Quantitative anatomical analysis of lumbar interspaces based on 3D CT imaging: optimized segment selection for lumbar puncture in different age groups

We retrospectively enrolled patients aged 10–80 years who underwent lumbar CT scans (0.6 mm slice thickness) at our hospital (Fujian Medical University Union Hospital, China) from January to December 2019. Lumbar CT images containing at least L3 to S1 levels were randomly selected.

Inclusion criteria were age 10–80 years and body mass index (BMI) of 18.5–40 kg/m2 [11, 12]. Exclusion criteria were: 1) ankylosing spondylitis; 2) lumbar scoliosis with Cobb angle ≥ 20° [13]; 3) severe lumbar anatomical variations; 4) congenital spinal defects; 5) history of lumbar surgery.

Patients were categorized into 4 age groups: Group A (10–20 years, n = 20), Group B (21–40 years, n = 20), Group C (41–60 years, n = 20), and Group D (61–80 years, n = 20). CT images were exported in DICOM format for 3D reconstruction.

The study was approved by the Institutional Review Board. Written informed consent was obtained from all participants.

The collected lumbar CT images (in DICOM format) containing at least L3-S1 levels were imported into Mimics Research 20.0 software (Materialise, Belgium). Based on CT grayscale and anatomical knowledge, target structures including vertebral bone, dura mater, nerve roots, and skin were delineated on transverse, sagittal, and coronal planes by adjusting the window width and level to enhance the region of interest (ROI) contrast. 3D reconstruction was performed to create distinct 3D models of the bone (green), dural sac (blue), and L3-S1 nerve roots (orange) (Fig. 1).

The conventional midline approach technique (MAT) for lumbar puncture was simulated using a virtual camera model. The needle insertion point and trajectory were modeled as the camera axis. The visible target area in the simulated camera view represented the permissible range for needle planning, where a larger area indicates greater error tolerance.

The area of the lumbar dural sac (ALDS) within the interlaminar space represents the puncture target. Camera angles were screened in the mid-sagittal plane to maximize ALDS, implying higher error tolerance and success rate. Images were captured at the optimal angle for further analysis (Fig. 2). This process was repeated for the L3-4, L4-5, and L5-S1 segments.

The captured images of the L3-4, L4-5, and L5-S1 segments at the optimized simulated puncture angles were imported into Image-Pro Plus 6.0 software (IPP, Media Cybernetics, USA). The following parameters were measured on each image: effective area of the lumbar dural sac (ALDS), area of the traversing nerve root (ATNR), and area of the lumbar interlaminar space (ALILS) (Figs. 3 and 4).

The puncture efficacy ratio of the interlaminar window for each segment was calculated as Puncture efficacy ratio (%) = (ALDS/ALILS) × 100%. Where ALDS is the effective area of the lumbar dural sac, and ALILS is the area of the lumbar interlaminar window.

The nerve injury risk ratio for each segment was calculated as Nerve injury risk ratio (%) = (ATNR/ALILS) × 100%. Where ATNR is the area of the traversing nerve root, and ALILS is the area of the lumbar interlaminar window.

ALDS, ALILS, ATNR, puncture efficacy ratio (ALDS/ALILS), and nerve injury risk ratio (ATNR/ALILS) were compared among the L3-4, L4-5, and L5-S1 segments. Subgroup analysis was also conducted across the four age groups.

The study included 80 patients, with 20 cases in each age group: 10–20 years, 21–40 years, 41–60 years, and 61–80 years. There was no significant difference in gender distribution among the groups (P > 0.05) (Table 1).

In the overall age group, no significant differences in ALDS were observed among L3-4, L4-5, and L5-S1 segments (P > 0.05). However, in the 61–80 year age group, the ALDS of L5-S1 (69.4 ± 37.8 mm²) was markedly larger than L3-4 (43.6 ± 24.3 mm²) and L4-5 (41.6 ± 19.1 mm²) (P < 0.05).

