Intermittent and Constant OsteoArthritis Pain

Abstract

The Intermittent and Constant OsteoArthritis Pain (ICOAP) questionnaire is a multidimensional, disease-specific patient-reported outcome measure (PROM) developed by an international working group under the auspices of the Osteoarthritis Research Society International (OARSI) and the Outcome Measures in Rheumatology (OMERACT) initiative. Conceptualized by Gillian A. Hawker and colleagues in 2008, the ICOAP was engineered to address a fundamental psychometric limitation of legacy osteoarthritis (OA) instruments—such as the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC)—which fail to delineate between background continuous aching and severe, unpredictable episodic pain flares. The ICOAP is an 11-item self-administered scale comprising two distinct, psychometrically robust subscales: the 5-item Constant Pain Subscale and the 6-item Intermittent Pain Subscale. Each item is scored on a 5-point Likert-type metric ranging from 0 to 4, evaluating pain intensity, sleep impairment, affective impact (frustration, annoyance, feeling upset), impact on overall quality of life, and the unique psychological burden of pain unpredictability. Subscale and total raw scores are normalized to a 0 to 100 metric, where higher scores reflect greater pain severity and functional-emotional disruption. Psychometric evaluations across diverse knee and hip osteoarthritis populations have demonstrated outstanding internal consistency (Cronbach’s alpha spanning .86 to .95), excellent test-retest reliability (intraclass correlation coefficients generally exceeding .85), and robust convergent validity against global pain visual analogue scales (VAS), functional impairment indices, and health-related quality of life inventories. Confirmatory factor analytic investigations consistently support a two-factor oblique structure, with emerging evidence for a bifactor framework incorporating an overarching general osteoarthritis pain dimension. The ICOAP exhibits high sensitivity to pharmacological and surgical interventions, making it an indispensable instrument in both longitudinal clinical trials and orthopedic outcome assessments.

Keywords

Intermittent and Constant OsteoArthritis Pain, ICOAP, osteoarthritis, pain measurement, psychometrics, chronic pain, intermittent pain, constant pain, OARSI-OMERACT, Patient-Reported Outcome Measures

Authors

The ICOAP was developed through an extensive multi-center international qualitative and quantitative initiative coordinated by leading investigators in rheumatology, epidemiology, and health outcomes research:

• Gillian A. Hawker, MD, MSc: Professor of Medicine and Health Policy, Management and Evaluation, University of Toronto; Senior Scientist at the Women’s College Research Institute, Women’s College Hospital, Toronto, Ontario, Canada.
• M. Robynne Stewart, PhD: Division of Health Care and Outcomes Research, Toronto Western Research Institute, University Health Network, Toronto, Ontario, Canada.
• Deborah A. Marshall, PhD: Department of Community Health Sciences, Cumming School of Medicine, University of Calgary, Calgary, Alberta, Canada.
• Aileen M. Davis, PhD: Senior Scientist, Krembil Research Institute, University Health Network; Professor, Department of Physical Therapy and Rehabilitation Sciences Institute, University of Toronto, Toronto, Ontario, Canada.
• International Adaptation Contributors: Prominent cross-cultural investigators including Jean-Francis Maillefert, MD, PhD (Department of Rheumatology, Dijon University Hospital, and INSERM U1093, Dijon, France), who spearheaded the psychometric validation of the Dutch and French linguistic adaptations under the OARSI/OMERACT umbrella.

Correspondence regarding the original development of the instrument is historically directed through the OARSI Initiative publications and the University of Toronto Health Sciences research divisions.

Purpose

The primary clinical and psychometric purpose of the Intermittent and Constant OsteoArthritis Pain (ICOAP) questionnaire is to comprehensively capture the dual-phenomenological experience of pain in individuals diagnosed with symptomatic osteoarthritis of the hip or knee. For several decades, clinical trials and rheumatological registries relied predominantly on instruments such as the WOMAC pain subscale or the Lequesne Algofunctional Index. While these legacy tools possess historical utility, they predominantly focus on activity-related or movement-evoked mechanical pain (e.g., walking on a flat surface, negotiating stairs, rising from a seated position) while neglecting the complex temporal dynamics and qualitative variations inherent to chronic degenerative joint disease.

