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IKDC Subjective Knee Form and Marx Activity Rating Scale are suitable to evaluate all orthopaedic sports medicine knee conditions: a systematic review
  1. Ajay C Kanakamedala1,
  2. Allen F Anderson2,
  3. James J Irrgang1
  1. 1Department of Orthopaedic Surgery, University of Pittsburgh, Pittsburgh, Pennsylvania, USA
  2. 2Tennessee Orthopaedic Alliance, Nashville, Tennessee, USA
  1. Correspondence to Dr James J Irrgang, Department of Orthopaedic Surgery, University of Pittsburgh, Room 911, Kaufman Building, 3471 Fifth Avenue, Pittsburgh, PA 15213, USA; jirrgang{at}


Importance Given the increasing importance and number of knee-related patient-reported outcome measures (PROMs), it would be desirable to have one PROM for assessing the outcomes of all patients with orthopaedic sports medicine knee conditions.

Objective The purpose of this review was to evaluate whether the existing literature supports the use and interpretation of the International Knee Documentation Committee Subjective Knee Form (IKDC-SKF) and Marx Activity Rating Scale (MARS) as the primary PROMs in the setting of orthopaedic research or clinical practice. We hypothesised that the reported data for the psychometric properties of the IKDC-SKF and MARS would meet accepted standards for interpretation and use of PROMs.

Evidence review A systematic search of MEDLINE was performed using PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines to identify articles reporting the development and psychometric properties of the IKDC-SKF and MARS. References of included articles were hand-searched to identify additional articles for inclusion. The results of these studies were extracted to determine if the reported psychometric properties for the IKDC-SKF and MARS met established standards.

Findings The reported test–retest intraclass correlations (ICCs) for the IKDC-SKF and the MARS respectively range from 0.85 to 0.99 and 0.81 to 0.97 (ICC>0.70 considered acceptable). The reported effect sizes (ES) and standardised response means (SRM) ranged from 0.76 to 2.11 for the IKDC-SKF and from 0.57 to 1.5 for the MARS (ES/SRM >0.5 or >0.8 are respectively considered moderate and large). The IKDC-SKF has been reported to show moderate/excellent correlations with 60/72 (83%) concomitantly administered measures of physical health and poor/fair correlations with 28/31 (90%) measures of mental health, thus demonstrating its convergent and divergent validity. The MARS has shown moderate/excellent correlations with 3/3 (100%) concomitantly administered measures of level of sports activities, which demonstrates its convergent validity. Standards for interpreting the IKDC-SKF, including the minimal detectable change, minimum clinically important difference, normative data and the patient acceptable symptom state, are also summarised.

Conclusions and relevance This review suggests that the IKDC-SKF and MARS have acceptable psychometric properties to support their use and interpretation to assess the clinical response of patients with a variety of knee conditions in clinical practice and research settings.

  • MeSH Terms: Questionnaires/standards
  • Knee
  • Knee Injuries/diagnosis
  • Knee Injuries/surgery
  • Treatment Outcome

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What is already known?

  • The IKDC Subjective Knee Form (IKDC-SKF) is a knee-specific patient-reported outcome measure that assesses an individual's knee-related symptoms, function, and sports activity. The Marx Activity Rating Scale (MARS) is a knee-specific patient-reported outcome measure of an individual's level of activity.

What are the new findings?

  • The IKDC-SKF and MARS have met accepted standards for the interpretation and use of a patient-reported outcome measure for individuals with a wide variety of sports-related knee injuries. Moreover, the IKDC-SKF has been culturally adapted and validated for world-wide use. These findings support the use of these measures for assessing all such patients in both clinical and research settings.


The past 20 years of orthopaedic practice and research have witnessed a paradigm shift in the way clinical success and patient-centred outcomes are defined. While patient evaluation traditionally relied on physician-assessed rating scales involving physical examination, there has been growing evidence that the results of the physician's examination often do not reflect or relate to a patient's perceptions of his/her symptoms, activity limitations and participation restrictions.1–5 In the past few decades, a consensus has formed that patient-reported outcome measures (PROMs) are essential to forming a complete picture of a patient's symptoms and ability to function.6–8 While PROMs were initially created to standardise the recording and publication of outcomes in research, many clinicians have begun to recognise that the integration of PROMs into routine clinical care might revolutionise the manner in which healthcare is delivered and improved.8 ,9

