A single-center, retrospective study was conducted at the Spine Center of Changzheng Hospital in Shanghai, China, with a focus on patients diagnosed with CESR due to LDH. The inclusion criteria comprised patients who were hospitalized from February 2017 to March 2022, aged 18–75 years, and diagnosed with CESR due to LDH. In addition, eligible patients had complete medical records, including X-ray and MRI data, and were candidates for surgical intervention. Exclusion criteria excluded patients with CESR caused by other etiologies (e.g., tumors, trauma, or fractures), a history of previous spinal surgery, insufficient medical data during follow-up, severe comorbidities that could influence surgical outcomes, and those who did not provide informed consent. This study adhered to the principles of the Helsinki Declaration and received approval from the Ethics Committee of Changzheng Hospital.

The sample size for this study was calculated to ensure adequate power to detect a clinically meaningful difference between the PLIF and CLIF groups. Assuming an expected effect size of 10 units and a standard deviation of 15 units, with a significance level (alpha) of 0.05 and a desired power of 0.8, the sample size per group was determined using the following formula, where Zα/2 is the critical value for the chosen alpha level (1.96 for a two-tailed test at 95% confidence), and Zβ is the critical value for the chosen power (0.84 for 80% power):

$$ n = \frac + Z_\beta )^2 *2*\sigma^2 }} $$

Thus, approximately 36 patients were needed per group to achieve 80% power to detect a difference of 10 units with a standard deviation of 15 units at a 5% significance level. This calculation ensures the study is adequately powered to detect meaningful differences between the groups.

All surgical procedures in this study were performed by a lead spine surgeon (JG. S., with 41 years of experience in spine surgery) and a specialized surgical team. PLIF surgery was performed as described in earlier publications [13, 14]. The PLIF procedure typically included the following steps: under general anesthesia, the patient was placed in a prone position. An incision was made in the lower back to expose the affected vertebrae, the intervertebral disc was removed to create space for the bone graft, and the bone graft and/or interbody cages were inserted into the disc space to promote fusion. Finally, pedicle screws and rods were used to stabilize the spine until fusion occurred. The whole procedure of CLIF is illustrated in Fig. 1 and the procedures of CLIF were detailed as follows:

Step 1: Patient positioning and exposure.

The patient is placed in a prone position under general endotracheal anesthesia. The surgical segment (L4/L5) is determined using intraoperative positioning with a C-arm machine (Fig. 1A). A midline incision is made in the lower back to expose the spinous processes, laminae, and facet joints of the L4–L5 vertebrae.

Step 2: Pedicle screw placement.

Bilateral pedicle screws are inserted into the L4 and L5 vertebrae using fluoroscopic guidance. The screws are placed in the optimal trajectory to ensure adequate purchase and minimize the risk of nerve root injury. There are potential variations: different screw sizes and designs can be used based on patient anatomy and surgeon preference; alternatively, a unilateral pedicle screw approach can be employed for less extensive decompression.

Step 3: Decompression.

The upper articular process of L5 and the lower articular process of L4 are partially excised, along with the ligamentum flavum, to expose the lateral recess and the connection between the dural sac and the L5 nerve root (Fig. 1B). The L4/5 intervertebral disc is carefully removed, and the nerve root canal is decompressed by removing any herniated disc material or bone spurs.

Illustration of the CLIF procedure and measurement of nerve tension. A Surgical segment (L4/L5) was determined; B the pedicle screws were inserted bilaterally in L4–L5 segments, and then the titanium rods were implanted and locked bilaterally. The upper articular process of L5 and the lower articular process of L4 were partially excised; C the nerve tension was measured by a nerve tension surveying instrument and recorded as the nerve tension before intraoperative decompression. The arrow indicates the measuring point; D resect L4/5 intervertebral disc, decompress of nerve root canal sufficiently, and compress the operated segment (L4/L5). Compression of the spine is the most important difference between CLIF and traditional posterior surgery; E implant interbody fusion cage in L4/5; F the nerve tension was measured again in the same location. The arrow indicates the measuring point

Step 4: Nerve tension measurement.

Before decompression, nerve tension is measured using a nerve tension surveying instrument at the point where the dural sac connects to the L5 nerve root on both sides (Fig. 1C).

Step 5: Intervertebral compression and cage placement.

