Western Ontario and McMaster Universities Osteoarthritis Index

Abstract

The Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) is a widely utilized, disease-specific patient-reported outcome measure (PROM) developed to evaluate clinically important, patient-relevant symptoms and physical disability in individuals suffering from osteoarthritis (OA) of the hip or knee (gonarthrosis and coxarthrosis). Originally conceptualized and validated by Nicholas Bellamy and colleagues in 1982 and formally published in 1988, the instrument has achieved international acceptance as an evaluative benchmark in orthopedic surgery, rheumatology, physical medicine, and pharmacological clinical trials. The questionnaire comprises 24 self-administered items distributed across three distinct symptom dimensions: Pain (5 items, assessing dynamic and static joint discomfort), Stiffness (2 items, capturing morning gel phenomenon and post-inactivity resistance), and Physical Function (17 items, assessing difficulty performing essential activities of daily living [ADLs]).

The instrument assesses symptom severity over a designated recall period, traditionally defined as the preceding 48 hours. Responses are captured utilizing either a 5-point Likert scale (ranging from 0 = None to 4 = Extreme) or a 100 mm Visual Analogue Scale (VAS). Subscale scores are obtained through simple summation or normalized to a 0–100 metric, wherein higher scores invariably denote worse pain, greater stiffness, and more profound functional impairment. Extensive psychometric evaluations following the COnsensus-based Standards for the selection of health Measurement INstruments (COSMIN) taxonomy demonstrate that the WOMAC possesses excellent internal consistency reliability (Cronbach’s alpha coefficients consistently exceeding 0.80 for Pain and 0.90 for Physical Function) and robust test-retest reliability (intraclass correlation coefficients [ICC] typically ranging between 0.70 and 0.95). Factor analytic investigations confirm a coherent multidimensional architecture, though modern item response theory and Rasch analyses highlight structural collinearity between pain and physical function domains. The WOMAC demonstrates superior responsiveness to surgical interventions such as total knee arthroplasty (TKA) and total hip arthroplasty (THA), solidifying its place as a gold-standard instrument in musculoskeletal outcome assessment.

Keywords

WOMAC, Osteoarthritis, Patient-Reported Outcome Measure, Knee Arthroplasty, Hip Arthroplasty, Joint Pain, Physical Function, Psychometrics, Rasch Analysis, Musculoskeletal Disability

Authors

The Western Ontario and McMaster Universities Osteoarthritis Index was developed in the early 1980s through an academic collaboration between clinical rheumatologists, epidemiologists, and biostatisticians based in Canada. The principal authors and contributors to the index’s foundational validation and cross-cultural adaptations include:

• Nicholas Bellamy, MD, MSc, MBA, FRACP, FRCPC: Principal developer of the index; formerly Professor of Medicine, Division of Rheumatology, Department of Medicine, University of Western Ontario (London, Ontario, Canada); later affiliated with the Centre of National Research on Disability and Rehabilitation Medicine (CONROD), The University of Queensland, Brisbane, Australia.
• W. Watson Buchanan, MD, FRCPC, FRCP: Co-developer and distinguished rheumatologist; formerly Professor of Medicine, Department of Medicine, McMaster University Health Sciences Centre, Hamilton, Ontario, Canada.
• Colin H. Goldsmith, PhD: Biostatistician and psychometrician; Department of Clinical Epidemiology and Biostatistics, McMaster University, Hamilton, Ontario, Canada.
• Jane Campbell, BA: Clinical research coordinator and investigator; Department of Medicine, University of Western Ontario, London, Ontario, Canada.
• Leo D. Roorda, MD, PT, PhD: Lead developer of the validated Dutch language adaptation; affiliated with the Amsterdam Rehabilitation Research Center | Reade, Amsterdam, The Netherlands, and Jan van Breemen Institute.