A PROM, however, is not very useful for comparison if it is not widely used. Feagin and Blake10 initially identified the futility of comparing treatments that used different standards to measure outcomes. This problem was epitomised by the findings in 1993 that, in a 10-year period, The American Journal of Sports Medicine and The Journal of Bone and Joint Surgery had published 52 articles on treatment of anterior cruciate ligament (ACL) of Injury with 38 different systems for scoring treatment outcomes.11 In the past few decades of clinical outcome measurement in orthopaedics, clinicians and researchers have begun to advocate standardising the use of PROMs, which has led to greater comparability between studies. Even so, a recent review reported that, from 1985 to 2010, there have been 24 different PROMs published and used for the knee.12

While this number may seem large, many of these instruments have different purposes and are applicable in unique situations. Many of these forms are disease specific, meaning that the instrument is targeted towards a particular pathology such as osteoarthritis or ligament injuries. Others are region or joint specific, intended to serve as a general measure for a variety of conditions affecting the knee.

Given the large number of PROMs just for the knee, it would greatly aid practitioners if there was agreement on the use of a single measure. Moreover, if a single ‘primary PROM’ could be regularly implemented into a clinical practice, it would provide clinicians and researchers with a large amount of data applicable across a variety of conditions and treatments. The use of any PROM requires evidence to facilitate the interpretation and use of the outcome measure. Ideal qualities of a PROM include extensively tested psychometric properties, widespread use to allow comparison, and minimal respondent and administrator burden, that is, the ideal survey is easy to administer and score and can be completed in a short amount of time.8 ,13 ,14

The purpose of this study is to systematically review whether the current literature supports the use of the International Knee Documentation Committee Subjective Knee Form (IKDC-SKF) and the Marx Activity Rating Scale (MARS) as the primary PROMs for assessing the outcome of patients with a variety of orthopaedic sports-related knee injuries and conditions in clinical and research settings. We hypothesised that the psychometric properties of the IKDC-SKF and MARS would meet established standards for PROMs, including adequate test–retest reliability, content and construct validity, and responsiveness. The IKDC-SKF is a knee-specific patient-reported measure of symptoms, function and sports activity that was initially developed and published in 2001.15 The MARS is a knee-specific patient-reported measure of the individual's level of activity. While they may seem to cover similar content, the IKDC-SKF and MARS are more accurately described as complementary knee-related PROMs. While the MARS measures how frequently a patient performs certain types of activities, the IKDC-SKF measures how well the knee functions and the symptoms that patients encounter when they try to perform such activities. Both PROMs are required to form a complete clinical picture for an individual patient. The pedi-IKDC-SKF and Hospital for Special Surgery Pediatric Functional Activity Brief Scale (HSS Pedi-FABS), which are versions of the adult IKDC-SKF and MARS that have been modified for use in children,16 ,17 will also be discussed.


A systematic search of MEDLINE was conducted using PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines.18 The search was designed to identify articles that described the development and psychometric properties of the IKDC-SKF and MARS. The terms ‘validity’, ‘reliability’, ‘responsiveness’, ‘normative data’, ‘sensitivity to change’, and ‘specificity to change’ were included as key words in the search and combined with the ‘OR’ Boolean operator. Either the ‘International Knee Documentation Committee’ or the ‘Marx Activity Rating Scale’ was then combined with the aforementioned search with the ‘AND’ Boolean operator. Multiple synonyms, such as the ‘IKDC Subjective Knee Form’, were used for each outcome measure. The titles and abstracts of each article were reviewed to identify articles that specifically reported the development and psychometric properties of the IKDC-SKF and MARS as well as the paediatric versions of these measures. Each article's reference section was hand-searched to identify additional articles for inclusion.

Publications were included if the paper explicitly investigated validity, reliability and/or responsiveness of the IKDC-SKF or MARS or if the paper provided normative data for these measures. Articles were only included if they were written in English; articles that described translated versions of the IKDC-SKF and MARS were included as long as they were written in English. Articles were excluded if they were not written in English or did not report psychometric properties of or normative data for the IKDC-SKF or MARS. Figure 1 details the screening process for this review. Included articles were then read and the results for relevant variables related to the psychometric properties of each PROM were recorded in an Excel spreadsheet. These variables include test–retest intraclass correlation (ICC) coefficients, minimal detectable change (MDC), SE of the mean, Cronbach's α, effect size (ES), standardised response mean (SRM), correlations to other variables and minimum clinical important difference (also known as the minimum clinically important change (MCIC)). The importance of each of these variables is summarised below. The values for each of the psychometric variables were then tabulated into a range, and the reported ranges were compared, when appropriate, to established standards.

