Abstract
Clinical management of patellofemoral (PF) instability is a challenge, particularly considering the wide range of contributing variables that must be taken into consideration when determining optimal treatment. An important outcome measure to consider in this patient population is disease-specific quality of life (QOL). The purpose of this study was to factor analyse and reduce the total number of items in the Banff Patellar Instability Instrument (BPII). Subsequent to the factor analysis, the new, item-reduced BPII 2.0 was tested for validity, reliability and responsiveness.
Disease-specific QOL was measured in patients with a confirmed diagnosis of PF instability (n = 223) at the initial consultation with the original BPII. Data from these BPII scores was used to employ a principal component analysis (PCA) to factor analyse and reduce the total number of items in the original BPII, to create the new BPII 2.0. The BPII 2.0 underwent content validation (Cronbach's Alpha, patient interviews and reading-level); construct validation (ANOVA comparing the initial consultation, 6, 12 and 24 month post-operative, Eta squared); convergent validation (Pearson r correlation to the original BPII); responsiveness testing (Eta squared, anchor-based distribution testing); and reliability testing (intra-class correlation coefficient (ICC) 2,k).
The original BPII was successfully reduced from 32 to 23 items. The new BPII 2.0 demonstrated excellent Cronbach's Alpha values: initial consult = 0.91; 6-months = 0.96; 12-months = 0.97; and 24-months post-operative = 0.76. Grade-level reading assessment for all items in the BPII 2.0 was below grade twelve. The ANOVA determined the BPII 2.0 was able to discriminate between the initial consultation, 6, 12 and 24 months post-operative assessments, with significant differences between each time-point (p < 0.05). Eta squared was 0.40, demonstrating a medium to large effect size. Convergent validity was established with the BPII 2.0 significantly correlated to the original BPII (initial consult = 0.82, 6-month = 0.90, 12-month = 0.90, and 24-month = 0.94). Anchor-based responsiveness was established with a significant correlation between the 7-point scale of patient-perceived improvement and 24-month post-operative BPII 2.0 scores. Strong reliability was established with an ICC (2,k) = .97.
The BPII has undergone a critical step in its psychometric and clinimetric evolution: structural validation. With the work completed in this study, the BPII and BPII 2.0 have completed assessment of seven of the nine Consensus-based Standards for the selection of health Measurement INstruments (COSMIN) properties including: 1) Internal consistency; 2) Reliability; 3) Standard error of measurement; 4) Content validation; 5) Structural validity; 6) Criterion validity; and, 7) Responsiveness testing. Completion of these assessments and the introduction of a structurally valid and shorter questionnaire, the BPII 2.0, provides a definitive level of credibility to this disease-specific outcome measure.
