|Year : 2021 | Volume
| Issue : 1 | Page : 34-41
Magnetic resonance imaging scan of the dimensions of normal lumbar intervertebral foramina - An observational study
Santanu P Mallick
Pain Physician and Anatomist, MGM Medical College and Hospital, Navi Mumbai, Maharashtra, India
|Date of Submission||24-Dec-2020|
|Date of Decision||24-Jan-2021|
|Date of Acceptance||16-Mar-2021|
|Date of Web Publication||27-Apr-2021|
Dr. Santanu P Mallick
Department of Anatomy, MGM Medical College, Navi Mumbai, Maharashtra
Source of Support: None, Conflict of Interest: None
Background: Intervertebral foramen (IVF) is an oval, round, or inverted teardrop shaped bilateral fibro osseous window throughout the entire vertebral column through which neurovascular structures pass. The lumber canal stenosis due to variations of narrowing of intervertebral foramina causes compression of exiting spinal nerve or other foraminal components, for which normal IVF dimensions must be known and understood thoroughly. Aims and Objectives: Although computed tomography scan remains very effective in the evaluation of osseous tissues, but being a fibro osseous canal, intervertebral foramina can be better evaluated by magnetic resonance imaging (MRI) scan for superior resolution. Materials and Methods: Normal morphometric database of right and left lumber IVF of 195 normal adult males (96) and females (99) from different age groups (20–69 years) has been taken to draw conclusions from the data relevant to clinical practice and future research. MRI scan in the sagittal plane gives a complete view of the fibro osseous canal, whereas horizontal imaging (axial view) does not demonstrate the entire IVF on a single image. That's why all the measurements were taken from the sagittal plane. Results: All the values are evaluated with the subjects' age, sex, weight, height, and also with the left versus right sides. The development of this normal database should further allow for more meaningful evaluation of the dimensions of the IVFs in pathologic states. Although diagnosis can be suspected from the history and physical findings, knowledge of the specific anatomy of this clinically significant area with radiological support is very important in the differential diagnosis which helps in proper management. Conclusion: Therefore, it is really convincing that, with regard to morphometry of the spine by means of MRI, the future is very promising.
Keywords: Back pain, lumbar intervertebral foramina, magnetic resonance imaging scan
|How to cite this article:|
Mallick SP. Magnetic resonance imaging scan of the dimensions of normal lumbar intervertebral foramina - An observational study. Indian J Pain 2021;35:34-41
|How to cite this URL:|
Mallick SP. Magnetic resonance imaging scan of the dimensions of normal lumbar intervertebral foramina - An observational study. Indian J Pain [serial online] 2021 [cited 2021 Jun 19];35:34-41. Available from: https://www.indianjpain.org/text.asp?2021/35/1/34/314692
| Introduction|| |
Intervertebral foramen (IVF) bridges between the intraspinal space and the extraspinal space. It is an oval, round, or inverted teardrop-shaped bilateral fibro-osseous window throughout the entire vertebral column through which neurovascular structures pass. IVF is unique in comparison to other foramina of the body carrying nerves and vessels due to its boundaries consisting of two movable joints: the ventral intervertebral joint and the dorsal zygapophyseal joint [Figure 1]. Hence, it is more of a neural canal although we call it foramen and that is why changes in size occur during spinal movements.
Since the beginning of the 20th century, the intervertebral foramen has been a region that has received much attention from those who are engaged in the study and care of the spine. The IVF is a region where the nerve roots and mixed spinal nerves are particularly vulnerable to compression from a variety of pathologies that can restrict foraminal width or height.
The lumber canal stenosis is one of the most common causes associated with radicular low back pain which may be due to normal as well as abnormal variations of narrowing of intervertebral foramina causing compression of exiting spinal nerve or other foraminal components. Although diagnosis can be suspected from the history and physical findings, knowledge of the specific anatomy of this clinically significant area with radiological support is very important in the differential diagnosis which helps in proper management. That is why normal IVF dimensions must be known and understood thoroughly before proper evaluation of this region.