Qualitative investigations conducting in-depth focus groups with individuals suffering from moderate-to-severe hip and knee OA revealed that patients experience their condition through two interconnected yet distinct pain narratives. The first narrative describes a persistent, underlying, dull, burning, or aching “background” discomfort that remains present almost continually, even at rest or during recumbency. The second narrative characterizes sudden, sharp, lancinating, and intensely severe pain episodes that punctuate daily existence. Critically, patients reported that these intermittent pain flares frequently occur without clear biomechanical precipitants, producing a distressing sense of unpredictability that drastically diminishes perceived control, compromises psychological wellbeing, interrupts nocturnal rest, and forces social and functional withdrawal.

The ICOAP was systematically engineered to address this gap by assessing not only pain intensity across both dimensions but also the associated emotional and cognitive burden. By incorporating items evaluating sleep disruption, frustration, annoyance, and generalized decrement in quality of life—alongside an item dedicated exclusively to the unpredictability of intermittent pain flares—the ICOAP serves multiple critical purposes:

• Clinical Trial Efficacy Evaluation: Dissecting whether therapeutic interventions (e.g., targeted analgesics, intra-articular injections, biological disease-modifying therapies, or total joint arthroplasty) preferentially attenuate constant neurogenic/inflammatory ache versus acute intermittent mechanical/paroxysmal pain spikes.
• Phenotyping and Patient Stratification: Assisting clinicians in characterizing pain phenotypes. For instance, predominant constant resting pain has been correlated with central sensitization and neuropathic-like features, whereas isolated intermittent pain often reflects mechanical stress and localized intra-articular pathology.
• Monitoring Disease Progression: Providing a longitudinal metric that tracks the transition from early-stage, purely activity-induced pain to advanced-stage disease accompanied by intractable, continuous background pain and severe sleep fragmentation.

Psychological Construct

The psychological and clinical constructs measured by the ICOAP are grounded in a multidimensional conceptualization of pain that synthesizes sensory-discriminative, affective-motivational, and cognitive-evaluative dimensions. Rather than treating pain as a unitary neurochemical or structural signal, the ICOAP decomposes the osteoarthritis pain experience into two core operational constructs:

1. Constant Pain

The construct of constant pain is operationally defined as an unrelenting, baseline sensory experience that persists across the observational window (the preceding week) regardless of physical activity or joint immobilization. In the neurobiology of osteoarthritis, constant pain is frequently associated with persistent nociceptive input driven by chronic synovitis, subchondral bone marrow lesions, and periarticular vascular remodeling. Furthermore, as chronic joint inflammation endures, sustained peripheral signaling may induce maladaptive neuroplastic alterations within the dorsal horn of the spinal cord and supra-spinal pain processing pathways—a phenomenon designated as central sensitization.

Within the ICOAP framework, constant pain is not evaluated merely as a millimeter measurement on an intensity spectrum; rather, it is measured alongside its pervasive secondary ramifications:

• Sensory Intensity: The magnitude of the continuous, unremitting aching sensation.
• Sleep Interruption: The degree to which background aching disrupts sleep architecture, preventing restful sleep and promoting the bidirectional cycle of sleep deprivation and hyperalgesia.
• Affective Distress: The continuous cognitive load that elicits persistent emotional fatigue, subjective annoyance, frustration, and feelings of helplessness.
• Global Quality of Life Disruption: The cumulative erosion of vitality, personal agency, and functional independence resulting from the continuous presence of somatic discomfort.

2. Intermittent Pain

The construct of intermittent pain encompasses sudden, transient, paroxysmal, or episodic surges of severe discomfort that “come and go.” Although intermittent pain can be triggered by specific mechanical overloading (e.g., unexpected twisting of the knee, weight-bearing on an irregular surface), it frequently erupts spontaneously. Phenomenologically, patients describe this dimension as “stabbing,” “shooting,” “grinding,” or “an electric shock.”

