title: The development, validation and internal consistency of the Norwich Patellar Instability (NPI) score.
abstract: OBJECTIVE
  No outcome measurements have previously been designed to assess patient-perceived patellar instability. The purposes of this study were to address this limitation and to describe the development and validation of the Norwich Patellar Instability (NPI) scores, a self-administered 19-item questionnaire to assess perceived patellar instability.
  
  
  METHODS
  A previous study assessed activities that aggravated symptoms in individuals with patellar dislocation and instability symptoms. These reported activities were ranked in order of severity, and a weighting system was calculated. The NPI score was introduced to routine clinical practice in three institutions. One hundred and two people who had experienced a lateral patellar dislocation completed 102 NPI questionnaires. The completed NPI score was correlated with the Knee injury and Osteoarthritis Outcome Score (KOOS) to evaluate divergent validity and with the J-sign, patellar mobility, apprehension test, Beighton score, Kujala Patellofemoral Disorder Score and Lysholm Knee Score to evaluate convergent validity. Internal consistency of the 19 items was also calculated.
  
  
  RESULTS
  The results indicated a moderate correlation between the NPI score and the Kujala Patellofemoral Disorder Score (rho = -0.66; p < 0.01) and Lysholm Knee Score (rho = -0.54; p = 0.03), suggesting good convergent validity. There was a little correlation between the KOOS and NPI score, indicating divergent validity (rho = -0.02 to -0.17; n.s.). There was high internal consistency (Cronbach's alpha = 0.93).
  
  
  CONCLUSIONS
  These results indicate the NPI score to be a valid tool to assess patellar instability for individuals following patellar dislocation. Further study is now required in order to assess the reliability and responsiveness of this new outcome measure.
authors.affiliation: Queen's Building, Faculty of Medicine and Health Sciences, University of East Anglia, Norwich Research Park, Norwich, NR4 7TJ, UK, toby.smith@uea.ac.uk.
authors.forename: Toby O
authors.initials: TO
authors.lastname: Smith
authors.forename: Simon T
authors.initials: ST
authors.lastname: Donell
authors.forename: Allan
authors.initials: A
authors.lastname: Clark
authors.forename: Rachel
authors.initials: R
authors.lastname: Chester
authors.forename: Jane
authors.initials: J
authors.lastname: Cross
authors.forename: Deiary F
authors.initials: DF
authors.lastname: Kader
authors.forename: Elizabeth A
authors.initials: EA
authors.lastname: Arendt
mesh_terms: D000328:Adult; D005260:Female; D006801:Humans; D007593:Joint Instability / Q000175:diagnosis* / Q000503:physiopathology; D008297:Male; D017063:Outcome Assessment, Health Care; D031222:Patellar Dislocation / Q000175:diagnosis* / Q000503:physiopathology; D057071:Patellofemoral Joint / Q000293:injuries / Q000503:physiopathology*; D015203:Reproducibility of Results; D012189:Retrospective Studies; D012720:Severity of Illness Index; D011795:Surveys and Questionnaires