Introduction
Back pain and its sequelae place an enormous burden on society, health care systems, and the economies of developed countries7,17. Degenerative changes in the spine can be a potential cause of back pain, and therefore an understanding of the relationship between pathoanatomic abnormality and advanced degeneration is of importance from a clinical and public health perspective. The purpose of this study was to investigate associations between facet joint tropism and facet joint orientation and two common lumbar degenerative spinal disorders: facet joint osteoarthritis (OA) and degenerative spondylolisthesis (DS).
The facet joints are the only synovial joints in the spine, with hyaline cartilage overlying subchondral bone, a synovial membrane and a joint capsule. Facet tropism and facet orientation may have a relationship to degenerative changes in the spine, either as the cause of degenerative changes or as the result of abnormal forces produced by degeneration. Facet orientation is the angle of the facet joint in the transverse plane relative to the sagittal plane (Figure 1). Facet tropism is defined as asymmetry of the left and right vertebral (zygapophyseal) facet-joint angles, with one joint having a more sagittal orientation than the other4. Despite extensive studies with some conflicting results2,3,6,9,11,12,26, the clinical significance of facet tropism and orientation is not yet well established.
Figure 1.
Schema of facet orientation measurement. Reference plane was defined by posterior aspect of vertebral body. Sagittal line was drawn though spinous process, 90° to the reference plane. Facet orientation (α) was measured automatically at each side by the software between sagittal line and line drawn though the margins of the facet joint.
Facet tropism and facet orientation have been proposed as factors associated with lumbar facet joint OA. One cadaveric study examining this relationship found an association between facet tropism and facet sclerosis, though no association between facet tropism and a composite score of cartilage degeneration and sclerosis12. Conclusions from this study are limited due to the very small sample size involved (n=22) and the somewhat arbitrary criteria used to grade degeneration. Another study examining the relationship between facet orientation and facet joint OA in 111 patients9 found a significant association between facet orientation and OA, but little association between facet tropism and OA. Prior studies also have found significant associations between facet orientation, facet tropism, and DS2,3,6,9,11,26 in simple univariate analyses, but have largely failed to account for simple covariates such as age, sex and BMI. The relationship between facet tropism and orientation, and facet joint OA and DS, while adjusting for important covariates, has yet to be clearly defined.
Due to precise demonstration of osseous details13,15,24 and relatively low cost, computed tomography (CT) is the preferred method for imaging lumbar facet joint OA. Abnormalities of the facet joints that can be demonstrated and categorized by CT include osteophyte formation, hypertrophy of articular processes, joint space narrowing, vacuum joint phenomenon, synovial and subchondral cysts, and calcification of the joint capsule5,25. CT also allows easy identification of spondylolisthesis, particularly when sagittal reformats are available.
The aims of the present study were: 1) to document the distribution of facet orientation and prevalence of facet tropism in the community based sample; 2) to evaluate the association between facet orientation and tropism and age, sex and BMI; and 3) to evaluate the association between facet orientation and tropism, and facet joint OA and DS.
Materials and Methods
Study design
Cross-sectional study.
Study Sample
This project was an ancillary project to the Framingham Heart Study. The Framingham Heart Study began in 1948 as a longitudinal population-based cohort study of the causes of heart disease. Initially, 5209 men and women between the ages of 30 and 60 years living in Framingham, Massachusetts were enrolled. Biennial examinations were conducted by trained research staff at the study clinic located in Framingham. In 1971, 5,124 offspring (and their spouses) of the original cohort were entered into the Offspring cohort. In 2002, 4095 men and women who were children of the Offspring cohort were enrolled in the Third Generation cohort. A description of the Offspring and Third Generation cohorts has been previously reported8,27. 3529 participants of the Framingham study (participants in both the Offspring and Third Generation cohorts) aged 40–80 years underwent abdominal and chest multi-detector MDCT to assess coronary and aortic calcification. The primary evaluation of the abdominal aorta, which lies immediately adjacent to the lumbar spine, offers excellent resolution for assessment of lumbar pathology; similar methods have been used previously in other studies of spondylolysis1. The recruitment and conduct of CT scanning have been previously reported14,22. During the latter part of the CT study, 191 participants were consecutively enrolled in this ancillary study to assess the association between radiographic features of the lumbosacral spine and LBP. Three individuals were not analyzed because of insufficient CT data.