A decreasing ALDS trend was noted after age 40 across segments. Within each segment, ALDS remained relatively stable between 10–40 years, followed by a gradual decrease. The decrease in ALDS occurred later and was smaller in magnitude at L5-S1 versus L3-4 and L4-5. In the overall study cohort, no significant ALDS differences existed among segments across the age range (P > 0.05) (Table 2 and Fig. 5).

Across all age groups, the ATNR of L5-S1 markedly exceeded L3-4 and L4-5 (P < 0.05). In the overall group and group B, L4-5 was greater than L3-4 (P < 0.05). Comparisons within subgroups showed a significantly larger L4-5 ATNR versus L3-4 only in group B (21–40 years) (P < 0.05), indicating distinct variance during this period.

The ATNR of L5-S1 markedly exceeded L3-4 and L4-5 in all age subgroups (P < 0.05). A statistically insignificant decreasing ATNR trend after age 40 was observed. No significant ATNR differences existed among segments within each age group (P > 0.05), implying minimal changes with aging.

The ATNR increased progressively from L3-4 to L5-S1, indicating the highest risks of nerve root injuries during L5-S1 puncture. In the overall group, the ATNR of L5-S1 significantly exceeded L3-4 and L4-5 (P < 0.05), and L4-5 was greater than L3-4 (P < 0.05) (Table 2 and Fig. 5).

Across all age groups and overall, the ALILS of L5-S1 markedly exceeded L3-4 and L4-5 (P < 0.05). No significant differences existed between L3-4 and L4-5 (P > 0.05).

In all age groups and overall, the ALILS of L5-S1 significantly exceeded L3-4 and L4-5 (P < 0.05) (Table 2 and Fig. 5).

The efficacy ratios remained relatively stable across aging. No significant differences existed among segments within each age group (P > 0.05) (Table 2 and Fig. 5).

However, the efficacy ratio showed a decreasing trend from L3-4 to L5-S1, indicating that given successful insertion into the interlaminar space, the probability of effective dural puncture was highest at L3-4, followed by L4-5 and lowest at L5-S1.

In group D (ages 61–80 years), L5-S1 exceeded L3-4 and L4-5 (P < 0.05), while L3-4 and L4-5 were comparable (P > 0.05). No significant differences existed among segments within each age group (P > 0.05) (Table 2 and Fig. 5).

However, the risk ratio showed an increasing trend from L3-4 to L5-S1, signifying the highest injury probability at L5-S1, followed by L4-5, and lowest at L3-4 given insertion into the interlaminar space.

Our results showed no significant ALDS differences among the L3-4, L4-5, and L5-S1 puncture segments across all age groups (P > 0.05), indicating comparable dural exposure. Analyses of subgroups aged 10–20 years, 21–40 years, and 41–60 years also showed no significant ALDS distinctions among the segments (P > 0.05), suggesting maintained similarities until age 60.

However, in the 61–80 year cohort, the L5-S1 ALDS (69.4 ± 37.8 mm²) markedly exceeded L3-4 (43.6 ± 24.3 mm²) and L4-5 (41.6 ± 19.1 mm²) (P < 0.05), implying superior L5-S1 outcomes in the elderly.

The ALDS remained stable from ages 10–40 years before declining, with the smallest decrease at L5-S1. Thus, L5-S1 may provide an alternative when traditional upper lumbar access becomes difficult for the elderly.

These findings align with age-related degeneration. Disc height loss and stenosis from osteophytes and thickened ligaments can compress the dural sac [17‐19]. The later and smaller ALDS decline at L5-S1 relates to its larger native window better accommodating changes before severe narrowing [20].

In summary, L5-S1 may be an alternative when traditional upper lumbar puncture sites become challenging in older patients, explaining potentially improved outcomes. Our quantitative data provides insights into the anatomical basis underlying these clinical observations.

Our results showed significant ATNR differences among the L3-4, L4-5, and L5-S1 puncture segments across all age groups (P < 0.05). The L5-S1 ATNR was 34.0 ± 27.8 mm², markedly exceeding 9.0 ± 14.0 mm² at L4-5. Meanwhile, the L4-5 ATNR also exceeded 1.1 ± 3.1 mm² at L3-4. Similar trends were seen in subgroups A, B, and C.

