Analysis of the relationship between morphology of intervertebral disc and some correlated factors following discography in patients with chronic low back pain

Abstract

Objective:  To evaluate the diagnostic effectiveness of discography in discogenic low back pain (LBP).

Methods:  Ninety‐six cases of chronic LBP with or without referred thigh pain were enrolled in this study. All these cases received CT scan following discography once conservative treatment for at least 6 months had failed. There were 42 men and 54 women, aged from 24 to 67 years (average 46.4). Discography was performed on 218 discs. The positive discs were classified morphologically according to the Dallas Discogram Description (DDD).

Results:  (i) The 56 cases (58.3%) which were positive on discography were divided into two subgroups of age less or more than 50 years. Positive rates for the two subgroups were 33.3% and 66.7%, respectively; (ii) one hundred and twenty‐two discs, of which 62 (50.8%) were positive on discography, showed morphologic abnormality, whereas all the discography positive discs showed morphologic abnormality. No complication related to discography was found in any case.

Conclusion:  (i) Compared with the younger patients, older LBP patients have a lower positive rate of discography despite the presence of more serious degenerative disc changes; (ii) outer layer disruption of the annulus fibrous correlates with positive discography; (iii) MRI intensity changes are not specific in diagnosing discogenic pain. Additional discography is needed to identify the painful disc; and (iv) the contrast volume injected into discs can be affected by a variety of factors which restrict its diagnostic value.

Introduction

The concept of discogenic low back pain (LBP) was first put forward by Park et al., who defined it as persistent LBP originating from intradiscal structural disarrangement and degeneration without signs of nerve root compression on radiology ¹ . Later, Crock raised the concept of internal disc disruption (IDD), and considered IDD to be the cause of LBP ² . Now discography is widely used as a tool for diagnosing discogenic LBP. The executive Committee of the North American Spine Society deemed discography to be the only valid method for diagnosing discogenic LBP ³ . However, controversies exist concerning its accuracy and clinical significance ⁴ , ⁵ . The present study will summarize the results of diagnostic discography in a group of patients with LBP, and analyze its accuracy and clinical significance.

Materials and methods

Clinical data

From April 2004 to December 2006, 96 cases were enrolled in this study. Criteria for inclusion: (i) chronic LBP with or without referred thigh pain for at least 6 months; and (ii) failure to respond to conservative treatment of least 6 months duration. Criteria for exclusion: radiculopathy as the main symptom; imaging data showing obvious lumbar disc herniation or stenosis. All our cases received CT (SIEMENS, Germany, KV120, MAS140, SL3mm) scan after discography. There were 42 men and 54 women aged from 24 to 67 years (average 46.4). Discography was performed on 218 discs, 6 of which were at L2‐3, 52 at L3‐4, 76 at L4‐5, and 84 at L5S1. Seventy‐two patients had involvement of 2 disc levels; 22 of 3, and 2 of 4. Six cases had had previous lumbar surgery history. No complication related to discography was noted.

Discography

In most cases discography, using inner discography needles (Stimuplex A, 0.9 mm × 150 mm, Chikuma‐Shi, Nagano‐Ken, Japan) following local infiltration anesthesia with 1% lidocaine, was performed in the prone position with a standard posterolateral approach. The contrast medium Omnipaque (GE Healthcare, Shanghai, China) was used for disc injections. Only two patients received discography in the lateral position due to intolerance of the prone position. CT scans were obtained during discography in all patients. During discography the degree, site and concordance of the patients' pain reactions, as well as volume of contrast used, were noted. After discography patients were asked to rest in bed for 24 h, and intravenous or oral antibiotics were administrated for 48 h to prevent intradiscal infection.

Criteria for a positive pain response during discography

As a provocative test, results of discography based on the patient's response, were classified into the following three categories: (i) concordant pain, which is an exact reproduction of the low back pain that the patient experienced previously in regard to quality, degree and location; (ii) non‐concordant pain, which differed from the patient's previously experienced LBP; and (iii) no pain. Concordant pain was deemed as positive and non‐concordant pain or no pain as negative.