Imaging parameters
Study participants were imaged with an eight-slice MDCT scanner (Lightspeed Ultra, GE, Milwaukee, WI, USA). Each subject underwent unenhanced abdominal CT that was performed using a sequential scan protocol with a slice collimation of 8 mm × 2.5 mm (120 KVp, 320/400 mA for 220 lbs body weight, respectively) during a single end-inspiratory breath hold (typical duration 18 s). For the abdominal scan, thirty contiguous 2.5 mm thick slices of the abdomen were acquired covering 150 mm above the level of S1.
Evaluation of facet orientation and tropism
The facet joint angle relative to the sagittal plane at L3–L4, L4–5 and L5–S1 was measured on the axial CT images using bone windows using the method described by Noren et al.21 (Figure 1). On an axial scan that bisected the intervertebral disk, one line was drawn in the midsagittal plane of the vertebra and one through each facet joint tangential to the superior articular process. Continuous facet joint tropism was defined as the absolute difference of the angles between the right and the left facet. In addition, according to the method described by Vanharanta et al. for categorical facet tropism28, moderate tropism was defined as a difference of 7° to 15°, and severe tropism as more than 15°.
Facet Joint OA evaluation
FJ OA evaluation was performed using eFilm Workstation (Version 2.0.0) software. All CT studies were read blinded to clinical information and to the hypothesis assessed here. Lumbar facet joints were graded on both the left and right side at levels L3–L4, L4–L5, and L5–S1. Four grades of facet joint OA were defined using criteria similar to those published by Pathria et al.23 and Weishaupt et al.29 (Figure 2). This system grades facet joint OA according to the grading of the individual subcategories of joint space narrowing (JSN), osteophytes, articular process hypertrophy, sclerosis, subarticular erosions, subchondral cysts, and vacuum phenomenon. Specific examples of facet OA grading are presented in Figure 2.
Figure 2.
Examples of CT-evaluated facet joint osteoarthritis grades. Numbers on the images indicate grades of facet joint OA. Letters a to e present different examples of the same grade.
Grade 1: Normal right L4–L5 and L5–S1 facet joints. There is no joint space narrowing, no sclerosis, no osteophytes, and no facet hypertrophy.
Grade 2: Mild osteoarthritis of the right L4–L5 and L5–S1 facet joints. Joint space narrowing (white arrows), sclerosis (white arrowheads), and facet hypertrophy (black arrowheads) are indicated.
Grade 3: Moderate osteoarthritis of the right L4–L5 and L5–S1 facet joints. Joint space narrowing is present throughout. Osteophytes (black arrows), facet hypertrophy (black arrowheads), subchondral cysts (white arrows), and vacuum phenomenon (white arrowheads) are indicated.
Grade 4: Severe osteoarthritis of the right L4–L5 and L5–S1 facet joints. Advanced joint space narrowing is present throughout. Large osteophytes (black arrows), facet hypertrophy (black arrowheads), subchondral cysts (white arrows), and vacuum phenomenon (white arrowheads) are indicated.
The following criteria were used for facet joint OA evaluation:
Grade I (Normal): No joint space narrowing (2mm or greater); no osteophytes or possible small osteophytes; no articular process hypertrophy; no sclerosis or doubtful sclerosis; no subchondral erosions; no subchondral cysts; no joint space vacuum phenomenon.
Grade II (Mild): JSN (joint space 1–2 mm); and/or definite small osteophytes; and/or mild articular process hypertrophy; and/or definite sclerosis; no subchondral erosions; no subchondral cysts; no joint space vacuum phenomenon.
