Psychometric properties of the Norwich Patellar Instability Score in people with recurrent patellar dislocation.

BACKGROUND: Patellar dislocations account for two percent of all knee injuries with a 17% chance of re-dislocation. There are few validated scores specific to instability. One of these is the Norwich Patellar Instability Score. There has been limited assessment of the validity, floor-ceiling effect and responsiveness of the Norwich Patellar Instability (NPI) Score from an independent centre. The purpose of this paper is to address this limitation. METHODS: Data from 90 patients referred to a tertiary referral patellofemoral clinic were analysed. All routinely completed data including NPI Score, Kujala Patellofemoral Disorder Score, and demographic outcomes during appointments, up to 36 months following initial clinical assessment was analysed. Convergent validity was assessed by correlating outcomes of the Kujala Patellofemoral Disorder Score to the NPI Score. Effect size (ES) was determined between baseline to 12-month and 12 to 36-month assessments to determine responsiveness. Floor-ceiling effect was evaluated at baseline, 12-month and 12 to 36-month follow-up. RESULTS: NPI Score demonstrated good convergent validity to the Kujala Patellofemoral Disorder Score (p < .001; 95% confidence interval (CI): -0.71 to -0.52). NPI Score demonstrated good responsiveness to change both between baseline to 12 months (ES: 0.43; 95% CI: 0.42 to 0.10) and 12 to 36 months (ES: 0.67; 95% CI: 0.60 to 0.15). Whilst the NPI Score did not demonstrate a ceiling effect, there was moderate risk of a floor-effect where 13% of the cohort reported the lowest levels of NPI scores. CONCLUSIONS: The NPI Score is a valid and responsive outcome for people with recurrent patellar dislocation.

What activities do patients with patellar instability perceive makes their patella unstable?

Patellar instability is a disabling musculoskeletal condition. Whilst previous texts have suggested that twisting activities may cause patients to experience instability symptoms, no studies have assessed which activities are related to the patient's perceived instability. The purpose of this study was to determine which activities and with what frequency patients with patellar instability symptoms, perceive their patella to be unstable. Ninety patients referred because of recurrent patellar instability were asked to assess the frequency with which they perceived patellar instability for 19 everyday and sporting activities. The results indicated that sporting and multi-directional twisting activities were more frequently related to patellar instability symptoms, compared to lower energy, uni-planar activities. Females and those without a family history of patellar instability reported more frequent patellar instability symptoms, compared to males, or those with a family history of this disorder. Further study is now recommended to determine whether these results reflect that of patients with milder subluxation disorders, and whether factors such as hypermobility have an impact on perceived patellar instability for this patient group.