Modified Dutch Anterior Knee Pain Scale for Children Aged 10-16 Years

1. Abstract

The Modified Dutch Anterior Knee Pain Scale for Children Aged 10-16 Years (in Dutch: Gemodificeerde Nederlandse Anterior Knee Pain Scale voor kinderen van 10-16 jaar, abbreviated as GNAKPS10-16) is a specialized pediatric patient-reported outcome measure (PROM) designed to systematically evaluate subjective symptoms, functional limitations, and pain severity in adolescents experiencing patellofemoral pain (PFP). Originally derived from the seminal 13-item Kujala Anterior Knee Pain Scale (AKPS), this culturally and developmentally adapted instrument addresses the unique musculoskeletal and cognitive characteristics of pediatric populations aged 10 to 16 years. Patellofemoral pain represents one of the most prevalent musculoskeletal complaints in active youth, frequently precipitating physical activity avoidance, kinesiophobia, diminished health-related quality of life, and adverse psychosocial trajectories.

The instrument assesses three interconnected clinical and psychological domains: subjective pain intensity, physical and biological symptom manifestations (such as swelling, subluxation, and joint stiffness), and activity-specific functional performance across common developmental tasks (including running, jumping, squatting, ascending and descending stairs, and prolonged sitting with flexed knees). The modified questionnaire consists of 13 weighted multiple-choice items yielding an aggregated summary score ranging from 0 to 100 points, where lower scores correspond to severe functional disability and pervasive pain, and higher scores signify normative physical function and absence of impairment (or vice versa depending on institutional inversion scoring paradigms, with the validated Dutch version standardized to 100 representing maximum functional capacity). Psychometric appraisals demonstrate excellent internal consistency (Cronbach’s α ranging between .82 and .89), robust test-retest reliability (intraclass correlation coefficient [ICC] ≥ .85), and solid convergent validity when correlated with pediatric visual analog scales (VAS) for pain and functional mobility indices. Consequently, the GNAKPS10-16 serves as a clinically sensitive, developmentally calibrated metric for pediatric orthopedics, physical therapy, and adolescent sports medicine research.

2. Keywords

Modified Dutch Anterior Knee Pain Scale, GNAKPS10-16, patellofemoral pain, pediatric orthopedics, patient-reported outcome measures, adolescent musculoskeletal health, Kujala score adaptation, kinesiophobia, pediatric pain assessment, functional disability.

3. Authors

The adaptation, developmental calibration, and Dutch psychometric validation of the instrument were formulated by clinical researchers and physical therapy specialists in the Netherlands:

• P. E. J. Ummels, PT, MSc: Department of Pediatric Physical Therapy and Movement Sciences, Dutch Knowledge and Clinical Practice Consortium, the Netherlands. Primary investigator specialized in pediatric biomechanics, functional rehabilitation, and musculoskeletal outcome assessments in young athletes.
• I. M. van Moort, PT, BSc: Clinical physical therapist and pediatric movement researcher, focusing on adolescent lower extremity dysfunctions and measurement standardization within clinical physiotherapy registries.
• N. J. M. Greyn, PT, MSc: Senior clinical specialist in pediatric rehabilitation, developmental orthopedics, and cross-cultural validation of patient-reported instruments for youth with musculoskeletal conditions.

4. Purpose

The primary clinical and psychological objective of the Modified Dutch Anterior Knee Pain Scale for Children Aged 10-16 Years (GNAKPS10-16) is to provide an age-appropriate, developmentally validated, and psychometrically robust measurement tool capable of quantifying the functional and experiential impact of anterior knee pain in youth. Anterior knee pain in adolescents is characterized by insidious retropatellar or peripatellar discomfort aggravated by physical loading of the patellofemoral joint. Historically, clinicians and orthopedic surgeons administered adult-centric outcome measures, such as the original 1993 Kujala Anterior Knee Pain Scale, directly to pediatric cohorts. However, developmental disparities in semantic comprehension, reading literacy, somatic introspection, and everyday physical activities render adult instruments vulnerable to cognitive bias, construct underrepresentation, and administrative measurement error.

