Knee complaints and consequences on work status; a 10-year follow-up survey among floor layers and graphic designers.

Abstract

The purpose of the study was to examine if knee complaints among floor layers predict exclusion from the trade.
In 1994/95 self-reported data were obtained from a cohort of floor layers and graphic designers with and without knee straining work activities, respectively. At follow-up in 2005 the questionnaire survey was repeated. The study population consisted of 81 floor layers and 173 graphic designers who were presently working in their trades at baseline (1995). All participants were men aged 36-70 years in 2005. We computed the risk of losing gainful employment in the trade according to occurrence of knee complaints at baseline, using Cox proportional hazard regression adjusted for a number of potential confounding variables. Moreover, the crude and adjusted odds risk ratio for knee complaints according to status of employment in the trade were computed, using graphic designers as reference.
A positive but non-significant association between knee complaints lasting more than 30 days the past 12 months and exclusion from the trade was found among floor layers (Hazard Ratio = 1.4, 95% CI = 0.6-3.5).The frequency of self-reported knee complaints was lower among floor layers presently at work in the trade in year 2005 (26.3%) compared with baseline in 1995 (41.1%), while the opposite tendency was seen among graphic designers (20.7% vs. 10.7%).
The study suggests that knee complaints are a risk factor for premature exclusion from a knee demanding trade. However, low power of the study precludes strong conclusions. The study also indicates a healthy worker effect among floor layers and a survivor effect among graphic designers.

Full text

Page 1

BioMed Central
Page 1 of 7
(page number not for citation purposes)
BMC Musculoskeletal Disorders
Open Access
Research article
Knee complaints and consequences on work status; a 10-year
follow-up survey among floor layers and graphic designers
Søren Rytter*1, Lilli Kirkeskov Jensen2 and Jens Peter Bonde3
Address: 1Department of Occupational Medicine and Orthopaedics, Region Hospital Skive, Viborg, Denmark, 2Department of Occupational
Medicine, Region Hospital Skive, Denmark and 3Department of Occupational Medicine, Aarhus University Hospital, Aarhus, Denmark
Email: Søren Rytter* - soeren.rytter@sygehusviborg.dk; Lilli Kirkeskov Jensen - lilli.kirkeskov.jensen@sygehusviborg.dk;
Jens Peter Bonde - jpbon@as.aaa.dk
* Corresponding author
Abstract
Background: The purpose of the study was to examine if knee complaints among floor layers
predict exclusion from the trade.
Methods: In 1994/95 self-reported data were obtained from a cohort of floor layers and graphic
designers with and without knee straining work activities, respectively. At follow-up in 2005 the
questionnaire survey was repeated. The study population consisted of 81 floor layers and 173
graphic designers who were presently working in their trades at baseline (1995). All participants
were men aged 36–70 years in 2005.
We computed the risk of losing gainful employment in the trade according to occurrence of knee
complaints at baseline, using Cox proportional hazard regression adjusted for a number of potential
confounding variables. Moreover, the crude and adjusted odds risk ratio for knee complaints
according to status of employment in the trade were computed, using graphic designers as
reference.
Results: A positive but non-significant association between knee complaints lasting more than 30
days the past 12 months and exclusion from the trade was found among floor layers (Hazard Ratio
= 1.4, 95% CI = 0.6–3.5).
The frequency of self-reported knee complaints was lower among floor layers presently at work in
the trade in year 2005 (26.3%) compared with baseline in 1995 (41.1%), while the opposite
tendency was seen among graphic designers (20.7% vs. 10.7%).
Conclusion: The study suggests that knee complaints are a risk factor for premature exclusion
from a knee demanding trade. However, low power of the study precludes strong conclusions. The
study also indicates a healthy worker effect among floor layers and a survivor effect among graphic
designers.
Background
Musculoskeletal disorders are very common in the general
population and are the predominant cause of disability
among construction workers [1]. Previous studies have
shown an increased frequency of self-reported and clini-
cally diagnosed knee disorders and radiological diag-
Published: 18 September 2007
BMC Musculoskeletal Disorders 2007, 8:93doi:10.1186/1471-2474-8-93
Received: 11 December 2006
Accepted: 18 September 2007
This article is available from: http://www.biomedcentral.com/1471-2474/8/93
© 2007 Rytter et al; licensee BioMed Central Ltd.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0),
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Page 2

