Abstract
Study design
A retrospective case-control design.
Objective
This study proposed a novel MRI finding termed the “Flying owl sign” to characterize a specific type of lumbar disc herniation (LDH) and examined the correlation between this image finding and the patient-reported outcomes (PROs) following full-endoscopic lumbar discectomy.
Methods
A retrospective case-control study was conducted involving LDH patients who underwent full-endoscopic lumbar discectomy in our institution. The propensity score matching method was employed to categorize patients into a “Flying owl sign” group and a control group. Multivariable logistic regression analysis was performed to evaluate the relationship between the “Flying owl sign” and the PROs.
Results
A total of 431 patients who underwent full-endoscopic lumbar discectomy between November 2015 and October 2022 in our institution were enrolled. After a mean follow-up of 32.24 ± 8.60 months, 377(87.47%) patients were satisfied (NASS score of 1 or 2). 10.21% (44/431) patients exhibited “Flying owl sign”, and experienced significantly greater improvements in VAS(P = 0.022), higher patient satisfaction index (P = 0.016), and a lower incidence of residual low back pain (P = 0.013) than the control group. Propensity score matching produced a final study cohort of 88 patients. The multivariate analysis revealed that “Flying owl sign” was a protective factor for satisfaction (OR = 3.544, P = 0.018).
Conclusion
The presence of “Flying owl sign” reflected a specific LDH type, which was the combination of giant disc herniation and the integrity of the posterior longitudinal ligament. “Flying owl sign” correlated with improved postoperative efficacy and satisfaction in patients with LDH, and may contribute to reducing residual low back pain rates.
Keywords
Introduction
Lumbar disc herniation (LDH) is a prevalent spinal disorder characterized by the herniation of the nucleus pulposus of the lumbar intervertebral disc, which compresses the nerve roots or spinal cord, leading to low back pain and radiating pain in the lower limbs. Endoscopic spine surgery has become an important treatment for LDH due to its advantages of less trauma and faster recovery. 1 In recent years, advancements in endoscopic technology have led to growing attention towards applying fully visible spinal endoscopy in the clinical management of LDH. However, the residual back pain and leg numbness, the recurrence of the LDH affected the satisfaction of full-endoscopic lumbar discectomy.2,3
For endoscopic spinal surgery, preoperative imaging evaluation plays a crucial role in formulating surgical plans and assessing prognosis. Magnetic resonance imaging (MRI), a key method for evaluating LDH, provides detailed information regarding the degree, location, and relationship between disc herniation and surrounding structures. The effects of radiological factors, such as adjacent-level disc degeneration, disc height index, sagittal range of motion, facet orientation, and facet tropism in full-endoscopic lumbar discectomy have been analyzed. 4
Modic change, Pfirrmann grade, annulus fibrous calcification, and fat inflation of paravertebral muscle have been found to be risk factors for LDH recurrence and low back pain 5-8. However, few studies focus on the effect of integrity and morphology of the posterior longitudinal ligament (PLL). Lee found that a concomitant PLL tear during initial LDH was significantly related to subsequent reherniation, which suggested that an intact PLL is a protective factor of full-endoscopic lumbar discectomy for LDH. 9
In clinical practice, we have observed a distinctive preoperative MRI imaging feature in which the herniated intervertebral disc elevates the posterior longitudinal ligament at both ends, creating an owl-like appearance in the sagittal plane, which we have termed the “Flying owl sign”. This sign reflects the intact PLL and giant size of the herniated nucleus pulposus. Thus, we conducted this study to investigate the relationship between the “Flying owl sign” and patient satisfaction, of individuals with lumbar disc herniation who have undergone spinal endoscopic surgery, to identify the clinical outcome of this specific type of LDH. This finding may offer valuable clinical insights, optimize surgical planning, and improve treatment outcomes.
Methods
Study Design
This study employed a retrospective case-control design. Patients who underwent full-endoscopic lumbar discectomy at our hospital between November 2015 and October 2022 were retrospectively analyzed. All procedures were performed by a single surgical team, and the study received approval from the ethics committee of our hospital (IRB approval number: 2023KYLL055, 960th Hospital of PLA, Jinan, China).
