Purpose: 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). Conclusion: 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. Level of evidence: III.
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Acknowledgments We would like to thank Jo Stephens from the Queen Elizabeth Hospital, Gateshead, UK, and Julie Agel from the University of Minnesota, Minneapolis, USA, for their assistance during the data collection phase of the validation exercise. We would also like to thank Dr Christina Jerosch-Herold from the University of East Anglia, UK, and Julie Agel for their advice during the preparation of this paper. Whilst no licence fee is required before using the NPI score, the developers request that institutions register their use of this score for each patient population or study, by emailing the corresponding author (firstname.lastname@example.org). This study forms part of a PhD undertaken at the University of East Anglia by one author (TS), which has been partly funded by Action Arthritis.
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- Outcome measure