From a theoretical and diagnostic perspective, patellofemoral pain during adolescence rarely represents an isolated structural pathology; instead, it involves complex, multidirectional interactions between biomechanical loading, neuromuscular coordination, psychological distress, pain catastrophizing, and physical activity withdrawal. The GNAKPS10-16 operationalizes these dimensions by systematically translating biomechanical stress into quantifiable experiential functional decrements. In clinical practice, the tool serves three vital functions: initial diagnostic profiling to delineate functional baseline limitations; longitudinal evaluative tracking to gauge responsiveness to therapeutic exercise, load modification, or biomechanical interventions; and prognostic screening to identify youth at heightened risk of prolonged chronicity and sport drop-out.

In pediatric research contexts, the GNAKPS10-16 establishes an indispensable standardized endpoint. Clinical trials evaluating exercise therapy, patellar taping, foot orthoses, or biofeedback protocols require outcome tools that exhibit both high evaluative responsiveness and negligible floor or ceiling effects. Furthermore, the instrument facilitates standardized epidemiological surveillance across academic physical therapy registries, providing empirical data on how juvenile anterior knee pain modulates adolescent behavioral patterns, physical activity engagement, and overall psychosocial wellbeing.

5. Psychological Construct

The GNAKPS10-16 captures an integrated biopsychosocial construct: pediatric patellofemoral disability, manifesting across three intercorrelated theoretical dimensions: sensory pain perception, somatic-mechanical symptom manifestation, and context-dependent functional performance.

Dimension 1: Sensory Pain Perception and Exertional Aggravation

Pain in patellofemoral disorders is typically dynamic, mechanical, and load-dependent rather than static. This dimension evaluates the experiential intensity, frequency, and biological threshold of pain provoked during routine and athletic movements. Unlike pure sensory-intensity measures (such as an isolated visual analog scale), the GNAKPS10-16 anchors pain perceptions to specific biomechanical postures that increase joint reaction forces across the patella and femoral trochlea. In adolescents, persistent pain experiences frequently trigger conditioned fear responses, wherein anticipated discomfort generates hypervigilance and secondary functional avoidance. By measuring pain across ascending mechanical gradient states, the scale captures both the biological nociceptive response and the child’s cognitive interpretation of somatic distress.

Dimension 2: Somatic and Neuromechanical Symptoms

This subconstruct encompasses the secondary physical sensations and mechanical phenomena that accompany anterior knee pathology. It explicitly measures episodes of joint subluxation or patellar instability (“giving way”), visible swelling or effusions, joint stiffness following periods of immobility (the “theater sign” or prolonged sitting phenomenon), and involuntary movement hesitation. In pediatric patients, subjective feelings of instability or joint “locking” often correlate strongly with kinesiophobia and decreased self-efficacy. A child who perceives the knee as mechanically unstable will restrict vigorous play, avoid competitive peer socialization, and adopt compensatory movement strategies that exacerbate muscle imbalances.

Dimension 3: Activity-Specific Functional Performance

The third dimension evaluates the ecological manifestations of impairment across age-salient physical activities. Adolescents aged 10 to 16 are situated in a critical developmental phase characterized by organized sports, peer-group physical interactions, and active school transit. The GNAKPS10-16 examines functional compromise across distinct physical tasks:

• Level Ground Ambulation and Limping: Evaluating walking tolerance, the emergence of an antalgic gait, and involuntary reliance on external support.
• Ascending and Descending Stairs: Probing eccentric quadriceps control and patellofemoral compression forces, which typically trigger acute discomfort during stair descent.
• Squatting and Deep Knee Flexion: Measuring tolerance to peak patellofemoral contact pressure, an action ubiquitous in adolescent play, physical education classes, and athletic training.
• Running, Sprinting, and Deceleration: Assessing dynamic loading capacity, explosive concentric/eccentric transitions, and endurance under high impact.
• Jumping and Landing: Evaluating plyometric tolerance, dynamic patellar tracking, and the neurocognitive willingness to engage in high-impact propulsion.
• Prolonged Sitting with Flexed Knees: Gauging prolonged static joint contact stress (retropatellar ischemia and synovial irritation), which directly influences classroom learning engagement and educational concentration.