Purpose

The primary purpose of the WOMAC is to provide a standardized, clinically meaningful, and psychometrically robust measurement of health status and functional limitation specifically tailored to individuals suffering from hip and knee osteoarthritis. Prior to the establishment of the WOMAC in the 1980s, musculoskeletal clinical trials and orthopedic outcome assessments relied heavily on generic health status surveys (such as the Short Form-36 [SF-36] or the Sickness Impact Profile) or physician-assessed composite indices (such as the Harris Hip Score or the American Knee Society Score). While generic instruments offer cross-disease comparability, they frequently lack the granular sensitivity and responsiveness necessary to capture subtle yet clinically pivotal shifts in lower-extremity joint pathology. Conversely, observer-rated scales introduce substantial inter-rater variability and often place disproportionate weight on physical signs (such as passive range of motion or joint deformity) rather than the patient’s subjective lived experience of functional handicap and distress.

Bellamy and Buchanan formulated the WOMAC to resolve these methodological limitations by constructing a disease-specific PROM centered purely on patient self-report. In clinical research, the instrument functions as an evaluative end-point tool in phase II, III, and IV pharmacological trials testing non-steroidal anti-inflammatory drugs (NSAIDs), cyclooxygenase-2 (COX-2) inhibitors, slow-acting disease-modifying osteoarthritis drugs (DMOADs), and intra-articular corticosteroid or hyaluronic acid injections. It is endorsed as a core outcome measure by the Osteoarthritis Research Society International (OARSI) and the World Health Organization (WHO) for OA clinical trials.

In routine orthopedic and rehabilitative clinical practice, the WOMAC serves three essential clinical functions: baseline triage and stratification, progress tracking during conservative interventions (physical therapy, exercise regimens, weight-reduction protocols), and perioperative outcome monitoring for patients undergoing total hip or knee arthroplasty. By focusing on a precise recall window of the preceding 48 hours, the instrument minimizes long-term recall bias while insulating the evaluation from brief hour-to-hour symptomatic fluctuations. The theoretical rationale rests on the premise that effective disease management must directly target and alleviate the triad of joint symptoms that fundamentally erode independence: nociceptive discomfort during weight-bearing and non-weight-bearing states, viscoelastic stiffness following periods of immobility, and performance decrements in activities essential to community ambulation and self-care.

Psychological Construct

Although the WOMAC is traditionally categorized as a clinical functional assessment, it inherently evaluates complex, overlapping psychological, psychophysiological, and behavioral constructs. Lower-extremity osteoarthritis is not simply a degenerative mechanical breakdown of articular cartilage; it represents a chronic, painful condition wherein somatic tissue damage interacts with cognitive appraisal, fear avoidance, somatosensory amplification, and behavioral coping mechanisms. The 24 items of the WOMAC map directly onto three dimensional constructs:

1. Pain (5 Items)

The WOMAC Pain subscale captures the multidimensional experience of joint discomfort across distinct biomechanical and behavioral contexts. Rather than operationalizing pain solely as a unidimensional sensory intensity (as done by a standard unidimensional numeric rating scale), the WOMAC measures pain across dynamic weight-bearing tasks (Item 1: walking on a flat surface; Item 2: going up or down stairs; Item 5: standing upright) as well as static, non-weight-bearing states (Item 3: at night while in bed; Item 4: sitting or lying). The inclusion of nocturnal pain and resting pain is theoretically crucial: nighttime discomfort reflects advanced joint pathology, intraosseous engorgement, sustained inflammatory processes, and central sensitization, which are potent drivers of sleep disruption, depressive affect, and psychological distress. Weight-bearing pain, by contrast, interfaces closely with kinesiophobia (fear of movement), catastrophic appraisal of somatic sensations, and anticipatory behavioral restriction.

2. Stiffness (2 Items)

Stiffness within the WOMAC is operationalized as the patient’s subjective perception of restricted joint mobility, resistance to initial movement, and mechanical sluggishness. The construct is evaluated across two distinct clinical manifestations: Item 6 measures stiffness upon first awakening in the morning (the classic rheumatologic “morning gel phenomenon”), while Item 7 measures stiffness occurring after periods of sitting, lying, or resting later in the day. Psychologically and physiologically, stiffness serves as an internal interoceptive cue that signals disease activity to the patient. Morning stiffness reflects transient periarticular edema and viscoelastic changes in the synovial membrane and capsular tissues that temporarily impede kinetic fluidity. Experiencing profound morning or post-inactivity stiffness amplifies perceived vulnerability, reinforces sedentary behavior, and precipitates anticipatory anxiety regarding the initiation of daily motor tasks.

