurto, Bilbao; 2Hospital of Galdakao, Vizcaya; 3Hospital of Mendaro, Mendaro; and 4Hospital of Santiago, Vitoria-Gasteiz, Spain

Abstract: The aim of this study was to validate atranslated version of the Western Ontario and McMasterUniversities Osteoarthritis Index (WOMAC) question-naire in Spanish patients with hip or knee osteoarthritis(OA). The WOMAC questionnaire and the SF-36 wereadministered to a sample of 269 patients on the waitinglist for hip or knee replacement. We studied theconvergent validity and the item-scale correlation usingPearson’s correlation coefficient and Spearman’s p. Forthe reliability study we used another sample of 58patients who received the WOMAC twice within 15days. The Pearson’s, Spearman’s p, and intraclasscorrelation coefficients were calculated. Internal con-sistency was measured by Cronbach’s a. The respon-siveness study was carried out by resending the twoquestionnaires to all patients 6 months after surgicalintervention; responsiveness was measured by means ofthe paired t-test, the effect size I and the standardisedresponse mean. The Pearson’s coefficients for theconvergent validity ranged from 70.52 to 70.63. Thecoefficients obtained for the item–scale correlation of thepain area were 0.74 or higher, 0.91 or higher forstiffness, and 0.61 or higher for function. Whenmeasuring the test–retest reliability, the coefficientsranged from 0.66 to 0.81. Internal consistency yieldeda Cronbach’s a ranging from 0.81 to 0.93. Theresponsiveness showed an effect size I ranging from1.5 to 2.2 in patients who underwent hip replacement;for those who underwent knee replacement the rangewas 1 to 1.8. The standardised response mean rangedfrom 1.3 to 1.9 for patients with hip OA; those with knee

OA ranged from 0.8 to 1.5. The Spanish version ofWOMAC is a valid, reliable and responsive instrumentin patients with hip or knee OA.

Keywords: Hip replacement; Knee replacement; Osteo-arthritis; Quality of life; WOMAC validation

Introduction

Despite the fact that health-related quality of life(HRQoL) has long been the concern of healthcareclinicians and managers, over the last 30 years morepublications have begun to appear on the subject in themedical literature [1].

Although different types of outcome measures havebeen used to evaluate the effectiveness of both medicaland surgical interventions, the patient’s point of view isnow increasingly being considered. This is particularlytrue with regard to chronic pathologies that are basicallydirected at improving or relieving symptoms. Instru-ments for measuring the HRQoL are one means toevaluate the outcome based on patients’ opinions.

Various instruments have been created and validated[2–5] to evaluate both the symptomatology and functionon osteoarthritis (OA) of the hip or knee. However, thedisease-specific questionnaire Western Ontario andMcMaster Universities (WOMAC) is the most widelyused instrument for this purpose [6,7], and it has provedto be the best for studies evaluating HRQoL after kneereplacement [8,9].

Considerable effort has been made to standardise theevaluation of the psychometric properties (validity,reliability and responsiveness) of quality-of-life ques-

Clin Rheumatol (2002) 21:466–471� 2002 Clinical Rheumatology Clinical

Rheumatology

Correspondence and offprint requests to: Dr Antonio EscobarMartınez, Unidad de Investigacion, Hospital de Basurto, Avenida deMontevideo, 18, 48013 Bilbao, Spain. Tel: +34 944006000 ext. 5307;Fax: +34 944006180; E-mail: aescobar@hbas.osakidetza.net

tionnaires to ensure that the versions of the questionnairethat have been adapted for use in another language areequivalent to the original questionnaire [10].

The goal of the present study was to evaluate thevalidity, reliability and responsiveness of the Spanishversion of the WOMAC questionnaire.

Patients and Methods

The sample consisted of 269 patients who underwent hipreplacement (142) or knee replacement (127) surgery atthe Department of Orthopedics, Hospital of Basurto,between March 1999 and February 2000. This is atertiary hospital covering a population of approximately350.000 inhabitants of the Bilbao area. All patientsfulfilled clinical and radiographic criteria for thediagnosis of primary OA, with just one joint affected.We excluded patients who were unable to complete thequestionnaires either because of languge difficulties, apsychiatric disorder or sensory impairment. All patientsreceived a disease-specific questionnaire, WOMAC, anda generic one, SF-36, as well as another series ofdemographic and clinical questions by mail. Patients hadto complete both questionnaires while they were on thewaiting list for surgery and 6 months after surgery.

