does a towbar increase the risk of neck injury in rear-end … · the modern collision zone curls...
TRANSCRIPT
DOES A TOWBAR INCREASE THE RISK OF
NECK INJURY IN REAR-END COLLISIONS?A STUDY BASED ON A MERGER OF DANISH
ADMINISTRATIVE REGISTRIES
A N N E V I N G A A R D O L E S E N
H A R RY L A H R M A N N
T H E T R A F F I C R E S E A R C H G R O U P AT A A L B O R G U N I V E R S I T Y
D E N M A R K
The tow-bar
• A common sight in Denmark especially in rural areas
• Prevalence of 60% in the Motor Vehicle Registry in the period 2003-12
• From seller’s car database on the internet 37% in all of Denmark (2017)
• A count in typical parking lot gave a prevalence of 48% with 90% towbars
actually on the car (2017)
• Prevalence has dropped recently because of an increase in small cars (as
cars number two and else if mounting is not possible)
The study aim
• To investigate the association between the risk of neck injury in occupants
of the struck car in a rear-end collision
The registry merger
• The National Registry of Motor Vehicles: registration plates, towbar status
• Police reports: personal identification numbers, registration plates, accident
type
• The Central Person Registry: personal identification numbers, vital status,
nationality
• The National Hospital Discharge Registry: personal identification numbers,
diagnoses
• Socio-economic databases at Statistics Denmark: personal identification
numbers, income, highest attained education, socioeconomic status
• All information available at date of accident
• The personal identifier and the registration plate are the central merger
variables
The modern collision zone curls in rear-end collisions
• In order to absorb the force and protect occupants in the cabin
• Mounting of a stiff tow-bar frame is highly likely to inflate the curling
mechanism
• May increase the risk of neck injury in the struck car with a towbar
• Several ways to mount the tow-bar
• And now possible to demount when not using it
• All of which may ”mix” the impacts of a towbar registered
in a general Motor Vehicle Registry
Strong experimental evidence:
Krafft et al. (2000). AAP 32: 187-95.
• Two Volvo 240s one with and one without a towbar
• Car acceleration higher in towbar equipped car with peak at 9.6g
• Car acceleration in car without towbar had peak at 8.0g
• Dummy in the car with towbar experienced 33% higher peak acceleration in
lower neck, 8.9g
• Compared with 6.7g for dummy in the car without towbar
• Increased risk of 22% for long-term consequences of neck injury in struck
cars with a towbar (p<0.001)
• One other study found no effect Linder et al.??
• Few other sources cite Krafft et al.(2000) ??
Negative finding: no effect of tow-bar on neck injury
• Could be explained by measurement error on the information on tow-bar in
the Motor Vehicle Registry: imprecise exposure erases the effect if present
Counts, person-years, and neck injury rates. Public registry data from Denmark, 2003–2012
N N P-years IR Variable
Variable Persons Events
in
1000s
per
1000 95% CI significance
All 9370 1519 7.810 194.5 185.0 204.5
Tow-bar on struck car
Yes 5019 827 4.168 198.4 185.3 212.4 p < 0.0001
No 3244 575 2.652 216.8 199.8 235.3
Unknown 1112 117 0.990 118.2 98.6 141.6
Unadjusted
IRR=0.92; p=0.11
1-year prevalence
of 16%
Estimated hazard ratios in multivariate Cox proportional hazard regression.
Hazard ratios can be interpreted as incidence rate ratios. Public registry data from Denmark, 2003–
2012
Multivariate analysis
Hazard Robust Variable
Variable ratio SE 95% CI P-value significance
Tow-bar on struck car
Yes 0.915 0.0523 0.818 1.024 0.122 0.111
No 1.000
Unknown 0.820 0.0892 0.662 1.015 0.068
Adjusted for: gender, age, seat, income, education, accident
type, calendar year, weight of car, first registration year
Sex
Male 1.000 p < 0.0001
Female 1.538 0.0852 1.380 1.714 0.000
Age in years
0–17 0.936 0.2096 0.604 1.452 0.769 p < 0.0001
18–29 1.022 0.0766 0.882 1.183 0.775
30–39 1.000
40–49 0.904 0.0665 0.783 1.045 0.172
50–59 0.688 0.0613 0.577 0.819 0.000
60–69 0.546 0.0677 0.428 0.696 0.000
70+ 0.285 0.0659 0.181 0.448 0.000
Females have increased risk
Older persons tend to underreport?
