Translation and cross-cultural adaption
Permission to translate and validate the PIPS (English version, see also Rodero et al. [11]) was obtained from the original authors. The translation and cross-cultural adaption process followed the guidelines of Beaton and colleagues [14]. Prior to the study, the pre-final version was administered to a group of five patients suffering from chronic back pain. They provided general remarks about the questionnaire and were interviewed about potential difficulties in understanding the items. The German and the back-translated versions of the questionnaire were also sent to the original authors of the PIPS, who approved the changes that were made. The German version of the PIPS can be found in the (Additional file 1).
Participants
Persons with chronic back pain (defined as back pain that has persisted three months or more) and German mother tongue were recruited via the internet. The survey was promoted on websites of several patient organisations and support groups for chronic pain patients in Germany as well as in an inpatient rehabilitation centre (MediClin Klinik am Hahnberg, Germany). 182 persons (54 men and 128 women) with a mean age 51.0 (±10.5 years) participated in the survey. The participants’ mean pain duration was 12.7 ± 10.9 years.
All participants provided informed consent to participate, and the study was approved by the Ethics Committee of the Department of Psychology, Philipps-University Marburg, Germany.
Instruments
Psychological inflexibility towards pain
The PIPS consists of 12 items rated on a 7-point scale from 1 (never true) to 7 (always true). In the original, 8 items are related to avoidance behaviours and form the subscale Avoidance. Four items concern cognitive fusion and form the subscale Fusion.
Pain intensity
The current and the average pain intensity over the last four weeks were assessed with a numeric rating scale (NRS-P) from 0 (no pain) to 10 (worst pain imaginable).
Pain acceptance
Pain acceptance was measured using the German version of the CPAQ [7, 15]. The CPAQ consists of 20 items rated on a 7-point scale from 0 (never true) to 6 (always true). The subscale Activities Engagement assesses the extent to which the person engages in activities despite their pain (e.g. ‘There are many activities I do when I feel pain.’), whereas the subscale Pain Willingness is an inverted measure of how much the person feels the need to avoid or control pain (e.g. ‘I need to concentrate on getting rid of my pain.’).
Pain catastrophizing
For the assessment of pain catastrophizing, the German version of the PCS was used [16, 12]. The PCS consists of 13 items scored on a 5-point scale ranging from 0 (not at all) to 4 (all the time). The items form three subscales: The subscale Rumination concerns the inability to stop thoughts about pain (e.g., ‘When I am in pain, I keep thinking how badly I want the pain to stop’). The subscale Magnification measures the tendency to exaggerate the threatening nature of the pain (e.g., ‘When I am in pain, I become afraid that the pain will get worse’). The subscale Helplessness captures feeling unable to deal with the pain (e.g., ‘When I am in pain, I feel I cannot go on’). The total score ranges from 0–52, with higher scores indicating higher levels of pain catastrophizing. The PCS has excellent psychometric properties [17].
Fear of movement
Fear of movement was measured using the German version of the Tampa Scale of Kinesiophobia (TSK) [18, 19]. The TSK is a self-report measure of fear of movement (e.g., ‘It is really not safe for a person with a condition like mine to be physically active’) and re-injury (e.g., ‘Pain always means I have injured my body’). It consists of 17 items scored on a 4-point scale from 1 (strongly disagree) to 4 (strongly agree). The resulting scores range from 17–68, with higher scores indicating higher levels of fear. Recent studies have suggested a two-factor structure, including somatic focus and activity avoidance [20]. The TSK has good psychometric properties [4].
Pain-related disability
Disability was measured using the German versions of the Pain Disability Index (PDI) [21, 22], and the Quebec Back Pain Disability Scale (QBPDS) [23]. The PDI measures the extent to which pain interferes with the person’s ability to engage in everyday activities (family/home responsibilities, recreation, social activities, occupation, sexual behaviour, self-care, and life-support activity). Each of the seven areas of activity are rated on an 11-point scale from 0 (no disability) to 10 (total disability). The total score ranges from 0–70, with higher scores indicating higher pain-related disability. The PDI has very good psychometric properties [24].
The QBPDS, more specifically, measures functional disability in persons with back pain. It assesses disability related to basic daily activities and postures (bed/rest, sitting/standing, ambulation, movement, bending/stooping, and handling of heavy objects). The patients are asked to rate the difficulty of performing the respective activity on the current day, using a 6-point scale ranging from 0 (not difficult at all) to 5 (unable to do). The total score is calculated as the sum of all the items with higher scores indicating higher disability.
Anxiety and depression
Depression and anxiety were assessed with the German version of the HADS [25, 26]. The scale was specifically designed for a clinical population with somatic symptoms and measures depressive and anxious symptoms in the past week. The 14-items are answered on a 4-point scale with item-specific response categories. The internal consistencies for both subscales, Depression and Anxiety, were good (Cronbach's α = .80 for each subscale).
Statistical analysis
Item analyses to determine mean item scores and standard deviations, item difficulty, and corrected item-total correlations were computed for each item. Mean inter-item correlations, mean item difficulty and internal consistency (standardized Cronbach’s α) were calculated for the whole scale and each subscale separately. In order to investigate whether the factor structure corroborated the original version, a confirmatory factor analysis was conducted. For measures of fit, we reported χ2 test and the χ2/df ratio, the root mean square error of approximation (RMSEA), the standardized root mean square residual (SRMR) and the goodness of fit index (GFI).
Pearson correlations between the scores of the PIPS subscales and age, current pain intensity, average pain intensity, months since pain onset, and the TSK, the PCS and its subscales, the PDI, the QBPDS, the CPAQ and its subscales and the HADS-D and HADS-A, were calculated. For sex and PIPS score, a point biserial correlation coefficient was computed.
In addition, hierarchical multiple regressions were calculated to assess whether the PIPS made a unique contribution to the prediction of pain-related disability (PDI, criterion) over and above firstly the TSK and secondly the CPAQ. For this we used the method of blockwise forced entry and tested the incremental gain from adding the PIPS when the TSK (or CPAQ, respectively) were already part of the model, and vice versa.
We also computed a multiple regression (Enter method) with the criterion PDI and the predictor variables sex, age, average pain intensity in the last month, PCS, TSK, HADS-A, HADS-D, and the subscales Avoidance and Fusion. Multicollinearity was assessed according to the recommendations of Menard (1995) suggesting that tolerance values below .20 should be of concern. The data were analysed using Statistica version 10 (Statsoft Inc, Tulsa, USA). For the confirmatory factor analysis, AMOS version 21 (IBM SPSS, Meadville, USA) was used.