A CT-based radiographic analysis of parameters of congenital lumbar neuroforaminal stenosis

Introduction

Congenital lumbar stenosis (CLS) is a debilitating condition characterized by the developmental narrowing of the spinal canal as well as intervertebral foramina, and is associated with pain and a decreased quality of life (1-4). As lumbar neuroforaminal stenosis poses a significant clinical challenge due to its diverse etiology and variable clinical presentations, accurate diagnosis and timely intervention are crucial to mitigate the associated morbidities and improve patient outcomes (5,6). Diagnosis of neuroforaminal stenosis should integrate clinical assessment, patient-reported symptoms, and radiographic imaging. However, interpretations of radiographic findings are typically not made based on quantitative methods, as objective criteria for diagnosing congenital lumbar neuroforaminal stenosis have yet to be commonly accepted.

A thorough understanding of anatomy, including normative guidelines for lumbar neuroforaminal dimensions (LNFD), is the foundation for successful intervention in CLS. Changes in LNFD have important implications for the diagnosis and treatment of spinal stenosis (7). There has been an investigation into standardized definitions of what constitutes “normal” LNFD, but there remains a lack of criteria for diagnosing CLS via LNFD measurements (8,9). Furthermore, understanding the influences of patient characteristics, such as sex, race, and ethnicity, on anatomic assessments is essential. To address these issues, this study initiates a second phase of analysis of anatomic parameters for diagnosing CLS, initially started by Shin et al. (10). This prior study sought to determine normative measurements of interpedicular distance (IPD), pedicle length, and canal diameter in a large cohort to further delineate the diagnostic parameters for CLS, as well as evaluate for race and ethnicity based anatomical similarities and differences. Similarly, the purpose of this study was to utilize computed tomography (CT) to solidify the diagnostic parameters of neuroforaminal stenosis via normative measurements of LNFD, as well as to evaluate for anatomical similarities or differences based on patient sex, race, and ethnicity. We present this article in accordance with the STROBE reporting checklist (available at https://jss.amegroups.com/article/view/10.21037/jss-25-85/rc).

Methods

Patient selection

This study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. Study approval was received from Loma Linda University Institutional Review Board (IRB No. 5240121) and informed consent was waived due to the nature of this retrospective chart review study. Radiographic and clinical data were collected from patients aged 18 to 35 who underwent abdominal/pelvic or lumbar CT scans between January 2015 and March 2023. Imaging eligible for analysis was restricted to studies obtained for indications unrelated to spinal pathology. Demographic variables, including patient height, weight, body mass index (BMI), sex, race, and ethnicity, were recorded as documented at the time of imaging. Racial and ethnic categories were defined based on the Office of Management and Budget and National Institutes of Health (NIH) National Institute on Minority Health and Health Disparities parameters (11). Similar to Shin et al., we excluded patients with any clinical or radiologic history of spinal abnormalities, including degenerative disc disease, scoliosis with coronal deformity >10°, spondylolisthesis, traumatic injury (bony or soft tissue), malignancy, infection, prior spinal surgery or instrumentation, or symptoms of back/leg pain or numbness (10).

Data collection

CT scans were analyzed using the IMPAX6 picture archiving and communication system (Agfa-Gavaert, Mortsel, Belgium) with window and level settings set to 2,000 and 500 Hounsfield units (HU), respectively, consistent with prior methods of Shin et al. (10). LNFD from L1–S1 were assessed for three parameters: height, width, and area. Foraminal height was defined in the sagittal plane as the vertical distance between the inferior margin of the upper pedicle and the superior margin of the lower pedicle. Width was defined on axial images as the minimal distance between the posterior-inferior vertebral corner of the superior vertebra and the anterior aspect of the superior articular process of the vertebra below. The area was manually traced in the sagittal view using the freeform tool within IMPAX6, outlining the bony margins of the neuroforamina. Coronal, sagittal, and axial views were utilized as visual aids to assess for accuracy regarding off-plane measurements, though measurements of width, height, and area were performed using solely the axial or sagittal plane. Figure 1 illustrates the technique for measuring neuroforaminal dimensions (NFD). Although the oblique orientation of lumbar foramina poses a measurement challenge, we followed the precedent of Harianja et al., who demonstrated that axial measurements reliably represent foraminal width without clinically significant deviation from straight sagittal-derived measurements (8).

