Background
Neck and low back pain is a common cause of long term sickness absence as well as exclusion from the labor market, both in Sweden and internationally [
1]. In Sweden alone, the cost of neck and low back pain has been estimated to 1.3% of GNP [
2]. Thus, the prevention of chronicity has become important, in order to reduce costs and to lessen the suffering for individuals with neck pain (NP) and low back pain (LBP) [
3]. It has been suggested that early preventive interventions may reduce future problems as well as selection criteria are of outmost importance for the outcome [
4].
Psychological factors have long been associated with chronic pain and they also seem to exacerbate the clinical component of pain [
5,
6]. In fact, psychosocial factors have shown not only to be pivotal in the transition from acute and subacute NP and LBP to chronicity but also have a strong influence on the onset of pain [
5,
7,
8]. Furthermore, coping or elements of coping have been shown to be a strong to moderate predictor for future LBP [
9,
10].
Attempts have been made to classify patients into subgroups to better understand which subtypes of patients would benefit from what particular treatment [
11‐
13]. The Multidimensional Pain Inventory (MPI) [
14] was originally developed for chronic pain patients and is widely used to derive subgroups of patients [
15]. Three different subgroups derived empirically from the MPI have been labeled: dysfunctional (DYS), interpersonally distressed (ID) and adaptive copers (AC) [
15]. The DYS subgroup are characterized by high pain severity, disability and affective distress, and ID individuals are characterized by low levels of social support, while the AC subgroup report a more successful adjustment to chronic pain.
The overall objective of the study was to evaluate the predictive validity of a subgroup classification based on the Swedish version of the MPI (the MPI-S) [
16,
17] among gainfully employed workers with NP and LBP during a follow-up period of 18 and 36-month.
Due to poorer coping ability and higher pain severity in both ID and DYS individuals, compared to AC individuals, it is hypothesized that DYS and ID subgroups should have more sickness absence than the AC subgroup. Secondly, it was hypothesized that DYS and ID individuals should score worse in regard to bodily pain, mental and physical health compared to the AC subgroup at the 18 and 36-month follow-ups.
To the best of the authors' knowledge, this is the first study that uses the Swedish version of the MPI (MPI-S) on a gainfully employed working population.
Discussion
The aim of this study was to evaluate the predictive validity of the MPI-S with regard to sickness absence, bodily pain, and mental and physical health on a gainfully employed working population with NP and/or LBP at 18 and 36-month follow-ups. As hypothesized, individuals with more pronounced psychosocial difficulties (DYS) demonstrated statistically significant more sickness absence compared to AC patients at 36 months. DYS patients also had a threefold increase in the risk ratio of long-term sickness absence (>30 days) at 18 months follow-up. Individuals belonging to the ID group showed overall more sickness absence compared to the AC patients at the 36-month follow-up. Furthermore, the ID subgroup had a twofold increase in the risk ratio of long-term sickness absence at 18 months. Figure
2 gives a visual overview of the trend regarding sickness absence throughout the study period for all of the MPI-S subgroups. It is noteworthy that the AC subgroup had consistently less sickness absence compared to the other MPI-S groups.
The secondary hypothesis was also confirmed demonstrating a significant difference in mental and physical health as well as bodily pain for ID and DYS individuals compared to the AC subgroup at both 18 and 36-month follow-ups. This further supports the definition of these three MPI-S groups in a working population.
In comparison with studies analyzing individuals with chronic pain using the MPI-S questionnaire [
35], the proportion of participants in the current study's AC subgroup is high. This seems reasonable, as this population probably includes a large proportion of individuals who may not have developed inappropriate coping strategies or pain behavior. In other studies, the AC subgroup report a high level of social support, and relatively low levels of pain. They also seem to remain active despite pain [
36,
37]. Further, it is evident that AC individuals in this study had consistently less sickness absence compared to both DYS and ID individual. This might possibly be due to important determinants for claiming sick leave due to LBP [
38], i.e. AC individuals are characterized by better coping abilities, a more positive outlook in regard to LBP, less pain and co-morbidities.
LBP is not a self-limiting condition [
39‐
41] as a large proportion (42-75%) still experience pain after 12 months and a majority (44-78%) experiences relapses of pain [
39]. Further, recurrence of LBP is strongly correlated with previous episodes of LBP [
39,
42]. The population in this study consisted of gainfully employed workers with NP/LBP, and the majority of the individuals could be considered non-chronic in nature. Nevertheless, due to previous episodes many individuals with recurrent NP and LBP may have similar experiences of pain as chronic patients, thus making the MPI-S questionnaire a useful tool in this kind of population as well.