To evaluate and compare variability of the dimensions of the IVFs in living individuals, different radiological techniques can be used, i.e., X-ray, computed tomography (CT) scan, magnetic resonance imaging (MRI) scan, myelography, ultrasonography, three-dimensional CT, or it can also be directly seen through cadaveric dissection. Plain radiographic (X-ray) measurements are misleading due to angulations and superposition as the foramina change orientation from horizontal and lateral in upper lumbar spine to anterior and caudal in lower lumbar and sacral levels. Similarly, myelography and ultrasonography have limited relevance for lateral spinal pathology. Cadaveric study may not support normal values accurately as we do not know their spine pathologies and anatomy may get distorted after death.
MRI scan is one of the newest advanced imaging techniques which has rapidly gained acceptance as a very important component of spinal imaging [Figure 2]. Although CT scan remains very effective in the evaluation of osseous tissues, but being a fibro-osseous canal, intervertebral foramina can be better evaluated by MRI scan for superior resolution.
|Figure 2: Parasagittal image of magnetic resonance imaging and computed tomography scan of Lumber vertebral column|
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A quantitative morphometric database of the normal lumber IVFs would aid clinicians in determining the relative patency of the lumber IVFs in their patients with suspected IVF stenosis secondary to disc degeneration, disc protrusion, zygapophyseal joint pathologies, or other pathologies.
Here, linear morphometric measurements of several parameters of right and left lumber IVFs were measured and comparatively studied through lumber parasagittal MRI scan of normal adult male and female population in Navi Mumbai region.
Aims and objectives
The purpose of this study is to develop a normal morphometric database of right and left lumbar IVFs of 195 normal adult male and female subjects (20 years to 69 years) and to draw conclusions from the data relevant to clinical practice and future research.
The following two parameters are measured for intervertebral foramina from first lumbar (L1) to fifth lumbar (L5) on both sides [Figure 3]:
In parasagittal view:
- Greatest superior-to-inferior (SI) dimension
- Anterior-to-posterior diameter of the intervertebral foramina at the upper border of the superior articular process (AP).
All the values are used to evaluate the relationship between the lumbar IVF dimensions with the subjects' age, sex, weight, height, and also with the left versus right sides.
| Materials and Methods|| |
A total of 195 asymptomatic patients (96 males and 99 females; age range 20–69 years) were selected during this 1-year study period (March 2015 to March 2016). Asymptomatic patients were selected from patients referred to the MRI unit of D. Y. Patil Hospital and Research Centre, 1200-bedded multispecialty hospital, attached to MCI recognized D Y Patil Medical College, Navi Mumbai, for abdominopelvic problems. On direct discussion or telephonic discussion, patients with a history of spine problems or those whose MRI showed spine deformity, disc degeneration, bony rotation or scoliosis, and previous spine surgery were excluded. The patients were stratified by age and sex so that each decade from 20-29 to 60-69 (10 patients per group) is evenly represented as possible, and each decade has as close to an equal number of men and women as possible. The study was approved by the hospital ethical committee
MRI was performed on a magnetic vision plus scanner with a 1.5 Tesla magnet (Siemens, Germany) using a spine array coil Sagittal T1-weighted single spin-echo images with a repetition time of 500 ms and echo time of 15 ms of the lumbosacral spine showed L1 to S1 region. Images were taken using 5 mm slice thickness and 30 cm field of view with a 0.10 distance factor. The most medial, best parasagittal images that showed the borders of the IVF precisely were taken
These were the following two measurements taken in this study [Figure 3]:
- Greatest SI dimension between superior and inferior pedicle
- Another anteroposterior dimension from the level of inferior vertebral endplate to the superior articular process of the vertebra below, which is called anteroposterior dimension (AP).
