title: Evidence-based classification of low back pain in the general population: one-year data collected with SMS Track.
abstract: BACKGROUND
  It was previously assumed that low back pain (LBP) is a disorder that can be classified as acute, subacute and chronic. Lately, the opinion seems to have veered towards a concept of it being a more recurrent or cyclic condition. Interestingly, a recent review of the literature indicated that LBP in the general population is a rather stable condition, characterized as either being present or absent. However, only one of the reviewed studies had used frequent data collection, which would be necessary when studying detailed course patterns over time. It was the purpose of this study to see, if it was possible to identify whether LBP, when present, is rather episodic or chronic/persistent. Further, we wanted to see if it was possible to describe any specific course profiles of LBP in the general population.
  
  
  METHODS
  In all, 293 49/50-yr old Danes, who previously participated in a population-based study on LBP were invited to respond to 26 fortnightly text-messages over one year, each time asking them the number of days they had been bothered by LBP in the past two weeks. The course patterns for these individuals were identified through manual analysis, by observing the interplay between non-episodes and episodes of LBP. A non-episode of LBP was defined as a period of at least one month without LBP as proposed by de Vet et al. A fortnight with at least one day of pain was defined as a pain fortnight (FN). At least one pain FN surrounded by a non-episode on each side was defined as an episode of LBP. After some preliminary observations of the spread of data, episodes were further classified as brief (consisting of only one pain FN) or longer (if there were at least 2 pain FNs in a row). An episode of at least 6 pain FNs in a row (i.e. 3 months) was defined as a long-lasting episode.
  
  
  RESULTS
  In all, 261 study subjects were included in the analyses, for which 7 distinct LBP subsets could be identified. These could be grouped into three major clusters; those mainly without LBP (35%), those with episodic LBP (30%) and those with persistent LBP (35%). There was a positive association between number of episodes and their duration.
  
  
  CONCLUSIONS
  In this study population, consisting of 50-yr old persons from the general population, LBP, when present, could be classified as either 'episodic' or 'mainly persistent'. About one third was mainly LBP-free throughout the year of study. More information is needed in relation to their relative proportions in various populations and the clinical relevance of these subgroups.
authors.affiliation: Research Department, The Spine Center of Southern Denmark, Hospital Lillebælt and Institute of Regional Health Research, University of Southern Denmark, Østre Hougvej 55, DK5500 Middelfart, Denmark ; Complexité, Innovation et Activités Motrices et Sportives, Bâtiment 335, UFR STAPS, Université Paris Sud-11, Orsay Cedex 91405, France.
authors.forename: Charlotte
authors.initials: C
authors.lastname: Leboeuf-Yde
authors.affiliation: Complexité, Innovation et Activités Motrices et Sportives, Bâtiment 335, UFR STAPS, Université Paris Sud-11, Orsay Cedex 91405, France ; Institut Franco-Européen de Chiropraxie, 72 Chemin de la Flambère, 31300 Toulouse, France.
authors.forename: Nadège
authors.initials: N
authors.lastname: Lemeunier
authors.affiliation: Orthopaedic Department, Center for Spine Surgery, Hospital of Lillebaelt, Institute of Regional Health Service Research and Center for Research in Childhood Health, University of Southern Denmark, Østre Hougvej 55, DK5500 Middelfart, Denmark.
authors.forename: Niels
authors.initials: N
authors.lastname: Wedderkopp
authors.affiliation: Research Department, The Spine Center of Southern Denmark, Hospital Lillebælt and Institute of Regional Health Research, University of Southern Denmark, Østre Hougvej 55, DK5500 Middelfart, Denmark ; Department of Sports Science and Clinical Biomechanics, University of Southern Denmark, Campusvej 55, DK5230 Odense M, Denmark.
authors.forename: Per
authors.initials: P
authors.lastname: Kjaer
keywords: Epidemiology; Episodes; Low back pain; Subgroups; Text-messages; Trajectories