Figure 1 Lumbar neuroforaminal dimension measurements. Neuroforaminal height was measured as illustrated by line AB. Sagittal width was measured as illustrated by line CD. Axial width was measured with the same methodology, though in the axial view. Foraminal area was measured as illustrated by the foramen contained within the hatched line.

To establish interobserver reliability, 100 CT scans were independently reviewed by two separate raters, and intraclass correlation coefficients (ICCs) were calculated using a two-way mixed-effects model based on absolute agreement (12-14). ICC values were interpreted using standard thresholds: <0.40 (poor), 0.40–0.59 (fair), 0.60–0.74 (good), and ≥0.75 (excellent) (13,14). The resulting ICC was 0.842 [95% confidence interval (CI): 0.816–0.882], indicating excellent reliability. The remaining images were then measured by a single trained reviewer per scan. CLS was defined per Bajwa et al., using a threshold of two standard deviations (SDs) below the cohort mean to designate a measurement as congenitally stenotic (15).

Statistical analyses

Statistical analyses were performed using SPSS v28 (IBM Corporation, Armonk, NY, USA), with significance defined as P<0.05. Data normality was assessed using Kolmogorov-Smirnov tests and Q-Q plots. Levene’s homogeneity of variance test and regression residual plots were used to determine homoscedasticity (16-18). Pearson ICCs were calculated to examine relationships between NFD and patient characteristics, with correlation strength categorized as weak (r=0–0.4), moderate (r=0.4–0.7), or strong (r=0.7–1) (19-21). Paired t-tests were conducted to evaluate side-to-side differences in pedicle lengths. Differences across sex, race, ethnicity, and spinal levels were tested using one-way analysis of variance (ANOVA) with Bonferroni and Tukey post hoc corrections when applicable.

Power calculations were performed for partial correlation and one-way ANOVA analyses. For partial correlation with a sample size of 1,000, P=0.05, and a partial r=0.3 (with four covariates), power was determined to be 100%. Similarly, power for ANOVA across race/ethnic groups (n=472, 319, 109, and 84) with assumed SD =1.0 and P=0.05 also yielded 100% power. Reference population size assumptions were based on U.S. Census data indicating a population of approximately 331.9 million (22-24).

Results

Cohort description

Of the 1,208 patients initially reviewed, 208 were excluded: 78 due to suboptimal image quality, 44 with prior spinal surgery, 28 with scoliosis, 26 with vertebral fractures, 22 scanned for back pain, 7 with spondylolisthesis, and 3 with neoplastic disease. This resulted in 1,000 included patients, comprised of 511 females and 489 males. The mean age was 27.4 years, the mean height was 1.59±0.11 meters, the mean weight was 80.47±23.64 kilograms, and the mean patient BMI was 28.32±7.4 kg/m². Demographic distribution by race and ethnicity was as follows: Hispanic (47.2%, n=472), White (31.9%, n=319), Black (10.9%, n=109), Asian (8.4%, n=84), and other (1.6%, n=16).

CLS threshold measurements

Mean LNFD measurements, regardless of vertebral level, were: 8.66±2.1 and 8.76±3.14 mm for left and right widths, 17.76±2.74 and 17.7±3.26 mm for left and right heights, and 133.12±34.72 and 133.4±33.86 mm² for left and right areas. Threshold values for neuroforaminal stenosis, regardless of vertebral level, were: 4.46 and 2.48 mm for left and right widths, 12.28 and 11.18 mm for left and right heights, and 63.68 and 65.68 mm² for left and right areas. The substantial left-right asymmetry in foraminal width threshold values resulted from applying a threshold defined as two SDs below the cohort mean. Tables 1-5 report stenosis threshold values for LNFD per vertebral level, respectively. Table S1 reports mean anatomic measurements based on vertebral level. Table S2 reports differences in anatomic measurements based on vertebral level.

