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Low Back Pain in Relation to Lumbar

Disc Degeneration

Katariina Luoma, MD,* Hilkka Riihima¨ki, DMedSc, MSc,* Ritva Luukkonen, PhD,* Raili Raininko, MD, DMedSc,† Eira Viikari-Juntura, MD, DMedSc,* and

Antti Lamminen, MD, DMedSc‡

Study Design. Cross-sectional magnetic resonance imaging (MRI) study.

Objectives. To study the relation of low back pain (LBP) to disc degeneration in the lumbar spine.

Background Data.Controversy still prevails about the relationship between disc degeneration and LBP. Classi-fication of disc degeneration and symptoms varies, ham-pering comparison of study results.

Methods. Subjects comprised 164 men aged 40 – 45 years—53 machine drivers, 51 construction carpenters, and 60 office workers. The data of different types of LBP, individual characteristics, and lifestyle factors were ob-tained from a questionnaire and a structured interview. Degeneration of discs L2/L3–L5/S1 (dark nucleus pulpo-sus and posterior and anterior bulge) was assessed with MRI.

Results.An increased risk of LBP (including all types) was found in relation to all signs of disc degeneration. An increased risk of sciatic pain was found in relation to posterior bulges, but local LBP was not related to disc degeneration. The risks of LBP and sciatic pain were strongly affected by occupation.

Conclusions.Low back pain is associated with signs of disc degeneration and sciatic pain with posterior disc bulges. Low back pain is strongly associated with occu-pation. [Key words: intervertebral disc abnormalities, work-related risk factors, low back pain, epidemiology, occupational load]Spine 2000;25:487– 492

Disc degeneration of the lumbar spine is considered as one of the underlying factors of low back pain (LBP), but controversy still prevails about their relationship. An as-sociation between radiographic film findings and LBP has been found in many cross-sectional population stud-ies, but not in many case-referent studstud-ies, and there is no firm evidence of the presence or absence of a causal re-lationship between radiographic findings and nonspe-cific LBP.29In some magnetic resonance imaging (MRI) studies an association has been found,6,19,27but degen-erative changes have been found to be common in asymptomatic people as well.2,10,20

In most studies the temporal relationship between the radiologic findings and the symptoms is obscure. Little is

known about the effect of occupation on the association between LBP and disc degeneration. There is no gener-ally accepted classification for LBP. Variation in the cri-teria for disc degeneration and nonspecificity of the symptoms may partly explain differences between the results of epidemiologic studies.

In this study, we have analyzed three signs of disc degeneration separately: signal intensity of the nucleus pulposus and anterior and posterior bulges of the disc. We have studied the associations of disc degeneration of the lumbar spine with different types of LBP.

Subjects and Methods

Subjects.The primary cohort consisted of 1832 25– 49-year-old men— 688 machine drivers, 553 carpenters, and 591 office workers, who had participated 7 and 4 years before the present study in questionnaire studies concerning occupational effects on LBP.21,23

The subjects of this study were a subgroup of the cohort selected, using age (40 – 45 years) and place of residence as inclusion criteria. The participation rate was 71%. The final study group comprised 53 machine operators, 51 construction carpenters, and 60 municipal office workers. Occupational load imposed on the back was distinctly different in each group. The machine drivers (earth mover operators, longshoremen) were exposed to whole-body vibration and prolonged sitting. Occasionally, they were exposed to manual material handling, particularly in maintenance work. The carpenters were ex-posed to dynamic physical work, including material handling and postural load, and to a high accident risk due to climbing, walking on rough surfaces with obstacles,etc.The office work-ers had sedentary work but were free to change posture and move around.

Questionnaire and Interview.A self-administered naire included the same questions as the previous question-naires about occurrence, duration, and type of low back symp-toms during the preceding 12 months and 4 years, about occupational history (e.g., years in occupation and exposures at work), individual characteristics (e.g., anthropometric mea-sures and level of education), and lifestyle factors (e.g., annual car driving, smoking). A structured interview with complemen-tary questions about LBP and potential risk factors was per-formed by a nurse, who also checked the questionnaire for the completeness of the answers.

The questions concerning LBP were as follows: Have you had low back pain during your lifetime; during the past 4 years; during the previous 12 months? What kind of low back pain have you had: lumbago (meaning sudden attack of low back pain); sciatic pain (meaning low back pain radiating to the lower limb); other low back pain? The concept of sciatic pain is From the *Finnish Institute of Occupational Health, Helsinki, Finland;

†Uppsala University, Uppsala, Sweden; ‡Helsinki University Central Hospital, Helsinki, Finland.

