Methods
Study Design
The study had a prospective, longitudinal, and correlative design to study changes over time and prediction. Assessments were performed at baseline (2–4 weeks after the accident) and at 3, 6, and 12 months follow-ups. In addition, a cross-sectional comparison with another subsample was done at baseline. According to Swedish law studies using questionnaires without any intervention were not required to be reviewed by the ethic board at this point in time. However, The Regional Ethics Committee in Uppsala, Sweden approved the study protocol (2005:098). Information to participants, obtainment of consent, and other research procedures adhered to the Declaration of Helsinki.
Setting and Participants
Ninety-eight participants were recruited from the emergency wards at two hospitals in Uppsala (University hospital) and Västerås (Regional county hospital) in Sweden between January 2007 and December 2009. Follow-ups were completed in December 2010. Eligibility criteria were; age 18 to 65 years, fulfilled criteria for the diagnosis of WAD grade I and II established by a physician on the emergency ward within 72 hours from the accident, satisfactory Swedish language skills, and subjective report of not being in need of further treatment due to mild pain and disability 2–4 weeks after the accident. For more details is referred to Figure 1.
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Figure 1.
Diagram over the participant flow through recruitment, baseline assessment, 3-, 6- and 12-month follow-ups.
The sample size was determined by the number of patients giving informed consent to be contacted for research purposes within two weeks from the accident and subjectively reported being in no need of treatment due to mild pain and disability. We estimated a priori that a sample size of 98 participants would allow for testing regression with 6 predictors when assuming a medium-size relationship between independent and dependent variables and α = 0.05 and β = 0.20.
Selection and Procedures
A consecutive selection was done meaning that all individuals attending the emergency wards during the scheduled time period underwent a physical examination to establish the WAD grade. Those eligible i.e. WAD grade I and II, received oral and written information about the RCT from an emergency nurse. They were given standardized, written self-management instructions for handling of common physical symptoms in WAD. Within two weeks from the accident, individuals giving consent for being contacted by the study co-ordinator (second author) were contacted by telephone. Those who declined participation in the RCT due to no need of further treatment and mild residual symptoms were informed about the current study. Provided verbal agreement, written information and baseline measures were distributed by ordinary mail. Participants were encouraged to return the questionnaires immediately, but no later than 4 weeks after the accident for the reason of capturing baseline data from the acute stage. Forty of 145 eligible individuals failed to return baseline questionnaires despite two reminders. The mail procedure was repeated at 3, 6, and 12 months for all participants who returned the baseline questionnaires (Figure 1).
Variables and Measures
Pain-related disability was measured with the Swedish version of The Pain Disability Index (PDI) that is a 7-item inventory designed to measure interference with role-functioning due to persistent pain. A general disability score ranging from 0 to 70 was calculated by summing scores of the seven items. Higher scores indicate higher disability. Acceptable psychometric properties have been reported for samples with persistent pain and WAD.
Pain intensity was operationalised as the average pain intensity experienced over the past two weeks, which was scored on a numerical rating scale (NRS) with anchors 0 (no pain) and 10 (worst pain imaginable/unbearable pain). The validity of NRS for pain intensity ratings is well documented and findings include positive, significant correlations with other measures of pain intensity.
Self-efficacy in performing common everyday life activities (functional self-efficacy) was measured by the Swedish version of The Self-Efficacy Scale (SES). The SES measures the strength of perceived self-efficacy in performing 20 common everyday life activities. A general self-efficacy score ranging from 0 to 200 was calculated computed by summing ratings of the 20 activities. Higher scores indicate higher self-efficacy. The Swedish version of SES has shown good reliability in patients with whiplash associated disorder WAD.
Fear of movement and (re)injury was measured by the Swedish version of the Tampa Scale of Kinesiophobia (TSK). A total score ranging from 17 to 68 was calculated where a higher total sum indicates more fear. The Swedish version of TSK has shown good reliability in patients with whiplash associated disorders.
Pain catastrophising was measured with the catastrophising subscale (6 items) from the Coping Strategies Questionnaire (CSQ). The sum of the 6 items was calculated to a sum score ranging from 0 to 36. Higher scores indicate higher frequency of catastrophic thinking. The Swedish version of CSQ has shown high internal consistency.
Post-traumatic stress symptoms was measured with the Impact of Event Scale (IES). The IES consists of 15 in which the patient is asked to report the occurrence of symptoms during the past seven days on four-point scales. High values indicate severe symptoms. A total IES-score was calculated, ranging from 0 through 75. The IES has been reported a valid measure of post traumatic stress reactions.
Data Management and Statistical Analysis
All data were analysed in the IBM SPSS Statistics© version 20.0. Included in the analyses were those with completed questionnaires from all time points. To avoid 'mass imputation', it was decided to exclude questionnaires where >25% of the items were missing. Missing values within the separate questionnaires were substituted with the median of each individual's observed item scores. The total amount of questionnaires with occasional missing items for all measures during all assessments were n = 7. Absolute p-values are reported and the level for statistical significance was set at ≤ .05. The two samples were described and compared using descriptive statistics, chi-square tests and Mann Whitney U tests. The Friedman test was used to analyse statistical changes over time. The cut-off for a clinically relevant deterioration in pain-related disability over the first year was set to ≥ 11 points on the PDI according to a previous study. Pearson's product–moment correlation (r) was used to examine the associations between the outcome (pain related disability) assessed at 12 months follow-up, and the potential predictors assessed at baseline. Core assumptions of linearity were checked before performing the linear regression analysis i.e. independence of the residuals, normally distributed residuals and constant variance of the residuals. Variables were then statistically checked for multicollinearity, which resulted in variance inflated factors (VIF) between 1.4 and 1.8. Hence no severe multicollinearity was supposed to hazard the planned regression models. Multiple linear regression analysis with backward selection was performed to regress pain-related disability at the 12-month follow-up (y1:2) on baseline assessments of pain-related disability (y1:1), pain intensity (y2:1), functional self-efficacy (y3:1), fear of movement/(re)injury (y4:1), pain catastrophising (y5:1), and post-traumatic stress symptoms (y6:1). A forward selection was also performed. Both methods resulted in equal results, and the backward selection method is presented in the results section. Finally, a diagnostic check of the distribution of the residuals was done with the Cook's distance tests. One participant had an extreme Cook's distance (4.7) and was excluded from further analyses.