Inclusion and Exclusion Criteria
Inclusion criteria: (1) patients diagnosed with LDH below L3/4; (2) sciatica with or without low back pain and failure to conservative treatments for over 6 weeks; (3) patients who underwent full-endoscopic lumbar discectomy; (4) patients with complete preoperative MRI imaging data suitable for evaluating the “Flying owl sign”; (5) patients with complete information and had a minimum 2-year follow up.
Exclusion criteria: (1) patients with a history of spinal surgery, infections, tumors, or other spinal structural lesions; (2) presence of 2 or more levels of disc herniation; (3) patients with severe systemic diseases that could impact postoperative recovery; (3) patients with incomplete follow-up data that precluded postoperative efficacy evaluation.
Surgical Procedures
Both procedures were performed under local anesthesia with the patient placed in a prone position. The L5/S1 intervertebral space was localized under fluoroscopic guidance.
TELD
The entry point was selected 8-12 cm lateral to the spinous process, based on the patient’s body habitus and preoperative imaging, while ensuring avoidance of the iliac crest. The puncture trajectory was oriented as horizontally as possible. Under continuous fluoroscopic monitoring, an 18-gauge needle was advanced into the intervertebral foramen, followed by insertion of a guidewire. Sequential dilation was then performed to establish the working channel, through which the endoscope was introduced after confirmation of its position. Foraminoplasty was carried out to obtain sufficient operative space. The herniated nucleus pulposus and associated nerve roots were clearly visualized and subsequently excised. Adequate decompression was confirmed endoscopically by observing unrestricted neural mobility and the absence of residual compression under irrigation.
IELD
The entry point was located approximately 1-2 cm lateral to the midline of the spinous process at the L5/S1 level. An 18-gauge needle was advanced through the ligamentum flavum into the epidural space under fluoroscopic guidance. Once correct placement was confirmed, 5-8 ml of a mixed solution of ropivacaine (0.15%) and lidocaine (0.67%) was injected into the epidural space. As in the TELD procedure, a working channel was subsequently established and the endoscope was introduced. Under direct endoscopic visualization, part of the ligamentum flavum was excised to expose the herniated disc, thecal sac, and nerve root. The nerve root was carefully medialized and protected within the working channel, after which discectomy was performed using grasping forceps to achieve adequate decompression.
Postoperatively, patients were encouraged to ambulate within 4 to 12 h, and the use of a lumbar brace was recommended for a duration of 3 weeks.
Data Collection
Data collection encompassed demographic characteristics, medical history, preoperative imaging data, surgical data, and postoperative follow-up information. Demographic characteristics included gender, age, height, weight, body mass index (BMI), cigarette smoking, alcohol consumption, and history of diabetes. In terms of clinical manifestation, the main clinical complaints were categorized into 3 types, including low back pain, radiculopathy, and low back pain combined with radiculopathy. The symptom duration was categorized into an acute subgroup (less than 3 months), a subacute subgroup (greater than 3 months and less than 1 year), and a chronic subgroup (greater than 1 year). Preoperative radiological images included standard anterior-posterior and lateral fluoroscopy, CT, and MRI of the lumbar spine. Modic sign, annulus fibrous calcification, and lesion site in the sagittal line and axial line were recorded. Clinical outcomes were assessed preoperatively and at final follow-up by Visual Analog Scales (VAS), the minimum clinically important difference (MCID) for VAS was established at 2 points. Patient satisfaction was evaluated using the PSI proposed by the North American Spine Society (NASS), with a score of 1 (“the treatment met my expectations”) or 2 (“I did not improve as much as I had hoped, but I would undergo the same treatment for the same outcome”) defined as satisfactory and a score of 3 (“I did not improve as much as I had hoped, and I would not undergo the same treatment for the same outcome”) or 4 (“I am the same or worse than before treatment”) was defined as dissatisfactory. 10 Residual symptoms were characterized by low back pain (LBP) or leg numbness observed at a minimum 2-year follow-up. Patients with an LBP-VAS score of ≥2 were considered to have postoperative residual LBP. 11 The presence of residual leg numbness was defined as a Japanese Orthopedic Association (JOA) score of 1, according to the JOA scoring system. 12 Recurrence was defined as recurrent lumbar disc herniation (LDH) that necessitated either a discectomy or fusion procedure. Patients were interviewed by telephone or face-to-face at out-patient during the 2-5 years postoperative period.
