Left facet joint OA and right facet joint OA significantly increased with the progression of the disc degeneration grade (both P < .001; Spearman = 0.824 and 0.806, respectively) (Tables 1 and 2). There were no significant differences based on sex regarding the grading of disc and facet joint degeneration for each lumbar 3-joint complex.
3.2 Disc degeneration and facet joint tropism
Table 3 presents the relationship between facet tropism and disc degeneration for each 3-joint complex. A total of 227 motion segments had tropism in this study. One hundred (61%) motion segments had tropism with disc degeneration grade IV. Forty (63%) motion segments had tropism with disc degeneration grade V. Facet tropism was significantly different (P < .001) with disc degeneration.
3.3 Disc degeneration and intervertebral height
The intervertebral disc height was measured at each segment with different disc degeneration grades. The mean disc height was 12.65 ± 0.69 mm when the disc degeneration was grade I, 12.05 ± 1.08 mm when the disc degeneration was grade II, 11.51 ± 1.23 mm when the disc degeneration was grade III, 9.97 ± 1.17 mm when the disc degeneration was grade IV, and 7.34 ± 1.31 mm when the disc degeneration was grade V.
The intervertebral height significantly decreased with the progression of disc degeneration grade, but there was no significant difference between grade I and grade II (I vs III and II vs III: P < .05), (I vs IV, I vs V, II vs IV, II vs V, III vs IV, III vs V, and IV vs V: P < .001). Figure 4 shows the associations between disc degeneration and intervertebral height. The intervertebral disc height significantly decreased with disc degeneration (Pearson r = −0.728).
3.4 Facet joint degeneration and intervertebral height
The intervertebral disc height was also measured at each segment with different facet joint degeneration grades. The mean intervertebral height was 12.44 ± 0.56 mm when the facet joint degeneration was grade 0, 11.63 ± 1.24 mm when the facet joint degeneration was grade 1, 9.88 ± 1.65 mm when the facet joint degeneration was grade 2, and 8.92 ± 1.80 mm when the facet joint degeneration was grade 3.
When the facet joint degeneration grade increased, the intervertebral disc height significantly decreased, except between grade 0 and grade 1 (0 vs 2, 0 vs 3, 1 vs 2, 1 vs 3, and 2 vs 3: P < .001). Figure 5 shows the association between facet joint degeneration and intervertebral height. The intervertebral disc height significantly decreased with facet joint degeneration (Pearson r = −0.573).
3.5 Reliability of variance for radiographic evaluation
The weighted kappa coefficient for the interobserver agreement regarding the disc degeneration evaluation was 0.729. The weighted kappa coefficient for the interobserver agreement regarding the facet degeneration evaluation was 0.679.
This study demonstrates that each individual joint degeneration influences the other 2 in the lumbar 3-joint complex. The results of this study provide evidence of a strong positive correlation between disc degeneration and facet joint degeneration. The study did not reveal a relatively high proportion of degenerated facet joints of moderate to severe grades (grades 2–3) accompanied by milder grades of disc degeneration (grades 1–2), which may not be in complete agreement with results reported in previous studies.[6–9,12] This discrepancy could be solved by designing a prospective long-term follow-up study and creating uniform degeneration grading systems for both intervertebral discs and facet joints.
After analyzing the radiographic parameters of 456 lumbar motion segments, we found a strong positive correlation between the severity of facet OA and disc degeneration. We found 139 lumbar motion segments with grade 3 disc degeneration. A total of 110 (79.1%) lumbar motion segments had grade 1 left facet joint degeneration, while 105 (75.5%) lumbar motion segments had grade 1 right facet joint degeneration. Most facet joints undergo mild degeneration when the lumbar disc is moderately degenerated. Therefore, disc degeneration could potentially occur prior to degeneration of the facet joint. This concept is in agreement with the previous findings of Fujiwara. Vernon-Roberts and Pirie dissected more than 100 lumbar spines and found that disc degeneration was always accompanied by degenerative changes of the facet joint. Based on their observations, there was a negative correlation between the severity of OA and disc structure (the lesser the degeneration, the lesser the degree of OA).