This work has been supported by the Work Environment Fund, Fin-land.

Acknowledgment date: March 17, 1999. Acceptance date: May 26, 1999. Device status category: 1. Conflict of interest category: 14.

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a nonspecific one including symptoms associated with true sci-atica but also,e.g.,referred pain felt in the thigh. In the analy-sis, lumbago and other LBP were combined as local LBP.

Job title was used as the measure of occupational load. The subjects had worked for 26 years on average in the present occupation. The other potential confounding factors and the percentages of the subjects according to each variable are pre-sented in Table 1.

MRI Examination.The lumbar spines were examined with a 0.1 T Imager (Mega4, Instrumentarium, Helsinki, Finland) and a surface coil. Proton density and T2-weighted images in sag-ittal plane were obtained using a dual echo technique (TR 2000/TE 25-86) with two acquisitions. The first echo was a gradient echo and the second one a spin echo. Slice thickness was 7 mm, field of view 410 mm⫻ 410 mm (imaging area L1–S1), matrix 256⫻256, and pixel size 1.6 mm. There were no gaps between the slices.

All examinations were included in the study, even though the general image quality was not optimal in 16 cases (9.8%). The peripheral regions are the most vulnerable for technical reasons. Disc L1/L2 was excluded from the analysis, as the image quality was low in the majority of the examinations. Observation was recorded as missing if the image quality was not high enough for reliable evaluation, which was the case in 9-25/164 observations of the L5/S1 discs, depending on the sign of disc degeneration.

Signal Intensity of the Nucleus Pulposus. Signal intensity of the nucleus pulposus of the discs L2/L3–L5/S1 on T2-weighted images was visually estimated by three radiologists on a four point scale (1 bright, 2 grey, 3 dark, 4 black), using the cere-brospinal fluid (CSF) in the adjacent dural sac as an intensity reference.11,12 Interobserver agreement (weighted kappa) be-tween each pair of the radiologists ranged from 0.59 to 0.83 for individual disc levels L2/L3–L5/S1. Twenty-two of the images were reevaluated by one of the radiologists. Intraobserver agreement rate was 57% to 81%. The interpretation of the most experienced radiologist was used in data analysis. The signal intensity of the nucleus pulposus was dichotomized be-fore the final data analysis; intensity lower than that of the adjacent CSF (grades 3 or 4) was considered as a positive find-ing, called dark nucleus pulposus (Figure 1A).

Anterior and Posterior Bulge of the Intervertebral Disc. All images were visually evaluated by the three radiologists. None of the subjects had a major asymmetric bulge or herniation. The magnitude of disc bulges was measured by one of the radiologists in the middle line of the disc from the proton den-sity-weighted sagittal images (Figure 1B), with the facility of the MRI device as presented earlier.13

A bulge measuring 3.2 mm or more was considered as a positive finding. The measure-ments were repeated for 25 of the images to estimate intraob-server agreement. Agreement rate was 86% to 100% for ante-rior bulges and 81% to 92% for posteante-rior bulges, lowest for L5/S1 disc.

Statistical Analysis.The association of LBP with disc degen-eration and the covariates was analyzed by multivariate mod-eling. The dependent variables were the 12-month and the 4-year prevalence of LBP (including all types of LBP) and sciatic pain. The numbers of discs with different signs of degeneration were used as determinants, and occupation, height, history of overweight, smoking, and car driving were adjusted for as con-founders.

Logistic regression analysis was performed using Genmod pro-cedure of statistical analysis system (SAS) to estimate the odds ratios (OR) and their 95% confidence intervals (CI). For each sign of degeneration a separate analysis was made. The number of discs either with dark nucleus pulposus or with posterior or

ante-Figure 1. Decreased signal intensity of the nucleus pulposus in the L5/S1 disc in a T2-weighted image(A). The posterior bulge in the L5/S1 disc was more clearly demonstrated in the proton density-weighted image(B).

