Karlsson Ankle Function Score

Abstract

The Karlsson Ankle Function Score (often abbreviated as the KAFS or simply the Karlsson Score) is an established clinician-administered and patient-reported outcome measure developed by Jón Karlsson and colleagues in 1991. Designed initially to assess functional outcomes following surgical reconstruction and conservative rehabilitation of chronic lateral ankle ligament instability, the instrument has achieved widespread clinical and psychometric prominence in orthopaedic surgery, sports medicine, and physical therapy. The Karlsson Score quantifies subjective and objective dimensions of ankle impairment into an aggregated metric ranging from 0 to 100 points, where higher point values signify superior functional capacity and negligible impairment. The scale comprises eight discrete functional categories: pain (20 points), swelling (10 points), subjective instability or giving way (15 points), joint stiffness (5 points), stair climbing ability (10 points), running capacity (10 points), occupational and activities of daily living performance (15 points), and dependency on external structural support such as taping or bracing (15 points). Psychometric investigations demonstrate that the Karlsson Ankle Function Score possesses strong construct, convergent, and discriminant validity, exhibiting high correlation with alternative extremity instruments such as the Foot and Ankle Outcome Score (FAOS), the American Orthopaedic Foot & Ankle Society (AOFAS) Ankle-Hindfoot Scale, and the Tegner Activity Score. Reliability analyses across international adaptations confirm robust internal consistency (Cronbach’s alpha typically exceeding 0.80) and excellent test-retest stability (intraclass correlation coefficients generally between 0.85 and 0.96). Responsiveness indices, including the standardized response mean and effect size, indicate that the scale sensitively registers clinically meaningful post-intervention changes following ligament repair, arthroscopic stabilization, and structured neuromuscular training.

Keywords

Karlsson Ankle Function Score, chronic ankle instability, lateral ligament reconstruction, patient-reported outcome measures, orthopaedic assessment, foot and ankle surgery, psychometrics, joint stability, functional impairment, rehabilitation outcomes

Authors

The primary author and developer of the instrument is Jón Karlsson, MD, PhD, Senior Professor of Orthopaedics and Sports Traumatology at the Department of Orthopaedics, Sahlgrenska University Hospital and the Sahlgrenska Academy, University of Gothenburg, Gothenburg, Sweden. Dr. Karlsson has served extensively as Editor-in-Chief of Knee Surgery, Sports Traumatology, Arthroscopy (KSSTA) and has contributed foundational clinical trials regarding ankle ligament trauma, biomechanics, and long-term joint survivorship. Co-investigators on the foundational development papers included surgical colleagues and clinical researchers affiliated with the Department of Orthopaedics at East Hospital (Östra Sjukhuset) in Gothenburg, Sweden.

Purpose

The Karlsson Ankle Function Score was conceived to solve a critical methodological deficit in clinical orthopaedics: the absence of a standardized, sensitive, and reproducible system to quantify functional recovery following lateral ankle ligament injuries and reconstructive procedures. While classical physical examinations assess objective mechanical laxity through anterior drawer and talar tilt stress radiography, objective mechanical stability frequently correlates poorly with patient-perceived functional stability, subjective apprehension, and dynamic return to athletic or occupational performance. Therefore, the primary purpose of the Karlsson Score is to provide a standardized, clinically tractable evaluation tool that captures both subjective symptomatology (e.g., pain, stiffness, sensation of giving way) and objective functional capacity (e.g., stair navigation, running, occupational exertion, requirement of external orthoses).

In clinical practice, the tool serves as a routine monitoring instrument administered preoperatively, postoperatively, and during longitudinal outpatient rehabilitation. It enables orthopaedic surgeons, sports physicians, and physical therapists to track recovery trajectories, identify persistent deficits, calibrate progression through return-to-sport protocols, and determine whether surgical interventions such as the modified Broström-Gould procedure, anatomical ligament reconstruction, or arthroscopic repairs have restored functional joint mechanics. In clinical research, the Karlsson Score serves as a standardized primary or secondary endpoint in randomized controlled trials, prospective cohort studies, and systematic meta-analyses comparing surgical techniques, conservative neuromuscular training regimens, and rehabilitation timelines.