The surgeon applies slow, controlled compression between the intervertebral bodies using a compression device or manual manipulation (Fig. 1D). This step aims to relieve tension on the cauda equina nerves by shortening the spinal segment. There is potential variation: the degree of controlled compression can be adjusted based on intraoperative nerve tension measurements and surgeon experience.

Step 6: Interbody fusion cage implantation.

An interbody fusion cage filled with bone graft material is implanted into the L4/5 disc space (Fig. 1E). The cage is positioned to maintain disc height and restore spinal alignment. There are potential variations: different types of interbody cages (e.g., PEEK, titanium) and graft materials (e.g., autograft, allograft, synthetic) can be used based on surgeon preference and patient factors.,Footnote 3Footnote 4

Step 7: Post-decompression nerve tension measurement.

After decompression and cage placement, nerve tension is measured again at the same points as before (Fig. 1F) to assess the effectiveness of the procedure.

Step 8: Closure and postoperative care.

The wound is closed in layers, and the patient is monitored for postoperative complications. Patients are advised to wear a lumbar brace for 3 month post-surgery to support the spine during fusion.

The innovation of CLIF lay in its comprehensive approach to addressing both degeneration-associated compression and nerve tension, potentially leading to improved outcomes for patients with CESR due to LDH.

The Oswestry disability index (ODI) is a widely validated and commonly used instrument for assessing functional disability in patients with pain. It covers various aspects of daily living, including pain intensity, personal care, lifting, walking, sitting, standing, sleeping, social life, and traveling. This comprehensive assessment allows us to evaluate the overall impact of the surgical procedures on patients’ quality of life and functional status. The recovery of neurological function was assessed by ODI score [17] and its recovery rate (RR of ODI: (ODI before operation − ODI at the follow-up)/(ODI before operation) × 100%). The ODI is a widely used questionnaire designed to measure a patient’s permanent functional disability. It consists of 10 sections, each with 6 statements. The sections include pain intensity, personal care, lifting, walking, sitting, standing, sleeping, social life, and traveling. Each section is scored from 0 to 5, with 0 indicating no disability and 5 indicating maximum disability.

The pain symptoms were assessed by visual analogue scale (VAS) [18]. The VAS is a simple and reliable tool for measuring the intensity of pain. Since pain is a significant symptom in CESR, the VAS provides a quantitative measure to assess the effectiveness of the surgical procedures in alleviating pain. The VAS is a tool used to measure the intensity of pain. It is typically a horizontal line, 10 cm in length, anchored by two descriptors at each end: “no pain” (score of 0) and “worst imaginable pain” (score of 10). Patients mark on the line the point that they feel represents their perception of their current state of pain. The VAS score is determined by measuring the distance in centimeters from the “no pain” end to the mark made by the patient.

The bladder function was assessed by international consultation on incontinence questionnaire short form score (ICI-Q-SF) and urodynamics [19]. The ICI-Q-SF is a brief, self-administered questionnaire used to evaluate the impact of urinary incontinence on quality of life. It includes questions about the frequency, severity, and impact of incontinence, with each item scored from 0 to 4. Higher scores indicate more severe symptoms. This allows us to assess the impact of the surgical procedures on bladder function, a crucial aspect of CESR.

Furthermore, the Rintala score was used to evaluate bowel function, particularly in patients with anorectal malformations or other bowel dysfunctions [20]. The Rintala score is specifically designed to evaluate bowel function in patients with anorectal malformations or other bowel dysfunctions. Given that bowel dysfunction is a common symptom in CESR, the Rintala score allows us to assess the impact of the surgical procedures on bowel function recovery. It includes parameters such as stool frequency, constipation, soiling, and the need for enemas. Scores were assigned based on the severity of symptoms, with higher scores indicating better bowel function.

Meanwhile, the intraoperative nerve tension, preoperative symptom duration, operation duration, intraoperative blood loss, operation related complications were recorded.

The main parameters of urodynamics assessment included residual urine, maximum urinary flow rate, bladder compliance (bladder compliance refers to the ratio of changes in bladder volume to changes in pressure, which can affect the ability of urine to enter the bladder and hold back urine), external sphincter coordination (external sphincter coordination is expressed by TL value as follows: compare the EMG amplitude T value before urination with the minimum amplitude L value during urination, and then take the logarithm of the above ratio. The greater the TL value, the better the external sphincter coordination function) [21].