3. Physical Function (17 Items)

The Physical Function subscale evaluates perceived difficulty and behavioral disability encountered while executing activities of daily living (ADLs) that require lower-extremity integrity, dynamic balance, muscular strength, and endurance. The items traverse basic self-care activities (putting on and taking off socks or stockings, getting in or out of the bath, getting on or off the toilet), domestic maintenance (light and heavy domestic duties), postural transitions (rising from sitting, rising from bed, bending to the floor), and locomotor mobility (ascending and descending stairs, walking on flat surfaces, getting in or out of a car, going shopping). From a psychological standpoint, functional impairment represents the behavioral manifestation of physical illness. When patients score high on functional difficulty, they are not merely reporting physical limitation; they are articulating a loss of functional autonomy, self-efficacy, and social role engagement. Prolonged functional dependence frequently triggers external locus of control, depressive symptom clusters, and systemic reductions in health-related quality of life (HRQoL).

Theoretical Framework

The theoretical architecture of the WOMAC is anchored in classical health status models, contemporary behavioral medicine, and the biopsychosocial framework articulated by George L. Engel (1977). Furthermore, its conceptual underpinnings correspond systematically with the International Classification of Functioning, Disability and Health (ICF) established by the World Health Organization, as well as the health-related quality of life causal pathway delineated by Wilson and Cleary (1995).

Under the ICF framework, the consequences of a chronic health condition are structured into three hierarchical yet interacting levels: Body Functions and Structures (impairments), Activities (limitations), and Participation (restrictions). The WOMAC cleanly operationalizes this structure. The Pain and Stiffness subscales assess impairments in Body Functions (specifically, neuromusculoskeletal and movement-related functions, ICF codes b280 for sensation of pain and b780 for sensations related to muscles and movement functions). The Physical Function subscale directly targets Activity Limitations (ICF Chapter 4: Mobility, including d450 walking, d455 moving around, d410 changing basic body position, and ICF Chapter 5: Self-Care, including d510 washing oneself and d540 dressing). By mapping directly onto impairments and activity limitations, the WOMAC provides a granular index of the direct kinetic impacts of joint degradation before those limitations necessarily cascade into complete societal and vocational participation restrictions.

The Wilson and Cleary (1995) model of HRQoL posits a unidirectional yet modifiable continuum connecting five levels of health outcomes: (1) Biological and physiological variables, (2) Symptom status, (3) Functioning, (4) General health perceptions, and (5) Overall quality of life. The WOMAC strategically situates itself across tiers 2 and 3 of this continuum. Osteoarthritis begins biologically with cartilage matrix fibrillation, chondrocyte apoptosis, osteophyte development, and subchondral sclerosis. However, biological measures (such as Kellgren-Lawrence radiographic grading) correlate notoriously poorly with clinical symptoms. Bellamy recognized that patient-centered decision-making requires evaluating Symptom Status (nociceptive input and joint stiffness) and its immediate behavioral sequelae—Functional Status. By omitting broader, non-specific constructs such as financial well-being or existential life satisfaction, the WOMAC retains exceptional sensitivity to localized biomechanical and pharmacological interventions.

From a cognitive-behavioral perspective, the WOMAC interfaces with the Gate Control Theory of pain (Melzack & Wall, 1965) and the Fear-Avoidance Model of chronic musculoskeletal pain (Vlaeyen & Linton, 2000). Pain experienced during dynamic loading (such as stair ambulation or walking) stimulates cognitive appraisals of harm. When patients interpret joint pain as an indicator of progressive mechanical destruction, hypervigilance and fear of movement ensue. This psychological anticipation leads directly to task modification, avoidance of physical activities (e.g., shopping, domestic chores), muscular deconditioning, and increased subjective ratings of functional difficulty on the WOMAC questionnaire.