Influence of sex, race, and ethnicity

Although there were a few significant differences based on patient sex, they were not consistently observed across all vertebral levels. Males demonstrated larger neuroforaminal stenosis threshold values with respect to left and right LNFD height from L1–L2, while our female cohort demonstrated a larger neuroforaminal stenosis threshold with respect to left LNFD height from L2–L3. Tables 1-5 additionally report neuroforaminal stenosis threshold values based on patient sex per vertebral level, respectively. Table S3 reports differences in LNFD measurements based on patient sex per vertebral level. Significant differences were observed based on patient race and ethnicity when analyzed per vertebral level from L1–S1, while no significant differences were found when analyzed irrespective of vertebral level. Table 6 reports neuroforaminal stenosis threshold values based on patient race and ethnicity per vertebral level. Table 7 reports mean differences in measurement values amongst racial and ethnic groups irrespective of vertebral level. Table S4 reports mean LNFD measurements based on race and ethnicity per vertebral level. Table S5 reports mean differences in measurement values amongst racial and ethnic groups per vertebral level. NFD were generally greater among Asian and Caucasian individuals compared to Hispanic and African American individuals, with the exception of height, where Caucasians and Hispanics demonstrated the largest values, followed by Asians and African Americans. However, these findings were not always significant.

Discussion

Lumbar spinal stenosis has been reported to increase in prevalence and severity with age (25). As CLS is present in approximately 9% of cases of spinal stenosis, it is essential to establish clear anatomic parameters for diagnosing CLS (26). While research regarding CLS has often focused on narrowing of the central canal, neuroforaminal stenosis and central canal stenosis are closely related (27,28). Congenital neuroforaminal stenosis has demonstrated increased incidence of foraminal protrusions as well as narrowing of the neural foramina (1). As such, a thorough understanding of lumbar neuroforaminal stenosis thresholds is imperative. Current categorization of CLS and neuroforaminal stenosis primarily utilizes qualitative grading of radiographic imaging via magnetic resonance imaging (MRI) (29-32). While Lee et al. showed how MRI can be an accurate method of grading lumbar neuroforaminal stenosis, diagnosing stenosis still relies primarily on multi-planar and qualitative interpretation of imaging (30). Notably, Senoo et al. conducted a detailed morphometric analysis of the lumbar foramen using CT-derived measurements in the axial and sagittal planes, formulating a 3D model which was accurately validated against cadaveric lumbar specimens (33). In terms of diagnosing foraminal stenosis via quantitative, anatomic parameters, current literature is limited primarily to a cadaveric study by Hasegawa et al., in which it was determined that a neuroforaminal height of less than 15 mm was indicative of stenosis (34). Our results support these findings, with neuroforaminal stenosis thresholds of left and right neuroforaminal height measuring 12.28 and 11.18 mm, respectively, when measured irrespective of vertebral level. Our study, which used a patient population of 1,000, also expands upon Hasegawa’s sample size of 100. Furthermore, while Hasegawa’s study measured neuroforaminal width and area, it did not dictate parameters of stenosis via foraminal width and area. Our measurements found neuroforaminal stenosis thresholds of 4.46 and 2.48 mm for left and right foraminal widths and 63.68 and 65.68 mm² for left and right foraminal areas. Current literature proposes that the normal lumbar foramen area varies from 40 to 160 mm², width varies from 8 to 10 mm, and height varies from 19 to 23 mm (35-38). While our measurements of area and width were within normative findings, our measurements of heights were a bit lower than normal parameters, which could explain the slightly lower neuroforaminal stenosis cutoff when compared to the Hasegawa study. While clinical diagnosis and management of spinal stenosis involves multiple factors, with symptomology being a key factor, the baseline anatomic parameters established by our findings may help in elucidating differences regarding prevalence and outcomes of congenital neuroforaminal stenosis amongst differing populations. Additionally, we hope that our measurements may aid in guiding early identification of symptomatic nerve compression, refine current anatomical classification thresholds, and potentially serving as inputs for predictive modeling when combined with clinical presentations.

Influence of patient sex, race, and ethnicity on CLS thresholds

The effects of sex on spinal stenosis are still under investigation. While women have demonstrated increased severity of symptoms of stenosis, reports have listed no differences in prevalence of lumbar spinal stenosis between men and women (25,39,40). Notably, a study of 42,355 cases demonstrated that neuroforaminal stenosis is more common in men (41). However, our findings didn’t showcase any significant differences between men and women in regards to LNFD and stenosis thresholds. While left and right foraminal areas seemed to be larger for males from L1–S1, male neuroforaminal stenosis thresholds based on area tended to be smaller, due to an increased SD of foraminal area values, but these findings were not statistically significant. Additionally, the finding of males having larger stenosis thresholds based on left and right LNFD heights from L1–L2 does not seem to be clinically significant when compared to the lack of significant differences found in every other aspect of NFD. Our lack of significant sex-based differences in NFD and corresponding stenosis thresholds goes hand in hand with current findings in literature (42,43).