Table 1. Distribution of Subjects by Explanatory Variables (N164)

% Number of discs with dark nucleus pulposus

0 12.6

1 37.7

2–4 49.7

Number of discs with posterior bulge

0 39.1

1 34.0

2–4 26.9

Number of discs with anterior bulge

0 50.0 1 34.5 2–4 15.5 Occupation Office worker 36.6 Carpenter 31.1 Machine driver 32.3 Height ⬍175 cm 29.2 175–179 32.3 ⱖ180 cm 38.4 History of overweight No overweight* 21.3 Others 59.1 Constant overweight† 19.5 History of smoking No smoking ever 31.7

Smoking in the past 32.9

Present smoking 35.4

History of car driving

⬍15 000 km/year 64.4

ⱖ15 000 km/year‡ 35.6

* BMI⬍24.0 kg/m2at 25 years and currently.

† BMIⱖ27.0 kg/m2at 25 years and currently.

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rior bulges was handled as a count term (0,1,2,3,4) in the analysis, assuming that the risk of LBP or sciatic pain for each unit change is increased with the same magnitude. This means that if the OR is,e.g.,1.5, and the person has one disc affected, the odds in favor of them having LBP are 1.5 times greater than the odds of them not having LBP. If the patient has three discs affected, the odds are 4.5 times (3⫻1.5) in favor of them having LBP.

Results

The 12-month prevalence of LBP was 74.4% and the 4-year prevalence was 81.1%. For sciatic pain the prev-alence rates were 29.9% and 39.0%, respectively. In Fig-ure 2, the prevalences are presented by occupation. Low back pain was more common among machine drivers and carpenters than among office workers. Sciatic pain was most common among machine drivers and local LBP among carpenters.

The prevalence of sciatic pain increased with increas-ing numbers of degenerated discs for each sign of degen-eration (Figure 3). The prevalence of local LBP was sim-ilar across all signs of disc degeneration and did not seem to depend on the number of degenerated discs.

All signs of disc degeneration were associated with an increased risk of LBP during the past 12 months, ad-justed odds ratios ranging from 2.0 to 3.4 for one unit change in the degeneration variable (Table 2). The

rela-tionship between LBP and posterior or anterior bulge was weaker for the 4-year prevalence of LBP, but regard-ing dark nucleus pulposus it was of the same magnitude as for the 12-month prevalence. Low back pain and sci-atic pain were distinctly related to occupation. The ad-justed OR of machine drivers (8.1) was considerably higher than the crude OR (4.7), suggesting a negative confounding effect. The confounding effects of other ex-planatory variables were small.

Sciatic pain had a weaker relation to all signs of disc degeneration than LBP (Table 3). The association was most distinct between 12-month prevalence of sciatic pain and posterior disc bulge (adjusted OR 2.0, 95% CI 1.3–3.1 for one unit change).

Discussion

In this study, LBP during the preceding 12 months was found to be significantly associated with disc degenera-tion, assessed either as dark nucleus pulposus or poste-rior or anteposte-rior disc bulge. Sciatic pain was associated with posterior disc bulge. Weaker associations were found regarding LBP during the preceding 4 years.

Oc-Figure 3. Twelve-month and 4-year prevalence of low back pain (LBP) (%) in relation to the number of degenerated discs L2/L3– L5/S1, with three signs of degeneration.

Figure 2. Twelve-month and 4-year prevalence of low back pain (LBP) (%) by occupation and type of LBP.

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cupation proved to be a strong predictor of LBP and sciatic pain.

To study the association between LBP and disc degen-eration, we chose the nonspecific concept of LBP, includ-ing all types, and sciatic pain as dependent variables. Local LBP includes pain,e.g.,caused by distensions and other traumas of soft tissues, in addition to possible dis-cogenic pain. Sciatic pain includes pain radiating below the knee, including pain symptoms associated with true sciatica, but also other pains such as referred pain thought to originate from the deep structures surround-ing the disc and felt in the thigh. Eighty percent of men

with sciatic pain in the questionnaire also reported hav-ing had LBP radiathav-ing below the knee in the interview.