Figure 1

Diagram of systematic literature search and screening process.

History of the IKDC-SKF

In 1997, owing to the popularity and success of the initial IKDC Evaluation Form as well as improvements in the evaluation of clinical outcomes, the IKDC began to revise the questionnaire component of the original IKDC Evaluation Form into what would become the IKDC-SKF.15 Questions from the previous edition of the IKDC Evaluation Form, the MODEMS Lower Limb Instrument, and the Activities of Daily Living and Sports Activity Scales of the Knee Outcome Survey were considered in the initial draft of the IKDC-SKF.15 After going through two rounds of pilot testing with two different versions of the IKDC-SKF, it was reduced to 18 items based on a review of the frequency of unanswered items as well as each item's statistical properties. The final version of the IKDC-SKF was published in 2001 and is available, along with detailed instructions for scoring, at

The IKDC-SKF includes 18 items covering the domains of symptoms (7 items), sports activities (10 items) and function (1 item). The IKDC-SKF ranges from 0 to 100; higher scores indicate higher levels of function and fewer symptoms. It generally takes <10 min to complete the form.19–22 The IKDC-SKF is endorsed by the American Orthopaedic Society for Sports Medicine (AOSSM); the European Society of Sports Traumatology, Knee Surgery, and Arthroscopy; and the International Cartilage Repair Society.23

Wera et al24 conducted a systematic review from 2005 to 2012 to investigate the prevalence of use of the IKDC-SKF in English language studies of patient outcomes after ACL reconstruction. They found that the IKDC-SKF was the third most frequently used outcome measure, after the IKDC Evaluation Form and the Lysholm Knee Score.24 Six investigators involved in the study assessed the methodological quality of each included article using the Modified Coleman Methodology Score (MCMS) and found that studies in the highest quartile (≥75%) for the MCMS score were more likely to have used the IKDC-SKF and the IKDC Evaluation Form (p=0.002) than studies in the lower quartile of quality.24 On the basis of the prevalence of its use as well as its association with higher methodological quality, the authors recommended the use of the IKDC-SKF and Evaluation Form along with the Tegner Activity Level Scale.24

The IKDC-SKF has been translated into Brazilian (Portuguese), Simplified and Traditional Chinese, Czechoslovakian, Dutch, French, German, Greek, Italian, Japanese, Korean, Norwegian, Persian, Polish, Spanish, Swedish, Thai and Turkish languages. It has been cross-culturally validated in the Chinese,25 Brazilian (Portuguese),26 Greek,27 Italian,19 Korean,28 Polish,20 Thai,29 Persian30 and Turkish22 languages.

Psychometric properties of the IKDC-SKF


Traditionally, validity refers to the degree to which a PROM accurately measures the construct it is intended to measure.13 ,14 ,31 However, recently, validity is defined as the degree to which the evidence supports the interpretation and use of an outcome score for its intended purpose.32 The type of evidence needed to support the interpretation and use of a PROM is dependent on the purpose of the outcome measure.33 When used for clinical practice and research, the primary purpose of the IKDC-SKF is to detect changes in an individual's level of symptoms, function and sports activity as the individual's knee condition improves or worsens. As such, the primary evidence to support the interpretation and use of the IKDC-SKF to assess treatment outcomes should demonstrate both that the score remains stable when the individual's condition has not changed (ie, test–retest reliability) and that the score improves as the individual's condition improves (ie, responsiveness). Other information, such as normative data, the MDC, as well as the minimum clinically important difference (MCID) and determination of the patient acceptable symptom state (PASS), can facilitate interpretation by providing standards against which individual scores can be compared or judged. Descriptions of the practical importance of each of these types of validity evidence are provided below, followed by a comprehensive systematic review of the literature investigating the IKDC-SKF in each respect.

It is important to note that the psychometric properties of any PROM are not fixed properties of the PROM itself; they are determined for a specific population under specific circumstances. For a PROM to be widely used, it needs to be validated in an equally wide range of contexts. Accordingly, the following literature review encompasses a large and diverse range of studies examining a diverse array of patients with a variety of conditions.