All the measurements were performed on a computer screen from the most medial and best parasagittal MRI images that revealed the best IVF dimensions. All the measurements were made on both sides (left and right) and each for three times. The procedures were performed on two different days and the averages of the three measurements of each sitting were used in future calculations. Measurements were made by using the linear measurement capability of the software package. The software automatically calculated the scale factor of the scan and the distance in millimeters was displayed on the computer screen.
The study developed a database from the IVFs of 195 subjects' MRI scans. The subjects were comprised 95 men and 99 women whose ages ranged from 20 to 69 and the demographic data were stratified into five groups of 10 years difference [Table 1].
|Table 1: Demographic data stratified into five groups of 10-year difference|
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[Table 2] shows the values obtained for the three different types of measurements (SI, U-AP, L-AP), for males and females, for the left and right L1/L2 through L5/S1 for five different age groups [Figure 4] and [Figure 5]. The values are in millimeters with standard deviations in parentheses.
|Figure 4: Graphic form of the data presented for left-sided intervertebral foramen in Table 2|
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|Figure 5: Graphic form of the data presented for right-sided intervertebral foramen in Table 2|
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|Table 2: Values of three different types of measurements (superior-inferior, superior anteroposterior, and inferior anteroposterior) on both left and right side in males and females|
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| Results and Observations|| |
[Figure 4] and [Figure 5] summarizes in the graphic form of dimension of left and right IVFs from the data represented in [Table 2].
[Figure 6] summarizes the graphical representation of the mean dimension of the lumbar IVFs.
|Figure 6: Graphic form of the average size lumbar intervertebral foramen|
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It is noticed from [Figure 5] that the SI distance is larger at the L2 and then diminished until L5 where it is smallest. It is also noticed that two anteroposterior measures (U-AP and L-AP) remained almost the same from L1 to L5, with the more superior measure being slightly larger than the inferior one.
Therefore, the general shape of the IVFs from L1 to L4 is that of a “comma” (on the left side, the mirror image of a comma on the right side) or as some have described, an “inverted pear.” The L5 IVF has the shape of an “egg”, with the narrow portion of the egg pointing inferiorly
The U-AP and L-AP dimensions were not significantly different between female and male subjects.
Hence, the typical size of an IVF from L1 to L4 is found to be:
- ~20 mm from superior to inferior
- ~10 mm across the level of the Z joint
- ~7.5 mm across the level of the vertebral endplate.
The overall regression analysis for all three of the measurements is found to be statistically significant (SI: F = 26.33, P = 0,000; U-AP: F = 11.16, P = 0,000; IAP: F = 4.46, P = 0.000).
The SI dimension was found to be minimally larger in males than in females (mean males = 19.8 ± 2.6 mm; mean females = 19.4 ± 2.5 mm) (t = 2.09, P = 0.04).
There was no difference of SI (t = −0.14, P =0.89), U-AP (t = 0.05, P = 0.96), and L-AP (t = 0.69, P = 0.49) measurements made at the same levels on the right side compared with those made on the left side. The ratio between the vertical IVF dimension (SI) and the horizontal dimension at the level of the Z-joint superior articular process (SAP) was 20.2:10.2 mm, almost exactly 2:1. Therefore, the horizontal dimension of the IVF at the level of the Z-joint is about half of the greatest vertical dimension.
The ratio between the IVF vertical dimension (SI) and the horizontal dimension at the level of the vertebral endplate (L-AP) was 20.2:7.9 mm, approximately 2.5–1. Another way of saying this is that the inferior dimension is a little over a third the size of the greatest vertical dimension.
Relationship among IVF size with age, height, weight, sex, and side (left and right) was also observed:
The vertical height of the IVFs (SI measurement) decreased as age increased in this study. We can only speculate that this might be due to subtle disc narrowing or slight vertical narrowing of the vertebral bodies with age. U-AP dimension increased in size as age increased, and the L-AP measurement did not change with age. Therefore, the shape of the IVFs changed as age increased.
