PREMUS 2001, Amsterdam A Case-Control Study of Biomechanical and Psychosocial Risk Factors for Occupational Low-Back Pain
Michael S. Kerr, PhD Institute for Work & Health Toronto, Ontario, CANADA
Kerr et al. (2001) Am J Pub Health 91:1069-1075
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Study Collaborators
Institute for Work & Health: Kerr MS, Frank JW, Shannon HS, Bombardier C
University of Waterloo: Norman RW, Wells RW, Neumann P General Motors: Mr. Elmer Beddome CAW: Mr. John Graham
The Ontario Universities Back Pain Study (OUBPS) Group Andrews D, Beaton DE, Dobbyn M, Edmonstone E, Ferrier S, Hogg-Johnson S, Ingelman P, Mondlock M, Peloso P, Smith J, Stanfield SA, Tarasuk V, Woo H 8 Institute for Work & Health, 2001
Why a case-control study?
• Uncertainty about role of physical demands necessitated concentration of effort on developing valid workplace measures • Cohort model required multiple measures at multiple times to assess valid exposure
Does a prospective cohort study with limited (e.g. once only) exposure assessment really provide more rigorous evidence than a case-control study with more detailed assessments? 8 Institute for Work & Health, 2001
Research Question
After controlling for individual characteristics, what are the main work-related biomechanical and psychosocial risk factors for reported low-back pain?
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STUDY SETTING
GM-Oshawa autoplex
65 km east of Toronto Total hourly-paid work force – approx. 3 Divisions: Car Assembly Truck Assembly Fabrications
12,000 (7,000) (3,000) N/A
Nursing stations – handle ALL WCB reports as well as most other health problems occurring at worksite Truck = 1 station; Car = 4 stations
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The GM Study of Low-back Pain
Incidence density sampling
workers without LBP
random selection
(job-matched)
STUDY POPULATION
10,000 car and truck plant workers (hourly-paid) accrued via workplace health stations conducted at home
(n=137)
CASES
workers with LBP
baseline questionnaire and physical exam
CONTROLS
(n=179)
(n2=65)
at work doing usual job
physical demands assessment
video, EMG, checklists, posture
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SUBJECT ENROLMENT
179 Random controls A total of 381 subjects enrolled in the study (Voluntary)
Cases 65 137 Job-matched controls (used for proxy data)
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CASE Definition
• Full-time, hourly-paid worker with “sprainstrain” LBP (reported to nursing stations) • No lost-time or WCB claim requirement • No previous worksite LBP report (90 days)
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sd from male reference population
Were cases and controls comparable except for LBP?
SF-36, Health-related quality of life
Physical Role – Bodily Social Vitality Role – Mental General Function Physical Pain Function Emotional Health Health Ref: Garrat et al. (1993) BMJ 306:1440-4 8 Institute for Work & Health, 2001
STUDY VARIABLES - 1
INDIVIDUAL Characteristics
Age, height, weight, body mass index, sex education, marital status, preschool children, main wage earner, nonoccupational physical activity, smoking, alcohol consumption
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STUDY VARIABLES - 2
BIOMECHANICAL Factors
(reduced a priori to about 20 key variables, NO EMG)
• Peak Forces: compression; shear; hand force • Cumulative Forces: average and integrated compression;
• Low-level (static) Forces: compression
• Posture and Movement: peak flexion; time non-neutral; trunk kinematics
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STUDY VARIABLES - 3
PSYCHOPHYSICAL Factors
self-rated physical exertion measures distinct from "objective" measurements of physical demands analyses, but may also have additional role to play over and above the measured demands
e.g. tolerance levels, "job stress", etc.
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STUDY VARIABLES - 4
PSYCHOSOCIAL Factors
• Karasek-Theorell Job Content Instrument
• "psychological demand", decision latitude, supervisor support, coworker support, workplace social environment, job self-identity
• - job dissatisfaction, mastery, empowerment, status inconsistency ("over-education")
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Summary of Key Risk Factors
(p<0.05)
Peak Shear Peak hand f orce Disc Compression (Cumulativ e) Series 4 Self -rated demands (Exertion) Series 6 Poor Social Env ironment Ov er-education High Co-worker support High Job Satisf action Low Job Control (* p=0.07)
3
Odds Ratio
2
1
0
Biomechanical Psychosocial 8 Institute for Work & Health, 2001
(after adjusting for individual characteristics)
Study Strengths
• Directly measured physical demands data combined with (basic) physical exam and interview-assisted psychosocial data for individual subjects • Comprehensive workplace job demands assessments (generalizable – i.e. not specific to automobile manufacturing) • Well defined study base for subject selection
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Study Weaknesses
• Modest participation rate (approx 60%) • Relied on workplace reporting mechanism to identify cases • Self-report only for psychosocial factors • Case-control design But … does a prospective cohort study with limited once only exposure assessment really provide more rigorous evidence than a case-control study with more detailed assessments?
8 Institute for Work & Health, 2001
How did we control for Bias?
• Directly measured physical demands data rather than self-report • Job-matched controls used to examine potential recall bias (none observed) • Used newly incident cases rather than prevalent cases • Compared cases with non-participating compensation claimants (no differences) • A priori reduction in variables eligible for regression modeling (multi-methods)
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Possible Biological Mechanisms
DIRECT EFFECTS
BIOMECHANICAL LOAD
TOLERANCE REDUCED TOLERANCE EXCEEDED
INDIRECT EFFECTS
e.g. job control
PSYCHOSOCIAL LOAD
TISSUE RESPONSE
HORMONES e.g. cortisol
MUSCLE TENSION
PSYCHOSOCIAL LOAD
e.g. cumulative compression
BIOMECHANICAL LOAD
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Main Conclusions
• • • Consistency and strength indicates biomechanical load increases LBP risk Psychosocial factors also shown to be associated with reporting LBP Job dissatisfaction not a risk factor for reporting LBP in this setting Psychosocial instruments (especially Karasek’s demands scale) may require further refinement for MSK studies
•
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For additional information please contact:
Mickey Kerr The Institute for Work & Health 481 University Ave., Suite 800 Toronto, ON M5G 2E9 Phone: (416) 927-2027 Fax: (416)-927-4167 Website: http://www.iwh.on.ca E-mail: info@iwh.on.ca The Institute for Work & Health operates with the support of the Ontario Workplace Safety & Insurance Board
Kerr et al. (2001) Am J Pub Health 91:1069-1075
8 Institute for Work & Health, 2001