MRI Evaluation
The herniated intervertebral disc elevates the posterior longitudinal ligament at both ends, creating an imaging appearance reminiscent of a flying owl in the sagittal plane, which we have termed the “Flying owl sign” (Figure 1). One experienced radiologist and one senior surgeon independently assessed the preoperative MRI images to determine the presence or absence of the “Flying owl sign”, and a third senior surgeon would confirm if disagreement existed. The Preoperative MRI of a 46-Year-old Female. A-C was the Sagittal T1-Weighted, T2-Weighted, and T2 Fat-Suppressed Images, D was the Axial T2-Weighted Image, E was the Schematic Image of the “Flying Owl Sign” in the T2 Fat-Suppressed Image, the Herniated Nucleus Pulposus was the Head of an Owl and the Posterior Longitudinal Ligament was the Wings, Similar With F
Propensity Score Matching (PSM)
The restricted calculation of the propensity score involves constructing a binary logistic regression model, where the presence or absence of the “Flying owl sign” serves as the dependent variable. Interference factors, such as age, gender, BMI were treated as independent variables to compute the propensity score for an individual. Subsequently, matching was performed, aligning samples from the treatment group and the control group based on the calculated propensity score. We will employ the 1:1 nearest neighbor matching method and consider utilizing a caliper value of 0.1 to restrict the differences in matching propensity scores, thereby enhancing the quality of the matches. Once matching is completed, we will analyze the matching effect, which includes examining the basic characteristics of the PSM, assessing changes in standardized difference post-matching.
Statistical Analysis
For multivariable analysis, logistic regression was applied to assess the independent correlation between the “Flying owl sign” and patient satisfaction, recurrence rate and residual symptoms. IBM SPSS Statistics Version 27 was used to perform all descriptive and comparative statistics. Continuous variables were described as means and standard deviations for normally distributed data. Categorical variables were represented as absolute numbers and component proportions. In this study, the primary outcome variable was patient satisfaction, which was measured on a rank variable (1 = very satisfied, 2 = satisfied, 3 = average, 4 = dissatisfied). To evaluate the impact of various indicators on the outcome variable, ANOVA was employed for continuous variables, while the Wilcoxon Rank Sum test or Kruskal-Wallis H test was utilized for categorical variables. Ordered logistic regression analysis was performed to identify independent variables for patient satisfaction. The covariates with values of P < 0.25 in univariate analysis were entered in the multivariate regression model. The c statistic was calculated for the regression model to assess its discriminative ability. Additionally, the standardized difference in covariates between the treatment and control group was compared before and after matching to determine whether it was significantly reduced, followed by a balance test to evaluate the quality of the matching process.
Results
Patient Selection Process
A total of 431 patients with full-endoscopic lumbar discectomy in our department were included in the current study. Among these included patients, 117 (27.15%) patients experienced residual LBP, and 129 (29.93%) patients experienced residual leg numbness at the last follow-up. Three hundred and seventy-seven patients were satisfied (NASS score of 1 or (2) and 54 patients were dissatisfied (NASS score of 3 or 4), resulting in a patient satisfaction rate of 87.47%. 44 patients exhibited “Flying owl sign”, accounting for 10.21% of all patients.
Baseline Data
Baseline Characteristics and Patient-Reported Satisfaction of Enrolled Patients
Patients with “Flying owl sign” exhibited a higher incidence of patient satisfaction (95.5% vs 86.6%, P = 0.016), a lower incidence of residual low back pain (13.6% vs 31.8%, P = 0.013) and a higher MCID (8 vs 7, P = 0.022), and a tendency of lower recurrence rate (2.3% vs 7.8%, P = 0.305) and lower incidence of residual leg numbness (15.9% vs 28.4%, P = 0.077).
