Significant morphometric differences in the L4-L5 intervertebral foramen: A cross-sectional study

ABSTRACT

Introduction:

The present study involves employed a cross-sectional analysis of lumbar spine magnetic resonance imaging (MRI) scans. The purpose of the study is to quantify the variations of diameters of the intervertebral foramen (IVF) at the level of L4/L5 vertebrae. The IVF serves as a crucial gateway through which nerve roots and spinal nerves exit the spine. This area is particularly susceptible to compression due to a reduction in foraminal width or height that occurs with age. Frequent compression of the nerve roots can lead to radiculopathy, resulting in pain in the lower back and legs. Understanding the variations of the IVF at the L4/L5 region as we age is essential.

Methodology:

A purposive sampling method was used for data collection in this study, which consists of two parts: a reliability study and a subject study. To measure the reliability consistency, an intraclass correlation coefficient and Pearson’s correlation coefficient were calculated. MRI scans of the lumbar spines of 149 subjects were selected for these measurements.

Results:

In assessing intraobserver reliability for measurement W1, Cronbach’s alpha values were found to be 0.500 for the right IVF and 0.902 for the left IVF, indicating moderate-to-high reliability. The most common shape observed in Cluster 2 for the right IVF is pyriform, which appears 10 times (45.5%). In addition, the IVF in male’s transitions to a teardrop shape as age increases.

Conclusion:

This research shows variations in the diameter of IVF as age increases which enhances our understanding of age-related anatomical variations in the lumbar region between genders and adds to the overall knowledge of spinal disorders.

INTRODUCTION

The intervertebral foramen (IVF) is an area where nerve roots and mixed spinal nerves are particularly susceptible to compression from pathologies that can narrow foraminal width or height. A small anterior–posterior parameter of the lumbar IVFs has been linked to mobility constraints.[1] The lumbar IVF houses the spinal nerve and the dorsal root ganglia, which consist of sensory neurons. A person’s functional capacity, health, quality of life, and symptom intensity are all affected by foraminal dimensions. Degenerative changes in the structures that form the walls of the IVF can change its dimensions and shape, resulting in narrowing or stenosis of the foramen. When diagnosing foraminal constriction and radiculopathy, the diameters of the IVFs are highly significant from a clinical standpoint.[2] Stenosis involves compression caused by various factors, including a herniated intervertebral disc, overgrowth of facet joints and ligaments, or unstable displacement of one vertebral body relative to the vertebra below. The IVF plays a critical role in studies related to the development of low back pain. Radiculopathy is a common cause of pain in the lower back and legs due to nerve root compression brought on by foramen stenosis.[3] The morphology of the IVF can be altered by various physiological or pathological risk factors for instance a bony callus formation. The structural changes in IVF also noticed in aging. However, there is limited understanding of how the IVF undergoes morphological changes with aging. It is possible to diagnose lumbar foramen stenosis radiologically utilizing a variety of radiologic modalities, including active X-ray, computed tomography, diffusion tensor imaging, and magnetic resonance imaging (MRI).[4] In many instances, IVF stenosis has traditionally been diagnosed by the obliteration of perineural fat surrounding the nerve root on parasagittal MRI. Research on the potential effects of degenerative changes and constitutional factors in the IVF region could have important implications for clinical practice.[5]

The association between the foraminal region and a medical condition and demographic factors including age and sex is not well-established. The main objective of this study was to evaluate the morphometric changes of the right and left IVFs across different age groups and to determine whether there are significant differences between sides and among age categories. Further, this investigation has characterized and compared the morphological distribution of the right and left IVF shapes with potential relation to foraminal stenosis risk or nerve root impingement.

METHODOLOGY

Study design

It was a retrospective cross-sectional study of the lumbar spine using accessible MRI images. The estimated sample group was calculated using single population proportions to determine the sample size. Data were collected from the databases between 2021 and 2023. The study population consisted of data from patients aged 20–45 years who visited the Imaging Department of the Management and Science Medical Centre (MSUMC) in Selangor between 2021 and 2023. Relevant data were retrieved from the MSUMC database for patients who underwent MRI for lumbar spine investigation.

An established approach for this research[6] was to evaluate the measurement’s accuracy and consistency, a reliability study using Cronbach’s alpha was conducted with a sample size of n = 1.