Grade III (Moderate): JSN (joint space ≤1 mm); and/or moderate osteophytes; and/or moderate articular process hypertrophy; and/or mild subchondral erosions; and/or mild subchondral cysts; and/or joint space vacuum phenomenon.
Grade IV (Severe): Severe JSN (bone to bone); and/or large osteophytes; and/or severe articular process hypertrophy; and/or severe articular erosions; and/or severe subchondral cysts; and/or joint space vacuum phenomenon.
Degenerative spondylolisthesis evaluation
Grading (I to IV) of spondylolisthesis (Meyerding20 classification) was estimated using sagittal reformations. Spondylolisthesis observed in the absence of spondylolysis and presence of at least Grade III facet joint OA was considered degenerative spondylolisthesis, after confirmation of absence of fracture, tumor or other primary pathology of posterior elements of the spine.
Reliability of CT readings
The reader (LK) was trained by an experienced research musculoskeletal radiologist (AG). A reading protocol for evaluation of facet joint OA based on the above outlined grading scheme was developed. Using this protocol, the intra- and inter-rater reliability was calculated for two readers (AG and LK). All CT scans were then analyzed in a blinded fashion. To evaluate for reader-drift, intra-rater reliability was periodically reassessed by inserting one repeated “reliability” scan for every 10 new scans. Before analyzing each new set of CT scans, 5 previously analyzed CTs were reevaluated to “recalibrate” the readings to a standard. All atypical cases were read by two readers. The intra-observer reliability for grading different facet joint OA indices varied between 0.64 and 0.91. The inter-observer reliability ranged from 0.59 to 0.94. For spondylolisthesis the intra-observer reliability varied at different levels between 0.95 and 1.00, and the inter-observer reliability ranged from 0.75 to 0.98. The intra-observer reliability for identification of spondylolysis, which was used for ascertaining DS, was 1.00. The inter-observer reliability was 0.98. This range of kappa statistics represents fair to excellent reproducibility.
Body mass index (BMI)
BMI was computed as the ratio of weight (in kg) divided by height (in square meters).
Statistical analysis
We initially calculated the mean±SD angle of facet orientation and facet tropism by spinal level and sex. We also calculated the prevalence of facet tropism (difference>7°) at least at one spinal level and prevalence of facet joint OA in a studied sample.
We then evaluated the association of facet orientation and tropism with age and sex using univariate analyses; and with age, sex and BMI using multivariate (multiple regression) analysis.
At the third stage we evaluated the association between facet joint OA and facet orientation and facet tropism at the corresponding level using two types of analyses. In the first analytic approach we compared the facet orientation and tropism (continuous) in joints with and without OA at each of studied spinal levels. In the second analytic approach we used logistic regression models with facet joint OA at each specific level and side as the dependent variable and facet orientation of the same joint as well as facet tropism at the corresponding level and age, sex, and BMI, as independent variables.
The same approach was used to evaluate the association between DS and facet orientation and tropism at the corresponding level. Initially, the t-test was used to compare the facet joint orientation, and χ2-test was used to compare facet tropism at the level of listhesis between the patients with DS and the normal subjects. We also used a logistic regression analysis with DS as the dependent variable and mean facet orientation as well as facet tropism at the corresponding level as well as age, sex and BMI, as independent variables.
All statistical analyses were performed using SAS software, (SAS Institute Inc, Cary, North Carolina, release 9.1).
Results
Descriptive statistics of the studied sample are shown in the Table 1. The mean age of the males and females was 51.90±11.25 and 53.61±10.20 respectively. Individuals were slightly overweight on average: BMI=27.95±4.16 for males and 27.71±5.98 for females. Facet orientation was similar in males and females, and became more coronally oriented in lower spinal segments. Prevalence of facet tropism (difference in facet angles >7°) was 76.7% in males and 66.3% in females, and facet joint osteoarthritis was prevalent in 60.2% of males and 70.0% of females.
Table 1.