Multivariate Analysis of Prognostic Factors of Patient Satisfaction
PSM produced a final study cohort of 88 patients (Figure 2). The “Flying owl sign” group included 44 patients (38.23 ± 11.91 years; 21 male [47.7%], 23 female [52.2%]; BMI: 24.73 ± 3.75), and the control group included 44 patients (38.18 ± 11.60 years; 23 male [52.2%], 21 female [47.7%]; BMI: 24.37 ± 3.89). Table 3 presents the distribution of propensity scores before and after matching. Table 4 presents the baseline characteristics of enrolled patients after matching. Flowchart of Patient Eligibility and Enrollment The Distribution of Propensity Scores Before and After Matching Baseline Characteristics and Patient-Reported Satisfaction of Enrolled Patients After Matching
Propensity-based matching produced 44 matched pairs with standardized differences in patient characteristics of less than 0.1, indicating that the balance among the variables was satisfactory.
Multivariate Analysis of Prognostic Factors of Patient Satisfaction After Matiching
Discussion
In this retrospective clinical study, we reviewed real-world data and proposed a sign that can be a predictor of good clinical outcomes of full-endoscopic lumbar discectomy for LDH. LDH patients with the “Flying owl sign” exhibited a better PROs (P = 0.016), a higher MCID (P = 0.022) and a lower incidence of residual low back pain (P = 0.013) than those without. As the most meaningful examination for LDH diagnosis, there have been many studies on the imaging characteristics of MRI, which focused anatomical structures of the lumbar vertebral body, intervertebral discs, articular processes, endplates, and paravertebral muscles, and proposed Modic typing of the endplates, the Pirfmann type of the degenerated intervertebral discs, and fat infiltration of paravertebral muscles.
Few studies focused on the integrity and morphology of the posterior longitudinal ligament (PLL), an intact PLL might help to reduce the risk of postoperative disc herniation recurrence, thereby alleviating the patient’s clinical symptoms. Studies have demonstrated that PLL integrity is associated with postoperative pain relief and neurological recovery. Ahn et al 13 emphasized that the integrity of the PLL correlated with improved clinical prognosis. Kawaguchi et al 14 found that the integrity of the PLL was associated with the resorption of intervertebral disc tissue. Their research suggested that an endplate and intact PLL might facilitate the resorption of postoperative residual intervertebral disc material, thereby reducing postoperative discomfort and residual symptoms. Lee found that a concomitant PLL tear during initial LDH was significantly related to subsequent recurrence, which suggested that an intact PLL is a protective factor of full-endoscopic lumbar discectomy for LDH. 9 In this study, we found a characteristic sign of intact PLL, in T2 fat-suppressed MRI, a herniated intervertebral disc elevated the posterior longitudinal ligament at both ends, creating an owl-like appearance in the sagittal plane, which we have termed the “Flying owl sign”. The intact PLL restricted the compression of the disc to the posterior side and protects the nerve roots within the dural sac.
The integrity of PLL might be also crucial for preventing the recurrence of LDH. Zhang et al demonstrated that PLL integrity is linked to a lower recurrence rate, and recommended retaining the PLL. 15 As a stabilizing structure of the spine, any compromise to the integrity of the PLL may result in the displacement and herniation of intervertebral disc material, thereby increasing the risk of recurrence and low back pain. Conversely, an intact PLL can enhance the stability of the remaining intervertebral disc, thereby reducing the low back pain. Consistent with this mechanism, the patients with “Flying owl sign” exhibited a significant lower incidence of residual low back pain than those without (P = 0.013).