Evidence related to content

Content validity refers to the extent to which a PROM covers all relevant aspects of the construct of interest. The questions for the IKDC-SKF were selected by a panel of experts based on content thought to be relevant to measuring the construct of function, symptoms and sports activities in patients affected by any of a variety of knee conditions.14

There were no patients directly involved in the selection of the content or items for inclusion in the IKDC-SKF.15 However, the patient-perceived importance of questions on the IKDC-SKF has been assessed and also compared with other similar PROMs. Tanner et al34 compiled a list of questions from 11 commonly used knee-specific PROMs into a single questionnaire and administered it to patients who had an ACL injury, isolated meniscus tear or osteoarthritis. Patients rated each question on the basis of how frequently they experienced the event referred to in each question as well as their opinion of the question's importance. On the basis of these data, a relative score was calculated for each question. The IKDC-SKF had the second highest relative score both in patients with an ACL rupture and patients with meniscal tears, but was ranked fourth in patients with osteoarthritis. In the first two cases, the only PROM with a higher score than the IKDC-SKF was a disease-specific PROM that was intended for the group's condition.

van Meer et al35 sent surveys to orthopaedic experts (surgeons, residents, physical therapists and sports physicians) at two medical centres as well as patients with a history of an ACL rupture, asking them to score every question on the Knee injury and Osteoarthritic Outcome Score (KOOS) or IKDC-SKF as relevant or irrelevant. A total of 19 orthopaedic experts and 26 patients completed the surveys. A question was judged as relevant if at least 75% of respondents judged the question to be relevant. The results indicated that 89% (16/18) of the questions on the IKDC-SKF were judged to be relevant, while only 60% (25/42) of questions on the KOOS were judged to be relevant to patients with a history of an ACL injury.

Hambly and Griva36 ,37 administered a questionnaire with items representing the content included on the IKDC-SKF and the KOOS to patients who had either undergone ACL reconstruction or articular cartilage repair. Patients were asked to rate the importance and frequency of occurrence of each question on a Likert scale of 0–5 (5=experienced and extremely important, 0=not experienced). In patients who had undergone ACL reconstruction or articular cartilage repair, the IKDC-SKF overall, as compared with the KOOS, had significantly higher average mean importance rating (MIR) and frequency-importance product (FIP) values for the items. The authors concluded that the IKDC-SKF outperformed the KOOS in most of their established criteria.36 ,37 They also pointed out that it is sometimes necessary to consider the length of each PROM when concerned about the respondent and administrator burden.37 The IKDC-SKF is shorter (18 items) than the KOOS (42 items) and simpler to interpret; it provides one total score while the KOOS provides five subscores which may be hard to interpret when they conflict.

Evidence for content validity of the IKDC-SKF has also considered its floor and ceiling effects. Floor and ceiling effects <15% are generally acceptable evidence for content of a PROM.38 All studies that have assessed the floor and ceiling effects of the IKDC-SKFs have found acceptable levels of floor and ceiling effects (<15%).15 ,22 ,25 ,26 ,28 ,29 ,35 ,39–43

Evidence related to reliability

Reliability refers to the ability of a measure to consistently measure a construct without random errors.13 ,14 ,31 Reliability is generally assessed by determining a PROM's test–retest reliability and internal consistency.

Test–retest reliability is assessed by administering an instrument to the same patient cohort at two time points during which the patient's condition is not expected to change and calculating an ICC between the two administrations. The duration between the test and retest for studies investigating the test–retest reliability of the IKDC-SKF has ranged from 3 days to 12 months. This wide range of test–retest duration is important because it is important to know that the IKDC-SKF is reliable over the wide range of possible times that one might realistically follow-up with a patient. The reported ICCs for the IKDC-SKF range from 0.85 to 0.99.15 ,19 ,22 ,25 ,26 ,28–30 ,35 ,40–42 An ICC of above 0.70 is considered satisfactory, thus suggesting that the IKDC-SKF has sufficient levels of test–retest reliability.

Internal consistency is typically assessed by calculating Cronbach's coefficient α, which represents the extent to which the questions in a single administration of the survey all consistently measure the construct of interest, or how related the questions are to each other.44 The reported value for Cronbach's α ranges from 0.77 to 0.97.15 ,19 ,22 ,25 ,26 ,28–30 ,41 ,42 ,45 ,46 A value of 0.7 to 0.9 is generally considered acceptable, thus suggesting that the IKDC-SKF is internally consistent.44

Evidence based on relationship with other variables

Evidence based on relationships with other variables includes convergent and divergent validity. Convergent validity refers to the extent to which a measure is associated with other PROMs that measure the same or similar constructs. For example, higher scores on the IKDC-SKF should be associated with higher scores on other PROMs that measure knee-related function or general physical health. Divergent validity refers to the extent to which a measure is unrelated to other PROMs it should not be related to. In other words, the IKDC-SKF would be expected to show weaker or no relationships with measures of emotional health.

