BLOG: Which outcome measures are valid for patellofemoral instability?
Patellofemoral instability is a challenging condition, with an incidence of up to 77/100,000 person/years in a high-risk population. Research on the rates of recurrence, sequelae and results of treatments for patellofemoral instability requires outcome measures that have been psychometrically validated in the patient population.
The recent interest in, and growth of, orthopedic outcome measures has included patient-reported outcome measures (PROMs) specific to patellofemoral instability. In addition, a number of studies have assessed the psychometric properties of PROMs commonly used to assess patients with patellofemoral instability and have addressed validity, reliability, responsiveness and cross-cultural adaptation. Patellofemoral instability occurs largely in an adolescent population. Therefore, it is vital that patellofemoral instability outcome measures are valid for assessing these younger patients. PROMs designed for and tested on adults may not consider matters that are relevant and appropriate for younger patients. The level and structure of language used in PROMs might also make questions difficult for younger patients to understand, which could result in incomplete or incorrect questionnaire responses.
This review will assess some of the common PROMs used for patellofemoral instability outcomes assessment in adults and children, with an emphasis on the testing of validity, reliability and responsiveness in the patellofemoral instability patient population.
Kujala Anterior Knee Pain Scale
The Kujala Anterior Knee Pain Scale was originally published in 1993. The Kujala scale has been widely used in patellofemoral instability studies, however only one question directly assesses instability symptoms. A moderate level of validity has been established for the Kujala scale, however the bulk of research has been completed in non-patellofemoral instability populations. Reliability has been demonstrated via high internal consistency (Cronbach’s alpha coefficient = 0.82 to 0.93), and high test-retest reliability (intra-class correlation coefficient (ICC) = 0.86 to 0.96) for patients with patellofemoral instability. the Kujala scale also demonstrated strong responsiveness to change when tested in short-term patellofemoral instability studies (d=0.96; SRM = 1.3).
The Kujala scale has demonstrated some ceiling effects. The Kujala Anterior Knee Pain Scale has not been validated for use in a pediatric population. There does not appear to be a published minimal clinically important difference (MCID) for patients with patellofemoral instability.
Translations of the Kujala scale are available in Italian, French, German, Greek, Thai, Spanish, Dutch, Persian, Brazilian Portuguese, Turkish, Chinese and Arabic. Readability of the English version is rated at a grade 5 reading level, however some of the terminology used in the Kujala scale is difficult for patients to understand and may result in unanswered questions.
Lysholm Knee Scoring Scale and Fulkerson Scale
The Lysholm Knee Scoring Scale was published in 1982 for the assessment of knee ligament instability. Fulkerson and colleagues modified the scale in 1990 by altering the instability and pain item weighting to make the questionnaire more specific to patellofemoral disorders. The Lysholm scale has shown moderate validity in a patellofemoral instability population but with ceiling effects of up to 21%. The Lysholm and Fulkerson scales have demonstrated moderate to high internal consistency (Cronbach’s alpha coefficient = 0.70 to 0.91), and high test-retest reliability (ICC = 0.88) for patellofemoral instability. The standard error of the mean (SEM) for patients with patellofemoral instability does not appear to have been reported. Both the Lysholm and the Fulkerson scales had excellent responsiveness to change in short-term follow-up. The Lysholm scale has not been specifically validated in a pediatric population. No MCID has been reported for patients with patellofemoral instability.
Readability of the English version is rated at a grade 5.7 reading level. The Lysholm has been translated and published in Dutch, Chinese, Turkish, Italian, German and Polish.
IKDC subjective knee evaluation form
The IKDC subjective evaluation form was first developed by an international committee of experts in 1993 and revised in 2001. The IKDC has been validated in patients with patellofemoral instability but has demonstrated significant ceiling effects (47%) indicating poor content validity for the patellofemoral instability population. The IKDC did not correlate well to the Lysholm, Kujala or Fulkerson scales indicating that it may not be measuring the same construct, namely patellofemoral instability. The IKDC has demonstrated moderate to high internal consistency (Cronbach’s alpha coefficient = 0.84) and a test-retest reliability of 0.82 in patients with patellofemoral instability.
The SEM for patients with patellofemoral instability does not appear to have been reported. The responsiveness of the IKDC for patients with patellofemoral instability does not appear to have been reported. MCID has not been determined for the patellofemoral instability population.
Readability of the English version is rated at a grade 8.7 reading level. Published translations for the IKDC are available in Chinese, Arabic, Swedish, Greek, Turkish, German, Thai, Korean, Brazilian Portuguese, Dutch, Italian and French.
Pediatric IKDC subjective knee evaluation form
The Pediatric IKDC subjective knee evaluation form (Pedi-IKDC) was developed in 2011. The Pedi-IKDC was the first knee-specific PROM score to be rigorously validated in a pediatric population. Content and construct validity were confirmed in the original publication with no evidence of floor or ceiling effects. The Pedi-IKDC has demonstrated high internal consistency (Cronbach’s alpha coefficient = 0.9-0.91). The test-retest reliability was excellent (ICC = 0.9), and the SEM has been reported at 4.1. The MCID was reported as 12. (SD 13.5). A recent study on the outcomes of patients after patellofemoral stabilization utilized the Pedi-IKDC as the PROM.
Readability of the English version is rated at a grade 5.6 reading level. The Pedi-IKDC has been published and formally validated in Danish.
The KOOS was published in 1998. The KOOS has not been separately evaluated in a patellofemoral instability population. The five KOOS subscales have demonstrated convergent validity with the Kujala score (r = 0.65-0.84; P<.001). The KOOS demonstrated divergent correlation to the Norwich Patellar Instability (NPI) instrument in patients with patellofemoral instability, suggesting that the KOOS may not be a first-line choice for studying patellofemoral instability. Reliability of the KOOS has not been reported for patellofemoral instability. In patellofemoral instability patients, the KOOS quality of life subscale showed responsiveness to change from preoperatively to 12 months postoperatively.
