Page 131 - THE PERCEPT STUDY Illness Perceptions in Physiotherapy Edwin de Raaij

All three primary outcome were assessed with an 11-point numeric rating scale (0-10). High scores for these three primary outcome measures mean respectively 1) higher levels of pain intensity, 2) stronger limitations in physical function and 3) greater interference of pain in daily activities. The physical function measure was adjusted to patients’ specific limitation in physical function (i.e. bending forward).
Illness Perceptions secondary outcome
The Brief Illness Perception Questionnaire was used to assess patients’ Illness Perceptions representation on LBP10,16. This questionnaire contains nine questions, of which the questions IP1 – IP8 were used in this study. Each item represents a different dimension of IPs. In order to ensure that all higher scores signify stronger dysfunctional IPs, data of the IP3-4 and IP7 were reversed before entering into the analyses.
Co-variates
The selection of co-variates was based on research showing these factors being associat- ed with treatment outcome of LBP. They have also previously been used in a SCED study on persistent LBP5. The co-variates are: fear of movement39, catastrophizing34, avoidance39, somberness24 and sleep37. For all these co-variates we hypothesized that the higher their scores, the more negative impact they will have on the primary outcome.
Statistical analysis
To investigate whether primary outcomes change during and after matched-care physiother- apy treatment, linear mixed model analyses were performed, including all repeated meas- urements as outcome, and ‘phase’ as independent variables. First a crude analysis was per- formed. In a next analysis we controlled for the co-variates.
To investigate whether IPs mediate the effect of matched-care physiotherapy on primary out- comes, these adjusted analyses were performed including the IPs. Based on the change in the coefficient for treatment phase (two dummies, with phase A as reference category) the mediating role of each IP was evaluated independently. The magnitude of the mediation ef- fect, the Indirect Effect, was calculated by subtracting the Direct Effect from the Total Effect. Finally, to investigate whether baseline IPs moderate the effect of matched-care physiother- apy on primary outcomes, effect sizes were calculated for treatment phase and post-treat- ment phase (two dummies, with phase A as reference category) by adding the baseline IPs to the adjusted linear mixed models. The importance of the moderation was evaluated on significance (p<0.05) of the interaction terms.
In addition to statistical significant effects, we evaluated the outcomes on their clinical mean- ingful effect using a threshold of ≥ 30% change in phase A’ on primary outcome from baseline scores phase A31. All analyses were performed with STATA® (version 15).
A multiple baseline single-case experimental design
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