Convergent and divergent validity have been assessed by concurrently administering the IKDC-SKF with the Short-Form-36 Health Survey (SF-36). In 12 studies concomitantly administering the SF-36 and the IKDC-SKF, the IKDC-SKF has shown moderate (0.5≤r<0.75) or excellent (r≥0.75) correlations in 80% (35/44) of the associations that were evaluated with the SF-36 physical function, role limitations to physical problems and bodily pain subscale scores as well as the physical component summary scores. Conversely, the IKDC-SKF has shown poor (r<0.25) or moderate (0.25≤r<0.5) correlations with 90% (28/31) of the associations with the SF-36 mental health and role limitations due to mental problems subscale scores as well as the mental component summary scores.

The IKDC-SKF has also been concurrently administrated with various validated knee-specific PROMs including the KOOS, Lysholm Knee Scoring Scale, Kujala Knee Score, Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC), modified Cincinnati Knee Rating Scale and Oxford-12 item knee questionnaire. Ninety-six per cent (23/24) of correlations with these other knee scales were moderate or excellent.22 ,26 ,28 ,35 ,39 ,41 ,47–49 The IKDC-SKF's consistent demonstration of higher correlations with measures of physical health (60/72 or 83% moderate or excellent) than with measures of mental health (28/31 or 90% poor or fair) provide evidence that the IKDC-SKF is a knee-specific measure of symptoms, sports activities and function.15 ,20 ,22 ,25–28 ,30 ,35 ,39 ,41 ,42 ,47 ,49 ,50


Responsiveness refers to the ability of a measure to detect changes in the construct of interest over time.13 ,14 ,31 Studies assess responsiveness by comparing pretreatment and post-treatment PROM scores for an intervention of proven efficacy. Responsiveness is most commonly assessed by calculating an ES and/or SRM. The ES is equal to the average change between the two time points divided by the SD of the initial scores.31 As such, the ES relates the amount of change to the variability between participants at the time of the baseline administration of the PROM. ES values of 0.2, 0.5 and 0.8 have generally been considered to be small, moderate and large, respectively.51 The reported ES's for the IKDC-SKF have ranged from 0.76 to 2.11.22 ,27 ,35 ,40 ,42 ,43 ,52

The SRM is equal to the average change between the pretreatment and post-treatment scores divided by the SD of the change scores.31 The SRM relates the average amount of change to the variability in the change that each individual participant experienced. Similar to ES, values of 0.2, 0.5 and 0.8 have generally been considered to be small, moderate and large, respectively, for the SRM.51 Five studies have reported a SRM for the IKDC-SKF, which have ranged from 0.57 to 1.5.28 ,39 ,40 ,42 ,43 ,52

Interpreting an IKDC-SKF score for individual clinical decision-making

Minimal detectable change

The MDC (also known as the minimal detectable difference (MDD) or smallest detectable change (SDC)) is a value that is dependent on the magnitude of a PROM's measurement error. Its calculation considers the SE of measurement (SEM) and the standard normal (z) score associated with the level of confidence for the MDC. The SEM is often determined on the basis of the ICC coefficient for the test–retest reliability of the PROM (SEM=SD×√(1-ICC)), where SD is the SD of the scores. Using the SEM, the MDC at the 95% CI is calculated as SEM×1.96×(√2). The reported MDCs for the IKDC-SKF range from 8.8 to 16.4.15 ,22 ,35 ,40 ,42 ,52 These MDCs represent the amount of difference between repeated measurements with the IKDC-SKF that is necessary to ensure that the difference is beyond measurement error and that a true change in the individual's IKDC-SKF score has occurred.