BMC Musculoskeletal Disorders 2007, 8:93http://www.biomedcentral.com/1471-2474/8/93
Page 2 of 7
(page number not for citation purposes)
nosed knee osteoarthritis among workers within certain
trades in the construction industry [2-8].
There are about 900 skilled floor layers in Denmark and
they spend on average more than half of their daily work-
ing time in knelling, knee supporting or squatting work
positions. Floor layers are highly exposed to various knee-
stressing work positions in their work tasks, defined as
work in kneeling positions, completely on both or partly
on one knee using the other as support, and as squatting
work positions. They install linoleum, carpet and vinyl
floorings and their work tasks involve removal of old
floorings, grinding, filling, installing underlay, measuring
and cutting materials, gluing, welding and installing skirt-
ing board (plastic). Graphic designers handle the layout
of text and use visual display units. Their work tasks did
not include any knee-demanding work positions.
Compared with other skilled construction workers, floor
layers have a high frequency of musculoskeletal com-
plaints, particularly anterior knee pain. This has been
observed among Danish, as well as American, Finnish and
Swedish floor layers [8-12]. Two earlier studies have
shown that floor layers have an increased risk of pro-
longed sick leave and premature retirement from the trade
[9,13]. This might be due to the high prevalence of knee
disorders in this trade.
Therefore the primary aim of the current study was to ana-
lyse whether knee complaints cause subsequent exclusion
from the trade. Furthermore self-reported knee com-
plaints and status of employment through a 10-year fol-
low-up period were analysed.
Methods
The study is a follow-up of a cohort of 129 floor layers and
302 graphic designers that in 1994/95 responded to a
questionnaire survey at an age less than 60 years and who
were alive in 2005 (Figure 1) [4,9]. Graphic designers were
included as references due to the fact that their work does
not include any knee demanding work tasks. Non-
respondents at the follow-up survey constituted 20 floor
layers and 61 graphic designers, whereas 8 floor layers and
4 graphic designers had deceased (Figure 1). This leaves
109 and 241 respondents among floor layers and graphic
designers, respectively. Among those 81 floor layers and
173 graphic designers were presently at work in their
trades in year 1995. All participants were men, living in
the area of Copenhagen.
A postal questionnaire was mailed to the cohort. Prior, the
Danish Data Protection Agency and the regional ethics
committee approved the investigation. In lack of response
the questionnaire was filled in by a structured telephone
interview, if willing to participate. The method and the
Diagram showing the selection of the final study population
Figure 1
Diagram showing the selection of the final study population.
FLOOR LAYERSGRAPHIC DESIGNERS
Respondents to baseline survey in 1995 (n) 161 389
Exclusions:
- Aged >60 in 1995 (n)
- Died between 1995 and 2005 (n)
24
8
83
4
Mailed follow-up survey in 2005 (n)
Non-respondents (n)
129
20
302
61
Respondents to follow-up survey in 2005 (n) 109 241