ID and DYS individuals may appear similar when comparing pain, disability and emotional distress, but their coping ability seems to differ, i.e. DYS patients often rely more on fear-avoidance coping strategies compared to both ID and AC patients [
37,
43]. Further, DYS patients have been found to be significantly more depressed compared to ID patients and are more likely to dwell on somatic symptoms or suffer from hypochondriasis [
37]. However, there may be different factors associated with depression in these two groups [
23], i.e. depression for ID patients could possibly more connected to marital and interpersonally difficulties compared to DYS patients. In addition, ID patients reported a lack of support from their significant other and rated their interpersonal relationships to be of lower quality compared to both DYS and AC patients [
37]. However, the distinction between ID and DYS patient groups have recently been questioned in a recent study by Junhaenel et al [
36]. The study found no statistically significant difference in some measures regarding interpersonal relationship between the two groups [
36]. However, the sample size in the cited study was small which could have contributed to the non-significant results. Nevertheless, predictive results from previous studies have shown that ID and DYS patients do differ in the development of future sickness absence [
23,
35].
Recurrent and persisting symptoms of NP and LBP may be more related to psychosocial factors than medical aspects [
21,
44]. Thus, when trying to predict the likelihood of progression from acute/subacute NP/LBP into chronicity and in turn future long-term sickness absence, it is necessary to address psychosocial factors. Heitz et al suggests that a psychosocial intervention may be more effective in a subacute phase but that an interdisciplinary approach is justified in both subacute and chronic LBP stages [
45]. Thus, DYS and ID considered at-risk may be offered early vocational rehabilitation, counseling when needed, interventions related to improve interpersonal relationships for ID patients, and scheduled for a follow-up visit with the OHS. Collaboration with other caregivers outside the OHS could also be established to enable early and more customized intervention, hence reducing the risk of long-term sickness absence.
The present study shows that this multidimensional approach to the classification of individuals based on psychological and psychosocial characteristics can distinguish different groups in gainfully employed working population with NP/LBP. Furthermore, the MPI-S should be viewed as a tool to classify patients into valid subgroups matching treatment plans to subgroup characteristics. The logical corollary would be that early, customized interventions for patients with NP and LBP would improve health outcomes, which is an important objective for any individual as well as healthcare system. Hence, by using the MPI-S classification system together with other clinical data, customized treatments may enable patients to break negative patterns of pain coping strategies, and thus reduce future long-term sickness absence.
There are some methodological considerations in this study that should be acknowledged. Firstly, the sample is male-dominated and the vast majority of subjects were blue-collar workers in the Process and Engineering Industries in Sweden. Thus, this could decrease the generalizability of the results on a more evenly distributed population with regard to gender and among other working populations, e.g. health care and service sector. Furthermore, the reason for sickness absence was not known as this information was not provided from the companies' pay-roll. Consequently, data on sickness absence in this study may also mirror other health complaints among these employees. Non-responsiveness in regard to mental and physical health as well as bodily pain at 18 and 36-month follow-ups may have introduced some bias. However, the non-response at 18 and 36-month follow-ups was proportionally similar between the MPI-S groups. Nevertheless, the results show that the DYS and ID groups display higher sickness absenteeism than the AC group during follow-up.
The data material did not contain information in regard to severity and duration of the individuals back and neck complaint which are in and by itself a limitation. If information on severity, chronicity of neck and back complaints in combination with vocational and comorbid factors would have been available it would have enhanced the internal validity of the study. However, previous studies have confirmed the internal reliability, validity and generalizability of the MPI-S instrument in a chronic population [
23]. Furthermore, during the validation process of the MPI-S, the subgroups did not differ on pain duration or medical variables [
23].
Failure by an individual to respond to section two of the MPI questionnaire leads to an unclassifiable profile [
46,
47]. In this study, 41 individuals were excluded who could not be classified at the first assessment due to missing data on section two of the MPI-S, which requires a significant other. In addition, another 23 individuals classified as Hybrid (response pattern represents aspects from more than one of the three MPI-S profiles) were also excluded, as it has been argued that they do not render any valid clinical information [
34]. Unfortunately, this contributed to small MPI-S subgroups which reduced the statistical power and increased the risk for Type II error, hence increasing the risk of failing to reject the null hypothesis. This exclusion of individuals could probably have been reduced, if the refinement of instructions in section two of the MPI, as described by Okifuji et al [
47], had been used in this study. A general consideration is that it has been shown that approximately one third of patients classified to one of the three MPI patient groups change within a month [
46] and that the majority of patients who change classification belong to the Hybrid category [
47]. This could potentially have been detected by administrating the MPI-S questionnaire within this time period, hence further increasing the statistical power.
Competing interests
The authors declare that they have no competing interests.
Authors' contributions
All authors have read and approved the final manuscript. CB was involved in analysis and interpretation of the data, drafting and revising manuscript and has given final approval. JH analyzed and interpreted the data, revised manuscript and gave final approval. LB was involved in the interpretation of data and revision of manuscript and gave final approval. IJ was involved in design, data collection, and revision of manuscript and gave final approval. GB was involved in the design, data collection, revision, interpretation of data, and revision of manuscript and gave final approval.