The psychometric evaluation of intermittent pain encompasses:

• Peak Severity / Intensity: The maximum amplitude of acute pain spikes during the recall timeframe.
• Nocturnal Arousal: Abrupt awakenings caused by sudden positional changes triggering acute joint flares.
• Reactive Affective Burden: The acute emotional upheaval, distress, and irritation evoked by transient, intense pain spikes.
• Erosion of Quality of Life: The restriction of spontaneous social, recreational, and occupational pursuits due to fear of precipitating a painful episode.
• Unpredictability: A uniquely vital cognitive-affective parameter capturing the psychological distress of being unable to forecast when, where, or how intensely the next pain flare will materialize. This unpredictability impairs cognitive mastery, fosters pain catastrophizing, and drives profound fear-avoidance behaviors.

Theoretical Framework

The architectural design and theoretical underpinning of the ICOAP integrate several foundational models within behavioral medicine, psychometrics, and neurophysiology:

The Neuromatrix Theory of Pain

Pioneered by Ronald Melzack, the Neuromatrix Theory of Pain posits that pain is a multidimensional experience produced by a widely distributed neural network—the body-self neuromatrix—rather than a passive readout of peripheral tissue damage. This network integrates sensory inputs, cognitive-evaluative processes, and motivational-affective mechanisms. The ICOAP explicitly operationalizes this tripartite paradigm. Items assessing pure intensity map to sensory-discriminative processing; items querying frustration, annoyance, and upset capture motivational-affective alterations; and the item evaluating the burden of unpredictability taps directly into the cognitive-evaluative appraisal of threat, somatic vigilance, and perceived uncontrollability.

The Biopsychosocial Model of Chronic Illness

Formulated by George Engel, the Biopsychosocial Model emphasizes that structural pathological manifestations (e.g., Kellgren-Lawrence radiographic grading of osteophytes and joint space narrowing) correlate poorly with perceived pain intensity, functional disability, and subjective distress. The ICOAP embodies this model by evaluating how structural degradation interacts with systemic sleep impairment, mood regulation, and life satisfaction. By contextualizing the pain experience within the patient’s overall life space, the instrument quantifies the psychological and biological burden imposed by the illness.

Cognitive Appraisal and the Fear-Avoidance Model

The Fear-Avoidance Model, popularized by Johan Vlaeyen and colleagues, posits that when pain is perceived as an uncontrollable and catastrophic threat, individuals develop pain-related fear, hypervigilance, and behavioral avoidance, leading to physical deconditioning and heightened depression. The intermittent subscale of the ICOAP explicitly addresses this paradigm through its measurement of “unpredictability.” Unpredictable adverse stimuli are known in experimental psychology to induce higher levels of learned helplessness, anxiety, and autonomic arousal than predictable stimuli of equal intensity. By quantifying the cognitive impact of unpredictability, the ICOAP measures the driver that sustains fear-avoidance cycles in osteoarthritis.

Validity

The psychometric validity of the ICOAP has been rigorously corroborated across diverse cross-sectional and prospective longitudinal validation cohorts globally, encompassing both knee and hip osteoarthritis populations.

Construct and Convergent Validity

Extensive studies published by Hawker et al. (2008), Davis et al. (2009), and Maillefert et al. (2009) have demonstrated strong convergent validity. When correlated with legacy joint-specific and generic health status measures, the ICOAP shows robust, hypothesized correlations:

• WOMAC Pain and Function Subscales: ICOAP Constant, Intermittent, and Total scores display high positive correlations ($r = .65$ to $.83$) with the WOMAC pain subscale and physical function subscale, reflecting shared variance regarding mechanical functional limitations.
• Short Form-36 (SF-36) Health Survey: The ICOAP subscales demonstrate strong inverse correlations ($r = -.60$ to $-.78$) with the SF-36 Bodily Pain domain, and moderate-to-strong inverse correlations ($r = -.45$ to $-.62$) with the Physical Functioning and Vitality subscales.
• Visual Analogue Scale (VAS) Pain: Correlations between the ICOAP Total Score and global pain intensity on a 100-mm VAS consistently range between $r = .68$ and $.81$, affirming that the instrument captures core nociceptive intensity while adding valuable multidimensional depth.