A device designed for nerve tension measurement was employed to quantify the tension [12]. The tension of the cauda equina nerve in the dural sac could not be directly evaluated intraoperatively, as the dural sac was not opened in any of the cases. Consequently, the tension of the cauda equina nerve was indirectly estimated by measuring the tension of the dural sac. Meanwhile, to eliminate bias in case comparisons, we decided to measure the dural sac at the onset of the nerve root of the compressed nerve root in all instances. The nerve tension measurement apparatus was maintained at a consistent angle, and zero-point calibration was performed after the measurement site (the initial interface between the dural sac and the nerve root) was fully exposed. Subsequently, the measuring needle was incrementally inserted into the measurement site and directed toward the midline. The tension reading from the device was recorded when the dural sac exhibited distortion. Each site was measured thrice consecutively, with the average value documented as the pre- and post-decompression nerve tension. Moreover, the nerve roots at all compressed levels on both sides were measured, and the mean value was recorded as the patient’s nerve tension. The rate of change in nerve tension was calculated as follows: (nerve tension before decompression − nerve tension after decompression)/(nerve tension before decompression) × 100%.

The study parameters included the anterior intervertebral height (AIH), posterior intervertebral height (PIH), foramen height (FH), and lumbar lordosis (LL), as determined from X-ray images [22, 23]. AIH refers to the vertical distance measured between the anterior edges of the adjacent vertebrae at the midpoint on a lateral X-ray view, indicating the space available at the front part of the intervertebral disc. PIH is the vertical distance between the posterior edges of the adjacent vertebrae at the midpoint on a lateral X-ray view, reflecting the space available at the back part of the intervertebral disc. FH measures the vertical height of the intervertebral foramen from the upper to the lower margin on a lateral X-ray view, indicating the space available for the exiting nerve roots. LL is defined as the angle formed between a line parallel to the upper edge of the endplate of the L1 vertebral body and a line parallel to the upper edge of the endplate of the S1 vertebral body, measured on a lateral X-ray view to indicate the curvature of the lumbar spine. Figure 2 illustrates these measurements, showing the positions and methods for measuring AIH, PIH, FH, and LL on lateral X-ray images of the lumbar spine. These parameters were chosen to evaluate the structural changes and alignment of the lumbar spine preoperatively and postoperatively, providing insight into the efficacy of the surgical interventions.

Imaging measurements according to the X-ray image: (a) anterior intervertebral height (AIH); (b) posterior intervertebral height (PIH); (c) foramen height (FH); (d) line parallel to the upper edge endplate of the L1 vertebral body; (e) line parallel to the upper edge endplate of the S1 vertebral body. LL was defined as the angle between (d) and (e)

Statistical analysis was conducted using GraphPad Prism 9 (GraphPad Software Inc., La Jolla, CA). Data in this present study were presented as mean (SD). The independent t test was used to detect the statistical differences of demographic parameters (patients’ age, duration of symptoms, duration of follow-up, operation time and intraoperative blood loss), clinical scores (ODI score, VAS score, ICI-Q-SF score, and Rintala score), urodynamics outcomes (residual urine, maximum urinary flow rate, bladder compliance, and TL value), nerve tension, and radiological outcomes (AIH, PIH, FH, and LL) between the two groups. The Mann–Whitney U test was used to detect the statistical differences of RR of ODI and change rate of nerve tension between the two groups. The Fisher’s exact test was used to compare the gender, surgical segments, and comorbidities between the two groups. Spearman’s correlation analysis was performed to assess whether there was any significant relationship between relevant parameters of nerve tension and ODI. The data analysis was conducted blinded to the treatment group assignments. This ensures that the analysis was not influenced by knowledge of which patients received CLIF or PLIF. In addition, the study data were independently verified by a second researcher to ensure accuracy and consistency. Besides, the study used objective outcome measures, such as the ODI, VAS, ICI-Q-SF, Rintala score, and urodynamic parameters, to assess patient outcomes. These measures are widely validated and minimize the potential for subjective bias. Furthermore, the follow-up data were collected through clinical visits and phone calls. The primary outcome measures were assessed at 3, 12, and 24 month post-surgery. This systematic approach to data collection and outcome assessment helps to ensure the reliability of the study outcomes. Values that were less than 0.05 (p < 0.05) were considered statistical significance.