The classification into groups of severity (slight,moderate and severe) was established through questionsnot included in the WOMAC questionnaire, concerningpain and the type of daily activities the person couldcarry out.

The WOMAC is a disease-specific self-administeredquestionnaire developed to study patients with hip orknee OA and requires about 5 min to complete. It has amultidimensional scale made up of 24 items groupedinto three dimensions: pain (five items), stiffness (twoitems) and physical function (17 items). We used theLikert version with five response levels for each item,representing different degrees of intensity (none, mild,moderate, severe or extreme) that were scored from 0 to4. The final score for the WOMAC was determined byadding the aggregate scores for pain, stiffness andfunction.

The higher the score, the worse the patient’scondition; therefore, an improvement was achieved byreducing the overall score. The data were standardised toa range of values from 0 to 100, where 0 represents thebest health status and 100 the worst possible status. Theoriginal questionnaire is reliable, valid and sensitive tothe changes in the health status of patients with hip orknee OA [2,11]. Moreover, several internationalorganisations have recommended this questionnaire forevaluating OA [12,13], and it has been translated intoSpanish and adapted for the population of Spain [14].

The SF-36 is a generic questionnaire on HRQoL thathas been translated into Spanish and validated [15]. Thequestionnaire evaluates eight dimensions regarding bothphysical and mental health and is widely used in HRQoLstudies [16,17].

Statistical Analysis

Descriptive data are expressed as percentages andmeans, with a standard deviation for the populationstudied.

Psychometric Properties

Validity To evaluate differences in mean scores betweenthe different groups of clinical severity (slight, moderate,and severe), a one-way analysis of variance was carriedout in the three WOMAC dimensions. We usedScheffe’s test for multiple comparisons.

To determine the convergent and divergent validitieswe calculated Pearson’s correlated coefficient andSpearman’s p between the WOMAC dimensions andthe SF-36 scales, when the questionnaire was adminis-tered before surgery. Likewise, we evaluated theWOMAC item–scale correlation using Pearson’s corre-lation coefficient and Spearman’s p.

Reliability The reliability of the internal consistency ofthe dimensions was assessed using Cronbach’s acoefficient.

To study test–retest reliability, we enrolled a differentsample of 92 patients who received the WOMACquestionnaire while they were on the waiting list forsurgery, and then again 15 days later while they werestill on the waiting list. They were explicitly askedwhether they had experienced any change in their healthstatus since completing the previous questionnaire, withno change being detected. We calculated Pearson’scorrelation coefficient, Spearman’s p coefficient and theintraclass correlation coefficient.

Responsiveness To evaluate the changes produced aftersurgery, the WOMAC questionnaire was sent again 6months [18,19] after discharge to the patients whounderwent hip or knee replacement surgery. The changeswere evaluated by three methods: the paired t-test; theeffect size I, or standardised effect size, defined as thedifference between the mean baseline scores and follow-up scores on the measure, divided by the standarddeviation of the baseline scores; and the effect size II orthe standardised response mean, defined as the meanscore change divided by the standard deviation of thatscore change [20]. According to the literature [21,22],values higher than 0.8 are proposed to represent highresponsiveness.

Results

Of the 269 patients who received both questionnaire, 203(75.5%) were included in the study because theycompleted both the WOMAC disease-specific ques-tionnaire and the generic SF-36 questionnaire (Table 1)and underwent knee replacement surgery (n = 103) orhip replacement (n = 100). The reliability study was

Validation of Spanish WOMAC 467

conducted based on a different sample of 58 patients outof 92 to whom were sent the WOMAC questionnairewhile on the waiting list (63%).

Validity

The descriptive data for the mean score, as well as thestandard deviation for each of the WOMAC dimensionsordered by anatomic location of the illness and itsseverity, are shown in Table 2. The results showed anincrease in the score for each domain as the severity of theillness increased for both anatomic locations. For patientswith knee OA, the differences were statistically sig-nificant between all levels of severity, except for thegroup classified as moderate on the stiffness dimension.In patients with hip OA, statistically significant differ-ences were observed between the severe and slight groupsand the severe and moderate groups; no differences wereseen between the moderate and slight groups.