Discriminant (Divergent) Validity

Discriminant validity is evidenced by significantly weaker correlations ($r = .20$ to $.38$) between the ICOAP subscale scores and mental health inventories measuring constructs distinct from joint-specific pain distress, such as the SF-36 Mental Health domain or generalized trait anxiety inventories. Furthermore, the ICOAP successfully differentiates between clinical subgroups:

• Patients awaiting total joint replacement surgery demonstrate substantially higher baseline ICOAP Constant and Intermittent scores (mean normalized scores often $> 60$) compared to conservative management cohorts in primary care (mean scores typically between $25$ and $45$).
• The scale effectively discriminates between patients with localized structural joint pathology versus those exhibiting generalized, widespread pain conditions like fibromyalgia, where the proportion of constant affective pain disproportionately overshadows mechanical intermittent pain.

Responsiveness and Longitudinal Validity

The ICOAP demonstrates exceptional responsiveness to clinical change following interventions. In prospective total knee and hip arthroplasty cohorts, the Standardized Response Mean (SRM) and Effect Size (ES) for both the Constant and Intermittent subscales consistently surpass $1.2$ to $1.8$ at 6 to 12 months post-surgery, demonstrating profound sensitivity to the alleviation of joint pain. Similarly, pharmacological clinical trials assessing nonsteroidal anti-inflammatory drugs (NSAIDs) or intra-articular corticosteroid injections demonstrate moderate effect sizes (ES $= 0.45$ to $0.70$), with the intermittent subscale often capturing rapid reductions in inflammatory flares before continuous background aching fully remits.

Reliability

The reliability of the ICOAP has been demonstrated across multiple languages and independent clinical populations, satisfying all criteria established by the COnsensus-based Standards for the selection of health Measurement INstruments (COSMIN) group.

Internal Consistency

Internal consistency estimates, evaluated via Cronbach’s alpha ($lpha$), consistently demonstrate high reliability without excessive item redundancy:

• Constant Pain Subscale (5 items): Cronbach’s $lpha$ spans $.86$ to $.93$ in knee osteoarthritis cohorts and $.88$ to $.94$ in hip osteoarthritis cohorts.
• Intermittent Pain Subscale (6 items): Cronbach’s $lpha$ spans $.87$ to $.94$ across knee and hip samples.
• Total ICOAP Score (11 items): Cronbach’s $lpha$ universally exceeds $.92$, frequently reaching $.95$ to $.96$.
• Item-total correlations for all 11 individual items routinely exceed $.60$, confirming that every item contributes meaningfully to its assigned domain.

Test-Retest Reliability

The temporal stability of the ICOAP over stable intervals (ranging from 48 hours to 2 weeks, in patients reporting no perceived change in clinical status) is excellent:

• Intraclass Correlation Coefficients (ICC): For the Constant Pain subscale, ICC values range from $.83$ to $.91$. For the Intermittent Pain subscale, ICC values range from $.80$ to $.89$. The Total Pain Score typically yields ICC values between $.85$ and $.93$.
• Standard Error of Measurement (SEM) & Minimal Detectable Change (MDC): Psychometric analyses indicate that the SEM for the normalized 0–100 scale approximates $6.0$ to $7.5$ points. The corresponding Minimal Detectable Change at the 95% confidence level ($MDC_{95}$) ranges from approximately $14.0$ to $18.0$ points for the subscales, establishing clear empirical thresholds for discerning true physiological change from random measurement error in clinical monitoring.

Factor Analysis

The latent structural integrity of the ICOAP was initially established through rigorous Exploratory Factor Analysis (EFA) during its developmental validation and has since been repeatedly corroborated via Confirmatory Factor Analysis (CFA) across international cohorts.

Exploratory Factor Analysis (EFA)

During initial scale development by Hawker et al. (2008), principal component analysis and maximum likelihood factor extractions utilizing oblique (Promax) rotation revealed a clear, stable two-factor solution across separate cohorts of individuals with knee and hip OA:

• Factor 1 (Constant Pain): Comprising Items 1 through 5, capturing intensity, sleep disturbance, frustration, upset, and overall quality of life impairment tied to continuous baseline pain. Factor loadings for these five items routinely exceed $.70$ (ranging from $.72$ to $.91$), with negligible cross-loadings ($< .25$) on the secondary factor.
• Factor 2 (Intermittent Pain): Comprising Items 6 through 11, capturing intensity, sleep interruption, frustration, upset, quality of life decrement, and the specific unpredictability of pain that comes and goes. Factor loadings range from $.68$ to $.89$, confirming that item 11 (unpredictability) loads directly onto this episodic dimension.
• The two latent factors exhibit a moderate-to-strong inter-factor correlation ($r pprox .60$ to $.75$), corroborating that while constant and intermittent pain represent discrete clinical phenomena, they stem from a shared underlying osteoarthritic degenerative syndrome.

