The Course of Low Back Pain From Adolescence to Adulthood

Eight-Year Follow-up of 9600 Twins

Lise Hestbaek, DC, PhD; Charlotte Leboeuf-Yde, DC, MPH, PhD; Kirsten Ohm Kyvik, MD, PhD; Claus Manniche, Dr. Med Sci

Disclosures

Spine. 2006;31(4):468-472. 

In This Article

Materials and Methods

Data Sources

The Danish Twin Register is the most comprehensive population-based twin register in the world, spanning a period of more than 100 years. The twins of interest for this study were born between 1972 and 1982. They were identified through the Danish civil registration system and represent 95% of the twins born during that period. The twins can be regarded as representative of the general population since they have previously been shown to have the same mortality rate[15] and the same prevalence of various diseases as the population at large, e.g., insulin-dependent diabetes,[16] hand eczema,[17] asthma and allergic rhinitis,[18] and LBP.[19] The database is described in detail elsewhere.[20] Since the population can be regarded as representative for the general population, the register provides a valuable population-based study population for various purposes. The fact that they are twins is of no interest for the purpose of this study. In 1994, when the sample was 12 to 22 years old, comprehensive questionnaires were sent to twins who previously had agreed to participate in future studies (96%). The questionnaires contained items relating to disease, health, and health-related behavior. Similar questionnaires were sent to the same population in 2002, when they were 20 to 30 years of age. The variable of interest for this paper was the number of days with LBP during the past year (LBP-days). The exact wording of the question was: How many days have you altogether had trouble with the lower part of your back during the past year? The possibilities for answering were: 0 days, 1-7 days, 8-30 days, more than 30 days, but not daily, or daily. The group with daily pain was too small for reasonable analyses; therefore, the two groups more than 30 days, but not daily and daily were combined into one group: more than 30 days. Furthermore, the question, Have you ever had trouble with the lower part of your back? (LBP-ever), was used for validation-purposes.

Validation

The questions regarding LBP were modeled on the Nordic Back Pain questionnaire,[21] which has been validated previously.[22] The reliability of LBP questions from the 1994 omnibus has previously been considered satisfactory through identification of logical errors.[23] Similarly, analyses of validity were performed on the data from the 2002 omnibus by cross-tabulating LBP-days with LBP-ever. Finally, to get an indication of the validity of lifetime recall in a young population, the response to LBP-ever in 1994 was compared with the response to the same question in 2002.

Representativeness

Responders and nonresponders at follow-up were compared with regard to age, gender, and LBP status at baseline.

Analyses

Persistent LBP (LBP-long) was defined as LBP for more than 30 days during the previous year and LBP during the past year (LBP-year) as LBP for more than zero days during the previous year. The distributions between LBP categories in 1994 and in 2002 are presented in Table 1 and Table 2 as proportions. Odds ratios for having LBP-year and, specifically, for having persistent LBP at follow-up were calculated for subgroups, based on LBP status in 1994, by means of logistic regression and adjusted for age and gender.

All analyses were done using STATA 8.0 statistical software package. We were not testing any hypotheses but merely describing the pattern of LBP in our population; thus, no significance testing was performed. Instead, all results are presented with 95% confidence intervals as they provide the key for referring from our sample back to the general Danish population of that age.[24]

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