The WOMAC pain dimension correlated best with theSF-36 bodily pain scale (r = 70.6, p = 70.55)compared with the rest of the scales (Tables 3). Thefunction dimension of the WOMAC questionnaireachieved the highest Pearson correlation coefficient of70.63 (p = 70.59) with the SF-36 pain scale, whereasthis coefficient was 70.52 (p = 70.59) with thephysical function scale of the same questionnaire. Thestiffness dimension obtained coefficients of r = 70.5(p = 70.48) with the bodily pain scale of the SF-36. Thecoefficients are negative, given the fact that for theWOMAC questionnaire, compared with the SF-36, thescores decrease with improvements in the HRQoL. Allcorrelations were significant (P<0.0001).

Regarding the item–domain correlation, the five itemsin the pain dimension obtained coefficients of more than0.74 (P<0.0001) with their dimension, which was lowerwith the other two dimensions (stiffness and function).The two items on stiffness obtained coefficients of morethan 0.91 (P<0.0001) with their own dimension, which

Table 1. Baseline characteristics of the study population

Hip replacement(n = 100)

Knee replacement(n = 103)

Mean age (yr) (SD) 68.6 (10.3) 70.9 (6)Gender (%)Men 44 (44) 30 (29.1)Women 56 (56) 73 (70.9)

Severity (%)Slight 12 (12) 8 (7.8)Moderate 12 (12) 25 (24.3)Severe 76 (76) 70 (67.9)

WOMAC*Pain 53.9 (19.4) 57 (18)Function 65.4 (17.5) 60.5 (17.7)Stiffness 58.3 (24.5) 56.7 (23.4)

*.Standardised data from 0 to 100. Mean score + standard deviation.A higher score indicates a worse state.

Table 2. Quality-of-life measure (mean + SD) using the WOMACscale, ordered by anatomic location and severity

Pain Function Stiffness n

Hip*SeveritySlight 32.9 (17.2) 44.2 (18.8) 34.4 (20) 12Moderate 36.7 (9.4) 53.6 (12) 40.6 (17) 12Severe 60.1 (16.9) 70.7 (14.6) 64.8 (22.7) 76

KneeSeveritySlight 29.4 (15.4){ 35.8 (15.1){ 34.4 (21.9){ 8Moderate 50.2 (17.3){ 54.8 (17){ 49.5 (18.6) 25Severe 62.6 (14.7){ 5.3 (15.4){ 61.8 (23.2){ 70

*.P<0.05 for differences between the severe–slight and severe–moderate groups.{.P<0.0001 for differences between the three groups, except forstiffness, where the difference is between slight and severe (P<0.05).

Table 3. Convergent and divergent validities: Pearson’s correlation coefficient (Spearman’s p)

WOMAC SF-36

Pain Function Stiffness BP PF RP GH VT SF RE MH

WOMACPain 71Function 70.79 71Stiffness 70.57 70.71 71SF-36BP 70.6 (70.55) 70.63 (70.59) 70.5 1PF 70.4 70.52 (70.59) 70.38 0.39 1RP 70.35 70.43 70.33 0.45 0.27 1GH 70.33 70.34 70.22 0.35 0.28 0.26 1VT 70.47 70.5 70.37 0.61 0.37 0.38 0.5 1SF 70.45 70.52 70.4 0.54 0.34 0.41 0.37 0.57 1RE 70.31 70.31 70.27 0.28 0.12 0.25 0.37 0.37 0.43 1MH 70.41 70.43 70.33 0.43 0.3 0.4 0.5 0.64 0.57 0.57 1

All the correlations were significant (P<0.0001).BP, bodily pain; PF, physical functioning; RP, role-physical; GH, general health; VT, vitality; SF, social functioning; RE, role-emotional; MH,mental health.

468 A. Escobar et al.

were lower with the other two dimensions. Finally, forthe 17 items in the function dimension, all but twoobtained coefficients of more than 0.61 (P<0.0001). Thetwo items were: What degree of difficulty do you havedescending stairs? and What degree of difficulty do youhave standing? and obtained 0.54 and 0.55, respectively,although they correlated better with the pain dimension(r = 0.59 and r = 0.63, respectively). The data forSpearman’s correlation coefficient gave the same results.

