The Norwich Patellar Instability Score: validity, internal consistency and responsiveness for people conservatively-managed following first-time patellar dislocation

Background: This paper assessed the validity, internal consistency, responsiveness and floor-ceiling effects of the Norwich Patellar Instability (NPI) Score for a cohort of conservatively managed people following first-time patellar dislocation (FTPD).  Methods: Fifty patients were recruited, providing 130 completed datasets over 12 months. The NPI Score, Lysholm Knee Score, Tegner Level of Activity Score and isometric knee extension strength were assessed at baseline, six weeks, six and 12 months post-injury.  Results: There was high convergent validity with a statistically significant correlation between the NPI Score and the Lysholm Knee Score (p < 0.001), Tegner Level of Activity Score (p < 0.001) and isometric knee extension strength (p < 0.002). Principal component analysis revealed that the NPI Score demonstrated good concurrent validity with four components account for 70.4% of the variability. Whilst the NPI Score demonstrated a flooring-effect for 13 of the 19 items, no ceiling effect was reported. There was high internal consistency with a Cronbach Alpha value of 0.93 (95% CI: 0.91 to 0.93). The NPI Score was responsive to change over the 12 months period with an effect size of 1.04 from baseline to 12 months post-injury.  Conclusions: The NPI Score is a valid tool to assess patellar instability symptoms in people conservatively managed following FTPD.  Level of evidence: Level II