This was a non-randomised controlled study embedded in the Danish Chiropractic Low Back Pain Cohort (ChiCo). All patients included were invited to complete baseline and follow-up questionnaires (see "Data Collection" below). Weekly SMS-tracking of pain in addition to follow-up questionnaires was planned for the first patients enrolled in the cohort until a level of 1,000 participants actively answering the text messages was reached, while the subsequent participants were followed by questionnaires only. Treatment was not affected by study participation, and the treating chiropractors were blinded to the patient reported information registered in the study. The ChiCo cohort and procedures for recruitment and data collection have previously been described in detail . The protocol for this study was not pre-registered. All methods were carried out in accordance with relevant guidelines and regulations.
Participants were recruited from a convenience sample of ten Danish chiropractic clinics. Most patients self-refer to chiropractic treatment in Denmark. Around 20% of costs of chiropractic care is covered by the national health insurance, while the rest is paid out of pocket, by the patients’ private insurance, or some combination of both.
Adults (> 18 years of age) consulting with a primary complaint of LBP with or without leg pain were eligible for inclusion. People who either could not read Danish, were in ongoing treatment for LBP, or with suspected serious pathology were not invited to the study.
Allocation to SMS-tracking
Based on a sample size estimation for investigating trajectories of LBP (unrelated to this study), 1,000 participants with weekly data collection using SMS-tracking was needed. Because of considerable costs related to the set-up of SMS-tracking, this was planned for the participants first enrolled, and to compensate for project participants without a mobile phone and expected drop-outs, 1,623 were enrolled in the SMS-tracking sample (hereafter referred to as the SMS group). Those recruited subsequently were not followed with SMS-tracking and constituted the control group (Fig. 1). We included the entire sample in the study which would be sufficient to demonstrate even a very small effect size.
Content of SMS-tracking
The SMS group received text messages to their mobile phone every week starting seven days after enrolment. The questions asked were (1) “How many days have you had back pain (or back-related leg pain) within the last 7 days? (please answer with one number from 0 to 7)”; (2) “How severe was the pain typically on a scale from 0 to 10?”; and (3) “How many days were you home last week from work or study because of your back pain? (please answer with one number from 0 to 7)”. In case the answer to the first message was “0” the two following messages were not sent. If no response was registered within 2 days, participants received one automated reminder.
Data collection in both groups
Questionnaire data were collected and stored using the online system Research Electronic Data Capture (REDCap) hosted by the Odense Patient data Explorative Network (OPEN). Record IDs and mobile phone numbers were exported to an SMS-Track service that automated weekly distribution of text message questions. Baseline data was collected in two parts. The initial part via an iPad in the clinic prior to the consultation and the second part via a link to an online survey sent by e-mail to the participant on the day of enrolment. Links to follow-up questionnaires were sent after 2 weeks, 3 months, and 12 months. Timing and content of the questionnaires did not differ between the SMS group and the control group.
The primary outcome measure for this study was LBP intensity at 1-year follow-up. Activity limitation and pain control after 1 year were secondary outcomes.
LBP intensity was measured on a 0–10 numeric rating pain scale (0 = no pain; 10 = worst imaginable pain) asking about typical pain in the previous week [14, 15]. Moderate to severe pain at follow-up was defined as NRS > 3 .
Activity limitation was measured by the 23-item Roland Morris Disability Questionnaire (RMDQ) converted to a proportional sum-score ranging from 0 to 100 (0 = No disability; 100 = Fully disabled) [15, 17]. Missing items in partly completed RMDQs were imputed by multiple imputation prior to calculating the sum-score. Non-improvement on RMDQ was defined as < 30% reduction in the sum score from baseline to 1-year follow-up .
Pain control was measured by a single coping item from the Örebro Musculoskeletal Pain Questionnaire (ÖMPQ) (Danish version) asking “Given an average day, to what extent can you handle or control your pain?” (0 = Not at all; 10 = Complete control) . As no formal cut-point exists for categorisation of the item, we defined “lack of control” as scoring < 5. The ÖMPQ was developed as a prognostic screening tool and this measure of pain control has not been validated as an outcome measure.
The two study groups were compared at baseline on the following parameters in addition to baseline values of outcome measures: Age (years), sex, education (higher or further education, vocational education, no qualifying education, other education), physical load at work (very strenuous, strenuous, somewhat strenuous, light, very light), episode duration, STarT Back Tool risk profile (low, medium, high risk), and recovery expectation (ÖMPQ “How certain are you that you will be able to return to ALL of your usual activities 1 month from today?”).
Patient characteristics were described as means with standard deviations (SD) and proportions (%).
Potential differences between the SMS group and the control group in the characteristics of those not completing 12-months follow-up were investigated in logistic regression models with SMS group, baseline characteristic (age, sex, LBP intensity, activity limitation, pain control, STarT risk profile, episode duration, recovery expectations) and an interaction between the two as independent variables and drop out as the dependent variable.
Group differences were investigated using the “intention to treat” principle, with all patients invited to SMS-tracking analysed in the SMS group even if not responding to any SMS questions. In a sensitivity analysis we included only those responding to SMS in more than 26 weeks, i.e. more than half of the SMS follow-ups, in the SMS group. The control group did not differ between the two analysis.
Group differences were estimated by linear (continuous outcomes) and logistic (binary outcomes) mixed models with clinician as random effects to account for dependence between observation from the same chiropractor and reported as beta coefficients and Odds Ratios (OR) with 95% confidence intervals (CI). Group differences were reported as crude estimates and after adjustment for baseline values that were observed to differ slightly between groups: LBP intensity, age, sex, duration of present episode, educational level, and workload.
Analyses were performed using Stata 16.1 (StataCorp, Texas, USA).