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Abstract

Purpose:

To determine the orientation of lumbar zygapophyseal joints and prevalence of facet tropism (FT) identified by computerized tomographic (CT) scans.

Methods:

In a cross-sectional study, 124 CT scans of patients with spinal injuries, seen between 2011and 2015, were retrospectively studied. A total of 566 uninjured motion segments were analysed. The right and left zygapophyseal joint angles (ZJAs) from L1–L2 to L5–S1 were measured on axial sections at uninjured levels. The absolute difference between the right and left ZJA was calculated to determine the prevalence and severity of FT at each level.

Results:

The mean ZJA (mean ± SD) at L1–L2, L2–L3, L3–L4, L4–L5 and L5–S1 was 23.55 ± 7.21, 27.97 ± 6.95, 39.47 ± 6.07, 48.32 ± 6.38 and 53.45 ± 6.67, respectively. Prevalence of FT at L1–L2, L2–L3, L3–L4, L4–L5 and L5–S1 was 22.42%, 25%, 27.19%, 47.82% and 38.5%, respectively. One-way repeated analysis of variance test showed a statistically significant difference in mean ZJA (p < 0.05) and prevalence of FT (p < 0.05) at different levels of the lumbar spine.

Conclusions:

This study confirms that ZJA is not the same at different levels of the lumbar spine. The mean ZJA progressively increases from L1–L2 to L5–S1. This could explain the greater range of movements in the lumbar spine at the lower levels. The higher prevalence of FT at L4–L5 and L5–S1 seen in this study could explain the greater incidence of disc prolapse and other degenerative disorders at these levels.

Keywords

computerized tomographic scans facet tropism zygapophyseal joint zygapophyseal joint angle

Introduction

Zygapophyseal joints (ZJs), otherwise known as facet joints, are an integral part of the spinal motion segment. These synovial joints provide structural support to the posterior elements of the spine and protect the motion segment from excessive shearing, rotational and flexion forces.¹ Approximately 33% of the dynamic compressive load and 35% of the static load of the spine are borne by these joints.^2,3 Proper alignment of the right and left ZJ is essential for synchronized movements of the spine. In the lumbar region, an abnormal orientation of these joints in the form of facet tropism (FT) can be a potential cause of low back pain. FT can also be a predisposing factor in the development of early degenerative arthrosis, spondylolisthesis and intervertebral disc prolapse.^4

–7 Available literature regarding the orientation of lumbar ZJ and prevalence of FT is mainly based on studies carried out in the Western population,^4,8,9 and few studies have been done in the Asian population.^5,10 This study was undertaken to determine the orientation of ZJ and prevalence of FT in the lumbar spine, identified by computerized tomographic (CT) scans, in a South Indian population.

Materials and methods

In a cross-sectional retrospective study, 151 consecutive adults with spinal injuries, who underwent CT scans of the whole spine as a routine investigation at the time of presentation, between January 2011 and December 2015, were retrospectively studied. Only preoperative CT scans were evaluated. As the ZJ morphology changes with age, only skeletally mature individuals below the age of 50 years were included. Twenty-seven patients who had multiple fractures of the lumbar spine, pathological fractures, scoliosis, spondylolisthesis and spondylodiscitis and other anomalies were excluded from the study. Thus, 124 CT scans of the lumbar spine formed the basis of this study. Spinal levels with fractured ZJ were excluded. A total of 1132 ZJ belonging to 566 motion segments (n = 107 at L1–L2, n = 116 at L2–L3, n = 114 at L3–L4, n = 115 at L4–L5 and n = 114 at L5–S1) were available for analysis. Approval from the institutional ethical committee was obtained.

Evaluation of zygapophyseal joint angle and FT

The digitalized CT scans were analysed using image analysis software Digimizer version 4.6.1. The orientation of the right and left ZJ was measured from an appropriate 4 mm CT bone window cut parallel to the inferior end plate of the superior vertebra (Figures 1 and 2). The zygapophyseal joint angle (ZJA) with respect to the sagittal plane at L1–L2, L2–L3, L3–L4, L4–L5 and L5–S1 was measured on axial CT images as described by Noren et al. (Figure 3).⁹ A reference line (AA′) was drawn along the posterior border of the vertebral body in the coronal plane. A sagittal line (BB′) was drawn through the spinous process perpendicular to the reference line. Intersecting lines were drawn connecting the two end points of each facet on the right (RR′) and left (LL′) side. The ZJA on either side, at each level, was determined by measuring the angle between the intersecting lines and the sagittal line (θ _R, θ _L). Different criteria have been used to categorize FT.^9,11 An absolute difference of more than 5° between the right and left ZJA was defined as FT.⁹ Combining the definitions of Noren et al. and Vanharanta et al., we graded FT as mild (5–7°), moderate (7–15°) and severe (>15°).^9,11

Figure 1.

