In the present study we found a positive, although non-significant trend, between knee complaints and premature exclusion from a knee demanding occupation. We did not follow floor layers from their first year in the trade but re-examined a cross-sectional sample including floor layers that had worked in the trade for many years. A bias towards the null because of the well-described healthy worker effect is very likely. The fact that 28 floor layers already had left their trade in year 1994/95 is consistent with this assumption. Questionnaire reports in 1994/95 from these individuals showed that 20% of the floor layers and 4% of graphic designers had been re-educated in another occupation due to knee troubles. Exclusion from the trade may occur at a lower seniority among many floor layers compared to participants included in this study (seniority (average) in 2005 29.4–34.9 years). Therefore, having knee complaints it may not be easy to survive in a knee demanding trade. In earlier studies physical work strains such as lifting and uncomfortable work postures has been found to be associated with early retirement, which support the results from the current study [17, 18].
As demonstrated in this follow-up survey, floor layers have a high frequency of knee complaints compared with workers without knee demanding work activities in their work tasks. Earlier studies among floor layers have shown similar results with high frequencies of musculoskeletal complaints, especially knee complaints [8–12]. However, in spite of the increasing age of the cohort the frequency of self-reported knee complaints was in general lower among floor layers presently at work in the trade in year 2005 compared with the frequency at baseline (1995), while the opposite tendency was seen among graphic designers. This could indicate a healthy worker effect among floor layers and moreover, that graphic designers may have an increased possibility to survive in the trade with knee troubles . Knee complaints may therefore be a risk factor of premature resignation from a trade, which involves knee demands and/or other physical work demands and a surviving factor in trades with few physical demands (graphic designers).
In order to prevent occupational musculoskeletal disorders, wearing-down and exclusion from the trade, major efforts have been made during the last ten years to reduce the daily amount of knee straining work activities among Danish floor layers. Among other things, innovations such as the development of tools that can be used in the upright working position have been enhanced and to some extent implemented in the floor layers work tasks . Among those using the equipment, results have shown that particularly severe knee complaints can be reduced and furthermore the effect is greater the longer the equipment have been used . In the light of the reduced frequency of knee complaints observed among floor layers presently at work in the trade in year 2005, this may indicate a certain extent of efficient prevention.
Apart from occupational hazards, it is evident that other aspects also affect the risk of exclusion or early retirement from the labour marked. Previous studies have ruled out the importance of other determinants that may contribute to the disability risk, such as individual factors (physical and psychological health status) and socio-economic conditions [17, 22].
During the period 1994/95–2005 the graphic subject area has been affected by structural changes, which have caused a high degree of unemployment and forced many graphic designers into other jobs and trades. Among graphic designers, these structural changes had a strong implication on future exclusion from the trade.
The overall response rate to the questionnaire was acceptable but there may be a risk of interview bias among those contacted by phone. To minimize this, a trained interviewer made all the interviews using a structured guide closely corresponding to the postal questionnaire. Using self-reported data there may additionally be a risk of information bias, e.g. subjects with a previous history of knee problems may have a tendency to respond more readily and accurate than those without . By comparing the answers from questionnaire and interview respondents there was a slightly lower frequency of knee complaints among those who were contacted by phone (but not significant), why the influence of interview bias is considered to be negligible.
In regard to potential confounders we have adjusted the results for some important determinants of self-reported knee complaints, such as seniority, earlier knee injuries and weight (BMI).
We have managed to obtain information's from the majority of the baseline cohort despite a very high occupational mobility among study subjects in both trades. However, a limitation of the present study is the small size of the study population, which may affect the precision of the results and confidence intervals. Subjects aged > 70 years in 2005 were excluded from the follow-up study. In Denmark workers can retire at the age of 60 years. Even if possible to stay at work after the age of 60 years it happens very seldom among construction workers and the risk of missing information owing to these reasons may be small.