A purposive sampling method was employed to collect 149 DICOM images from the database. All images were transferred to a Macintosh operating system (MacBook Pro 13-inch, M1, 2020; Apple Inc., California) with Horos Software installed. The MRI images were preprocessed to obtain precise measurements of the selected landmarks. The pixel size in the image was determined based on the reference scale included in the MRI. The area of interest was defined using a freehand selection tool, enhancing the contrast of the entire image and highlighting the prominent landmarks.

Image selection for measurement

MRI scans of the lumbar spine were selected using standardized imaging protocols that showcase the parasagittal midline of the pedicles, which form the roof and floor of the IVF. The image that clearly displayed the IVF margins in the midpedicular area was selected. This process was carried out at every level on both the left and right sides. T1-weighted and T2-weighted MRI sequences were utilized to visualize IVF. The imaging parameters were optimized to achieve high spatial resolution and enhanced tissue contrast. Measurements were taken bilaterally for each participant, and both intra- and interrater reliability assessments were performed to ensure consistency and accuracy in the measurements. The principal investigator and radiologist received training before the data collection procedure. Images were magnified to 200–400 times their normal size for enhanced visualization, and measurements were taken twice before recording the final value.

Lumbar intervertebral foramen measurement

The anatomical landmarks on both sides of the lumbar vertebrae cannot be observed in the same axial cut of the MRI images due to vertebral rotation. To achieve precise measurements of the selected landmarks, MRI images were first preprocessed. The pixel size of the images was determined using the reference scale provided. A contrast enhancement was applied to equalize the histogram, utilizing contrast stretching with a saturating pixel percentage of 70. Next, an area of interest was outlined with a freehand selection tool to adjust the contrast across the entire image and highlight the landmarks.

In this study, the anatomical topography dimensions of the IVF were assessed, which included the anteroposterior width, under which the following measurements [Figure 1] were conducted; W1: IVF width measured from the cranial vertebra’s pedicle to the superior aspect of the caudal pedicle. W2: IVF width at the upper margin of the disc. W3: IVF width at the midpoint of the disc. W4: IVF width at the lower edge of the disc. For the craniocaudal height (H1): IVF Height at the posterior margin of the superior vertebral body and superior articular facet were measured.

Figure 1.

The specific measurements of L4-L5 intervertebral foramen

Inclusion and exclusion criteria

The patient databases were extracted, and each image was evaluated according to the established inclusion and exclusion criteria. The exclusion criteria included repeated examinations of the same individual, degenerative disc disease, disc pathology, and spinal deformities or pathologies, as determined by the consultant radiologist.

Ethical approval

Ethical approval was obtained from the University Ethics Committee at the Postgraduate Centre with reference code E-L1-01-IMS-2024-04-001. All patients gave written consent before receiving treatment at MSUMC.

Statistical evaluation

To evaluate the data gathered, this study used statistical analysis, both descriptive and comparative. Statistical Package was used for Social Sciences version 29 for comparison analysis and Microsoft Excel version 2021 for descriptive analysis and graph production. For categorical data, percentages and frequencies were computed, and for continuous variables, means and standard deviations. The identification of relevant correlations between the numerous variables in this research was made easier by these techniques.

RESULTS

Reliability

In assessing intraobserver reliability for measurement W1 [Table 1], the Cronbach’s alpha values were 0.500 for the right IVF and 0.902 for the left IVF, indicating moderate-to-high reliability. In addition, Pearson’s correlation coefficients were 1.000 for the right IVF and − 0.064 for the left IVF.

Table 1.

Intraobserver reliability of measurement

	Intraobserver
	Right IVF		Left IVF
	Cronbach’s Alpha	Pearson’s	Cronbach’s Alpha	Pearson’s
W1: IVF width at cranial vertebra of the cranial pedicle and superior of the caudal pedicle	0.500	1.000	0.902	-0.064
W2: IVF width at the superior margin of the disc	0.714	0.208	0.899	-0.066
W3: Width at mid-point of the disc	0.494	0.673	0.875	-0.006
W4: Width at the inferior margin of the discs	0.454	0.587	0.899	-0.073
H1: IVF height along the posterior margin of the superior vertebral body and superior articular facet	0.734	0.303	0.866	-0.074
IVF Shape	1.000	0.206	0.923	-0.052