Morphological classification of disc following discography

The positive discs were classified morphologically according to the Dallas Discogram Description (DDD) ⁶ as follows: (i) DDD fibrous annulus degeneration grading: grade 0, contrast fills the normal space of the nucleus pulposus; grade 1, contrast fills less than 10% of the fibrous annulus; grade 2, contrast filled area 10%–50%; grade 3, contrast filled area > 50% (Fig. 1); and (ii) DDD fibrous annulus rupture grading: grade 0, no annulus cleavage; grade 1, the annulus cleavage reaches between 1/3 and 2/3 of the whole annulus; grade 2, the cleavage breaks through 2/3 and remains within the rim of the annulus; grade 3, the cleavage breaks through the outer rim of the annulus (Fig. 2).

Figure 1.

DDD fibrous annulus degeneration grading.

Figure 2.

DDD fibrous annulus rupture grading.

Statistical analysis

Between groups, quantitative data were compared by one‐way analysis of Variance (ANOVA), and qualitative data by the Pearson‐χ² test. All analyses were performed on SPSS 10.0 software.

Results

Relationship between age and positive discography

The 56 patients (58.3%) who were positive on discography were divided into two subgroups by age: less or more than 50 years. Positive rates for the two subgroups were 33.3% and 66.7%, respectively (P < 0.05, Table 1).

Table 1.

Patients' age and rates of positive discography

Age (years)	Negative	Positive	Total	Positive rate (%)
<50	24	48	72	66.7
≥50	16	8	24	33.3
Total	40	56	96	58.3

Chi‐square test: χ² = 8.229, P < 0.05.

Relationship between morphology of disc and positive discography

One hundred and twenty‐two discs showed morphologic abnormality, of which 62 (50.8%) were positive on discography and 60 (49.2%) negative. In positive discs, DDD fibrous annulus degeneration grade 1, 2, 3 accounted for 45.2%, 35.5%, and 19.4%, respectively; in negative discs, they accounted for 30.0%, 33.3%, and 36.7%, respectively, (P > 0.25, Table 2). In positive discs DDD fibrous annulus rupture grade 1, 2, 3 accounted for 32.3%, 67.7%, and 0, respectively; in negative discs, they accounted for 23.3%, 20.0%, and 56.7%, respectively (P < 0.005, Table 3).

Table 2.

DDD fibrous annulus degeneration grade and rates of positive discography

Grade	Grade 1	Grade 2	Grade 3	Total
Positive	28 (45.2%)	22 (35.5%)	12 (19.4%)	62
Negative	18 (30.0%)	20 (33.3%)	22 (36.7%)	60
Total	46	42	34	122

Chi‐square test: χ² = 5.082, P > 0.25.

Table 3.

DDD fibrous annulus rupture grade and rates of positive discography

Grade	1	2	3	Total
Positive	20 (32.3%)	42 (67.7%)	0 (0)	62
Negative	14 (23.3%)	12 (20.0%)	34 (56.7%)	60
Total	26	62	34	122

Chi‐square test: χ² = 53.432, P < 0.005.

Relationship between age and disc morphology

One hundred and twenty‐two discs showed morphologic abnormality of which 78 discs were from younger (<50 years) and 44 from older patients (≥50 years). DDD annulus fibrous degeneration grade 1, 2, 3 in the younger group accounted for 51.3%, 33.3%, 15.4%, respectively, whereas those in the older group accounted for 13.6%, 40.9%, 45.5%, respectively (P < 0.005, Table 4); DDD annulus fibrous rupture grade 1, 2, 3 in the younger group accounted for 25.6%, 56.4%, 17.9%, respectively, meanwhile, those in the older group accounted for 13.6%, 36.4%, 50.0%, respectively (P < 0.005, Table 5).

Table 4.

Age and DDD fibrous annulus degeneration grade

Age(years)	Grade 1	Grade 2	Grade 3	Total
<50	40 (51.3%)	26 (33.3%)	12 (15.4%)	78
≥50	6 (13.6%)	16 (36.4%)	22 (50.0%)	44
Total	46	42	34	122

Chi‐square test: χ² = 22.732, P < 0.005.

Table 5.