It was assumed that IVF height will be increased with overall body height. Interestingly, the U-AP and L-AP measurements also increased as overall body height increased. Therefore, the IVFs became longer (from superior to inferior) and wider (from anterior to posterior) as body height increased. Since oval IVF shapes are less likely to be associated with foraminal stenosis and the radiculopathy, than auricular-shaped IVFs, it is speculated that this long, oval configuration might be considered healthy.
Both anterior and posterior (U-AP and L-AP) dimensions of the IVF were found to decrease as body weight increased. The result of this would be a more auricular-shaped IVF. Since auricular-shaped IVFs have been related to stenosis, perhaps, increased weight would predispose the IVFs to stenosis. The decrease in the U-AP and L-AP dimensions would presumably be the result of slight imbrication of facet surfaces with increased weight.
In this study, it was found that the anterior-to-posterior IVF dimensions (U-AP and L-AP) were not different between males and females. The height of the IVFs (SI dimension) of males and females was found to be different and reached statistical significance (t = 2.09, P = 0.04), with the SI measurement being larger in men. However, the difference in IVF height was <0.5 mm, which cannot be considered clinically significant.
Left versus right side
No difference was found between IVFs of the left and right sides. This is of particular clinical interest. If a clinician suspects stenosis of a particular IVF (for example, the left L3 IVF), a comparison can be made with the IVF of the same level on the opposite side (right L3 in our example), knowing that the dimensions are normally closely related.
[Figure 7] shows the relationship among IVF size with age, height, weight, sex, and side (left and right).
|Figure 7: Changes in intervertebral foramen dimensions with increasing age, height, and weight|
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[Table 3] shows the relationship between age, height, and weight with intervertebral foramen dimensions.
|Table 3: Summary of relationship between age, height, and weight with intervertebral foramen dimensions|
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| Discussion|| |
IVF is an oval, round, or inverted teardrop (auricular)-shaped window, bilaterally placed throughout the entire vertebral column between the axis and sacrum through which neurovascular structures pass. IVF provides an osteoligamentous boundary between the central nervous system and the peripheral nervous system. This foramen is unlike from any other foramen in the body, formed by two movable bones (vertebra) and two joints: anteriorly intervertebral joints (secondary cartilaginous joint) and posteriorly zygapophyseal joints (synovial joint). That is why the dimension of IVF varies with the movements of spine; it becomes larger in spinal flexion and smaller in extension. However, there are some quantitative and structural regional variations in between the cervical, thoracic, and lumbar regions. Sacrum has a series of paired dorsal and ventral foramina, but there is no IVF in between the atlas (C1) and axis (C2) vertebra. A foramen contains a segmental mixed spinal nerve and its dural sheaths, from two to four recurrent meningeal (sinuvertebral) nerves, lymphatic channel (s), variable number of spinal arteries, and plexiform venous connections between the internal and external vertebral venous plexuses (Rauschning et al., 1983).
The IVFs are smallest in the cervical region and generally, there is a gradual increase in IVF dimensions up to the L4 vertebra. The IVFs in between L5 and S1 are unique in size and shape. The L5 intervertebral foramen is longer and runs at a more oblique anterior angle than the rest of the lumber IVFs. The lateral recess of the L5 vertebra is the deepest laterally and often the narrowest from anterior to posterior of the entire vertebral column (Rauschning, 1984). Cervical IVFs are distinct, face anterolaterally (Newell 1999), whereas thoracic and lumbar IVF (1981) face laterally [Figure 8].
In addition, the anteroinferior boundaries of the first to tenth thoracic foramina are formed by the articulations of the head of a rib and the capsules of double synovial joints (with the demi-facets on the adjacent vertebra and the intra-articular ligament between the costocapitular ridge and the intervertebral symphysis). Lumber IVFs lie between the two principal lines of vertebral attachment of psoas major.
