Enhanced analysis of the German Spine Study (EPILIFT) aiming at deriving appropriate threshold values, part A: study centre Berlin

Status:

completed 11/2012

Aims:

The EPILIFT study – which constitutes the basis of the present “DWS-Richtwertestudie” – led to the fundamental conclusion that the risk for the development of lumbar disc diseases is elevated even when physical workload lies below specific threshold values of the Mainz Dortmund Dose Model (MDD). In the “original” EPILIFT study, dose models, threshold values for lumbar-disc compressive force, the degree of forward bending and for the daily dose were not varied separately, but in combination. However, to date the consequences on the model fit are unknown if single thresholds of the MDD are separately lowered – leaving other thresholds unchanged. In part 1) of this in-depth analysis of the original EPILIFT study (“DWS-Richtwertestudie”), lumbar-disc compressive force, degree of forward bending, and daily dose are changed separately (and afterwards simultaneously). Model fit is considered based on continuous (instead of categorized) exposure variables, applying several new statistical methods (Akaike Information Criterion AIC; fractional polynomias; multi model approaches). In part 2) of this study, the fit of simplified procedures is analyzed, that are applicable in practice.

Activities/Methods:

In an in-depth analysis of the original EPILIFT study, the following parameters are changed separately: part 1) daily dose, lumbar-disc compressive force, degree of forward bending part 2) furthermore, the fit of simplified procedures is analyzed, that are applicable in practice

Results:

Two fundamental aspects of the MDD approach could be confirmed in the present investigation: a) squared weighting of the lumbar-disc compressive force in relation to the duration of exposure and b) the inclusion of the whole amount of compressive force (versus considering only the compressive force above defined thresholds or above the compressive force when standing). Moreover, this reanalysis of the EPILIFT data based on continuous (instead of categorized in the original EPILIFT study) exposure variables confirmed a positive relationship between lifetime exposure and lumbar disc disease for the considered four case groups. As a result of this in-depth analysis, lowering the daily dose thresholds (from 5.5 to 2 kNh among men and from 3.5 to ½ kNh among women) as well as the threshold for the degree of forward bending (from 90 to 45 degree of forward bending) improved the MDD model fit. The threshold values for the lumbar-disc compressive force could be confirmed as “best estimates“ and therefore appear scientifically justifiable. As a further result of these threshold variations, the cumulative lifetime thresholds of the MDD appear considerably too high, however, methodological uncertainties have to be taken into consideration; this study in fact reveals as “best estimate” a doubling dose for lumbar disc diseases of about 7 MNh among men and 3 MNh among women. About 30 % of population reach or exceed these dose levels. When appropriate threshold values are to be established for use in recognition praxis, besides statistical-epidemiological knowledge also biological plausibility, practicability and the existence of appropriate measurement instruments has to be considered.

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