Minimum clinically important difference

Change beyond measurement error does not always translate to clinically meaningful change. This is the rationale behind determining a MCID (which is also sometimes called the MCIC). The MCID serves as the minimum value for change in the PROM score that patients would consider to be an important change in their condition. Based on testing of a general population of patients with impairment of the knee, the MCID for the IKDC-SKF has been reported to be between 11.5 and 20.5.53 For a value of 11.5, the MCID had a sensitivity of 0.82 and specificity of 0.64 and the MCID of 20.5 had a sensitivity of 0.64 and a specificity of 0.84. Based on testing of the IKDC-SKF in patients who had undergone an articular cartilage procedure, the MCID for the IKDC-SKF was reported to be 6.3 at 6 months (sensitivity=0.79, specificity=0.74) and 16.7 at 12 months (sensitivity=0.74, specificity=0.8).40

Normative data

Normative data for the IKDC-SKF have been reported53 previously by gender and age. In general, the goal of treatment is to return an individual to his/her preinjury level of sports activity and participation; however, preinjury scores for PROMs are typically not available since patients only present after they have sustained an injury. Thus, the availability of normative data for the IKDC-SKF provides the context for interpreting an individual's score relative to sex-specific and age-specific population averages. To facilitate this interpretation, it is possible to convert an individual's IKDC-SKF to a standard score that indicates how many SDs an individual's score is above or below the population average.


While the MCID indicates a change in score at which the patient feels better, healthcare providers are becoming increasingly aware of a patient's desire to not just feel ‘better’ but to feel well and satisfied with their state.54 It is thus important to know, for any given PROM, the score that indicates the patient has achieved the PASS, or the state at which the patient feels well or satisfied with their current condition.54 ,55 While there have been some data on the PASS score for patients who have undergone ACL reconstruction,56 PASS thresholds have not yet been determined for patients undergoing treatment for other conditions. Once the PASS is defined for these conditions, clinicians can more confidently determine which treatments returned patients to an acceptable symptom state.

Pedi-IKDC-SKF and the adult IKDC-SKF in paediatric populations

The initial IKDC-SKF (adult IKDC-SKF) was validated in an adult population (average age 36.1 years, range of 13–75 years) and not specifically in a paediatric population.15 Measures that are validated in adult populations with orthopaedic injuries do not, however, always have the same properties in paediatric or adolescent populations with orthopaedic injuries.57 ,58

In 2010, Iversen et al59 conducted cognitive interviews on 30 children with a primary knee injury to understand how they interpret the questions on the adult IKDC-SKF and to identify possible sources of response error. Based on their findings, modifications to the wording of several items were made to create a modified version of the adult IKDC-SKF, the pedi-IKDC-SKF.16 For example, one modification included changing ‘how often have you had pain?’ to ‘how much of the time did your knee hurt?’.

Subsequently, the pedi-IKDC-SKF was tested to provide evidence for reliability, responsiveness and validity in a population ranging from 10 to 18 years of age with a variety of knee conditions.16 Acceptable test–retest reliability (ICC=0.91), internal consistency (Cronbach's α=0.91), construct validity (correlation with 9 relevant domains of Child Health Questionnaire, p<0.0001 and 11 hypotheses were significant with p<0.0001), floor and ceiling effects (<30%) and responsiveness (ES=1.39, SRM=1.35) were demonstrated.16 Normative data for the pedi-IKDC-SKF have not as yet been obtained.

Oak et al21 conducted a study to determine if there was a significant difference between the adult and pedi-IKDC scores in adolescents. Both the pedi-IKDC and adult IKDC-SKF were given to 100 participants aged 13–17 years. Comparison between the adult and pedi-IKDC-SKF scores yielded a significant difference of 1.5 points (p=0.011), but the 95% CI (0.3 to 2.6) was still under the threshold of 5 points set for clinical significance. This value was chosen because it is slightly less than the published MCID for the adult IKDC-SFK.21 Noting the problems with the adult IKDC-SKF in children, the original study emphasised that much of the confusion arose in patients under 13 years of age.59 While this study demonstrates the lack of clinically relevant differences between the two versions of the IKDC-SKF for patients between 13 and 17 years of age, it does not provide evidence that the two forms of the IKDC-SKF are equivalent in the population with the most prior documented confusion with the adult-IKDC.

Marx Activity Rating Scale

The MARS was developed and published in 2001 as an alternative activity rating scale to the other most commonly used activity rating scale at the time, the Tegner Activity Score.60 The MARS is notably different from other activity scales in that, rather than focusing on participation in specific sports, it examines specific activities and functions utilised in various sports.61 It was designed to be a short (<1 min) questionnaire that could be used as a supplement to other PROMs that examine a patient's symptoms and level of function. The MARS consists of four items; higher scores indicate a higher frequency of participation in sports activities that place stress on the knee. In the context of tracking knee function and symptoms, it is important to understand a patient's level of sports activity and the frequency with which they perform activities such as running or pivoting. For example, in a prospective study comparing two forms of articular cartilage repair, it would augment the quality of the study if investigators could show that two treatment groups have similar pretreatment activity levels. Otherwise, the study might be comparing two significantly different groups.