Confirmatory Factor Analysis (CFA) and Model Fit

Subsequent CFA investigations across English, French, Dutch, German, and Spanish cohorts have formally compared competing structural models:

• Unidimensional Model: Forcing all 11 items onto a single general pain factor yields poor model fit indices ($\chi^2 / df > 5.0$, $CFI < .88$,$RMSEA > .11$), confirming that osteoarthritis pain cannot be adequately modeled as a unitary construct.
• Correlated Two-Factor Model: The two-factor oblique model demonstrates superior fit statistics, consistently meeting established psychometric benchmarks: Comparative Fit Index ($CFI$) $ge .96$, Tucker-Lewis Index ($TLI$) $ge .95$, Root Mean Square Error of Approximation ($RMSEA$) $le .055$ (90% CI: $.042–.068$), and Standardized Root Mean Square Residual ($SRMR$) $le .040$.
• Bifactor Model: Advanced structural equation modeling studies have also evaluated a bifactor model featuring a broad “General Osteoarthritis Pain” dimension alongside two orthogonal specific factors representing constant and intermittent variance. This model demonstrates excellent fit and provides theoretical justification for computing both the two separate subscale scores and the unified ICOAP Total Score.

Instrument / Measurement Tool

The operational administration, scoring algorithms, and technical properties of the ICOAP are detailed below:

• Instrument Designation: Intermittent and Constant OsteoArthritis Pain (ICOAP).
• Instrument Type: Self-report questionnaire / Patient-Reported Outcome Measure (PROM).
• Target Population: Adults and older adults diagnosed with osteoarthritis of the hip, knee, or both.
• Recall Period: The preceding 7 days (“In the past week”).
• Administration Modality: Paper-and-pencil, electronic clinical outcome assessment (eCOA), or web-based digital portals. Typically completed within 2 to 5 minutes.
• Total Item Count: 11 items.
  • Constant Pain Subscale: Items 1 to 5 (5 items).
  • Intermittent Pain Subscale: Items 6 to 11 (6 items).
• Response Metrics:
  • Items 1, 2, 6, 7 (Intensity & Sleep): 5-point Likert scale (0 to 4):
    • 0 = Not at all
    • 1 = Mildly
    • 2 = Moderately
    • 3 = Severely
    • 4 = Very severely
  • Items 3, 8 (Frustration / Annoyance Frequency): 5-point Likert scale (0 to 4):
    • 0 = Never
    • 1 = Rarely
    • 2 = Sometimes
    • 3 = Often
    • 4 = Very often
  • Items 4, 5, 9, 10, 11 (Upset, Quality of Life, Unpredictability Impact): 5-point Likert scale (0 to 4):
    • 0 = Not at all
    • 1 = Mildly
    • 2 = Moderately
    • 3 = Severely
    • 4 = Extremely
• Scoring and Normalization Rules:
  • Constant Pain Subscale Raw Score: Sum of Items 1 through 5 (Raw score range: 0 to 20).
  • Intermittent Pain Subscale Raw Score: Sum of Items 6 through 11 (Raw score range: 0 to 24).
  • Total Pain Raw Score: Sum of all 11 items (Raw score range: 0 to 44).
  • Score Linear Transformation (0 to 100 Scale): Subscale and total raw scores are normalized using the following standard mathematical formula:

    Normalized Score = (Raw Score / Maximum Possible Score) * 100

    Specifically:

    • Normalized Constant Pain Score: $(\text{Raw Score} / 20) \times 100$
    • Normalized Intermittent Pain Score: $(\text{Raw Score} / 24) \times 100$
    • Normalized Total Pain Score: $(\text{Raw Score} / 44) \times 100$
  • Interpretation: A normalized score of 0 indicates no pain, while a score of 100 indicates the worst possible pain severity and impact. Higher scores uniformly reflect worse symptom severity and functional-emotional degradation.
  • Missing Data Handling: If more than one item is missing on either subscale, that subscale score cannot be validly computed. If exactly one item is missing, intra-subscale mean imputation may be applied before linear transformation.

