Between November 2003 and July 2005 a total of 3184 consecutive patients with low back pain were screened by 170 trained clinicians (73 general practitioners, 77 physiotherapists, and 20 chiropractors). Of these patients, 973 who had non-specific low back pain for less than two weeks agreed to participate in the larger cohort of acute low back pain.11
Overall, 406 patients had not recovered by three months and agreed to join the inception cohort study of recent onset chronic low back pain (fig 1). Few data (0.1%) on prognostic markers were missing. In total, 377 patients (93%) were successfully followed up until they recovered or were censored at the time of their last follow-up. The remaining 29 patients were censored early. Completeness of follow-up was 97% of person time.18
Table 1 shows the baseline characteristics of the study population.
Fig 1Flow of participants through study
Table 1 Characteristics of study population. Values are numbers (percentages) unless stated otherwise
Figure 2 presents the Kaplan-Meier survival curves for the three outcomes. As the estimated survival did not fall below 50%, 25th centile survival times are presented: 178 days (95% confidence interval 153 to 209) for disability, 192 (170 to 222) for pain, and 197 (176 to 223) for complete recovery. The cumulative probability of being pain-free was 35% (141 events) at nine months and 42% (165 events) at 12 months. Of the 259 participants who had not recovered from pain related disability at entry to the chronic study, 39% (99 events) had recovered by nine months and 47% (118 events) by 12 months. The cumulative probability of complete recovery was 35% (139 events) at nine months and 41% (163 events) at 12 months (fig 2). Only 44 of 406 participants (11%) had not returned to work in their previous capacity at the onset of chronicity and, of these, 46% (20 events) had returned to work in their previous capacity by 12 months.
Fig 2Kaplan-Meier survival curves (95% confidence intervals) of recovery from pain (n=406) and disability (n=259) and complete recovery (n=406)
Table 2 shows the characteristics of the study population for pain intensity, pain related disability, and work status at the onset of symptoms (acute presentation), study entry (onset of chronicity), and the follow-up assessments. At the onset of chronicity the cohort had relatively low levels of pain and disability—for example, around two thirds reported pain levels as “mild” or less and disability levels as a “little bit” or less. At nine months, 32% (n=121) of participants reported being free of pain and 59% (n=223) free of pain related disability; respective values at 12 months were 37% (n=142) and 63% (n=238). Before the onset of the acute episode of low back pain, 76% (n=289) of the participants were working full time; this value had decreased to 43% (n=164) at the onset of acute low back pain. At the onset of chronicity, 88% (254/289) of the participants who were employed before the onset of the acute episode had returned to work.
Table 2 Pain, disability, and work status of 380 participants.* Values are numbers (percentages)
See web extra on bmj.com for the full results of univariate Cox regression analysis. Table 3 lists the variables associated with time to recovery from pain. The correlations between the variables feelings of depression and feelings of tension or anxiety and between the variables pain intensity and disability at chronic presentation were greater than 0.6 (Pearson’s r=0.65 for both correlations). Based on ease of assessment and on the univariate hazard ratio, the variables feelings of tension or anxiety and pain intensity were excluded from the multivariate analysis. Of the 15 variables entered into the multivariate model, Cox regression showed that only previous sick leave due to low back pain (likelihood ratio χ21=4.9, P=0.03), high disability levels at chronic presentation (χ21=20.1, P<0.001), low level of education (χ21=3.8, P=0.05), and greater perceived risk of persistent pain (χ21=10.0, P=0.002) were significantly associated with delayed recovery. The adjusted hazard ratio for previous sick leave due to low back pain was 0.69 (95% confidence interval 0.50 to 0.97) and for low level of education was 0.74 (0.54 to 1.00). Therefore participants who had taken previous sick leave due to low back pain and those with no education beyond secondary school were 31% and 26%, respectively, less likely to recover from pain at any time in the future compared with those without these characteristics. The adjusted hazard ratio for disability level was 0.68 (0.56 to 0.81) and for perceived risk of persistent pain was 0.91 (0.86 to 0.97). As these variables are continuous the interpretation is that for every unit increase in these scales, participants are 32% and 9% less likely, respectively, to recover from pain at some time in the future.
Table 3 Unadjusted and adjusted effects of variables on time to recovery from chronic low back pain (n=402)
Table 4 lists the variables associated with time to recovery from disability. Owing to the high correlation between the variables feelings of tension or anxiety and depression (Pearson’s r=0.66) and between pain intensity and disability (Pearson’s r=0.65) these were excluded from the multivariate model. Of the 15 variables entered into the multivariate model, Cox regression showed that only high levels of disability at chronic presentation (likelihood ratio χ21=13.5, P=0.0003), greater perceived risk of persistent pain (χ21=17.0, P<0.0001), and being born outside Australia (χ21=10.7, P=0.001) were significantly associated with delayed recovery as a result of disability. The adjusted hazard ratio for participants born outside Australia was 0.51 (95% confidence interval 0.33 to 0.78). Therefore participants who were born outside Australia were 49% less likely at some time in the future to recover from disability than those who were born in Australia. The adjusted hazard ratio for disability level was 0.69 (0.57 to 0.85) and for perceived risk of persistent pain was 0.88 (0.82 to 0.94). As disability level and perceived risk of persistent pain are continuous variables, for every unit increase in these scales participants were 31% and 12% less likely, respectively, to recover from disability at some time in the future.
Table 4 Unadjusted and adjusted effects of variables on time to recovery from low back pain related disability (n=256)
Because of the strong association between the variable pain intensity and delayed recovery in terms of pain and disability the regression analysis was repeated in both models with the variable pain intensity instead of the variable disability. This sensitivity analysis showed that the same set of variables (previous sick leave due to low back pain, low level of education, greater perceived risk of persistent pain, and being born outside Australia) plus higher pain intensity were significantly associated with delayed recovery owing to pain and disability. The adjusted hazard ratio for pain intensity in the pain model was 0.73 (0.62 to 0.85) and in the disability model was 0.78 (0.66 to 0.92). The adjusted hazard ratios for the other predictors were similar to the primary multivariate analysis (tables 3 and 4).