IVF - Intervertebral foramen

Measurements W2, W3, and W4 displayed different levels of reliability, as assessed by Cronbach’s alpha and Pearson’s correlation coefficients. For measurement H1, which focuses on the IVF height at specific points, the Cronbach’s alpha values were 0.734 for the right IVF and 0.866 for the left IVF, indicating moderate-to-high reliability [Table 1]. The corresponding Pearson’s correlation coefficients were 0.303 for the right IVF and − 0.074 for the left IVF. The assessment of the shape of the IVF demonstrated excellent reliability, with a Cronbach’s alpha of 1.000, and a low correlation as indicated by Pearson’s coefficient.

In interobserver reliability [Table 2], two statistical measures were used to assess reliability: the intraclass correlation coefficient (ICC) and Pearson’s correlation coefficient. These measures indicated the consistency of measurements between different observers.

Table 2.

Interobserver reliability of measurement

	Interobserver
	Right IVF		Left IVF
	ICC	Pearson’s	ICC	Pearson’s
W1: IVF width at cranial vertebra of the cranial pedicle and superior of the caudal pedicle
W2: IVF width at the superior margin of the disc
W3: Width at mid-point of the disc	0.736	0.087	0.903	0.082
W4: Width at the inferior margin of the disc
H1: IVF height along the posterior margin of the superior vertebral body and superior articular facet
IVF Shape

IVF - Intervertebral foramen; ICC - Intraclass correlation coefficient

For interobserver reliability, observers measured the IVF Width at midpoint of the disc (W3). Table 2 illustrates that from interobserver reliability, the ICC values was 0.736 for the right IVF, demonstrating a good interobserver reliability, showing that both observers obtained consistent measurements for W3. However, the corresponding Pearson’s correlation coefficient (r = 0.087) was low, reflecting a weak linear relationship between observers’ measurements. For the left IVF, the ICC value was 0.903, indicating excellent reliability and demonstrating a high level of agreement between observers for this parameter. This reflects that measurements on the left side were performed with greater precision and consistency. The low Pearson’s r (r = 0.082) again suggested limited linear association.

Overall, the Tables 1 and 2 illustrates the reliability of IVF dimensions and shape measurements, ensuring consistency and accuracy in the study’s findings.

Age- and sex-related differences

Table 3 illustrates that across all age groups and both sexes, the pyriform shape is clearly the most prevalent – right IVF: 67 (45%) and left IVF: 51 (34%). This consistency suggested that the pyriform configuration is the dominant and optimal morphology of the IVF during adulthood. The next most frequent shape was the teardrop, followed by reniform and elliptical shapes. The distribution between the right and left sides was generally comparable. The right side showed slightly higher pyriform and teardrop shapes, and the left side exhibited marginally higher reniform shape.

Table 3.

Distribution of right and left intervertebral foramen shape according to gender and age group

Gender	Age	Shape of Right IVF				Shape of Left IVF				Total
		Elliptical	Reniform	Pyriform	Teardrop	Elliptical	Reniform	Pyriform	Teardrop
Male	20–24	1	-	3	-	-	-	2	2	8
	25–29	1	1	4	5	3	1	2	5	22
	30–34	1	4	3	5	-	4	4	5	26
	35–39	2	3	7	3	-	4	7	4	30
	40–44	1	3	9	3	-	2	7	7	32
	44–49	-	-	-	2	-	-	-	2	4
Female	20–24	-	2	3	-	1	2	2	-	10
	25–29	3	2	6	2	2	2	6	3	26
	30–34	4	2	11	5	4	3	8	7	44
	35–39	5	5	10	4	7	6	5	6	48
	40–44	5	2	7	4	3	2	6	7	36
	44–49	-	1	4	1	1	1	2	2	12
Total		23	25	67	34	21	27	51	50	298

IVF - Intervertebral foramen

In relation to age and sex, in males pyriform predominates in the age range of 20–24 years. Pyriform remained common in the age group of 25–29 years, but teardrop frequency increased, reflecting early degenerative or postural changes. In the age group of 30–34 years, there was an increase in reniform and teardrop shapes, while there was a decline in elliptical shape, which indicated age-related morphological remodeling. Very few numbers of teardrop IVF shape were noticed in age range of more than 44 years. These findings highlighted degenerative changes, which may cause narrowing of the IVF.