Recent studies on the outcomes of patients after various patellofemoral stabilization procedures have utilized the KOOS as a PROM but the psychometric properties of the instrument were not specifically reported. KOOS scores have demonstrated variability according to age and gender. The MCID has not been reported for a patellofemoral instability population. Readability of the English version is rated at a grade 4.8 reading level. Translations are available online in 51 languages.
KOOS for children
The KOOS for children (KOOS-Child) was published in 2012. Content and construct validity have been confirmed for the KOOS-Child, with no significant floor or ceiling effects in a population that included patients with patellofemoral instability. The test-retest reliability is acceptable, with an ICC range of 0.78 to 0.91 for the subscales. Internal consistency was demonstrated for all subscales (Cronbach’s alpha 0.8 to 0.9) except for the “symptoms” subscale, which had a Cronbach’s alpha of 0.59. SEM has been reported as 5.28 to 8.14 depending on which subscale was tested. One recent study has used the KOOS-Child for pediatric patients undergoing both operative and nonoperative management for a first-time patellar dislocation. The MCID for patients with patellofemoral instability has not been established.
Readability of the English version is rated at a grade 4.7 reading level. Translations in French, Danish, Greek, Norwegian, and Persian, English and Swedish are available online.
The NPI score was originally published in 2014. The NPI is designed to assess patient-perceived patellofemoral instability symptoms. Face validity and construct validity for the NPI score have been established. The NPI score is the only disease-specific tool that has been validated specifically in a first-time dislocation population. The NPI score has exhibited some floor effects across all 19 items, especially in the conservatively managed population, which may limit its ability to demonstrate improvement in these patients over time. Internal consistency has been reported as high, with a Cronbach’s alpha = 0.93. Test-retest reliability has not been published. The NPI demonstrated parallel responsiveness in first-time dislocators to improvements in the Lysholm scale, the Tegner Activity Score and isometric knee extension strength.
The NPI has not been validated for a pediatric population. There is no reported MCID for the NPI.
Readability of the English version is rated at a grade 5.9 reading level. The NPI has been translated into German for research purposes, but a formally validated translation has not been published.
Banff Patella Instability Instrument
The Banff Patella Instability Instrument (BPII) was initially published in 2013 and underwent factor analysis and item reduction with the introduction of BPII 2.0 in 2016. Face and content validity have been established in patients with patellofemoral instability, for both recurrent instability as well as postoperative stabilization procedures. Construct validity has been established by demonstrating convergent validity of the BPII to the Kujala scale and the NPI score (P<.001). The BPII indicated a degree of predictive validity in an assessment of surgical failures. The BPII 2.0 has been further validated with the successful cross-cultural adaptation into German. For reliability, the BPII 2.0 has demonstrated a Cronbach’s alpha coefficient of greater than 0.90 from preoperative to 12-month postoperative. Test-retest reliability was also assessed for the BPII 2.0 (ICC 0.97). Responsiveness to change of both the BPII and BPII 2.0 has been demonstrated for preoperative and postoperative MPFL reconstruction. The BPII has been validated in adolescents, making it the only disease-specific PROM validated for use in the pediatric population. The BPII 2.0 has also undergone concurrent validation in an adolescent population to the Pedi-IKDC, demonstrating a moderate correlation (r = 0.65) between the outcome measures, as well high test-retest reliability (ICC = 0.94), and no floor or ceiling effects. The SEM was calculated as 2.13. The MCID was established using one half the standard deviation of the mean preoperative score, providing an estimated MCID of 6.2. Readability of the English version is rated at a grade 7.1 reading level. The BPII 2.0 has been formally translated and validated in German, and is currently undergoing validation in Dutch, Spanish, Portuguese, Finnish, Swedish and French.
Standardized PROMs are essential for assessing effectiveness of treatments. While numerous outcome measures have been adopted for use in the patellofemoral instability population, few have undergone rigorous psychometric testing. Generic outcome measures have previously been utilized due to a lack of disease-specific measures. However, it is important for new studies to explain the rationale for the selection of PROMs because generic tools have been shown to be less sensitive than disease-specific measures when repeated over time.
Ideally, PROM scores should be validated for the specific patient condition and population of interest. The BPII (BPII and BPII 2.0) and the NPI score have been specifically developed and subsequently validated for use in patients with patellofemoral instability.
Ensuring that outcome measures are age appropriate is also important because children with patellofemoral instability exhibit distinct risk factors, recurrence rates and outcomes compared with adults. Adult PROMs may not be appropriate for use with younger patients and tools validated for use in pediatric patients with patellofemoral instability should be selected whenever possible. The BPII and BPII 2.0 are currently the only disease-specific PROM validated for pediatric patients. Overall, the use of appropriate, critically validated, PROMs will contribute to stronger evaluation of treatments for the challenging clinical entity of patellofemoral instability.
At the present time, there are only two PROMs that have been designed and tested on large patient cohorts in the patellofemoral instability population, the BPII and NPI. The BPII is a holistic quality of life outcome measure, and the NPI is a symptom score.
Use of these tools in combination with generic knee outcome measures, activity assessment measures and/or psychological outcome measures that are been proven to be valid and reliable across the spectrum of the patellofemoral instability patient population, has the greatest potential to provide a well-rounded evaluation of patient-reported treatment outcomes.
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Laurie Anne Hiemstra, MD, PhD, FRCSC, is from Banff Sport Medicine in Banff, Canada and the Department of Surgery at University of Calgary in Calgary, Canada. Sarah Kerslake, MSc, BPhty, is from Banff Sports Medicine Foundation in Banff, Canada.