Recall error is known to affect reporting of LBP. Ac-cording to a previous report, recall error is differential across occupational groups so that it is highest among office workers and lowest among machine drivers.23

Sci-atic pain is remembered better than other types of LBP. Recall is also affected by the recentness and severity of symptoms.22

For the classification of disc degeneration we used three signs that often coexisted. The repeatability of the signs of degeneration of discs L2/L3–L5/S1 was good. Table 2. Low Back Pain (LBP) in Relation to Different Signs of Disc Degeneration (L2/L3–L5/S1) and Occupation; Odds Ratios (OR), and Their 95% Confidence Intervals (CI)

Determinant

12-Month Prevalence 4-Year Prevalence

Crude Adjusted* Crude Adjusted*

OR 95% CI OR 95% CI OR 95% CI OR 95% CI Posterior bulge† 2.2 1.4–3.7 2.7 1.5–4.8 1.6 1.0–2.7 1.7 1.0–3.0 Occupation Office worker 1.0 1.0 1.0 1.0 Carpenter 2.9 1.2–6.9 2.5 0.9–6.9 5.4 1.9–15.7 4.3 1.4–13.5 Machine driver 4.7 1.8–12.1 8.1 2.4–27.1 6.9 2.2–21.9 10.1 2.5–40.7 Anterior bulge† 3.8 1.7–8.1 3.4 1.4–8.2 2.6 1.2–5.6 1.9 0.8–4.3 Occupation Office worker 1.0 1.0 1.0 1.0 Carpenter 2.9 1.2–6.9 2.0 0.7–5.8 5.4 1.9–15.7 5.7 1.6–20.5 Machine driver 4.7 1.8–12.1 4.4 1.2–16.4 6.9 2.2–21.9 9.7 1.9–49.5

Dark nucleus pulposus† 1.7 1.1–2.5 2.0 1.2–3.1 1.9 1.2–3.1 2.1 1.2–3.6

Occupation

Office worker 1.0 1.0 1.0 1.0

Carpenter 2.9 1.2–6.9 4.1 1.4–12.4 5.4 1.9–15.7 7.2 2.0–26.0

Machine driver 4.7 1.8–12.1 4.0 1.3–12.6 6.9 2.2–21.9 6.2 1.7–23.0

* Adjusted for height and for history of car driving, smoking and overweight. † Change of OR per one unit change in the number of discs with the sign.

Table 3. Sciatic Pain in Relation to Different Signs of Disc Degeneration (L2/L3–L5/S1) and Occupation; Odds Ratios (OR), and Their 95% Confidence Intervals (CI)

Determinant

12-Month Prevalence 4-Year Prevalence

Crude Adjusted* Crude Adjusted*

OR 95% CI OR 95% CI OR 95% CI OR 95% CI Posterior bulge† 1.6 1.1–2.2 2.0 1.3–3.1 1.3 0.9–1.9 1.7 1.1–2.5 Occupation Office worker 1.0 1.0 1.0 1.0 Carpenter 1.7 0.7–4.6 1.0 0.3–2.9 1.5 0.6–3.6 1.1 0.4–2.74 Machine driver 6.4 2.6–15.5 7.2 2.5–20.6 5.6 2.4–13.2 7.4 2.6–20.8 Anterior bulge† 1.5 1.1–2.2 1.3 0.8–1.9 1.5 1.0–2.3 1.5 0.9–2.3 Occupation Office worker 1.0 1.0 1.0 1.0 Carpenter 1.7 0.7–4.6 1.1 0.4–3.2 1.4 0.6–3.2 1.0 0.4–2.7 Machine driver 6.4 2.6–15.5 4.3 1.5–12.0 5.0 2.2–11.1 5.2 1.8–15.3

Dark nucleus pulposus† 1.3 0.9–1.8 1.3 0.8–1.9 1.5 1.0–2.1 1.5 1.0–2.4

Occupation

Office worker 1.0 1.0 1.0 1.0

Carpenter 1.7 0.7–4.6 1.6 0.5–4.9 1.4 0.6–3.2 1.3 0.4–3.9

Machine driver 6.4 2.6–15.5 6.0 2.1–17.3 5.0 2.2–11.1 7.8 2.6–23.0

* Adjusted for height and for history of car driving, smoking and overweight. † Change of OR per one unit change in the number of discs with the sign.