Age and DDD fibrous annulus rupture grade

Age(years)	Grade 1	Grade 2	Grade 3	Total
<50	20 (25.6%)	44 (56.4%)	14 (17.9%)	78
≥50	6 (13.6%)	18 (40.9%)	20 (45.5%)	44
Total	26	62	34	122

Chi‐square test: χ² = 41.588, P < 0.005.

Relation between MRI and positive discography

Sixty‐two of 122 discs (50.8%) with morphological abnormality on MRI were positive on discography. All 62 discography positive discs in 56 patients demonstrated more or less degenerative changes on MRI (black discs), whereas none of the 96 discs which were normal on MRI was positive on discography. A total of 16 high intensity zones (HIZ) were noted in these cases, and 14 of them were positive on discography, accounting for 22.6% of all positive discs.

Relationship between injection volume and positive discography

The contrast volume injected into morphologically normal discs averaged 1.2 ml. As to morphologically abnormal discs, discography positive discs averaged 3.2 ml, and discography negative discs 3.4 ml. The difference between these two groups was not statistically significant.

Discussion

Discogenic LBP commonly occurs during the fourth decade ⁷ .A previous study has shown a very high prevalence rate, which increases with age, for abnormal morphology on discography in the LBP population ⁷ . Although signs of disc degeneration were more evident in older patients, the rate of positive discs was not increased in proportion to the degree of disc degeneration on discography, which indicates that age‐related disc degeneration is not closely related to discogenic LBP.

The normal intervertebral disc consists of an outer annulus fibrosus which is rich in collagens that account for its tensile strength, an inner nucleus pulposus which contains large proteoglycans that retain water for resisting loading by compression, and an intermediate zone between the annulus fibrosus and nucleus pulposus, the transitional zone. With disc degeneration, there is a net loss of proteoglycans and water from the nucleus, leading to poor hydrodynamic transfer of axial stresses to the outer annulus fibrosus. Abnormal distribution of forces across the disc results in cracking and fissuring of the annulus fibrosus, further decreasing its ability to distribute stresses properly during loading. The pathology responsible for chronic discogenic low back pain is within the properties of the disc itself. Common pathological findings are degeneration of the intervertebral disc, and annular tear of different patterns without disc herniation. The patterns of abnormalities of these discs on MRI and discography are indistinguishable from those of asymptomatic degenerated discs. The only feature that distinguishes these discs from asymptomatic degenerated discs is a positive pain response during discography. Peng et al. evaluated 285 cases with discography, according to DDD, and found that none of the discs in which contrast failed to reach into the outer annulus was positive ⁸ . Their results indicate a positive correlation between outer annulus rupture and concordant pain response. In our study concordant pain response was reproduced in 50.8% of the discs. Moreover, Antti‐Poika et al. have reported a 53% positive rate, which is similar to our finding ⁹ . Accordingly, we consider that disc degeneration and annular rupture may be two important factors related to discogenic pain. Colhoun et al. divided 162 patients into two groups according to their pain response, and found that the success rate of interbody fusion in the concordant pain group was 88%, and in the non‐concordant pain group only 52%. ¹⁰ Their results indicate that concordant pain response is related to the pathology of discogenic low back pain.

Intervertebral disc degeneration is a complex process characterized by biochemical and structural changes. Because these changes are similar to those that occur with increasing age, researchers are facing difficulty in differentiating degeneration of the intervertebral disc that occurs purely as an aging phenomenon and degeneration that occurs as part of a pathologic process. Disc degeneration usually appears on MRI T2‐weighted images as a decline in signal intensity, that is, the so‐called ‘black’ disc. Both a normal aging disc and a pathologically painful disc appear as an area of decreased signal intensity on T2‐weighted images. MRI cannot differentiate between these two disease entities ¹¹ . Recently, there has been interest in the presence of an area of HIZ within the posterior annulus seen on the T2‐weighted, sagittally oriented MRI sequence ¹² . Whether the presence of a HIZ is a reliable marker of painful disc has not yet been clarified. According to our study, we believe that an HIZ in the lumbar disc of a patient with LBP is a reliable marker of painful outer annular disruption.