Sagittal section of lumbar CT scan showing appropriate cuts parallel to the inferior end plate of the superior vertebra at different levels of the lumbar spine (a). Axial sections at L1–L2 (b) and L2–L3 (c) showing the ZJs. CT: computerized tomographic; ZJ: zygapophyseal joint.

Figure 2.

Axial sections at L3–L4 (a), L4–L5 (b) and L5–S1 (c) showing the ZJs. ZJ: zygapophyseal joint.

Figure 3.

Schematic diagram showing ZJA measurement. A reference line drawn along the posterior border of the vertebral body in the coronal plane (AA′). A sagittal line, drawn through the spinous process, perpendicular to the reference line (BB′). Intersecting lines connecting the two end points of each facet on the right and left side (RR′ and LL′). ZJAs on the right and left side (θ _R and θ _L). ZJA: zygapophyseal joint angle.

Statistical analysis

Two investigators (MPK and SK) independently measured the ZJA in a blinded fashion. When a difference was noted in the values, mean of the two readings was taken to be the true ZJA. The intra-class correlation coefficient was used to determine inter-observer agreement for value of ZJA. All statistical analyses were done using Statistical Package for Social Sciences for Windows version 15. The mean ZJA at L1–L2, L2–L3, L3–L4, L4–L5 and L5–S1 was calculated. Prevalence and severity of FT at each level was determined. One-way repeated analysis of variance (ANOVA) was used to determine whether there was any statistically significant difference in the mean ZJA and prevalence of FT at different levels. A p value less than 0.05 was considered to be statistically significant. In case ZJA at one or more levels were already known, a formula for calculation of unknown adjacent ZJA was obtained using regression analysis.

Results

The mean age of patients in this study was 35.12 ± 10.41 years. The intra-class correlation coefficient for ZJA was 0.95–0.99 which showed very good agreement between the two observers. Mean ZJA, mean FT and prevalence of FT in the lumbar spine is shown in Table 1. The mean ZJA was dissimilar at different levels. There was a statistically significant difference in the values of adjacent ZJA (p < 0.05). The mean ZJA showed a gradually increasing trend from L1–L2 to L5–S1. Prevalence of FT was found to be highest at L4–L5 followed by L5–S1. A statistically significant difference in the prevalence of FT was seen at different levels in the lumbar spine (p < 0.05). Twenty-five patients had no tropism at any level whereas 44 patients had FT at one level. Prevalence of FT at more than one level was seen in 55 patients. The severity of FT at different levels is shown in Table 2.

Table 1.

ZJA, FT and prevalence of FT at different levels of the lumbar spine.

Level	Mean ZJA (mean ± SD)^a (°)	FT (mean ± SD) (°)	prevalence of FT^b
L1–L2 (n = 107)	23.55 ± 7.21	9.17 ± 2.5	22.42% (n = 24)
L2–L3 (n = 116)	27.97 ± 6.95	9.83 ± 3.13	25% (n = 29)
L3–L4 (n = 114)	39.47 ± 6.07	9.19 ± 3.89	27.19% (n = 31)
L4–L5 (n = 115)	48.32 ± 6.38	9.6 ± 3.71	47.82% (n = 55)
L5–S1 (n = 114)	53.45 ± 6.67	9.13 ± 4.58	38.50% (n = 44)

ZJA: zygapophyseal joint angle; FT: facet tropism; SD: standard deviation; ANOVA: analysis of variance.

^aOne-way repeated ANOVA showed that the mean ZJA at different levels were statistically different p < 0.05.

^bOne-way repeated ANOVA showed that the prevalence of FT at different levels were statistically different p < 0.05.

Table 2.

prevalence and severity of FT at different lumbar levels.

Level	Mild tropism (5–7°)	Moderate tropism (7–15°)	Severe tropism (>15°)	Total tropism^a
L1–L2 (n = 107)	1.86% (2)	19.6% (21)	0.93% (1)	22.42% (n = 24)
L2–L3 (n = 116)	2.5% (3)	20.68% (24)	1.72%(2)	25% (n = 29)
L3–L4 (n = 114)	5.26% (6)	19.2% (22)	2.6% (3)	27.19% (n = 31)
L4–L5 (n = 115)	10.43% (12)	31.30% (36)	6.08% (7)	47.82% (n = 55)
L5–S1 (n = 114)	7.01% (8)	27.19% (31)	4.38% (5)	38.50% (n = 44)

FT: facet tropism; ANOVA: analysis of variance.

^aOne-way repeated ANOVA showed that the prevalence of FT at different levels were statistically different p < 0.05.