Seat in struck car
Driver 1.000 p < 0.0001
Passenger 1.524 0.1302 1.289 1.801 0.000
Household income
<1st quintile 0.827 0.0740 0.694 0.986 0.034 p < 0.0001
1st–2nd quintiles 0.990 0.0782 0.848 1.155 0.895
2nd–3rd quintiles 1.000
3rd–4th quintiles 0.974 0.0785 0.832 1.141 0.746
>4th quintile 0.843 0.0774 0.705 1.010 0.063
Unknown 0.076 0.0363 0.030 0.194 0.000
Passenger seat associated with increased risk of neck injury
Poor and wealthy have reduced risk
Highest level of education
Primary education 1.019 0.1418 0.776 1.339 0.891 0.497
Upper secondary education 0.992 0.1563 0.728 1.350 0.957
Vocational Education and Training (VET) 0.948 0.1279 0.728 1.235 0.694
Qualifying educational programs 2.703 1.9514 0.657 11.127 0.168
Short-cycle higher education 1.000
Vocational bachelors’ programs 1.096 0.1569 0.828 1.451 0.522
Bachelors’ programs 0.929 0.2552 0.543 1.592 0.790
Masters’ programs 0.866 0.1586 0.605 1.240 0.431
PhD programs 0.592 0.4267 0.144 2.430 0.467
Unknown 0.812 0.1716 0.536 1.228 0.323
Educational effect explained by income level etc.
Accident type
Hit directly from the back when driving
straight ahead 1.000 0.069
Hit from the back when turning right 0.730 0.1087 0.545 0.977 0.035
Hit from the back when turning left 0.943 0.0658 0.822 1.081 0.397
Calendar year
2003–2004 1.000 0.481
2005–2006 0.988 0.0770 0.848 1.151 0.880
2007–2008 0.899 0.0728 0.767 1.054 0.188
2009–2010 0.979 0.0835 0.828 1.157 0.806
2011–2012 0.882 0.0817 0.735 1.058 0.175
Reduced risk when turning right
No significant effect of calendar year
Weight of struck car in kilograms
500–1000 0.996 0.0681 0.871 1.139 0.950 0.985
1000–1500 1.000
1500–2000 1.019 0.1100 0.825 1.259 0.861
2000+ 0.853 0.2513 0.479 1.519 0.589
Unknown 1.029 0.6233 0.314 3.373 0.962
First registration year of struck car
1966–1989 0.946 0.1076 0.757 1.182 0.623 0.495
1990–1994 1.035 0.0940 0.867 1.237 0.701
1995–1999 0.962 0.0739 0.828 1.119 0.618
2000–2004 1.000
2005–2009 0.839 0.0784 0.699 1.008 0.060
2010–2015 0.827 0.1608 0.565 1.211 0.328
Unknown 1.079 0.6575 0.327 3.562 0.900
No effect of weight and first registration year of struck car
Conclusion
• 16% of occupants of struck cars were diagnosed with neck injury by doctors
in hospitals/emergency rooms after rear-end collisions
• No effect of tow bar on the struck car in large study of 9370 occupants
• Measurement error might induce result
• Count of cars in typical parking lot demonstrated that 90% of tow-bars were
actually on the car (and not taken off)
• More crash test needed in order to corroborate registry finding
• Could the tow-bar be associated with neglectible risk of neck injury in
modern cars
• Law of physics does somehow rule
What we learnt from this merge
• Remarkable possibilities for registry merge in Denmark and Scandinavia –
which is of course well-known
• Merge by registration plate in police reports and Motor Vehicle Registry
possible
• Merge by personal identifier in police reports and National Discharge
Registry (hospital registry) possible
• Denmark is only a novice regarding registry studies
Strengths and limitations of registry studies
• Complete populationbased studies with many cases (likely a high power)
• Facilitate assessment of various hypotheses such as mix between different
personal injuries and diagnoses and hospital courses
• But -
• Measurement error such as on information of tow-bar status in the Motor
Vehicle Registry
• Lack of information on confounders such as (precise) personal
characteristics, attitudes and psychological profiles
• Missing values in some variables ”unknowns”
Experiences with registry data in Scandinavia?
• Which kinds of merges are possible in the different countries?
• Experiences with different sources such as police reports with registry data?
• Experiences with various sorts of sources of error?
• How to overcome the limitations: sensitivity analyses and multiple
imputation or ?
• Other issues?