Descriptive Statistics of the Studied Sample. Mean ± SD (n = 188)
| Males | Females | Males vs. Females* P |
|
| N | 104 | 84 | |
| Age (yrs) | 51.90 ± 11.25 | 53.61 ± 10.20 | 0.3072 |
| BMI (kg/m2) | 27.95 ± 4.16 | 27.71 ± 5.98 | 0.7282 |
| Facet orientation L3–L4† | 37.48 ± 7.75 | 38.11 ± 9.12 | 0.6132 |
| Facet orientation L4–L5† | 46.59 ± 9.11 | 45.72 ± 9.93 | 0.5398 |
| Facet orientation L5–S1† | 47.95 ± 10.65 | 48.55 ± 10.99 | 0.7135 |
| Facet tropism L3–L4 | 5.95 ± 4.35 | 6.23 ± 5.61 | 0.7193 |
| Facet tropism L4–L5 | 7.48 ± 6.92 | 6.13 ± 4.75 | 0.1225 |
| Facet tropism L5–S1 | 7.67 ± 7.91 | 6.68 ± 8.68 | 0.4282 |
| Prevalence of facet tropism (difference >7°) in at least one spinal level | 79 (76.7%) | 53 (66.3%) | 0.5189 |
| Prevalence of facet joint osteoarthritis (grade ≥2 at L3–S1 level) | 62 (60.2%) | 56 (70.0%) | 0.1692 |
Results of t test continuous variables and χ2 test for categorical variables.
Relative to sagittal plane.
Results of comparisons between males and females (Table 1) showed no sex differences in facet orientation at any studied level (p=0.57 at L3–L4; p=0.24 at L4–L5; and p=0.71 at L5–S1 spinal levels). As well, there were no significant differences between males and females in mean facet tropism (continuous) at any studied spinal level (p=0.72 at L3–L4 level, p=0.12 L4–L5 level, and p=0.43 at L5–S1 level). There was no significant difference in prevalence of facet joint OA at studied spinal levels between males and females (p=0.17).
In the present study there was no significant difference between age groups in mean values of facet orientation and tropism at each studied spinal level in the univariate analysis (Table 2). We can mention signs of trend toward more sagittally oriented facet joints at L3–L4 and L4–L5 spinal levels with age, but the differences between age group were not significant (p=0.13 for L3–L4, and p=0.18 for L4–L5 levels). In the multivariate analysis we evaluated the associations between facet orientation at the L3–L4, L4–L5 and L5–S1 spinal levels and age, sex and BMI. Sex and BMI were not associated with facet orientation at any studied spinal level. Age showed a statistically significant negative association (β=−0.28, p=0.03) with facet orientation at L4–L5 spinal level, suggesting that in older age the L4–L5 facets are more sagittally oriented. Multiple logistic regression analysis showed that there were no statistically significant associations between moderate or severe facet tropism and age, sex and BMI at each studied spinal level.
Table 2.
Association Between Facet Orientation and Tropism and Age
| <50 | 50–59 | 60+ | Comparison* | |
|---|---|---|---|---|
| Facet Orientation L3–L4† | 39.07 ± 7.58 | 37.66 ± 7.49 | 35.94 ± 10.19 | 0.1329 |
| Facet Orientation L4–L5† | 47.70 ± 9.11 | 45.79 ± 9.61 | 44.48 ± 9.62 | 0.1838 |
| Facet Orientation L5–S1† | 47.98 ± 9.83 | 48.55 ± 9.80 | 48.09 ± 13.47 | 0.9518 |
| Facet tropism L3–L4 | 6.17 ± 4.71 | 5.73 ± 4.76 | 6.38 ± 5.51 | 0.7771 |
| Facet tropism L4–L5 | 7.54 ± 7.32 | 6.17 ± 4.94 | 6.91 ± 5.46 | 0.4335 |
| Facet tropism L5–S1 | 7.0 ± 6.78 | 6.78 ± 7.57 | 8.27 ± 10.99 | 0.6251 |
| Prevalence of facet tropism L3–L4 | 30 (42.3%) | 23 (35.9%) | 16 (33.3%) | 0.5766 |
| Prevalence of facet tropism L4–L5 | 26 (36.6%) | 24 (37.5%) | 19 (42.2%) | 0.8208 |
| Prevalence of facet tropism L5–S1 | 26 (36.6%) | 29 (46.0%) | 16 (36.4%) | 0.4639 |
Results of t test for continuous variables and χ2 test for categorical variables.