In the present study, we found that this sign was usually found in central disc herniation (36.36%), followed by left paracentral (31.82%) and right paracentral (31.82%), which is consistent with the anatomy of the PLL, which is thicker in the center and thinner at the sides. 16 Central LDH accounts for 33.8% of all types of LDH. 17 Previous studies have indicated that the clinical outcomes for central disc herniation are less favorable compared to other types of traditional fenestration surgery. 18 Yan found that leg numbness symptoms last longer in central disc herniation patients than in paracentral and foraminal disc herniation patients after surgery. 19 Choi found that 95 (33.6%) of the 283 patients with incomplete removal of the herniated disc had improperly positioned working access, 91 of which (32.2%) occurred in the central herniated discs in a study which followed 10228 patients with LDH that underwent full-endoscopic lumbar discectomy. 3 However, the PRO of central LDH patients with “Flying owl sign” was inconsistent with previous studies, only one patient experienced recurrence, and the recurrence rate (2.3%) was lower than control (7.8%) with no significant difference (P = 0.305).
The PLL is anatomically situated at the center of the anterior wall of the spinal canal. Due to the integrity of the posterior longitudinal ligament, the prolapsed nucleus pulposus tissue elevates the posterior longitudinal ligament at both the cranial and caudal ends, thereby creating a distinctive “Flying owl sign” on the sagittal plane of MRI. In cases with central LDH, the protruded disc is often more significantly obstructed by the dura mater and nerve roots. However, full-endoscopic lumbar discectomy can be performed through a lateral approach, which offers a different view and technique for dealing with disc tissue and may lead to similar or even better clinical outcomes. 20 Our technical tips are as follows: the puncture target for central disc herniation was close to the posterior midline. The key of this process is the full expansion and formation of the intervertebral foramen to provide enough space to remove the prominent nucleus pulposus.
We also found that this sign was common in giant disc herniation, which exceeded 50% of the sagittal diameter of the spinal canal. 20 The incidence of giant LDH varies from 8 to 22%.20-22 As the large number of inflammatory mediators leak from the interior of the disc to irritate the nerve roots, a large LDH often leads to more severe low back pain and is more likely to have bilateral neurologic symptoms, as well as lower-extremity weakness. After the removal of the herniated nucleus, the patient experienced a higher satisfaction. Thus, the patient with this sign experienced a lower incidence of residual low back pain and a higher MCID. This was consistent with previous studies, which confirmed that smaller-sized herniated discs are linked to early recurrences and the recurrence rate is lower for giant LDH (1.95%) compared to smaller herniations.20,23
This study employed a retrospective case-control design and utilized the PSM method to investigate the relationship between PRO and the “Flying owl sign” observed in preoperative MRI images. Before matching, the positive Lasegue sign, the “Flying owl sign” and the right paracentral disc were found to be prognostic factors of patient satisfaction, which was consistent with our previous report. 2 However, after matching, the “Flying owl sign” was found to be the only prognostic factor of patient satisfaction (OR = 3.533, P = 0.018, c statistic = 0.640).
While this study offers valuable insights into the “Flying owl sign”, it is important to acknowledge certain limitations. First, being a retrospective study, it may be subject to selection bias and information bias. Second, as the aim of this study is to analyze the effect of “Flying owl sign” on prognosis, the details related to the surgery, such as surgery time, intraoperative bleeding were not analyzed. Third, since this paper is a real data study, and the data was collected in one institution, the generalizability and external validity of our findings might be weakened. After the presentation of this sign, it is necessary for more centers to study this sign, and to diversify among different ethnic groups. Furthermore, although PRO was influenced by health status, depressed patients reported worse postoperative outcomes, including worse physical function, pain and disability, 24 the mental health data that was not collected for Chinese patients might deny depression and provide inaccurate data due to Chinese traditional culture.
Conclusion
The presence of the “Flying owl sign” in preoperative MRI is useful to predict a good patient-reported outcome, a higher MCID and a lower rate of low back pain. This finding offers clinicians valuable insights for predicting surgical outcomes and developing individualized treatment strategies.
Footnotes
Funding
The authors received no financial support for the research, authorship, and/or publication of this article.
Declaration of Conflicting Interests
The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.