Validity

The psychometric validity of the WOMAC has been rigorously scrutinized and established across hundreds of international empirical investigations, encompassing diverse ethnic populations, age brackets, surgical cohorts, and conservative care settings.

1. Content and Face Validity

Content validity was established during the instrument’s initial development through systematic patient interviews, clinical rheumatologist expert panels, and comprehensive item-reduction methodologies. Bellamy et al. (1988) conducted qualitative inquiries with knee and hip osteoarthritis patients to identify symptom manifestations and functional activities that patients themselves deemed most troublesome and intrusive. Items displaying universal relevance, strong discriminatory power, and high clinical frequency were retained. Face validity is exceptionally robust; patients suffering from gonarthrosis or coxarthrosis immediately recognize the 24 items as direct representations of their daily physical limitations.

2. Construct and Convergent Validity

Construct validity has been verified against an array of convergent and divergent health measures:

• Convergent Validity with Generic Instruments: When correlated with the Medical Outcomes Study Short Form-36 (SF-36), the WOMAC demonstrates strong, statistically significant correlations with conceptually convergent domains. Specifically, the WOMAC Physical Function subscale correlates strongly with the SF-36 Physical Functioning (PF) domain ($r = -0.65$ to $-0.84$) and Physical Component Summary (PCS) score ($r = -0.60$ to $-0.78$). The WOMAC Pain subscale correlates robustly with the SF-36 Bodily Pain (BP) domain ($r = -0.68$ to $-0.82$).
• Convergent Validity with Disease-Specific Measures: High correlations are universally observed between the WOMAC and the Knee Injury and Osteoarthritis Outcome Score (KOOS), the Hip Disability and Osteoarthritis Outcome Score (HOOS), and the Lequesne Algofunctional Index ($r = 0.72$ to $0.89$). Because the KOOS and HOOS directly incorporate the authentic 24 WOMAC items within their subscale structures, convergence within those common dimensions approaches unity ($r > 0.95$).
• Convergence with Objective Physical Performance: Moderate, physiologically coherent correlations exist between WOMAC Physical Function scores and objective biomechanical performance measures, such as the 6-Minute Walk Test (6MWT, $r = -0.45$ to $-0.62$), the Timed Up and Go (TUG) test ($r = 0.48$ to $0.65$), and the 30-Second Chair Stand Test ($r = -0.42$ to $-0.58$).

3. Discriminant (Divergent) Validity

The WOMAC successfully demonstrates discriminant validity when compared against non-somatic and affective dimensions. Correlations between the WOMAC subscales and the SF-36 Mental Health (MH) subscale, Role-Emotional (RE) subscale, or the Mental Component Summary (MCS) are consistently weak to modest ($r = -0.15$ to $-0.35$). This divergence confirms that the WOMAC specifically isolates physical symptoms and mechanical disability rather than non-specific global psychological distress.

4. Criterion and Predictive Validity

While no definitive single “gold standard” laboratory marker exists for subjective functional impairment, the WOMAC demonstrates excellent predictive validity. Preoperative WOMAC scores significantly predict postoperative satisfaction, time to discharge, inpatient rehabilitation utilization, and long-term functional recovery following primary joint arthroplasty. Patients displaying severe baseline impairment (WOMAC > 70 on a normalized 0–100 scale) who achieve score reductions of 50% or more consistently report high subjective treatment success.

5. Responsiveness to Change

The WOMAC is celebrated for its extraordinary responsiveness (evaluative validity). In surgical cohorts undergoing total hip or knee replacement, the Standardized Response Mean (SRM) and Effect Size (ES) post-surgery frequently range from $1.20$ to $2.50$, indicating massive clinical improvement. In pharmacological trials examining intra-articular steroid injections, oral NSAIDs, or exercise therapy, the SRM typically ranges between $0.40$ and $0.85$, reliably capturing small-to-moderate therapeutic benefits. The Minimal Clinically Important Difference (MCID) has been established in multiple consensus studies: a relative reduction of 15% to 20% from baseline, or an absolute reduction of 9 to 12 points on a 0–100 normalized scale, reliably reflects clinically meaningful patient improvement (Tubach et al., 2005).