Relative to sagittal plane.
Table 3 presents the relation of facet OA to facet orientation and facet tropism at each studied spinal level. At each spinal level the facet joints with OA were more sagittally oriented, but the difference was statistically significant only at L4–L5 spinal level (p=0.0007). Facet tropism showed no significant difference between facet joints with and without OA at each studied spinal level.
Table 3.
Facet Orientation in Joint With and Without OA
| Spinal Level | Facet Joint OA (Present) | Facet Joint OA (Absent) | Comparison (t Test) |
|---|---|---|---|
| Facet Orientation L3–L4* | 36.04 ± 10.07 | 38.61 ± 7.32 | 0.0865 |
| Facet Orientation L4–L5* | 43.76 ± 9.49 | 48.51 ± 9.09 | 0.0007 |
| Facet Orientation L5–S1* | 47.10 ± 13.13 | 48.84 ± 9.13 | 0.3417 |
| Facet tropism L3–L4 | 6.74 ± 5.23 | 5.77 ± 4.75 | 0.2150 |
| Facet tropism L4–L5 | 7.67 ± 7.21 | 6.35 ± 5.06 | 0.1668 |
| Facet tropism L5–S1 | 8.24 ± 11.01 | 6.72 ± 5.97 | 0.3017 |
| Prevalence of facet tropism L3–L4 | 44 (37.0%) | 26 (38.8%) | 0.8045 |
| Prevalence of facet tropism L4–L5 | 48 (41.0%) | 23 (34.9%) | 0.4102 |
| Prevalence of facet tropism L5–S1 | 44 (38.3%) | 28 (42.4%) | 0.5817 |
Statistically significant association at P < 0.05 level marked bold.
Relative to sagittal plane.
Results of the multivariate analysis of the association between facet joint OA and facet orientation at the same side level, and facet tropism at the same level, while adjusting for age, sex, BMI are shown in Table 4. Facet orientation was significantly associated with facet joint OA only at the left side of the L4–L5 (OR(95%CI)=1.06(1.00, 1.13)) and L5–S1 (1.06(1.01–1.11) spinal levels. Facet tropism did not show an association with facet joint osteoarthritis at any spinal level.
Table 4.
Association Between Facet Joint OA and Facet Orientation and Tropism (Results of Multiple Logistic Regression Analysis)
| Dependent Variable | Independent Variable | OR | 95% Cl | |
|---|---|---|---|---|
| Facet joint osteoarthritis right L3–L4 | Age | 1.09 | 1.06 | 1.13 |
| Sex* | 0.57 | 0.29 | 1.12 | |
| BMI | 1.06 | 0.99 | 1.14 | |
| Facet orientation L3–L4 (right) | 0.98 | 0.95 | 1.02 | |
| Facet tropism (continuous) L3–L4 | 1.03 | 0.97 | 1.11 | |
| Facet joint osteoarthritis left L3–L4 | Age | 1.10 | 1.06 | 1.14 |
| Sex* | 1.26 | 0.65 | 2.43 | |
| BMI | 1.02 | 0.95 | 1.09 | |
| Facer orientation L3–L4 (left) | 0.98 | 0.95 | 1.02 | |
| Facer tropism (continuous) L3–L4 | 1.00 | 0.93 | 1.07 | |
| Facet joint osteoarthritis right L4–L5 | Age | 1.11 | 1.07 | 1.16 |
| Sex* | 2.16 | 1.04 | 4.47 | |
| BMI | 1.12 | 1.02 | 1.22 | |
| Facer orientation L4–L5 (right) | 0.97 | 0.94 | 1.00 | |
| Facet tripism (continuous) L4–L5 | 1.02 | 0.96 | 1.08 | |
| Facet joint osteoarthritis left L4–L5 | Age | 1.09 | 1.05 | 1.13 |
| Sex* | 1.11 | 0.57 | 2.19 | |
| BMI | 1.05 | 0.98 | 1.13 | |
| Facet orientation L4–L5 (left) | 1.06 | 1.00 | 1.13 | |
| Facet tropism (continuous) L4–L5 | 0.97 | 0.94 | 1.00 | |
| Facet joint osteoarthritis right L5–S1 | Age | 1.07 | 1.03 | 1.10 |
| Sex* | 3.03 | 1.52 | 6.05 | |
| BMI | 1.00 | 0.93 | 1.07 | |
| Facet orientation L5–S1 (right) | 0.98 | 0.95 | 1.01 | |