This study also showed a significant correlation between facet tropism and disc degeneration. The role of facet tropism in the pathogenesis of disc degeneration is a controversial. Vanharanta et al reported that the correlation between facet tropism and disc degeneration was not significant. However, Noren et al concluded that facet joint tropism is an important risk factor in the development of disc degeneration. Additionally, Dai performed a study of 106 subjects and reported that a significant correlation existed between the degree of disc degeneration and facet joint tropism for patients with degenerative spondylolisthesis. Kong et al found a higher incidence of highly degenerated discs at L4/5, which was observed within the facet tropism group. Schleich et al indicated that facet tropism and sagittal orientation of the facet joint represent risk factors for the development of early biochemical alterations of lumbar intervertebral discs. Mohanty et al performed a study of 426 subjects and confirmed the existence of a significant association between lumbar intervertebral disc prolapse and facet tropism. However, the previous studies[28,29] found that facet tropism did not exhibit an association with facet joint OA at the L3/4, L4/5, and L5/S1 levels. In this study, a total of 197 (61%) of lumbar motion segments had facet tropism with disc degeneration IV and 63 (63%) of lumbar motion segments had facet tropism with disc degeneration V. The results of this study showed that there is a significant correlation between facet tropism and disc degeneration. These findings are consistent with those of previous reports.
Many biomechanical studies reported that 2 facet joints may carry parallel loading in the lumbar spine.[2,3] Furthermore, facet tropism may alter the biomechanics of the lumbar spine. Cyron and Hutton deduced that the annulus fibrosus on the side with greater coronal orientation may undergo a higher compressive load and increased rotational stress. Therefore, the loading imbalance would accelerate the degeneration of the facet joints and intervertebral discs.
In this study, we found that for increasing grades of disc degeneration (grades 2–5), there was a significant intervertebral height loss at each level. Teichtahl et al performed a study of 72 community-based individuals not previously examined for low back pain and reported that the estimated marginal means of the intervertebral disc height were 13.2 mm, 11.5 mm, 10.3 mm, and 4.2 mm for disc degeneration grades 2, 3, 4, and 5, respectively, at the L4/5 level. They demonstrated narrowing of 0.98 to 1.60 mm in the disc at various levels within the lumbosacral spine. We assessed intervertebral disc height for each grade of disc degeneration. We found that the average disc height associated with grade 2 disc degeneration was 12.05 mm. The average disc height associated with grade 3 disc degeneration was 11.51 mm. For grade 4 disc degeneration, the average disc height was 9.97 mm and for grade 5 disc degeneration, the average disc height was 7.34 mm. In our study, the intervertebral disc height differences between disc degeneration grades 2 and 3 were less than 0.98 mm. The intervertebral disc height showed no significant differences between disc degeneration grades 1 and 2. The difference between disc degeneration grades 1 and 2 was equivalent to the signal intensity according to Pfirrmann scores.
Our findings showed that intervertebral height loss probably leads to deterioration of the facet joint. Using a large sample, we found that the correlation coefficient of the correlation between facet joint degeneration and disc height was −0.573. Only a few studies have reported the correlation between facet joint degeneration and intervertebral disc height. Suri et al found that some individuals experience facet degeneration without any loss of disc height. To our knowledge, the facet joints play an important role in load transfer because they provide posterior load-bearing assistance. The percentage of segmental load transmitted through the posterior elements can increase up to 47% in degenerated facets with greater losses of disc space height.
Some limitations of the present study should be acknowledged. Since the total number of samples was not large enough, this study did not differentiate between different segments when discussing degeneration of the lumbar 3-joint complex. However, our study revealed an overall trend. As this was a retrospective study, we did not consider the relationship between symptoms and radiographic degeneration.
This study discussed various aspects of degeneration of the lumbar 3-joint complex. We found that the mechanical consequences of disc degeneration, including decreased disc height, increase facet joint degeneration.
The authors thank the assistance of Editage in Shanghai, which provided English language editing.
Conceptualization: Min Dai.
Data curation: Quanwei Song, Xuqiang Liu, Qi Lai.
Funding acquisition: Zongmiao Wan.
Investigation: Quanwei Song, De-jian Chen, Benyu Tang.
Methodology: Bin Zhang.
Project administration: Zongmiao Wan.
Software: Benyu Tang.
Supervision: Bin Zhang, Min Dai.
Writing – original draft: Quanwei Song.
Writing – review & editing: Zongmiao Wan.
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Keywords:Copyright © 2019 the Author(s). Published by Wolters Kluwer Health, Inc.
degeneration; facet tropism; intervertebral disc height; intervertebral disc; lumbar complex joints degeneration; lumbar facet joint