Discussion

The morphology of the ZJ reflects its biomechanical requirements. They play an important role in maintaining the stability and are intricately involved in mobility of the spine.¹² The range of movements of a motion segment is determined by the orientation of the articular surfaces of the ZJ, which vary at each level of the vertebral column. In the cervical spine, the plane of the ZJ is more horizontal and thus provides greater mobility. In the thoracic spine, they are almost vertical and provide more stability than mobility. In the lumbar spine, the plane of the ZJ changes from one level to the next. The value of mean ZJA noted in this study is similar to other studies.^8,9,10,13 The association between the orientation of the ZJ and spinal movements in various planes is not well defined. However, White and Punjabi demonstrated a correlation between ZJA and spinal movements at different levels.¹³ This could probably be explained by the findings of the current study which showed a gradual increase in the ZJA from L1–L2 to L5–S1. The ZJA showed a considerable segmental variation. This is comparable to earlier studies (Table 3). We tried to correlate the ZJA between adjacent motion segments by regression analysis. However, it was found to be statistically dissimilar. Therefore, it seems unreasonable to use ZJA of adjacent motion segments as controls. Greater understanding of the normal orientation of the ZJ at different spinal levels will help identify the spinal levels that are predisposed to develop degenerative spondylolisthesis.¹⁰

Table 3.

Comparison of ZJA reported in different studies.

Level	Present study (°) (mean ± SD), n = 124	Noren et al.⁹ (°) (mean and range), n = 54	White and Panjabi¹³ (°)^a (mean and range)	Ahmed et al.¹⁵ (°) (median and range), n = 35	Van Schaik et al.⁸ (°) (mean and range), n = 100	Farfan et al.¹⁴ (°) (mean and range), n = 66
L1–L2	23.55 ± 7.21	–	25 (15–47)	–	–	–
L2–L3	27.97 ± 6.95	–	28 (17–51)	–	–	–
L3–L4	39.47 ± 6.07	39.6 (17.6–57)	37 (15–57)	–	37.1 (17–57)	–
L4–L5	48.32 ± 6.38	48.4 (30–64.5	48 (13–70)	52 (12.6–70.3)	48.2 (29.5–75)	43 (10–70)
L5–S1	53.45 ± 6.67	53.9 (29–77.5)	53 (36–70)	–	53.1 (36–70)	52 (20–90)

ZJA: zygapophyseal joint angle.

^aData compiled by White and Panjabi from Van Schaik et al., Ahamed et al. and Taylor et al.

FT is currently a subject of intense interest, as it can alter the pattern of movements that occur in the lumbar spine. Ultimately, this may contribute to some form of instability which may lead to different disorders such as degenerative spondylosis, spondylolisthesis and intervertebral disc prolapse.^4

–7 This study showed that FT frequently occurs in the lumbar spine. The highest prevalence of FT was seen at L4–L5 followed by L5–S1. This is similar to the findings of Noren et al.⁹ However, some studies have noted the greater prevalence of FT at L5–S1.⁴ A sagittal-oriented ZJ provides less mechanical resistance to axial torque, leading to a rotational strain on the intervertebral disc. Therefore, lumbar disc prolapse is believed to occur more commonly on the side of the more sagittal-oriented ZJ.^14,15 The greater prevalence of FT at L4–L5 and L5–S1 seen in this study could probably explain the higher incidence of disc prolapse occurring at these levels. Prevalence of moderate and severe degree of FT in at least one level found in this study may be a cause of non-specific low back pain seen in the general population.

The strengths of this study include the large sample size and the fact that we analysed the morphology of the ZJ and the prevalence of FT at all five lumbar levels. In addition, we classified the severity of FT and determined the prevalence of varying degrees of FT at all levels of the lumbar spine. A potential weakness of the study is that the sample was predominantly male. However, studies have shown, ZJA and FT are similar in males and females.⁴ It would have also been ideal to study flexion–extension radiographs of the lumbar spine to correlate the segmental movements with the ZJA and see how motion at each segment is affected by FT. However, as the study population was exclusively patients with spinal injuries, this could not be done. Finally, ZJ orientation measured by ZJA is only a two-dimensional representation of the three-dimensional morphology of the ZJ. Further studies on the shape and other three-dimensional characteristics are necessary to understand the complex morphology of the ZJ.

Conclusion

The orientation of the ZJ at each level of the lumbar spine is different from that at other levels. The mean ZJA progressively increases from L1–L2 to L5–S1. This could explain the greater range of movements in lumbar spine at the lower levels. Maximum prevalence of FT was noted at L4–L5 followed by L5–S1. This could explain the greater incidence of disc prolapse at these levels.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

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Morphometric study of the orientation of lumbar zygapophyseal joints in a South Indian population