In the older cohort D, the L5-S1 ATNR remained significantly higher at 25.9 ± 22.2 mm² versus 5.2 ± 7.6 mm² at L4-5 (P < 0.05). The L4-5 and L3-4 difference of 0.6 ± 1.3 mm² was insignificant (P > 0.05).

The larger lower lumbar ATNRs may relate to the inferior nerve root exit points from the dural sac, coupled with their wider diameters and smaller traversing angles, predisposing the S1 roots to injury during puncture [21].

The greater ATNRs in lower segments can be attributed to the acute angles and inferior origins of the L5 and S1 nerve roots. Studies show these roots emanate lower and have more oblique courses compared to upper roots [22]. The S1 roots demonstrate particular vulnerability at L5-S1, heightening damage risks [21].

Our quantitative data provides an anatomical basis for the substantially higher incidence of S1 versus other lumbar radiculopathies. The findings underscore the critical importance of extreme caution and optimal trajectory planning during L5-S1 puncture.

The interlaminar space area (ALILS) is clinically important, as the osseous structures outside the ALILS provide crucial tactile feedback during lumbar puncture, enabling adjustment of the needle trajectory to enter the ALILS.

Our results showed the L5-S1 ALILS significantly exceeded L3-4 and L4-5 across all age groups (P < 0.05), indicating L5-S1 allows the easiest canal access. Overall, the ALILS was 197.6 ± 90.3 mm² at L5-S1, 113.8 ± 65.3 mm² at L4-5, and 93.4 ± 51.0 mm² at L3-4, with L5-S1 exceeding L3-4 two-fold.

From ages 10–40 years, the ALILS remained relatively constant. After age 40, the ALILS declined across segments [20]. The consistently larger L5-S1 window was maintained at a greater size for longer, enabling its potential use for complex punctures. The greater L5-S1 ALILS may relate to the lumbosacral junction anatomy and biomechanics [23], with evolutionary enlargement to accommodate spinal-pelvic forces [20]. Our findings align with degeneration primarily affecting L3-4 and L4-5 [24]. Biomechanical stresses likely contribute to earlier upper-level stenosis [25].

The innately wider L5-S1 window better accommodates age-related changes before severe compression [26], allowing its potential use for difficult access when conventional sites narrow [7]. Our quantitative ALILS data provides anatomical support for this technique.

In clinical practice, we frequently encounter scenarios where the needle passes into the epidural space through the interlaminar window (ALILS), yet fails to penetrate the dura to access the subarachnoid space (ALDS). This nuance is often overlooked, and our quantitative analysis of ALILS and ALDS provides evidence-based guidance for optimal puncture segment selection to avoid such dilemmas. For instance, our data showed preferentially selecting L3-4 maximizes the chances of entering ALDS after penetrating ALILS, as it had the highest efficacy ratio. We also summarized the varying characteristics of ALILS and ALDS across lumbar segments, enabling clinicians to choose appropriate intervertebral spaces to increase overall puncture success. When facing ALDS access failure after entering ALILS, we can advise modifications based on our segmental measurements. Overall, delineating these anatomical differences quantitatively supports optimized, patient-tailored puncture techniques.

The efficacy ratios remained relatively stable across aging for each level, indicating the highest dural puncture likelihood at L3-4, followed by L4-5, and lowest at L5-S1 given successful interlaminar insertion [27].

When accessible, L3-4 and L4-5 should be preferred over L5-S1, as their bone provides superior tactile feedback during needle advancement. L5-S1 risks neural complications like hematoma, nerve injury, and disc damage [28].

Despite being statistically insignificant, the declining efficacy ratio order from groups B to D suggests age-related dural compression [29]. Studies also report higher L4-5 iatrogenic disc injury risks [30], potentially causing degeneration and herniation [31].

Ultimately, individual factors determine optimal segment selection, such as disc herniation or stenosis may necessitate alternate levels, unilateral radiculopathy favors a contralateral approach.

