The Norwegian version of the Norwich Patellar Instability score has good validity and moderate reproducibility

To translate and adapt the Norwich Patellar Instability (NPI) score into Norwegian, and second, to examine the psychometric properties of the Norwegian version (NPI-No).

NPI was translated according to international guidelines. A cohort of 107 patients surgically treated for recurrent patellofemoral instability completed NPI-No, related questionnaires and functional tests prior to and six months post-surgery. Validity (face, content and construct validity), internal consistency (Cronbach's alpha [α]), test-retest reliability (intraclass correlation coefficient [ICC]], measurement error (standard error of measurement [SEM] and smallest detectable change at individual [SDC_ind] and group level [SDC_group]) and construct validity (hypotheses testing; independent t tests, Pearson's r) were examined.

NPI-No had good face and content validity. Internal consistency was satisfactory (α = 0.88), test-retest reliability was moderate ICC_2.1 0.65 (95% confidence interval = 0.47-0.77) and measurement error low (SEM = 7.8). SDC_ind was 21.7 points and SDC_group was 2.8. Seven of the 10 hypotheses about construct validity were confirmed. While there was no ceiling effect pre- or post-operatively, a substantial floor effect (28%) was observed at the 6-month follow-up.

The NPI-No is valid for assessment of self-perceived patellar instability before and after surgery in Norwegian patients. However, reproducibility was found to be only moderate. This study adds further knowledge about the measurement properties of the NPI.

Level II.