Additionally, understanding racial differences in spinal stenosis is essential, as dimensions for stenosis have been reported to vary based on race (30). These variations may lead to differences in surgical outcomes for different populations; specifically, black patients have demonstrated differences in outcomes of lumbar stenosis surgery, such as longer length of stay (44). Our findings demonstrated how the African-American cohort had smaller left LNFD widths compared to Asian, Caucasian, and Hispanic cohorts at the level of L1–L2. Asians and Caucasians tended to have larger NFDs of width and area compared to Hispanics and African Americans, similar to findings by Harianja et al. (8). However, these findings were not significant when considered irrespective of vertebral level and did not translate to any significant differences in terms of neuroforaminal stenosis thresholds. Despite the lack of significant findings based on sex, race, and ethnicity, our findings warrant future investigation to further understand the implications of sex and race on radiographic imaging and clinical presentation of congenital neuroforaminal stenosis.

Limitations

Our study suffered from a few important limitations. As aging and spinal pathology can compress the central canal and foramina, we utilized a patient population of asymptomatic individuals from ages 18 to 35 years (38). However, our inclusion criteria excluded symptomatic and asymptomatic patients of ages below 18 and above 35 years. While this was performed to establish baseline parameters, future studies can investigate LNFD of symptomatic patients to allow for the comparison of pathological NFD measurements with the thresholds determined in this study, as well as determine LNFD for growing and adolescent patients to determine proper growth and detect early manifestations of neuroforaminal stenosis. Additionally, our study did not consider the effect of laterality in our interpretation of neuroforamina dimensions, as it was a factor already accounted for in literature (45,46). While we did measure both left and right LNFD, mean differences between the two foraminal areas, widths, and heights were not analyzed for significance.

Another limitation is the absence of patient-reported outcome measures (PROMs) of pain or functional disability, which could provide further insight into the clinical relevance of this study’s established anatomic parameters. While our radiographic analysis was intended to establish a baseline and possibly assist in identifying patients who are at higher risk for developing radiculopathy, without PROMs, it is difficult to correlate our findings with neurological symptoms associated with the identified anatomic parameters. Lastly, the use of CT diverges from other investigations which have purported MRI as the standard for measurement of anatomic measurements (47). However, prior investigations have demonstrated that CT scan measurements are highly correlated with direct anatomic measurements (33,48). Bajwa et al. previously demonstrated that a cohort of 1,066 participants yields a 95% confidence level with a 3% margin of error when estimating proportions for a U.S. population of 300 million (15,22). Given our adequate power and sample size, as well as our thresholds being in line with current literature, the use of CT for evaluation of lumbar neuroforaminal characteristics serves as an appropriate measurement tool.

Conclusions

This study reports measurements of LNFD via CT of 1,000 patients to establish quantitative thresholds for diagnosis of neuroforaminal stenosis. Mean anatomic LNFD measurements, regardless of vertebral level, were: 8.66±2.1 and 8.76±3.14 mm for left and right widths, 17.76±2.74 and 17.7±3.26 mm for left and right heights, and 133.12±34.72 and 133.4±33.86 mm² for left and right areas. Threshold values for neuroforaminal stenosis, regardless of vertebral level, were 4.46 and 2.48 mm for left and right foraminal widths, of 12.28 and 11.18 mm for left and right foraminal heights, and 63.68 and 65.68 mm² for left and right foraminal areas. While patient sex, race, and ethnicity highlighted some significant findings per vertebral level, when considered irrespective of vertebral level, patient characteristics did not translate to any significant differences in terms of neuroforaminal stenosis thresholds. These findings may help solidify anatomic thresholds of LNFD and foraminal stenosis via CT imaging and establish the foundation for future research on the diagnosis of spinal stenosis.

Acknowledgments

None.

Footnote

Reporting Checklist: The authors have completed the STROBE reporting checklist. Available at https://jss.amegroups.com/article/view/10.21037/jss-25-85/rc

Data Sharing Statement: Available at https://jss.amegroups.com/article/view/10.21037/jss-25-85/dss

Peer Review File: Available at https://jss.amegroups.com/article/view/10.21037/jss-25-85/prf

Funding: None

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://jss.amegroups.com/article/view/10.21037/jss-25-85/coif). The authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. This study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. Study approval was received from Loma Linda University Institutional Review Board (IRB No. 5240121) and informed consent was waived due to the nature of this retrospective chart review study.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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