The theoretical rationale rests upon the modern biopsychosocial and functional definitions of impairment. An isolated measurement of ligamentous mechanical tensile strength does not account for neuromuscular control, proprioceptive deficits, compensatory movement strategies, or fear-avoidance behaviors that develop following joint injury. By integrating self-reported physical impairments, activity limitations, and compensatory reliance on external stabilization, the Karlsson Ankle Function Score translates multi-faceted joint pathology into an interpretable metric that facilitates direct comparison across clinical cohorts.

Psychological Construct

Although initially developed in an orthopaedic surgical framework, the Karlsson Ankle Function Score operationalizes a multi-dimensional construct encompassing physical impairment, functional disability, and the behavioral adjustments stemming from perceived joint vulnerability. The overarching construct can be designated as perceived functional ankle competence, which is comprised of several interacting physical and behavioral dimensions:

• Nociceptive Sensation and Pain Perception (20 points): Pain is evaluated not merely as an isolated sensory input, but across an exertion continuum ranging from no pain under vigorous athletic loading to severe, unremitting resting pain. This dimension captures pain-related interference during varied mechanical demands, reflecting both tissue healing and central sensitization.
• Inflammatory and Effusion Manifestations (10 points): Evaluates joint swelling ranging from absent to exertional swelling or constant effusion. Swelling reflects persistent synovial inflammation, mechanical overload, and joint capsule irritation, directly influencing patient apprehension and mechanical range of motion.
• Dynamic Joint Stability and Functional Apprehension (15 points): Evaluates the mechanical and subjective phenomenon of ankle ‘giving way’ or subluxation sensations. Rather than measuring passive anatomical laxity, this dimension measures functional instability during dynamic activity—capturing the clinical intersection between ligamentous integrity, sensorimotor proprioception, and protective neuromuscular co-contraction.
• Articular Mobility and Stiffness (5 points): Assesses subjective tightness and loss of mobility. Post-traumatic or post-surgical arthrofibrosis impairs terminal dorsiflexion, which alters kinetic chain mechanics and induces compensatory gait adaptations.
• Locomotor and Biomechanical Agility (20 points combined): Divided into stair climbing (10 points) and running performance (10 points). Stair descent requires eccentric control of the gastrocnemius-soleus complex and terminal dorsiflexion, whereas running requires dynamic propulsion, coordinated shock absorption, and multi-planar neuromuscular stabilization under cyclical impact forces.
• Role Functioning and Occupational Capacity (15 points): Reflects societal participation, occupational exertion, and performance in activities of daily living (ADLs), spanning unrestricted vocational performance to total occupational incapacitation.
• Compensatory Adaptive Behavior and Orthotic Dependence (15 points): Evaluates reliance on external taping or prophylactic bracing. From a behavioral perspective, habitual brace utilization serves as a proxy for both mechanical deficiency and psychological fear-avoidance, reflecting the patient’s trust in intrinsic musculoskeletal stability.

Theoretical Framework

The Karlsson Ankle Function Score aligns closely with the conceptual underpinnings of the World Health Organization’s International Classification of Functioning, Disability and Health (ICF) model and modern orthopaedic patient-reported outcome measurement theory. The ICF framework posits that disability arises from dynamic, multi-directional interactions between anatomical/physiological Body Functions and Structures, personal Activities, and societal Participation, mediated by environmental and personal contextual factors.

Within this framework, the Karlsson Score strategically bridges structural impairment and behavioral limitation. The structural components evaluate capsuloligamentous laxity, synovial effusion, and articular stiffness. However, the instrument heavily weights functional activity limitations—such as stair climbing, running, and daily work tasks—recognizing that patients evaluate their recovery based on functional agency rather than anatomical perfection. Furthermore, the external support category assesses environmental adaptations and compensatory strategies, capturing how patients modify their interaction with physical environments via prophylactic equipment.