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The signal intensity of the intervertebral disc correlates with its water and proteoglycan content.15,26It is decreased

in most discs with a radial fissure,5,32which is also consid-ered as a sign of disc degeneration.8We estimated the signal

intensity in the nucleus pulposus, the region with the high-est water content. The effect of the arbitrary scaling of the MR images and the distance of the disc from the surface coil on the signal intensity was corrected by using adjacent CSF as an intensity reference.11,12

An association between disc bulge and radial fissure has been found,3,28,33in particular, for bulges of the size of 2.5 mm or more.33The size of 3.2 mm or more was

chosen as the criterion for a positive finding for a bulge. It was considered as the minimum size to be assessed reliably. Only a few of the posterior bulges were 4.8 mm or more in size, but none of the subjects had major asym-metric bulges or herniations. The measurement was per-formed in the midsagittal slice because the projectional distortion affects the measurements in the more lateral slices. Differentiation between diffuse bulging or focal bulge of the disc was not attempted. Part of the lateral or posterolateral disc bulges may not have been detected in sagittal slices. We did not attempt to assess a possible nerve root compromise. In the neutral supine position, even in axial slices, the degree of compression of the dural sac or nerve roots by disc bulges may be underes-timated as compared with the situation in the upright position with normal axial load on the discs. Thus, the true degree of compromise cannot be accurately mea-sured as recently suggested.7

Odds ratio is known to overestimate the risk of com-mon events. In our analysis we could not estimate the risk ratios (RR) because available multivariate methods were not applicable to the data. The crude OR versus RR of 12-month LBP was 2.9 versus 1.4 for carpenters and 4.7 versus 1.5 for machine drivers. For sciatic pain the figures were 1.7 versus 1.6 and 6.4 versus 3.5, respec-tively.

Pain felt in the low back may originate from the ver-tebrae, ligaments, fascias, muscles, facet joints, or inter-vertebral discs. Intradiscal pathology is assumed to play a major role in nonspecific LBP syndromes30and chronic

LBP.17There are pain sensitive nerve endings in the fibers of annulus fibrosus, even in the inner parts of a degener-ated disc.4 Nucleus pulposus tissue is known to have marked inflammatory properties.14,18

Neural compromise is a known cause of sciatic pain. Disc herniation or bulge or narrowing osteoarthritic changes may cause mechanical compression or disten-sion of the lumbar nerve root, dorsal root ganglion, or smaller nerves surrounding the disc. In recent studies, chemical irritation has been suggested to be a more im-portant cause of sciatic pain.24Most of the bulges in our

study were small to cause neural compromise, but of the size likely to be associated with a radial tear. Posterior disc bulge was a stronger risk factor for recent than for earlier LBP and sciatic pain as expected.

Low back pain has been found to be associated with changes in the annulus fibrosus,1,9,16,31with disc

protru-sions,27, and with the decrease of signal intensity of the

disc.6,19 An association between LBP and radiographic

film findings has been found in some studies, among them one follow-up study,25although van Tulder29

con-cluded that there is no firm evidence of a causal relation-ship.

The presence of degenerative changes is not diagnostic of low back pain. Indeed some 30% of asymptomatic individuals have such changes.2,10,20,27 However, the

present data indicate that these changes render low back pain to be more likely and more so the greater the num-ber of discs affected.

Occupation had a strong effect on the prevalence of LBP in our study. Machine drivers exposed to whole-body vibration and prolonged constrained sitting had the highest prevalence of sciatic pain. The prevalence of local LBP was highest among carpenters exposed to dy-namic physical work and high accident risk. Adjustment for the potential confounding factors had a minor effect on the relationships between LBP and the signs of disc degeneration, with the exception of occupation, which had a negative confounding effect on the relationship between LBP and posterior bulges. Even if the crude OR for machine drivers and carpenters was higher than that for the signs of disc degeneration, the association be-tween LBP and disc degeneration prevailed in multivar-iate modeling when the occupation and other confound-ing factors were added to the model.

In cross-sectional studies and also in case-referent studies the problem concerning the temporal relation-ship between radiologic findings and symptoms is present: the timing of the occurrence of LBP and the appearance of the signs of disc degeneration is not known. In the future, the association between LBP and disc degeneration should be studied with longitudinal study designs, using repeated assessment of the occur-rence of LBP and repeated MRIs in order to unravel the temporal relations between the incidence of low back symptoms and appearance of disc degeneration. It seems beneficial to differentiate between various types of LBP and various signs of disc degeneration in epidemiologic studies on low back disorders.

Conclusions

In this cross-sectional MRI study, LBP was associated with signs of disc degeneration and sciatic pain with pos-terior disc bulges. The risks of LBP and sciatic pain were strongly affected by occupation.

References

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Address reprint requests to

Katariina Luoma, MD

Finnish Institute of Occupational Health Topeliuksenkatu 41 a A FIN-00250 Helsinki, Finland E-mail: kluoma@dlc.fi

References

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