Questions for the MARS were generated by interviewing a group of 20 patients with knee disorders as well as orthopaedic surgeons, physical therapists and athletic trainers who all specialised in sports medicine. Four items were selected for the final version of the MARS after interviewing a group of 50 patients with knee disorders, thus suggesting that the MARS has content validity. In terms of reliability, the ICCs for two administrations of the MARS are reported to range between 0.81 and 0.97, thus suggesting acceptable test–retest reliability.61 ,62 The duration between testing and retesting ranged from 2 to 7 days. Marx et al61 tested the MARS for convergent validity by co-administering it with three other activity rating scales. The MARS demonstrated moderate or excellent correlation with 100% (3/3) of these scales (Tegner scale, r=0.66; Cincinnati scale, r=0.67; Daniel scale, r=0.52). The MARS also demonstrated a fair (r=−0.48) inverse correlation with age, indicating that activity level decreases with increasing age. The responsiveness of the MARS has not as yet been assessed. Normative data for the MARS has been reported for a young physically active population.63 Overall, the current literature does suggest that the MARS is a reliable and valid PROM to assess the frequency of participation in sports-related activities for an adult population.

While the MARS has been validated in an adult population, a recent study examining the MARS in a paediatric population found an unacceptable ceiling effect (50.6%), which limits the use of the MARS in this population.64 Owing to the lack of an activity rating scale specifically designed for paediatric populations, the HSS Pedi-FABS was developed and published in 2013.17 Although the original paper describing this PROM does not specifically describe it as a paediatric version of the MARS, its structure as well as the questions included are very similar. Fabricant et al17 ,65 found acceptable test–retest reliability (ICC=0.91), internal consistency (Cronbach's α=0.914), no ceiling or floor effects, and suitable construct validity. The responsiveness of the HSS Pedi-FABS has not as yet been reported and future studies are necessary to further explore its psychometric properties.


Traditionally, healthcare providers have relied on history and physical examination to assess the progress of their patients. Such means of assessing patient's progress complicated the research and reporting of results, as well as functional tests such as the Lachman often did not correlate with the sorts of concerns and functional deficits that patients were concerned with.1–5 These drawbacks paved the way for the use of PROMs in facilitating evidence-based practice in orthopaedics.14 A validated PROM can provide a standardised method for measuring the patient's perception of his/her symptoms, activity limitations and participation restrictions. In the treatment of knee conditions, there are a variety of PROMs available to assist in making clinical decisions and for clinical research.12 The IKDC-SKF and MARS, both published in 2001, have been extensively studied and stand together as validated, internationally used24 and patient-endorsed34–37 ,61 instruments that can measure symptoms, function and sports activities in patients with a variety of knee conditions. The IKDC-SKF has substantial evidence that can be used to facilitate interpretation of scores for individual patients, making it useful for monitoring patient status. With regard to limitations, this systematic review does not directly compare the IKDC-SKF to similar knee-specific PROMs. It is important to note, though, that the multiple articles summarised here did directly compare the IKDC-SKF to similar knee-specific PROMs and showed it to be more relevant to the issues that patients are most concerned about,34–37 as well as one of the most widely used.24 Given the role of healthcare providers to deliver personalised care that addresses each patient's specific concerns, the IKDC-SKF and MARS can both serve as valuable tools for tracking each patient's symptoms, activity limitations and participation restrictions enabling clinicians to more confidently apply the latest scientific research to their own practice.


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  • Correction notice This paper has been amended since it was published Online First. The competing interest statement was inadvertently removed by the production office. This has now been reinstated.

  • Contributors JJI conceived the original review. JJI and ACK were responsible for planning the review. ACK was responsible for conducting the systematic search, screening all articles, compiling all data, conducting the statistical analysis, and drafting the manuscript. JJI and ACK were responsible for revising the manuscript. JJI and AFA were responsible for conceptualisation of the study as well as final review and approval of the manuscript.

  • Competing interests JJI and AFA were both involved in the development and validation of the IKDC Subjective Knee Form.

  • Provenance and peer review Commissioned; externally peer reviewed.

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