The stable efficacy ratios from ages 10–40 years coincide with minimal degenerative changes during this period [32]. The subsequent declining trend after middle age aligns with reduced disc height, hypertrophic facets/laminae, and buckled ligaments [33, 34], preferentially narrowing the dural space [35].

Our data advocates preferential L3-4 and L4-5 puncture when feasible, as L5-S1 risks neural injury and requires subarachnoid space expertise [7]. However, L5-S1 should be considered as an alternative if upper lumbar stenosis necessitates more caudal access [11, 36].

While transient radicular symptoms during lumbar puncture may not be uncommon (13%) [37], more sustained nerve injury appears to be rare [38]. Our results demonstrated significant differences in the nerve injury risk ratios (ATNR/ALILS) among the lumbar segments across all age groups (P < 0.05). The highest ratio was at L5-S1 (17.6 ± 13.8%), followed by L4-5 (7.7 ± 9.9%), and lowest at L3-4 (1.2 ± 3.7%). Similar trends were seen in subgroups A, B, and C.

Currently, most LPs are performed at the L3-4 and L4-5 segments, with less frequent use of L5-S1. The lower inherent exposure of traversing nerve roots at L3-4 and L4-5 results in lower injury risks. However, L5-S1 puncture increases proportional nerve root exposure due to the larger interlaminar space, elevating injury risks – a key point of this study.

Furthermore, the effective dural puncture area (ALDS) decreased after age 40 and was markedly reduced at L3-4 and L4-5 in group D patients (ages 61–80 years). In such cases, L5-S1 may need to be used despite lower efficacy and higher injury risks, concerning for clinicians.

In response to calculating injury risks, while post-LP nerve injury is rare, our aim was to provide an additional metric for potential risks. This can help guide decisions between segments, particularly in borderline scenarios.

In the older cohort D, the L5-S1 risk ratio remained markedly higher at 16.6 ± 14.4% versus 7.6 ± 9.9% at L4-5 (P < 0.05), while there was no significant difference between L4-5 and L3-4 (1.9 ± 6.2%, P > 0.05).

The greater lower lumbar risk ratios may relate to the differing nerve root exit points, diameters, and angles originating from these segments [39], heightening S1 injury risks during attempted L5-S1 puncture.

The rising L3-4 to L5-S1 risk ratios parallel the progressively larger and more horizontal transverse nerve roots at the lower levels [40]. The wide diameters and tethering of S1 roots within the recesses predispose them to damage. Rampersaud et al. showed the S1 roots occupied over half the L5-S1 epidural space based on cadaver measurements [41].

Although the risk ratios were relatively stable across aging, caution is still warranted for caudal punctures. Precise alignment and controlled needle advancement are critical to prevent irreparable neural damage [8]. Ultrasonography can also improve difficult L5-S1 access safety [7].

This study has several limitations.

First, CT imaging was performed in supine rather than flexion position typical for lumbar puncture. Though supine imaging follows standard protocols, flexion positioning would better approximate puncture anatomy. Paired LP and CT imaging in all subjects was not feasible presently due to ethical constraints. Further studies with paired imaging could help validate our methodology.

Second, our cohort was restricted to participants meeting strict criteria. Further studies should evaluate more diverse populations, expanded age ranges, and high-risk groups like obesity and scoliosis to enhance generalizability.

Third, we performed computational measurements without clinical correlation. Future research could integrate anatomical assessments with lumbar puncture outcomes data to validate which parameters predict procedural success.

In this analysis, we isolated the effect of needle trajectory angle by examining the efficacy ratio at the optimal puncture angle, without considering the needle entry point margin of error and skin-dura distance. This is a limitation, as incorporating more variables would enable a more comprehensive model. In future studies, will incorporate entry point precision and skin-to-dura distances to provide a robust simulation of real-world puncture scenarios. This phased approach will enhance the clinical applicability of our modeling methodology.

Despite limitations, our study establishes a novel quantitative foundation for informing age-specific, patient-tailored lumbar puncture techniques balancing safety and efficacy. Advances in digital spine modeling and analytics will continue enhancing evidence-based procedural guidance.