Reliability

The reliability of the WOMAC has been extensively demonstrated across diverse clinical trials, cross-cultural adaptations, and longitudinal observational registries. Evaluated primarily through the prism of internal consistency and test-retest stability, the instrument meets and exceeds the stringent reliability benchmarks set by the scientific community (such as the COSMIN standards, which recommend reliability coefficients $ge 0.70$ for group-level research and $ge 0.90$ for individual patient decision-making).

1. Internal Consistency

Internal consistency, measured via Cronbach’s alpha ($\alpha$), reflects the degree of interrelatedness among items within each subscale. In the original landmark validation by Bellamy et al. (1988) and subsequent global validation initiatives (including the Dutch validation by Roorda et al., 2004; the German validation by Stucki et al., 1996; and the French validation by Tubach et al., 2005), internal consistency figures have proven exceptionally robust:

• Pain Subscale (5 items): Cronbach’s $\alpha$ consistently ranges from $0.82$ to $0.93$. The high alpha confirms that while the 5 items capture pain across distinct functional postures (walking, stairs, nocturnal, resting, standing), they reflect a unified underlying construct of osteoarthritic joint discomfort.
• Stiffness Subscale (2 items): Cronbach’s $\alpha$ typically ranges from $0.75$ to $0.88$. Although two-item scales mathematically suppress alpha values due to low item count, the inter-item correlation between morning stiffness (Item 6) and post-rest stiffness (Item 7) typically exceeds $r = 0.60$, reflecting high internal homogeneity.
• Physical Function Subscale (17 items): Cronbach’s $\alpha$ typically ranges from $0.93$ to $0.97$. This extremely high alpha denotes exceptional internal consistency, although psychometricians utilizing item response theory have occasionally noted that values exceeding $0.95$ may indicate a degree of item redundancy among certain mobility tasks.
• Total Score (24 items): When computed as a global index, the overall Cronbach’s $\alpha$ routinely exceeds $0.95$.

2. Test-Retest Reliability

Test-retest stability assesses the instrument’s capacity to yield consistent scores over time in clinically stable patients. When administered across intervals varying from 24 to 48 hours up to two weeks (prior to the initiation of active therapeutic interventions), the Intraclass Correlation Coefficients (ICC, two-way mixed effects model for absolute agreement) demonstrate high reproducibility:

• Pain: ICC values range from $0.73$ to $0.91$.
• Stiffness: ICC values range from $0.68$ to $0.86$. (Stiffness demonstrates marginally lower reproducibility, partly attributable to the genuine biological volatility of morning gel phenomena dependent on temperature, barometric pressure, and prior-day activity).
• Physical Function: ICC values range from $0.85$ to $0.96$, demonstrating exceptional stability in habitual ADL performance.
• Total Score: ICC values consistently fall between $0.88$ and $0.95$.

3. Measurement Precision: SEM and MDC

To differentiate true biological or therapeutic changes from random measurement error, psychometricians have established the Standard Error of Measurement (SEM) and the Minimal Detectable Change at the 95% confidence level ($MDC_{95}$):

• The SEM for the normalized WOMAC subscales generally ranges from $4.0$ to $7.5$ points on a 0–100 scale.
• The $MDC_{95}$ (calculated as $SEM \times 1.96 \times \sqrt{2}$) typically spans $11.1$ to $15.8$ points for the Pain subscale and $9.5$ to $13.2$ points for the Physical Function subscale. Any patient exhibiting a postoperative or post-treatment score reduction exceeding these thresholds can be classified with 95% certainty as having experienced true individual change beyond measurement noise.

Factor Analysis

The structural dimensionality of the WOMAC has served as a fertile subject of psychometric inquiry, spanning classical exploratory factor analysis (EFA), confirmatory factor analysis (CFA), and modern item response theory (IRT) / Rasch measurement models.