Reliability

To assess the test–retest reliability of the WOMACquestionnaire in the sample of 58 patients, we usedPearson’s correlation coefficient, Spearman’s p coeffi-cient and the intraclass correlation coefficient. Thedescriptive data for the pre and post scores, as well asthe corresponding coefficients, are given in Table 4. Thecoefficients ranged from 0.66 to 0.81, with the lowerones corresponding to stiffness and the higher ones tofunction. All coefficients were statistically significant(P<0.0001).

Internal consistency, evaluating using Cronbach’s acoefficient, gave values of 0.82 for pain, 0.93 forfunction, and 0.81 for stiffness.

Responsiveness

All patients underwent either hip or knee replacementsurgery. Six months after surgery they again receivedboth the WOMAC and SF-36. A significant statisticalimprovement was observed (P<0.001) in the threeWOMAC dimensions and in the SF-36 in patients whounderwent hip replacement (Table 5) and those whounderwent knee replacement (Table 6). The effect size Ivalues for patients with a hip replacement were 1.9 forpain, 2.2 for function and 1.5 for stiffness. For those whounderwent knee replacement surgery, the values were1.8, 1.5 and 1, respectively. The effect size II values, orthe standardised response means, for patients whounderwent hip replacement were 1.8 for pain, 1.9 forfunction and 1.3 for stiffness. For patients who under-

Table 4. Analysis of WOMAC’s test–retest reliability

WOMAC Test Retest Correlation coefficientX (D.E.) X (D.E.)

Pearson p Intraclass Cronbach’s a

Pain 53 (18.4) 51.7 (20) 0.78 0.79 0.78 0.82Function 62.1 (16.4) 62.6 (17.6) 0.81 0.81 0.81 0.93Stiffness 63.4 (24) 62.1 (21.2) 0.67 0.66 0.67 0.81

All coefficients were statistically significant (P<0.0001).

Table 5. Changes in the HRQoL measured by WOMAC in patients who underwent hip replacement

Variable Pre interventionX (D.E.)

Post interventionX (D.E.)

DifferenceX (D.E.)

P SRM* Effect size I

WOMACPain 53.9 (19.4) 16.2 (16) 37.7 (20.5) <0.0001 1.8 1.9Function 65.3 (17.5) 27.4 (17.7) 37.9 (19.9) <0.0001 1.9 2.2Stiffness 58.3 (24.5) 22.4 (18.4) 35.9 (26.8) <0.0001 1.3 1.5SE-36Bodily pain 30.7 (27.1) 58.4 (28.4) 27.3 (30.3) <0.0001 0.9 1Physical functioning 19.8 (19.1) 48.7 (22.4) 28.9 (25.7) <0.0001 1.1 1.5

Table 6. Changes in HRQoL measured by WOMAC in patients who underwent knee replacement

Variable Pre interventionX (D.E.)

Post interventionX (D.E.)

DifferenceX (D.E.)

P SRM* Effect size I

WOMACPain 53.0 (18.0) 24.4 (17.6) 32.6 (21.9) <0.0001 1.5 1.8Function 60.3 (17.7) 32.7 (18.7) 27.6 (21.8) <0.0001 1.3 1.5Stiffness 56.7 (23.4) 33.0 (22.2) 23.7 (31.1) <0.0001 0.8 1SE-36Bodily pain 34.7 (28.2) 51.1 (28.9) 15.6 (34.1) <0.0001 0.5 0.5Physical functioning 25.0 (21.6) 50.2 (24.5) 25.2 (27.1) <0.0001 0.9 1.2

*.Standardised response mean or effect size II.

Validation of Spanish WOMAC 469

went knee replacement, the values were 1.5, 1.3 and 0.8,respectively. The results for the SF-36 were lower for allindicators.