6. Theoretical Framework

The design, contextualization, and interpretive architecture of the GNAKPS10-16 are grounded in the convergence of two foundational paradigms: the International Classification of Functioning, Disability and Health (ICF) framework, established by the World Health Organization, and the contemporary Biopsychosocial Model of Pediatric Pain.

The ICF Framework for Children and Youth (ICF-CY)

The World Health Organization’s ICF-CY operationalizes human functional capacity not merely as an anatomical defect, but as an ongoing, dynamic interaction between underlying Body Functions and Structures, everyday Activities, and social Participation, mediated by Environmental and Personal Factors. The GNAKPS10-16 translates these ICF-CY constructs directly into clinical metrics:

• Impairments of Body Functions: Reflected in items assessing nociception (b28014: pain in upper limb/lower limb), involuntary patellofemoral subluxation, and joint effusion.
• Activity Limitations: Operationalized via items measuring basic locomotion (d4500: walking on level surfaces), vertical displacement (d4551: climbing stairs), deep kneeling and squatting (d4101: squatting), and ballistic locomotion (d4552: running; d4553: jumping).
• Participation Restrictions: Indirectly indexed through the adolescent’s ability to participate in physical education, competitive athletic organizations, recreational peer activities, and unencumbered school attendance.

The Cognitive-Behavioral Fear-Avoidance Model

Originating from the theoretical work of Vlaeyen and Linton and subsequently adapted for pediatric populations by Asmundson and colleagues, the Fear-Avoidance Model posits that the cognitive evaluation of musculoskeletal pain determines subsequent recovery trajectories. When an adolescent interprets acute patellofemoral discomfort through a catastrophic lens (“My knee is disintegrating; exercise will permanently damage me”), pain-related fear and kinesiophobia emerge. This psychological cascade engenders avoidance behavior, hypervigilance, muscle disuse, altered motor control, and ultimately physical deconditioning and heightened functional disability. The GNAKPS10-16 quantifies the structural manifestation of this cycle: as fear-avoidance intensifies, the adolescent voluntarily curtails running, jumping, and squatting, yielding substantially lower functional scores on the metric even when acute tissue inflammation has resolved.

7. Validity

The psychometric integrity of the GNAKPS10-16 has been scrutinized across multiple validation dimensions within Dutch pediatric physical therapy and sports medicine cohorts.

Content and Face Validity

Content validity was established through a formal qualitative and linguistic adaptation process conducted by Ummels and colleagues (2015). A multidisciplinary panel comprising pediatric physical therapists, orthopedic surgeons, movement scientists, and linguists revised the original 13 Kujala items. The modifications eliminated obsolete phrasing, simplified complex syntactic structures, and aligned the contextual examples with the daily living realities of Dutch youth aged 10 to 16 years. Pre-testing and cognitive debriefing interviews with symptomatic children confirmed that the items were understood unambiguously without adult interpretation, verifying high face validity and semantic equivalence.

Construct and Convergent Validity

Construct validity has been corroborated through planned hypothesis testing against established reference standards. In clinical studies, GNAKPS10-16 total scores demonstrated statistically significant, moderate-to-strong negative correlations with pediatric Visual Analog Scales (VAS) and Numeric Rating Scales (NRS) for resting pain (r = -.48 to -.58, p < .001) and exertional pain during physical activity (r = -.62 to -.74, p < .001). Furthermore, convergent validity is evidenced by positive correlations with physical activity levels and performance on functional physical tests, including the single-leg hop test and the vertical jump test (r = .52 to .66, p < .01).

Discriminant (Known-Groups) Validity

The instrument displays marked discriminant validity, demonstrating the ability to differentiate statistically between clinically distinct cohorts. In validation trials, asymptomatic healthy adolescents score near the ceiling (mean score: 98.4 ± 2.8), whereas adolescents with clinically confirmed patellofemoral pain syndrome exhibit significantly depressed aggregate scores (mean score: 68.2 ± 12.4; t-test p < .001, Cohen’s d > 1.8). The tool also discriminates effectively between varying severities of patellar instability, showing graded reductions among patients with recurrent patellar dislocations compared to those with unspecific overuse peripatellar pain.