1. Exploratory and Confirmatory Factor Analyses

In the original developmental analyses conducted by Bellamy and colleagues, exploratory factor analysis with orthogonal and oblique rotations supported the empirical separation of the 24 items into the three proposed clinical domains: Pain, Stiffness, and Physical Function. Eigenvalues and scree plot inspections revealed a dominant first factor accounting for the majority of the variance (frequently > 50%), with secondary and tertiary factors corresponding to the stiffness and static pain items.

Subsequent confirmatory factor analyses (CFAs) across international samples have tested whether the theoretical three-factor oblique model fits observed patient response matrices adequately. Typical findings from CFA studies (e.g., McConnell et al., 2001; Sun et al., 2003; Roorda et al., 2004) demonstrate:

• Three-Factor Oblique Model Fit: Standard goodness-of-fit indices generally demonstrate moderate to acceptable fit. Reported indices typically include: Comparative Fit Index (CFI) $= 0.88 – 0.94$; Tucker-Lewis Index (TLI) $= 0.87 – 0.93$; Root Mean Square Error of Approximation (RMSEA) $= 0.065 – 0.088$; Standardized Root Mean Square Residual (SRMR) $= 0.045 – 0.060$.
• Factor Loadings: In the three-factor model, standardized item factor loadings are uniformly high. For the Pain subscale, loadings range between $0.68$ and $0.88$. The two Stiffness items load heavily on their dedicated latent factor ($0.74$ to $0.89$). Within the Physical Function subscale, all 17 items demonstrate strong loadings ranging from $0.65$ to $0.91$, with stair ambulation, rising from sitting, and walking on flat surfaces exhibiting the highest communalities.
• Inter-Factor Correlations: A persistent finding across all CFA investigations is the extremely high latent correlation between the Pain factor and the Physical Function factor, with correlation coefficients frequently falling between $r = 0.80$ and $r = 0.92$. While clinically distinguishable (pain is a sensory/emotional perception, while function represents physical task execution), patients with lower-extremity OA rarely dissociate the difficulty of performing an activity from the pain provoked by that activity. Consequently, several psychometricians have proposed a second-order general factor representing overall “Osteoarthritic Disease Burden,” or alternative bifactor specifications where a general OA severity factor accounts for common variance alongside specific orthogonal group factors for stiffness and nocturnal symptoms.

2. Item Response Theory and Rasch Analyses

Modern Rasch and IRT modeling studies (e.g., Roorda et al., 2004; Davis et al., 2009) have subjected the WOMAC to rigorous testing of unidimensionality, item fit, differential item functioning (DIF), and response category ordering:

• Threshold Ordering: In both the 5-point Likert and numerical response formats, threshold ordering is generally preserved. As underlying latent trait disability increases, respondents systematically transition from selecting category 0 (“None”) through 1 (“Slight”), 2 (“Moderate”), 3 (“Severe”), to 4 (“Extreme”), confirming that the response scale operates monotonically without category collapse.
• Item Infit and Outfit Statistics: Rasch analysis indicates that most items adhere to acceptable mean-square infit and outfit boundaries ($0.70$ to $1.30$). However, some studies identify mild outfit distortion in Item 16 (“putting on socks/stockings”) and Item 18 (“taking off socks/stockings”) in knee osteoarthritis cohorts, because dressing the foot requires significant lumbar and hip flexion rather than pure tibiofemoral excursion, occasionally causing unexpected misfit in knee-specific models.
• Differential Item Functioning (DIF): Extensive testing indicates that WOMAC items are largely free of meaningful DIF across biological sex and age brackets, confirming measurement invariance. Although elderly individuals occasionally report slightly higher difficulty on high-demand domestic duties (Item 23) independent of joint degradation, this variance does not compromise global scale invariance.