In females, pyriform remained the dominant shape in the age range of 20–24 years. The age group from 25 years to 39 years showed gradual increase in pyriform and teardrop shapes, illustrating consistency related to aging and spinal adaptation. The IVF shapes in the age range of 40–44 years have even distribution of all the four shapes showing greater variability in aged females. Decrease in all shapes of the IVF, especially Pyriform are observed in females with age more than 44 years.

There was no major variation in overall shape distribution across age groups, indicating that age between 20 and 49 years has minimal impact on the relative proportions of IVF shape types.

Distribution of intervertebral foramen between clusters

The Chi-square test was performed to investigate if the distribution of IVF shapes significantly differs between the various clusters. The data from Table 4 demonstrate significant morphological variations in the shapes of the IVFs among the three identified clusters on both the right and left sides. The distribution of shapes was statistically different across clusters, as evidenced by highly significant Chi-square values for both sides (right: statistic = 70.339, P < 0.001; left: statistic = 92.434, P < 0.001).

Table 4.

Association between clusters, right and left intervertebral foramen

Shape	Cluster 1, Frequency (%)	Cluster 2, Frequency (%)	Cluster 3, Frequency (%)	Statistic	P
Right IVF
Elliptical	1 (2.6)	10 (45.5)	12 (13.5)	70.339	<0.001
Pyriform	7 (18.4)	10 (45.5)	50 (56.2)
Reniform	5 (13.2)	1 (4.5)	19 (21.3)
Teardrop	25 (65.8)	1 (4.5)	8 (9.0)
Left IVF
Elliptical	0	14 (63.6)	6 (6.7)	92.434	<0.001
Pyriform	8 (21.1)	6 (27.3)	47 (52.8)
Reniform	4 (10.5)	1 (4.5)	21 (23.6)
Teardrop	26 (68.4)	1 (4.5)	15 (16.9)

IVF - Intervertebral foramen

In the right IVF, Cluster 1 was chiefly marked by the teardrop-shaped foramina (65.8%). The most common shape in Cluster 2 was elliptical and pyriform with a frequency of 10 (45.5%) pointing out a transitional framework, while Cluster 3 was dominated by pyriform shape foramina with a frequency of 50 (56.2%).

In the left IVF, in Cluster 1 teardrop shape was the most prevalent with a frequency of 26 (68.4%). Cluster 2 by elliptical shapes (63.6%). Pyriform shape dominated in Cluster 3 with a frequency of 47 (52.8%). The high Chi-square value on the left side (statistic: 92.43, P < 0.001) indicates a more pronounced differentiation between clusters compared to the right side.

Intervertebral foramen diameter between age groups

Observations from Table 5 demonstrates that in both right and left foramina, W1 values were highest among all width parameters, while W4 consistently showed the lowest value. This pattern illustrates that at the inferior margin (W4), the IVF is naturally narrower and wider at superior margin (W1). This dimension reflects a typical teardrop foraminal shape. From 20 to 24 years (right W1 = 9.58 mm; left W1 = 9.72 mm) to 45–49 years (right W1 = 8.05 mm; left W1 = 8.76 mm), a slight reduction in W1 values was observed. Similar mild reductions are noted in W2–W4 values across age groups, suggesting progressive narrowing of the foramina with age. The narrowing trend was more apparent in the lower foraminal widths (W3–W4), may be due to degeneration of intervertebral disc causing height loss and facet joint hypertrophy, which encroach upon the lower borders of the foramen.

Table 5.