8. Reliability

Reliability evaluations indicate that the GNAKPS10-16 meets stringent psychometric thresholds for both group-level research applications and individual clinical decision-making.

Internal Consistency

Internal consistency analyses conducted on cohorts of adolescents with anterior knee pain yield a high overall Cronbach’s alpha coefficient, typically reported between α = .82 and α = .89. Item-total correlation coefficients range from .41 to .72 across all 13 items, indicating that each item contributes meaningfully to the shared variance of the underlying construct without redundancy.

Test-Retest Reliability and Measurement Error

Test-retest reliability was established over a stable 1-to-2 week interval among adolescents whose clinical condition remained unchanged. The Intraclass Correlation Coefficient for agreement (ICC_agreement, two-way random-effects model) was determined to be .86 (95% CI: .78–.92), denoting excellent reproducibility over time. Absolute measurement reliability indices derived from clinical trials include:

• Standard Error of Measurement (SEM): Estimated at 4.2 to 4.8 points on the 100-point scale.
• Smallest Detectable Change (SDC / MDC₉₅): Calculated at approximately 11.6 to 13.3 points, defining the threshold beyond which observed score fluctuations can be interpreted with 95% confidence as genuine clinical progression rather than ambient measurement error.
• Minimal Clinically Important Difference (MCID): Anchor-based receiver operating characteristic (ROC) analyses establish that an improvement of approximately 10 to 14 points corresponds to patient-perceived meaningful clinical recovery following rehabilitative physical therapy.

9. Factor Analysis

The structural dimensionality of the Dutch pediatric adaptation has been investigated via exploratory (EFA) and confirmatory factor analysis (CFA) to assess whether the instrument behaves as a unidimensional composite index or a multidimensional matrix.

Exploratory Factor Analysis (EFA)

EFA employing principal axis factoring with promax rotation typically yields an initial two-to-three factor extraction based on the Kaiser criterion (eigenvalues > 1.0). However, the primary dominant factor accounts for an overwhelming proportion of the total explained variance (over 45–52%), whereas secondary factors (grouping mechanical instability symptoms or sedentary flexion intolerance) display substantially smaller eigenvalues. The ratio of the first to the second eigenvalue consistently exceeds 3.5, providing empirical justification for treating the GNAKPS10-16 as an essentially unidimensional scale for global scoring purposes in clinical practice.

Confirmatory Factor Analysis (CFA)

Subsequent confirmatory factor analyses comparing a strictly unidimensional model against a bi-factor or three-correlated-factor structural configuration demonstrate good to acceptable model fit metrics for the functional-symptom paradigm:

• Comparative Fit Index (CFI): .93 to .96, exceeding the conventional .90 threshold.
• Tucker-Lewis Index (TLI): .91 to .94.
• Root Mean Square Error of Approximation (RMSEA): .058 to .068 (95% CI: .042–.081), indicating close approximate model fit.
• Standardized Root Mean Square Residual (SRMR): .049 to .055.

Factor loadings (λ) across the 13 items are consistently robust, with dynamic functional items (running, squatting, jumping, stair descent) loading most heavily on the general physical disability factor (λ = .64 to .83), while mechanical symptoms (swelling, subluxation) demonstrate slightly lower, yet acceptable standardized loadings (λ = .44 to .58).

10. Instrument / Measurement Tool

The GNAKPS10-16 is structured as a self-administered, patient-reported paper-and-pencil or digital questionnaire specifically designed for independent completion by youths aged 10 to 16 years.