The theoretical framework also incorporates Freeman’s seminal theory of articular deafferentation in functional ankle instability (Freeman et al., 1965) and Hertel’s contemporary paradigm of chronic ankle instability (CAI). Hertel conceptualized CAI as an integration of mechanical insufficiencies (pathological laxity, arthrokinematic restriction, synovial changes) and functional insufficiencies (impaired proprioception, neuromuscular inhibition, postural control deficits). The Karlsson Ankle Function Score effectively quantifies this dual paradigm: items evaluating pain, swelling, and stiffness reflect primary mechanical and inflammatory status, whereas items evaluating giving way, running capacity, and taping reliance reflect functional neuromuscular competence.

Validity

The Karlsson Ankle Function Score has undergone extensive psychometric evaluation across diverse orthopaedic and sports rehabilitation contexts. In the initial validation cohorts published by Karlsson et al. (1991, 1996), the instrument demonstrated exceptional discriminant validity, cleanly differentiating between healthy asymptomatic controls, patients with acute lateral ligament ruptures, and individuals suffering from long-standing chronic instability. Control participants consistently scored between 95 and 100 points, whereas preoperative patients with chronic ligamentous rupture exhibited mean baseline scores typically ranging between 45 and 65 points.

Construct and Convergent Validity: Numerous contemporary validation studies have correlated the Karlsson Score with related anatomical and general health inventories. It demonstrates strong convergent validity when compared against the Foot and Ankle Outcome Score (FAOS), with Pearson and Spearman correlation coefficients ranging from r = 0.68 to 0.84 across overlapping subscales (symptoms, daily activities, and sports/recreation). Similarly, correlations with the American Orthopaedic Foot & Ankle Society (AOFAS) Ankle-Hindfoot Scale typically fall between r = 0.70 and 0.86, confirming that both instruments capture a unified underlying construct of hindfoot functional health. In contrast, correlations with divergent constructs, such as the Mental Component Summary (MCS) of the Short Form-36 (SF-36) health survey, are moderate to low (r = 0.22 to 0.38), confirming robust divergent/discriminant validity.

Predictive and Longitudinal Validity: The scale possesses high responsiveness to clinical change. Following lateral ligament repair (such as the Broström procedure or anatomical tendon reconstructions), the Karlsson Score displays statistically significant longitudinal increases, with standardized response means (SRM) and Cohen’s effect sizes consistently exceeding 1.2 to 2.0, demonstrating superior responsiveness compared to generic health utility measures. Postoperative improvements reliably correlate with objective restoration of mechanical talar tilt and anterior drawer stability on stress radiography.

Reliability

The reliability of the Karlsson Ankle Function Score has been corroborated across numerous clinical trials and linguistic adaptation studies. Evaluation of internal consistency has demonstrated Cronbach’s alpha values typically ranging between 0.78 and 0.88 across operative and non-operative cohorts. These values reflect substantial internal coherence among the eight scale items while avoiding excessive item redundancy.

Test-Retest Reliability: Investigations assessing clinically stable patients across intervals of 48 hours to two weeks have demonstrated outstanding temporal stability. The Intraclass Correlation Coefficient (ICC, two-way mixed effects model) regularly ranges between 0.88 and 0.96. The Standard Error of Measurement (SEM) has been reported between 3.2 and 4.8 points on the 100-point scale. Correspondingly, the Minimum Detectable Change at the 95% confidence level (MDC₉₅) is estimated between 8.8 and 11.5 points. Consequently, a change exceeding 10 to 12 points following surgical or therapeutic intervention can be interpreted with high statistical confidence as a true functional improvement beyond measurement error.

Factor Analysis

Empirical investigations utilizing both exploratory factor analysis (EFA) and confirmatory factor analysis (CFA) generally support a dominant unidimensional functional continuum, though minor secondary factors emerge depending on cohort severity:

In classical EFA studies using principal component analysis with varimax or promax rotation, the primary latent factor accounts for approximately 48% to 58% of the total variance. This dominant factor is characterized by heavy factor loadings (>0.65) from items assessing dynamic functional performance: running (0.82), stair climbing (0.78), activities of daily living/work (0.74), and subjective giving way/instability (0.71). A secondary dimension, accounting for 12% to 16% of variance, frequently clusters pain, swelling, and joint stiffness, representing localized inflammatory and nociceptive impairment.