| Facet tropism (continuous) L5–S1 | 1.02 | 0.98 | 1.07 | |
| Facet joint osteoarthritis left L5–S1 | Age | 1.12 | 1.07 | 1.17 |
| Sex* | 3.58 | 1.72 | 7.45 | |
| BMI | 0.96 | 0.90 | 1.04 | |
| Facet orientation L5–S1 (left) | 1.06 | 1.01 | 1.11 | |
| Facet tropism (continuous) L5–S1 | 0.97 | 0.94 | 1.00 | |
Sex: Male as reference group; statistically significant association marked bold.
Table 5 shows the comparison of facet orientation and tropism at the level of listhesis between individuals with and without DS. The prevalence of facet tropism was not different in individuals with or without DS. However, facet orientation showed significant differences (p<0.0001) with more sagittal facet orientation in individuals with DS. Table 6 shows the associations between DS and facet orientation and tropism of the corresponding level while adjusted for age, sex and BMI. Facet orientation was significantly associated with DS (0.89 (0.84–0.94)), however facet tropism showed no association with DS.
Table 5.
Comparison of the Facet Angle and Tropism at the Level of Listhesis Between the Individuals With and Without Degeneration Spondylolisthesis
| Individuals With DS (n = 23) |
Individuals Without DS (n = 165) |
Comparison | |
|---|---|---|---|
| Facet orientation | 34.67 ± 11.79 | 48.74 ± 18.42 | P < 0.0001* |
| Prevalence of facet tropism (difference >7°) | 9 (39.1%) | 61 (38.1%) | P = 0.9261† |
Results of t test.
Results of χ2 test; statistically significant association at P < 0.05 level marked bold.
Table 6.
Association Between Degenerative Spondylolisthesis and Facet Orientation and Tropism at the Same Level, Adjusted for Age, Sex, and BMI. Results of Multiple Logistic Regression Analysis
| Dependent Variable |
Independent Variable |
OR | 95% Cl | |
|---|---|---|---|---|
| Degenerative spondylolisthesis | Age | 1.09 | 1.03 | 1.16 |
| Sex* | 3.05 | 1.02 | 9.18 | |
| BMI | 1.03 | 0.93 | 1.14 | |
| Facet orientation† | 0.89 | 0.84 | 0.94 | |
| Facet tropism | 0.99 | 0.90 | 1.09 | |
Sex: Male as reference group; statistically significant association marked bold.
Mean value of facet orientation, between left and right sides.
Discussion
To our knowledge, this work represents the largest study to date to evaluate the association between facet orientation and tropism, and facet joint OA and DS. This cohort also includes the largest population of patients over the age of 50.
This study demonstrates that more sagittally oriented facets are associated with a slightly higher occurrence of facet joint osteoarthritis at the L4–L5 and L5–S1 spinal levels. This is in agreement with previous findings by Fujiwara9. The fact that we found a significant association with facet joint OA and facet orientation only at the L4–L5 spinal level can be explained by the fact that this is the most common level of facet joint OA10,16,18. Our study showed a statistically significant association between facet orientation and presence of DS as well as the difference in facet orientation between individuals with and without DS. Individuals with DS have on average more sagittally oriented facet joints than individuals without DS. These results are in agreement with previously published studies2,3,6,9,11,26.