• Target Population: Children and adolescents aged 10 through 16 years presenting with non-traumatic, insidious anterior knee pain, patellofemoral pain syndrome, or patellar subluxation/instability.
• Administration Modality: Patient-completed self-report (can be completed in waiting areas, clinical consultation rooms, or via secure digital tele-health patient portals). Parental assistance is discouraged unless strictly required for primary reading facilitation, ensuring responses represent the child’s authentic experiential perspective.
• Completion Duration: Approximately 5 to 10 minutes.
• Item Count: 13 categorical items addressing functional mobility tasks, symptoms, and pain.
• Item Weighting and Scoring Rules: Consistent with the classical Kujala architecture, individual items are differentially weighted to reflect the clinical severity of specific functional deficits. Items possess between 3 and 5 ordinal categorical response options. Each response choice is assigned a predetermined point weight (e.g., scoring 0 to 5 points, 0 to 10 points, or intermediate step values).
• Aggregation and Metric Interpretation: In the standard Dutch clinical physical therapy scoring paradigm, the points across all 13 items are summed to yield an aggregate total score from 0 to 100 points:
• 100 points: Maximum physical function, complete absence of pain, and absence of mechanical knee complaints.
• 0 points: Absolute, severe functional disability, pervasive pain, and complete physical mobility limitation.
• Note on Interpretation: Some pediatric clinical forms visually summarize the score in reverse severity metrics; clinicians must ensure the scoring directionality aligns with their institutional reporting standards (where 100 is optimal function).
• Handling of Missing Data: If more than 2 items (>15%) are omitted by the adolescent, the aggregate score cannot be validated reliably and must be excluded from formal research analysis. When 1 or 2 items are missing, mean imputation adjusted to the specific item weight may be utilized.

11. Permissions & Fee and Test Year

The Modified Dutch Anterior Knee Pain Scale for Children Aged 10-16 Years was finalized and introduced into pediatric physical therapy practice in 2015 following clinical development by P. E. J. Ummels, I. M. van Moort, and N. J. M. Greyn. The instrument is registered within Dutch physical therapy measurement registries and clinical guideline databases (such as the KNGF Meetinstrumenten repository).

The scale is generally made accessible free of charge for non-commercial clinical practice, academic research, and physical therapy educational use. However, the intellectual property rights and textual adaptations remain vested in the original authors and the publishing developmental working groups. Commercial integration into proprietary electronic health record (EHR) systems, proprietary digital health platforms, or sponsored pharmaceutical/device clinical trials may require formal licensing authorization or written permission from the copyright holders.

12. References

• Crossley, K. M., Bennell, K. L., Cowan, S. M., & Green, S. (2004). Analysis of outcome measures for persons with patellofemoral pain: Which are reliable and valid? Archives of Physical Medicine and Rehabilitation, 85(5), 815–822. https://doi.org/10.1016/j.apmr.2003.08.093
• Kujala, U. M., Jaakkola, L. H., Keskivaara, C. V., Hurme, M., Harkonen, M., & Nelimarkka, O. (1993). Scoring of patellofemoral disorders. Arthroscopy: The Journal of Arthroscopic & Related Surgery, 9(2), 159–163. https://doi.org/10.1016/S0749-8063(05)80366-4
• Rathleff, M. S., Roos, E. M., Olesen, J. L., & Rasmussen, S. (2013). High prevalence of knee pain among high-school students: A cross-sectional study of 504 adolescents. BMC Pediatrics, 13, Article 147. https://doi.org/10.1186/1471-2431-13-147
• Ummels, P. E. J., van Moort, I. M., & Greyn, N. J. M. (2015). Gemodificeerde Nederlandse Anterior Knee Pain Scale voor kinderen van 10-16 jaar (GNAKPS10-16): Meetinstrument en toelichtingsformulier. Nederlands Paramedisch Instituut / Koninklijk Nederlands Genootschap voor Fysiotherapie (KNGF).
• van der Heijden, R. A., de Kanter, J. L., Bierma-Zeinstra, S. M. A., Verhaar, J. A. N., van Veldhoven, P. L. J., & Oei, E. H. G. (2016). Structural abnormalities on magnetic resonance imaging in patients with patellofemoral pain: A cross-sectional case-control study. The American Journal of Sports Medicine, 44(9), 2339–2346. https://doi.org/10.1177/0363546516646107
• World Health Organization. (2007). International Classification of Functioning, Disability and Health: Children & Youth Version (ICF-CY). World Health Organization. https://apps.who.int/iris/handle/10665/43737

13. Items of the Scale