Instrument / Measurement Tool

The Western Ontario and McMaster Universities Osteoarthritis Index is structured as follows:

• Test Type: Disease-specific, self-administered Patient-Reported Outcome Measure (PROM); clinician-administered structured interview formats are also validated.
• Target Population: Adults and elderly individuals suffering from primary or secondary osteoarthritis of the knee (gonarthrosis) and/or hip (coxarthrosis).
• Total Items: 24 discrete items.
• Subscale Architecture:
  • Subscale A: Pain — 5 items (Items 1 through 5; measuring pain experienced during walking, stairs, in bed at night, sitting/lying, and standing).
  • Subscale B: Stiffness — 2 items (Items 6 and 7; measuring morning stiffness upon waking and post-rest stiffness occurring later in the day).
  • Subscale C: Physical Function — 17 items (Items 8 through 24; measuring degree of difficulty executing diverse self-care, postural, locomotor, and domestic tasks).
• Recall Period: Symptom status and physical limitation experienced over the preceding 48 hours.
• Response Scale (Authentic):
  • 5-point Likert scale (0 = None, 1 = Slight, 2 = Moderate, 3 = Severe, 4 = Extreme) or 100 mm Visual Analogue Scale (VAS).
• Scoring Rules:
  • Raw Subscale Scores: Calculated by summing the individual item scores within each subscale:
    • Pain Subscale (Items 1–5): Score range from 0 to 20.
    • Stiffness Subscale (Items 6–7): Score range from 0 to 8.
    • Physical Function Subscale (Items 8–24): Score range from 0 to 68.
    • Total WOMAC Global Score: Score range from 0 to 96 (sum of all 24 items).
  • Directionality: Higher scores unconditionally indicate worse pain, greater joint stiffness, and more severe functional limitation. Lower scores indicate minimal symptoms and intact physical functioning.
  • Standardized / Normalized 0–100 Transformation: To facilitate clinical interpretation and comparative meta-analyses, raw subscale scores are frequently normalized to a 0–100 metric using the standard linear formula:
    $$\text{Normalized Score} = \left( \frac{\text{Raw Score}}{\text{Ma\ximum Possible Raw Score}} \right) \times 100$$
    (e.g., for Pain: $[\text{Raw Pain} / 20] \times 100$; for Function: $[\text{Raw Function} / 68] \times 100$). On this normalized scale, 0 represents no symptoms/disability, and 100 represents maximal possible symptoms/disability. (Note: When utilizing inverted versions like the KOOS/HOOS format, scores are inverted such that 100 denotes perfect health).
  • Handling of Missing Data: In accordance with Bellamy’s published guidelines, if a patient leaves 1 item missing in the Pain subscale, 1 item missing in the Stiffness subscale, or up to 3 items missing in the Physical Function subscale, the mean score of the completed items within that specific subscale may be imputed to substitute for the missing value. If more items are omitted, the subscale is considered invalid and must be recorded as missing data.
• Administration Time: Approximately 5 to 10 minutes for completion; scoring requires under 3 minutes.
• Administration Formats: Traditional pen-and-paper questionnaire, computer-assisted self-interview (CASI), web-based electronic Patient-Reported Outcome (ePRO) platforms, and interactive clinical tablets.

Permissions & Fee and Test Year

The Western Ontario and McMaster Universities Osteoarthritis Index was conceptualized and first validated in 1982 by Nicholas Bellamy and W. Watson Buchanan, with the seminal, comprehensive peer-reviewed validation publication appearing in the Journal of Rheumatology in 1988. The validated Dutch adaptation referenced in clinical guidelines was developed by Leo D. Roorda and colleagues in 2004.

Copyright and Licensing: The WOMAC is an internationally copyrighted instrument. The proprietary copyright resides with Nicholas Bellamy. While the index is widely documented in academic literature and has been translated into over 60 languages and dialects, the official, authorized use of the original WOMAC index (including certified electronic formats, validated linguistic translations, and standardized user guides) requires formal copyright licensing permissions.

Fee Structure:

• Academic and Non-Funded Academic Research: Academic researchers, unfunded postgraduate students, and registered non-profit clinicians can often obtain permission or licensing at reduced or waived fee schedules upon direct application through official licensing administrators.
• Commercial and Funded Clinical Trials: For-profit pharmaceutical enterprises, biotechnology companies, medical device manufacturers, and commercially sponsored clinical trials are required to pay standard copyright licensing, registration, and linguistic validation fees per protocol and language version.

Official licensing inquiries, user agreements, and translation rights are administered via the official WOMAC licensing channels and representative outcome distribution organizations.