Page 3

BMC Musculoskeletal Disorders 2007, 8:93 http://www.biomedcentral.com/1471-2474/8/93
Page 3 of 7
(page number not for citation purposes)
applied questionnaire was the same as used in the 1995
study and questions about musculoskeletal complaints
were consistent with the Nordic Musculoskeletal Ques-
tionnaire [14]. The questionnaire asked about present and
previous employment and the history of any knee com-
plaints or knee traumas. In regard to employment we
asked about their seniority (years of employment in the
trade) and to describe their work tasks. If no longer
engaged in the trade they were asked which year they
stopped and if this were due to illness, unemployment,
early retirement, old-age pension, disablement pension,
rehabilitation or change of occupation. In the question-
naire, knee complaints were defined as ache, pain or nui-
sance. In case of knee complaints they reported if
complaints had been present during the past 12 months
or daily or more than 30 days during the past 12 months.
Data was coded in the software package EpiData and sta-
tistical analysis conducted with Stata [15,16]. Using Cox
proportional hazard regression, we analysed the probabil-
ity of surviving in the trade with knee complaints (>30
days during the past 12 months at baseline) during the
10-year follow-up from 1994/95 to 2005. Furthermore,
logistic regression was used to analyse patterns of self-
reported knee complaints relative to employment in the
trade in year 2005. All analysis was conducted on floor
layers (N = 81) and graphic designers (N = 173) presently
working in their trades in year 1995. Models were
adjusted for effects of body mass index (BMI) and earlier
knee injuries. Seniority showed a stronger association
than age and was used in the model. The continuous cov-
ariates, BMI and seniority, were categorized into three
BMI groups (< 25, 25–29 & ≥ 30 kg/m2) and two seniority
groups (≤ 20 & > 21 years).
Results
Responses to the mailed follow-up survey were received
from 109 floor layers and 241 graphic designers (Figure
1). Among those 36% and 31% had answered the ques-
tionnaire by a structured interview. Graphic designers
were slightly older and had a higher seniority than floor
layers but in respect to body mass index the two groups
were comparable (Table 1).
Employment status in 2005 compared to 1995
Of the respondents to the follow-up survey, 81 (74%)
floor layers and 173 (72%) graphic designers had been
working in their trade at the time of baseline survey in
1995 (and hence "at risk" of leaving the trade during fol-
low-up). At the time of the follow-up survey in 2005, only
56 (51%) floor layers and 75 (31%) graphic designers
were still working in their respective trades (Table 1).
Knee complaints & survival in the trade 1995–2005
Analysis among floor layers (N = 81) and graphic design-
ers (N = 173) "at risk" of leaving the trade during follow-
up, indicated that the probability of surviving in the trade
was higher among floor layers without knee complaints
(> 30 days during the past 12 months) in 1995 than
among those with knee complaints, although the differ-
ence did not reach statistical significance (Figure 2).
Among graphic designers proportions were almost identi-
cal in the two groups. The Hazard Ratios adjusted for sen-
iority, BMI and earlier knee injuries were 1.4, 95% CI =
Table 1: Demographic and anthropometric characteristics of questionnaire respondents in 2005
Floor layers (N = 109)
1995
Graphic designers (N = 241)
1995 20052005
Mean Range MeanRangeMean RangeMean Range
Age (years)
Seniority * (years)
BMI‡(kg/m2)
43.5
22.0
25.5
26–60
2–40
53.5
29.4
26.7
36–70
3–49
48.6
27.6
25.0
30–60
1–45
58.6
34.9
26.1
40–70
2–54
19.6–40.120.0–51.1 16.8–37.316.6–42.0
Employment in the traden (%) n (%) n (%)n (%)
Presently at work
Presently off work
- Change of occupation
- Early retirement
- Old-age pension
- Disability pension
- Rehabilitated
- Long illness
81 (74.3%)
28 (25.7%)
18 (16.5%)
2 (1.9%)
56 (51.4%)
53 (48.6%)
27 (24.8%)
9 (8.3%)
5 (4.6%)
8 (7.3%)
1 (0.9%)
3 (2.7%)
173 (71.8%)
68 (28.2%)
37 (15.3%)
27 (11.2%)
75 (31.1%)
166 (68.9%)
69 (28.7%)
68 (28.2%)
19 (7.9%)
8 (3.3%)
--
7 (6.4%)4 (1.7%)
--
-
-
1 (0.9%)2 (0.8%)
*Duration of employment in the trade
‡BMI, body mass index