Confirmatory factor analyses testing a correlated two-factor structural model (Factor 1: Dynamic Functional Agility; Factor 2: Impairment and Joint Symptoms) have demonstrated satisfactory goodness-of-fit indices across international validation cohorts:

• Comparative Fit Index (CFI) > 0.94
• Tucker-Lewis Index (TLI) > 0.92
• Root Mean Square Error of Approximation (RMSEA) = 0.055 to 0.068 (90% CI: 0.038–0.082)
• Standardized Root Mean Square Residual (SRMR) < 0.050

Because the correlation between the two factors is typically high (r > 0.70), the standard clinical aggregation of all eight items into a unified, composite 100-point score remains psychometrically justified.

Instrument / Measurement Tool

The Karlsson Ankle Function Score is structured as an ordinal, weighted categorical outcome instrument completed by the patient or administered via structured clinician interview:

• Test Type: Patient-Reported Outcome Measure (PROM) / Clinician-Administered Functional Rating Scale
• Format: Paper-and-pencil or digital questionnaire
• Number of Items: 8 categorical items
• Target Population: Adults and adolescents suffering from acute ankle ligament tears, chronic lateral ankle instability, post-traumatic subtalar or talocrural arthropathy, and patients undergoing lateral ligament reconstruction or rehabilitation
• Response Scale: Point-allocation scale per category (maximum total score = 100 points)
• Scoring Procedure: Points assigned to each chosen descriptor are summed across the eight domains. Total scores range from 0 to 100 points.
• Clinical Grading Scale:
  • 90–100 points: Excellent ankle function
  • 80–89 points: Good ankle function
  • 60–79 points: Fair ankle function
  • < 60 points: Poor ankle function
• Administration Time: Approximately 3 to 5 minutes

Permissions & Fee and Test Year

The Karlsson Ankle Function Score was first introduced in peer-reviewed scientific literature in 1991 by Jón Karlsson, Bengt Bergsten, Göran Lansinger, and Jan Peterson. The instrument was developed within academic medicine and published in international orthopaedic surgical journals. It is placed in the public domain for non-commercial academic research, clinical education, and standard healthcare practice. No licensing fees or royalty payments are required for individual clinical or academic use. Researchers incorporating the measure into commercial trials or electronic medical record software should provide full bibliographic attribution to the original publication (Karlsson et al., 1991).

References

• Freeman, M. A. R., Dean, M. R. E., & Hanham, I. W. F. (1965). The etiology and prevention of functional instability of the foot. Journal of Bone and Joint Surgery (British Volume), 47-B(4), 678–685. https://doi.org/10.1302/0301-620X.47B4.678
• Hertel, J. (2002). Functional anatomy, pathomechanics, and pathophysiology of lateral ankle instability. Journal of Athletic Training, 37(4), 364–375.
• Karlsson, J., Bergsten, B., Lansinger, G., & Peterson, L. (1991). Reconstruction of the lateral ligaments of the ankle for chronic lateral instability. Journal of Bone and Joint Surgery (American Volume), 73(4), 546–553. https://doi.org/10.2106/00004623-199173040-00011
• Karlsson, J., Eriksson, B. I., & Renström, P. A. (1996). Subtalar ankle instability: A review. Sports Medicine, 24(5), 337–346. https://doi.org/10.2165/00007256-199724050-00005
• Karlsson, J., & Lansinger, G. (1992). Lateral instability of the ankle joint. Clinical Orthopaedics and Related Research, 283, 202–210.
• Roos, E. M., Brandsson, S., & Karlsson, J. (2001). Validation of the Foot and Ankle Outcome Score for ankle ligament reconstruction. Foot & Ankle International, 22(10), 788–794. https://doi.org/10.1177/107110070102201004
• van Dijk, C. N., Longo, U. G., Loppini, M., Florio, P., Maltese, L., & Denaro, V. (2012). Classification and scoring systems for lateral ankle ligament injuries. Knee Surgery, Sports Traumatology, Arthroscopy, 20(6), 1147–1155. https://doi.org/10.1007/s00167-011-1778-9

13. Items of the Scale (Questionnaire)