Discussion

Hip and knee OA is a chronic illness that, despite beingnon-life-threatening, does cause morbidity: it has animportant social impact, and in many cases it leads to ahip or knee replacement to improve the patient’s qualityof life [23].Various instruments have been used to measure

HRQoL in these orthopaedic patients [22,24]; however,the WOMAC questionnaire is the disease-specificinstrument most widely used in clinical practice foroutcome measurement [6,7].The advantages of disease-specific questionnaires

such as the WOMAC are derived from the fact thatthey are more closely focused on the illness orimpairment under study (in this case OA) and shouldbe better at detecting the changes resulting fromtreatment than questionnaires that are not diseasespecific. The disadvantge is that they cannot detectother types of impairment or complications in areas ororgans that are not specifically related to the illnessunder study [24] that could be important to the patient.To adequately compare the studies carried out in

different countries, the versions of the questionnairesshould be adapted and validated. Although the WOMACquestionnaire has been translated and adapted for aSpanish population [14] its psychometric properties havenot been studied, and this was the goal of this presentstudy.The psychometric properties of the questionnaires

measuring the HRQoL are established by studying theirvalidity, reliability and responsiveness.There are various ways of determining the validity of

a questionnaire, one of which is convergent anddivergent validity. This is generally done by comparingthe instrument under study with other instruments thathave already been validated. The SF-36 is a widely usedinstrument [25,26] and has been validated in Spanish[14]. Our data on validity present moderate correlationswhen the dimensions of both questionnaires werecompared, with correlation coefficients ranging from70.4 to 70.6, which agrees with other studies [27,28].As expected, for both Pearson’s coefficient andSpearman’s p the WOMAC pain dimension obtainedthe best correlation with the SF-36 bodily pain scalecompared with the rest of the scales. For the stiffnessdimension, the highest coefficient values were found inthe bodily pain scale with both coefficients. Whenanalysing the data for the function dimension, weoberved that it correlated best with the bodily pain andfunction scales of the SF-36 (r = 70.63 and r = 70.52,respectively), although with Spearman’s coefficient thisrelation changed slightly, with coefficients of 70.59 in

both cases. This could be a result of the fact that the datado not have a normal distribution and are similar to thosedescribed [24].

The divergent validity was studied according to levelsof severity. Regarding the patients who underwent hipreplacement surgery, no statistically significant differ-ences were found between slight and moderate severity,which could be the result of the small number of patientsin both groups.

Reliability is another psychometric characteristic to beevaluated in a questionnaire. A reliable measurement isone that produces the same results when it isadministered two or more times under the sameconditions. In the present study the questionnaire wassent to the patients a second time, 15 days after they hadreplied to the first one. They were asked whether theyhad experienced any change that could affect theirhealth; a negative response was obtained from allpatients. The values of all the reliability coefficientsindicate that the dimensions are coherent, they aresimilar to those obtained for the Swedish version of thequestionnaire [28], and are slightly lower than thoseobtained for the Hebrew version with regard to internalconsistency [29].

The instrument’s responsiveness presents more diffi-culties regarding both its concept and the way in whichto measure or quantify it. In our study we measured theso-called internal responsiveness, defined as the abilityof a measure to change over a specific period [20]. Allthe results of the WOMAC questionnaire showed goodresponsiveness and therefore a significant improvementin the patients’ health states after knee or hipreplacement surgery, with values higher than thoseshown by other authors [27]. As observed in Tables 5and 6, these changes were greater than those for the SF-36. This confirms that the disease-specific instrument ismore sensitive to changes experienced by patients thanthe generic questionnaire for the corresponding scales.These data confirm the data obtained in other studies thathave used the same questionnaires to make a comparison[9,24].

A possible limitation in the use of the WOMACquestionnaire is the age of the population to which it wasdirected, particularly in the case of patients whounderwent hip or knee replacement, as age does notlimit the indication for surgery [6,30], and for olderpeople it is possible to limit both the number ofresponses as well as the interpretation of some items.A further limitation is that the responses may beinfluenced by another type of problem, such as backpain [31], which is common in our practice.

In conclusion, the Spanish version of the WOMACdisease-specific questionnaire offers psychometric prop-erties that make it a valid, reliable and responsiveinstrument for patients with hip or knee OA who haveundergone a hip or knee replacement. A genericinstrument that measures different but complementaryhealth areas should accompany it.

470 A. Escobar et al.

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Received for publication 12 November 2001Accepted in revised form 21 May 2002

Validation of Spanish WOMAC 471