The dorsal root ganglion location in perspective to the foramen can be quite variable. The majority of DRGs in the lumbar levels are located within the anatomic boundaries of the IVF (1989).,,, Mostly commonly, the position of the DRG within the foramen is located directly beneath the foramen . Only at the S1 level, this rule is not applicable. Studies have reported that the S1 DRG exists (80% time) within the spinal canal. This intraspinal placement makes the S1 DRG at increased risk of injury from the herniation or degenerative changes of the L5–S1 intervertebral disc (1995).
Within the boundaries, there is an intrinsic network of ligaments that divide the intervertebral foramen into multiple subcompartments containing specific anatomic structures. It is conceivable that these ligaments may serve a protective role in preventing injury to the neurovasculature that pass through them (1981). Five major types of transforaminal ligaments are identified: [Figure 9].
|Figure 9: Transforaminal ligaments (picture taken from Clinical Anatomy of Spine, Spinal cord, and ANS by Cramer and Darby)|
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- Superior corporotransverse
- Inferior corporotransverse
- Superior transforaminal
- Mid transforaminal
- Inferior transforaminal.
The superior corporotransverse ligament was the most frequently observed ligament. These bands or ligaments are anomalous in origin and are a potential source of nerve root entrapment. These ligaments can be found in the fifth lumber intervertebral foramen.
The position of these ligaments creates multiple subcompartments just external to the foramen. A large central compartment was seen encasing the exiting ventral rami. Anterior and superior to this central compartment are two smaller openings through which the spinal artery, recurrent meningeal nerve, and a small branch of segmental artery travel. Inferior to the ventral rami, foramina are typically two or more compartments through which veins traverse (1981).
In the posterior aspect of the external foramen, superior and inferior compartments exist. The superior compartment contained the medial division of posterior primary rami and branches of the lumbar artery and vein. The inferior tunnel transmits the lateral division of the posterior ramus and branches of segmental artery and veins.
Quite contrary to the present study, that done on Nigerians showed a steady decrease in the transverse diameter in both sexes with a high degree of variation [Table 4], and another study showed a decrease in the transverse diameter up to L4 and thereafter an increase at L5.
|Table 4: Comparison of the transverse diameter of the intervertebral foramen in various studies|
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The integrity of the IVD has a bearing on the vertical diameter of the IVF, considering the fact that the IVD constitutes 40%–50% of the middle part of its anterior wall. Studies have demonstrated that removal of the IVD can reduce the foramen height on an average by 6.5 mm.
| Conclusion|| |
The results of this study may support clinicians in determining the relative patency of the lumbar IVFs in patients with suspected foraminal stenosis which is the most common cause associated with radicular low back pain that may be due to normal as well as abnormal variations of narrowing of intervertebral foramina causing compression of exiting spinal nerve or other foraminal components.
The IVF database of present study may yield an increased understanding of the dynamic relationships within the IVFs by being used as a source of comparison to assess changes of the IVFs following therapeutic interventions. The development of this normal database should further allow for more meaningful evaluation of the dimensions of the IVFs in pathologic states, such as IVF changes secondary to disc degeneration and disc protrusion or prolapse, facet arthropathy (using the U-AP measure), and discogenic spondylosis (using the L-AP measure). This work should also help with continued development of diagnostic ratios for clinical evaluation of IVF size. One must keep in mind, however, that compression of spinal nerves or nerve roots within the IVFs does not always result in symptoms.
Although diagnosis can be suspected from the history and physical findings, knowledge of the specific anatomy of this clinically significant area with radiological support is very important in the differential diagnosis which helps in proper management. That's why normal IVF dimensions must be known and understood thoroughly before proper evaluation of this region.
Therefore, it is really convincing that, with regard to morphometry of the spine by means of MRI, the future is very promising.
Financial Support and Sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9]
[Table 1], [Table 2], [Table 3], [Table 4]