Dose-response relationship between physical exposure and lumbar spine disease. Results of the EPILIFT Exposure Criteria Study

Aims. The EPILIFT case-control study which constitutes the basis of the present EPILIFT Exposure Criteria Study (EPILIFT2) led to the fundamental conclusion that the risk of developing lumbar disc diseases is elevated even when the physical workload lies below specific threshold values of the Mainz-Dortmund Dose Model (MDD). In the "original" EPILIFT study, threshold values for lumbar-disc compressive force, for the degree of forward bending, and for the daily dose were not varied separately but in combination. Therefore, to date, the consequences on the model fit are unknown if single thresholds of the MDD are lowered separately, leaving other thresholds unchanged. The aim of this in-depth data analysis of the original EPILIFT study was to derive a dose model (including appropriate threshold values) that best describes the dose-response relation-ship between physical exposure and lumbar spine diseases. Material and methods. Lumbar-disc compressive force, degree of forward bending, and daily dose were changed separately (and afterward simultaneously). Model fit was considered based on continuous (instead of categorized) exposure variables, applying several new statistical methods (Akaike information criterion, AIC; fractional polynomials; multimodel approaches). Results. The calculation of threshold values made use of the information of several dose models representing a broad range of threshold values. As a result of this in-depth analysis, the following thresholds were established as a "best estimate": lumbar-disc compressive force of 3.2 kN among men and 2.5 kN among women; 45 degree of forward inclination of the trunk; daily dose threshold of 2 kNh among men and 0.5 kNh among women; doubling (life time) dose of about 7 MNh among men and 3 MNh among women. Conclusions. This in-depth re-analysis of the EPILIFT case-control study data constitutes an important basis for the determination of the dose-response relationship between physical exposure and lumbar spine diseases. The statistical uncertainties of our study results as well as the statutory requirements of German occupational disease no. 2108 (lumbar spine disease) have to be considered before appropriate threshold values can be established for use in practice.