Diameter of intervertebral foramen across various age groups

	Right IVF					Left IVF
	W1	W2	W3	W4	H1	W1	W2	W3	W4	H1
20–24	9.58±1.31	7.32±1.13	5.63±1.37	4.46±1.49	18.15±2.33	9.72±1.38	6.85±1.15	5.80±1.12	4.45±1.46	18.01±2.04
25–29	9.07±2.35	7.17±1.89	5.80±2.06	4.83±1.94	18.37±1.77	9.74±1.43	7.16±2.04	6.30±2.45	5.39±2.14	18.91±1.95
30–34	11.90±13.23	6.68±2.01	5.47±2.21	4.62±1.63	17.95±1.77	9.29±1.47	6.40±1.96	4.98±2.05	4.42±2.07	17.77±1.87
35–39	9.61±1.40	6.66±1.94	5.55±2.01	5.01±1.76	18.49±1.49	9.29±2.16	6.30±1.88	5.02±2.06	4.82±2.08	18.46±3.22
40–44	12.01±14.32	6.29±2.06	4.96±1.67	4.30±1.55	18.66±1.93	9.11±1.05	5.59±1.84	4.34±1.99	4.42±1.87	18.87±1.36
45–49	8.05±0.74	5.96±1.07	5.06±1.86	3.87±1.21	17.37±2.36	8.76±1.30	5.22±0.70	4.25±1.08	3.91±0.71	16.81±2.16

IVF - Intervertebral foramen

The IVF height (H1) stays almost same for different ages, measuring about 17.3 mm to 18.9 mm on each side. However, slight reduction in the height was observed in the age range of 45–49 years (right IVF: 17.37 mm; left IVF: 16.81 mm). The decrease in the height in this age group reflects primary indication of vertical compression, probably due to intervertebral disc degeneration. The results reveal that the dimensions for the right and left IVF are largely symmetrical, with few nonsignificant differences.

In majority of the age ranges, the dimensions of left IVF are slightly higher than the right IVF. This asymmetry probably explains a natural postural or muscular dominance without any disease condition.

DISCUSSION

The bony boundaries of IVF are crucial, as they directly reflect the range of motion. Bony anatomical features, including pedicles, vertebrae, and processes, define the border.[7] To diagnose foraminal stenosis and radiculopathy, the foramen’s diameters are clinically significant.

Intraobserver reliability

Intraobserver reliability on the right IVF in this study reported Cronbach’s alpha values from 0.454 to 1.000, which signifies moderate-to-excellent reliability. The highest Cronbach’s alpha values were observed for IVF shape determination (α = 1.000), followed by height of the IVF (H1) (α = 0.734) and width at the superior margin of the disc (W2) (α = 0.714). These figures indicate that parameters associated with clearly defined anatomical demarcations are measured with greater consistency. Conversely, measurements with lower reliability (W3 = 0.494; W4 = 0.454) was possibly because of difficulty in outlining boundaries of the foramens at these levels. This may be due to ligamentous band running across lumbar IVF which can cause discrepancies in recognizing the margins of the foramina.[1,2] Pearson’s correlation value for W1 (1.00) in the right IVF shows perfect correlation and excellent repeatability of cranial-level width measurements.

The left IVF measurements reported uniformly high Cronbach’s alpha values (0.866–0.923), reflecting excellent internal consistency across all dimensions. However, the Pearson’s correlation values were low and negative ranging from − 0.006 to − 0.074. The derived Pearson’s r may exhibit a low value due to the insufficient variance present among the individuals, which hinders the ability to establish a strong correlation. Consequently, a high value of Cronbach’s alpha reflects effective measurement consistency.[8]

The study demonstrates good to excellent interobserver reliability. The excellent ICC on the left side (0.903) highlights the strength of the measurement protocol and suggests that the methodological approach minimizes observer-dependent error. The low Pearson’s correlation coefficients, however, highlight that when morphometric data shows low interindividual variance, its reliability might be misjudged by linear correlation.[9] ICC should be the preferred measure of reliability for continuous anatomical data, as it captures both agreement and consistency.[10]

Although previous research has categorized the IVF into different forms, most of these studies concentrate on the dimensional morphometry of the foramina. Cramer et al. developed a morphometric database of lumbar IVF dimensions using MRI, demonstrating significant inter- and intraobserver reliability. They identified associations between foraminal size and factors such as age, sex, vertebral level, side asymmetry, and concluded that the shapes of the IVF vary among individuals, which aligns with the distinct shape clusters identified in this study.[6]