References

• Angst, F., Aeschlimann, A., Steiner, W., & Stucki, G. (2001). Responsiveness of the WOMAC osteoarthritis index as compared with the SF-36 in patients with osteoarthritis of the walking disability undergoing total hip or knee replacement. Osteoarthritis and Cartilage, 9(8), 774–782. https://doi.org/10.1053/joca.2001.0471
• Bellamy, N. (1982). Osteoarthritis: An evaluative index for clinical trials (Master’s thesis). McMaster University, Hamilton, Ontario, Canada.
• Bellamy, N., Buchanan, W. W., Goldsmith, C. H., Campbell, J., & Stitt, L. W. (1988). Validation study of WOMAC: A health status instrument for measuring clinically important patient relevant outcomes to antirheumatic drug therapy in patients with osteoarthritis of the hip or knee. The Journal of Rheumatology, 15(12), 1833–1840. https://pubmed.ncbi.nlm.nih.gov/3068365/
• Bellamy, N. (1995). Outcome measurement in osteoarthritis clinical trials. The Journal of Rheumatology. Supplement, 43, 49–51.
• Bellamy, N. (2002). WOMAC: A 20-year milestone in osteoarthritis outcome measurement. The Journal of Rheumatology, 29(12), 2473–2476.
• Davis, A. M., Badley, E. M., Beaton, D. E., Kopec, J., Kurrasch, R., & Perruccio, A. V. (2009). Rasch analysis of the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) in patients with hip and knee osteoarthritis. Arthritis Care & Research, 61(10), 1334–1342. https://doi.org/10.1002/art.24823
• Engel, G. L. (1977). The need for a new medical model: A challenge for biomedicine. Science, 196(4286), 129–136. https://doi.org/10.1126/science.847460
• McConnell, S., Kolopack, P., & Davis, A. M. (2001). The Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC): A review of its utility and psychometric properties. Arthritis Care & Research, 45(5), 453–461. https://doi.org/10.1126/science.150.3699.971
• Roorda, L. D., Jones, C. A., Waltz, M., Lankhorst, G. J., Bouter, L. M., van der Eijken, J. W., Willems, W. J., & Heyligers, I. C. (2004). Satisfactory cross cultural equivalence of the Dutch WOMAC in patients with hip osteoarthritis waiting for arthroplasty. Annals of the Rheumatic Diseases, 63(1), 36–42. https://doi.org/10.1136/ard.2002.001966
• Stucki, G., Meier, D., Stucki, S., Michel, B. A., Tyndall, A. G., Dick, W., & Theiler, R. (1996). Evaluation of a German version of the WOMAC (Western Ontario and McMaster Universities) Osteoarthritis Index. Zeitschrift für Rheumatologie, 55(1), 40–49.
• Sun, Y., Sturmer, T., Gunther, K. P., & Brenner, H. (2003). Reliability and validity of clinical outcome measurements of osteoarthritis of the hip and knee: A systematic review. Osteoarthritis and Cartilage, 11(4), 283–298. https://doi.org/10.1016/s1063-4584(03)00029-4
• Tubach, F., Ravaud, P., Baron, G., Falissard, B., Logeart, I., Bellamy, N., Bombardier, C., Felson, D., Hochberg, M., van der Heijde, D., & Dougados, M. (2005). Evaluation of clinically relevant states in patient reported outcomes in knee and hip osteoarthritis: The Minimal Clinically Important Improvement (MCII) and the Patient Acceptable Symptom State (PASS). Annals of the Rheumatic Diseases, 64(1), 29–33. https://doi.org/10.1136/ard.2004.022996
• Vlaeyen, J. W., & Linton, S. J. (2000). Fear-avoidance and its consequences in chronic musculoskeletal pain: A state of the art. Pain, 85(3), 317–332. https://doi.org/10.1016/S0304-3959(99)00242-0
• Wilson, I. B., & Cleary, P. D. (1995). Linking clinical variables with health-related quality of life: A conceptual model of patient outcomes. JAMA, 273(1), 59–65. https://doi.org/10.1001/jama.1995.03520250075037

Items of the Scale