Permissions & Fee and Test Year

The development of the Intermittent and Constant OsteoArthritis Pain (ICOAP) questionnaire was concluded and formally published in 2008 by Dr. Gillian A. Hawker and an international consortium of clinical researchers under the co-sponsorship of the Osteoarthritis Research Society International (OARSI) and OMERACT. The validated Dutch adaptation led by Dr. Jean-Francis Maillefert was published in 2009.

Regarding licensing and copyright:

• Academic and Non-Commercial Clinical Use: The ICOAP is an open-access public domain instrument designed to advance global rheumatological research. It is typically available free of charge for non-commercial academic research, non-funded academic clinical trials, and routine personal clinical practice.
• Commercial and Funded Industry Trials: Commercial entities, pharmaceutical sponsors, or contract research organizations (CROs) wishing to integrate the ICOAP into funded clinical drug/device trials may be subject to licensing agreements and administrative user fees administered through official distribution channels (such as Mapi Research Trust / PROVIDE) or through direct authorization from the copyright holders (OARSI).
• Translations and Adaptations: Official linguistic adaptations and cross-cultural validations require adherence to standardized translation guidelines to ensure cross-cultural semantic equivalence and psychometric integrity.

References

• Davis, A. M., Perruccio, A. V., Canizares, M., Hawker, G. A., Roos, E. M., Maillefert, J. F., & Lohmander, L. S. (2009). Comparative, cross-cultural evaluation of the Dutch, French, Swedish and Canadian English versions of the Intermittent and Constant OsteoArthritis Pain (ICOAP) measure in knee osteoarthritis. Osteoarthritis and Cartilage, 17(11), 1432–1437. https://doi.org/10.1016/j.joca.2009.05.011
• French, D. J., Hawker, G. A., Wellsandt, E., & Hunter, D. J. (2011). Structural validity of the Intermittent and Constant OsteoArthritis Pain (ICOAP) measure in individuals with knee and hip osteoarthritis. Journal of Rheumatology, 38(8), 1709–1716. https://doi.org/10.3899/jrheum.101037
• Hawker, G. A., Stewart, L., French, M. R., Cibere, J., Jordan, J. M., March, L., Suarez-Almazor, M. E., & Gooberman-Hill, R. (2008). Understanding the pain experience in hip and knee osteoarthritis—An OARSI/OMERACT initiative. Osteoarthritis and Cartilage, 16(4), 415–422. https://doi.org/10.1016/j.joca.2007.12.017
• Hawker, G. A., Davis, A. M., French, M. R., Cibere, J., Jordan, J. M., March, L., Suarez-Almazor, M. E., Katz, J. N., & Dieppe, P. (2008). Development and preliminary psychometric testing of a new measure of intermittent and constant osteoarthritis pain (ICOAP) in knee and hip osteoarthritis: An OARSI/OMERACT initiative. Osteoarthritis and Cartilage, 16(4), 423–431. https://doi.org/10.1016/j.joca.2007.12.015
• Maillefert, J. F., Kloppenburg, M., Fernandes, L., Punzi, L., Günther, K. P., Martin Mola, E., Lohmander, S., Pavelka, K., Lopez-Olivo, M. A., Dougados, M., & Hawker, G. A. (2009). Multi-language translation and cross-cultural adaptation of the OARSI/OMERACT measure of Intermittent and Constant OsteoArthritis Pain (ICOAP) in hip and knee osteoarthritis. Osteoarthritis and Cartilage, 17(10), 1293–1296. https://doi.org/10.1016/j.joca.2009.04.017
• Melzack, R. (2001). Pain and the neuromatrix in the brain. Journal of Dental Education, 65(12), 1378–1382. https://doi.org/10.1002/j.0022-0337.2001.65.12.tb03497.x
• 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

13. Items of the Scale (Questionnaire)