Page 4

BMC Musculoskeletal Disorders 2007, 8:93 http://www.biomedcentral.com/1471-2474/8/93
Page 4 of 7
(page number not for citation purposes)
0.6–3.5 for floor layers and 0.8, 95% CI = 0.4–1.9 for
graphic designers.
Knee complaints & status of employment in 2005
In regard to status of employment there had been a high
employment turnover through the follow-up period. A
high percentage was engaged in other trades or had been
retired prematurely. Among those who reported change of
occupation in the period 1995–2005, 18 floor layers had
left the trade before 1995 and 9 in the period 1995–2005.
Among graphic designers 37 had changed before 1995
and 32 in the period 1995–2005.
Table 2 depicts the odds risk ratio of knee complaints
among 81 floor layers compared to 173 graphic designers
that were presently working in their trade in 1994/95.
Knee complaints, more than 30 days during the past 12
months, among graphic designers presently at work in the
trade, were twice as frequent (20.7%) in 2005 compared
with results at baseline (1995) (10.7%). On the contrary,
knee complaints > 30 days during the past 12 months,
were more frequent at baseline (41.1%) compared with
results at follow-up (26.3%) among floor layers presently
at work in the trade as well as for floor layers pensioned
(50.0% compared to 29.0%). Among those who had
changed occupation there were minor differences in the
frequency of these knee complaints. Floor layers had sig-
nificant more knee complaints than graphic designers, if
they were no longer engaged in the trade. Among floor
layers presently at work in the trade, there were no signif-
icant differences for knee complaints > 30 days during the
past 12 months compared with graphic designers (OR =
1.4, 95% CI = 0.6–3.3).
Discussion
In the present study we found a positive, although non-
significant trend, between knee complaints and prema-
ture 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 lay-
ers 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
Kaplan-Meier survival curve during a ten years follow-up period (1995–2005) according to self-reported knee com-plaints (> 30 days during the past 12 months) in 1995 among floor layers (N = 81) & graphic designers (N = 173) presently at work in the trade at baseline (1995)
Figure 2
Kaplan-Meier survival curve during a ten years follow-up
period (1995–2005) according to self-reported knee com-
plaints (> 30 days during the past 12 months) in 1995 among
floor layers (N = 81) & graphic designers (N = 173) presently
at work in the trade at baseline (1995).
0.40
0.50
0.60
0.70
0.80
0.90
1.00
Survival in the trade, %
1995 1997 1999 200120032005
Years
Floor layers
0.40
0.50
0.60
0.70
0.80
0.90
1.00
Survival in the trade, %
19951997 1999200120032005
Years
Knee complaints, noKnee complaints, yes
Graphic designers

Page 5

BMC Musculoskeletal Disorders 2007, 8:93
http://www.biomedcentral.com/1471-2474/8/93
Page 5 of 7
(page number not for citation purposes)
Table 2: Odds risk ratio of self-reported knee complaints in 1994/95 and 2005 among 81 floor layers presently working in the trade in 1994/95 relative to graphic designers
Knee complaintsEmployment in the trade in year 2005Knee complaints 1995Knee complaints 2005
Nn (%) ORcrude
95% CI ‡
ORadj†
95% CI‡
n (%)ORcrude
95% CI‡
ORadj†
95% CI ‡
Presently at work
- Floor layers
- Graphic designers
56
75
39 (69.6%)
16 (21.3%)
8.53.8–18.79.23.8–22.232 (56.1%)
28 (36.8%)
2.21.1–4.52.71.2–5.8
1-1-1-1-
During the past 12 monthsPresently off work *
- Floor layers
- Graphic designers
16
66
10 (62.5%)
16 (24.2%)
5.11.6–16.37.72.1–28.97 (43.8%)
19 (28.8%)
1.90.6–5.92.70.8–8.9
1-1-1-1-
Change of occupation
- Floor layers
- Graphic designers
98 (88.9%)
7 (21.9%)
27.4 2.9–258.426.92.7–265.07 (70.0%)
10 (31.3%)
7.71.4–43.911.71.1–124.8
321-1-1-1-
Presently at work
- Floor layers
- Graphic designers
56
75
23 (41.1%)
8 (10.7%)
5.8 2.3–14.25.52.2–13.915 (26.3%)
17 (20.7%)
1.30.6–.2.91.40.6–3.3
1-1-1-1-
Daily or > 30 days during
the past 12 months
Presently off work *
- Floor layers
- Graphic designers
16
66
8 (50.0%)
8 (12.1%)
7.1 2.1–24.39.12.4–34.45 (29.0%)
10 (16.7%)
2.6 0.7–8.93.8 1.0–15.0
1-1-1-1-
Change of occupation
- Floor layers
- Graphic designers
95 (55.6%)
3 (9.4%)
11.7 2.0–68.8 15.32.1–113.45 (50.0%)
4 (12.5%)
8.81.6–47.013.81.7–115.1
321-1-1-1-
* Off work due to early retirement, old – age pension, disability pension, rehabilitation or long illness
† OR, odds risk ratio adjusted for seniority, body mass index & earlier knee injuries
‡ CI, confidence interval