Table 4

Associations between levels of cumulated neck exposures and cervical herniated disc disorders, IR per 10 000 person-years, crude and adjusted IRR (95% CI)

	Level 1*	Level 2*	Level 3*	Level 4*	Level 5*	Total
Cumulated angular velocity
CDH, cases	1355	2152	3131	3655	3707	14 000
Person years	4 051 052	4 054 683	4 052 995	4 044 577	4 032 033	20 235 440
IR	3.3	5.3	7.7	9.0	9.2	6.9
IRR, crude	1(ref)	1.59 (1.48 to 1.70)	2.31 (2.17 to 2.46)	2.70 (2.54 to 2.88)	2.75 (2.58 to 2.93)
IRR, adjusted†	1(ref)	0.99 (0.92 to 1.06)	0.97 (0.91 to 1.04)	0.91 (0.85 to 0.97)	0.90 (0.84 to 0.96)
IRR, fully adjusted‡	1(ref)	0.99 (0.92 to 1.06)	0.98 (0.91 to 1.04)	0.92 (0.86 to 0.98)	0.90 (0.84 to 0.96)
Cumulated extension
CDH, cases	1305	2055	2973	3707	3960	14 000
Person years	4 047 233	4 057 605	4 054 817	4 047 237	4 028 549	20 235 440
IR	3.2	5.1	7.3	9.2	9.8	6.9
IRR, crude	1(ref)	1.57 (1.47 to 1.68)	2.27 (2.13 to 2.43)	2.84 (2.67 to 3.03)	3.05 (2.86 to 3.25)
IRR, adjusted†	1(ref)	1.06 (0.99 to 1.14)	1.02 (0.95 to 1.09)	0.94 (0.88 to 1.01)	0.93 (0.87 to 0.99)
IRR, fully adjusted‡	1(ref)	1.06 (0.99 to 1.14)	1.02 (0.95 to 1.09)	0.96 (0.89 to 1.02)	0.96 (0.90 to 1.03)
Cumulated flexion
CDH, cases	1360	2086	3037	3784	3733	14 000
Person years	4 015 039	4 077 672	4 065 870	4 048 840	4 028 019	20 235 440
IR	3.4	5.1	7.5	9.3	9.3	6.9
IRR, crude	1(ref)	1.51 (1.41 to 1.62)	2.21 (2.07 to 2.35)	2.76 (2.59 to 2.94)	2.74 (2.57 to 2.91)
IRR, adjusted†	1(ref)	1.03 (0.96 to 1.10)	1.00 (0.94 to 1.07)	0.93 (0.87 to 0.99)	0.85 (0.79 to 0.91)
IRR, fully adjusted‡	1(ref)	1.03 (0.96 to 1.10)	1.00 (0.94 to 1.07)	0.94 (0.88 to 1.00)	0.87 (0.81 to 0.93)

Estimates in bold are significant at P<0.05.
*Based on quintiles of cumulative exposure in the study population.
†Adjusted for age, age×age, sex and calendar-year.
‡Adjusted for age, age×age, sex, calendar-year, lumbar disc herniation and education.
CDH, cervical disc herniation; IR, Incidence rate; IRR, Incidence rate ratio.