Factor Analysis and Item Reduction of the Banff Patella Instability Instrument (BPII): Introduction of BPII 2.0.

BACKGROUND

Clinical management of patellofemoral (PF) instability is a challenge, particularly considering the number of variables that should be taken into consideration for treatment. Quality of life is an important measure to consider with this patient population.

PURPOSE

To factor analyze and reduce the total number of items in the Banff Patella Instability Instrument (BPII). Subsequent to the factor analysis, the new, item-reduced BPII 2.0 was tested for validity, reliability, and responsiveness.

STUDY DESIGN

Cohort study (diagnosis); Level of evidence, 2.

METHODS

Quality of life was measured for PF instability patients (N = 223) through use of the original BPII at their initial consultation. Data from the BPII scores were used in a principal components analysis (PCA) to factor analyze and reduce the total number of items in the original BPII, to create a revised BPII 2.0. The BPII 2.0 underwent content validation (Cronbach alpha, patient interviews, and grade-level checking), construct validation (analysis of variance comparing the initial visit and the 6-, 12-, and 24-month postoperative visits, eta-square), convergent validation (Pearson r correlation to the original BPII), responsiveness testing (eta-square, anchor-based distribution testing), and reliability testing (intraclass correlation coefficient [ICC]).

RESULTS

The BPII was successfully reduced from 32 to 23 items with excellent Cronbach alpha values in the new BPII 2.0: initial visit = 0.91; 6-month postoperative visit = 0.96; 12-month postoperative visit = 0.97; and 24-month postoperative visit = 0.76. Grade-level reading for all items was assessed as below grade 12. The BPII 2.0 was able to discriminate between all time periods with significant differences between groups (P < .05). Eta-square was 0.40, demonstrating a medium to large effect size. The BPII significantly correlated with the BPII 2.0 (0.82, 0.90, 0.90, and 0.94 at the initial visit and 6-, 12-, and 24-month postoperative visits, respectively), providing evidence of convergent validity. A significant correlation was found between the 7-point scale and 24-month postoperative BPII 2.0 scores, a sign of anchor-based responsiveness. ICC (2,k) was 0.97, indicating strong reliability.

CONCLUSION

The BPII 2.0 is valid, reliable, and responsive for assessment of patients with PF instability, both surgically and nonsurgically treated.