The Chi-square test was performed to check significant association between clusters and shape of IVF. The analysis demonstrated a highly significant association between IVF shape and the clusters, wherein the teardrop shape was prominently observed bilaterally in cluster 1. This observation aligns with previous studies indicating that the lumbar foramen typically presents as an oval, circular, or inverted teardrop-shaped opening on the lateral aspect of the lumbar spine.[11] The prevalence of elliptical foramina in Cluster 2 reflects transitional or adaptive morphologies resulting from postural modifications or initial degenerative changes of the IVF. This elliptical pattern was consistent with prior research suggesting that foraminal shape variation was not exclusively anatomical but could also indicate dynamic biomechanical responses to spinal loading and posture.[12,13] Pyriform and reniform foramina having greater frequency in Cluster 3 imply structural remodeling related to degenerative alterations or changes in mechanical stresses within the vertebral column. Such changes may result from age-related disc height reduction for instance in condition like spondylolisthesis which is a degenerative condition in middle age and elderly population that effects the lumbar vertebrae at L4/L5 level.[14,15] In this condition the facet joint hypertrophies which influences the configuration of the IVF that accelerate capsule of the joints between the vertebrae narrowing the IVF causing nerve injury.[16] Stephen et al. in their cadaveric study highlighted that the quantification of the IVF parameters relies on the accurate position of the body during MRI. Significant position-dependent differences were noted in the cross-sectional area of the IVF within the lumbar spine.[17] In an earlier study, the IVF grade was found to be higher when evaluated in extension as opposed to flexion and neutral positions, indicating that extension reduces the foraminal size and the space accessible for the exiting nerve root.[18,19]

Moreover, the notable bilateral consistency in the distribution of IVF shapes between the right and left sides indicates that the clustering reflects inherent anatomical variation. Overall, the results highlight that the classification of IVF shapes can be a crucial morphological indicator for the assessment of spinal health and degeneration. The significant statistical relationship between clusters and foramen shape emphasizes the potential of shape-based morphometric analysis in differentiating normal, adaptive, and degenerative spinal IVF.

In the present study, IVF width W1-W4 decreased with increasing age noticeably in the age group of 45–49 years while the H1 shows a slight declined after 40 years. These deformations pointed toward degenerative changes in the intervertebral disc and joints facet in intervertebral joints which may cause IVF narrowing promoting impingement of the concerned nerve root.[20] Decrease in W3 and W4 in older age range was associated with loss of height of intervertebral disc and formation of osteophytes.[21] Earlier studies indicated that in healthy subjects, the foraminal height and width both decline as age increases.[22] Moreover, considerable amount of research has demonstrated how the size of the lumbar IVF changes dynamically and these progressive changes in the IVF leads to the development of lumbar IVF stenosis, resulting in radiculopathy in older population group.[23] Lumbar nerve root compression most often occurs at the L5 level due to a higher nerve root diameter to foraminal area ratio compared to upper lumbar regions.[24] The segments of the spine that are most prone to degeneration are found in the lower lumbar region. Subsequently, as people age, changes in trunk muscle strength and postural instability worsen forward slippage of the vertebrae, particularly at the upper part of the spinal segment.[25] The soft tissue components, including the intervertebral discs and ligaments, play a significant role in the pathology of diseases associated with the IVF.[26] Studies have shown that the presence of abnormal intervertebral discs leads to a decrease in vertical intervertebral foraminal height at L4-L5 and L5-S1 levels.[27] The diameters of the IVF shrink when an intervertebral disc collapses by 4 mm.[28] It is vital for radiologists and spine surgeons to recognize these subtle changes that are influenced by age and side effects in practical terms.

The analysis of the IVF in the present study was restricted to its bony threshold value and did not include soft tissue. Another limitation is that moving and axial postures were not included in the original MRI data, which was solely obtained in the supine position. The body postures alter the IVF’s shape. Furthermore, a multi-Centre approach can enhance the generalizability of the findings and provide a larger, more diverse dataset.

CONCLUSION

This study provides a comprehensive investigation of the lumbar vertebrae at the L4/L5 level through MRI, emphasizing age-related variations. The consistent predominance of the pyriform type across groups strengthens the conclusion that this morphology is a fundamental feature of adult spinal architecture. The findings revealed significant differences in the IVF based on age. This research improves our understanding of age-related anatomical variations in the lumbar region between sexes and contributes to the broader knowledge of spinal disorders.

Conflicts of interest

There are no conflicts of interest.

Funding Statement

Nil.