We did not find an association between facet tropism and occurrence of facet joint OA or DS at the lower three spinal levels. This lack of association contradicts previously reported findings by Grogan et al.12 and, to some extent, by Fujiwara et al.9. The discrepancy between Grogan’s findings and ours may be explained by differences in methodology. Because previous studies and the preliminary calculations for the present study showed that facet joints with OA are more sagittally oriented, we decided to account for this by evaluating the association between facet tropism (at each spinal level) and side-specific facet joint OA separately in the multivariate analysis. If only one value of facet joint OA for each spinal segment is recorded9,12, this could lead to false positive association with facet tropism. The side that has more prevalent or more severe OA would, most probably, have more sagittally oriented facets. Therefore, if facet joint OA is not symmetrical, and we have only one value of facet joint OA, we would find a false positive association between facet tropism and facet joint OA.
Due to the cross-sectional design, we cannot conclude if sagittally oriented joints predispose to the development of facet joint OA, or if sagittal orientation of the facets are the result of a remodeling process of OA. We did demonstrate a relationship between age and facet orientation particularly at the L4–L5 level, suggesting that orientation could change with remodeling over time, and underlining the importance of appropriately adjusting for age. We found that in older age facets are more sagittally oriented. L4–L5 spinal level is most common site of development of facet joint OA16. Our hypothesis that can explain this association is that facet orientation changes as a result of articular process remodeling during development of osteoarthritis. This assumption will need to be further examined in a longitudinal study.
We found no significant difference between males and females in facet orientation and tropism at any studied spinal level in agreement with study of Love et al.19. BMI was also not associated with facet orientation and tropism at any spinal level.
Conclusions
The current study confirms a significant association between sagittal orientation and OA of the lumbar facet joints and DS, albeit the relation was not strong. Previous strong relations may have inadequately adjusted for important confounders, in particular age. Facet tropism was not associated with the occurrence of facet joint OA or degenerative spondylolisthesis. Additional, longitudinal studies are needed to understand the causal relationship between facet joint morphology and OA.
Acknowledgments
From the Framingham Heart Study of the National Heart Lung and Blood Institute of the National Institutes of Health and Boston University School of Medicine. This work was supported by the National Heart, Lung and Blood Institute's Framingham Heart Study contract (No. N01-HC-25195) for the recruitment, enrollment, and examination of the Offspring and Third Generation Cohorts, and the imaging by computed tomography scan.
L.K. is supported by an Arthritis Foundation Postdoctoral Grant.
P.S. is supported by the Rehabilitation Medicine Scientist Training K12 Program (RMSTP) and National Institutes of Health (K12 HD 01097).
Appendix 1
Grades of osteoarthritic features of facet joints.
| Joint Space Narrowing (JSN) | Subchondral Sclerosis | ||
| Normal | No JSN (2mm or greater) | Normal | No sclerosis |
| Mild JSN (1–2 mm) | Doubtful sclerosis | ||
| Moderate JSN (<1mm) | Abnormal | Definite sclerosis | |
| Severe | Severe JSN (bone-to bone) | Articular Process Hypertrophy | |
| Osteophytes | Normal | No hypertrophy | |
| Normal | No osteophytes | Mild hypertrophy | |
| Possible small osteophytes | Moderate hypertrophy | ||
| Definite small osteophytes | Severe | Severe hypertrophy | |
| Moderate osteophytes | Joint Space Vacuum Phenomenon | ||
| Severe | Severe osteophytes | Normal | No vacuum phenomenon |
| Subchondral Cysts | Abnormal | Present vacuum phenomenon | |
| Normal | No subchondral cysts | Subchondral Erosions | |
| Mild (focal) subchondral cysts | Normal | No erosions | |
| Severe (diffuse or multiple focal) subchondral cysts | Mild (focal) erosion | ||
| Severe | Severe | Severe (diffuse or multiple focal) erosion | |
Footnotes
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