Regression of Lumbar Ossification of the Ligamentum Flavum after Indirect Decompression via Full-endoscopic Trans-Kambin's Triangle Lumbar Interbody Fusion: A 3-year Case Report

Van Tuan NGUYEN; Takayuki KITAHARA; Yuji NAGAO; Takafumi OHSHIMA; Naoto ONO; Saori SOEDA; Makoto TAKEUCHI; Hiroaki MANABE; Masatoshi MORIMOTO; Fumitake TEZUKA; Hiroshi KAGEYAMA; Junzo FUJITANI; Koichi SAIRYO

doi:10.2176/jns-nmc.2025-0401

Abstract

Ossification of the ligamentum flavum is a pathological condition that can cause progressive myelopathy or severe radiculopathy. Standard surgical management involves direct decompression via posterior resection of the ossification. However, there is a substantial risk of complications, including dural tears and iatrogenic instability. We report a rare case of lumbar ossification of the ligamentum flavum with degenerative spondylolisthesis treated solely by full-endoscopic trans-Kambin's triangle lumbar interbody fusion, aiming for indirect decompression and stabilization, without direct resection of the ossification. A 68-year-old woman had progressive low back pain, bilateral lower extremity radiculopathy, and severe neurogenic claudication, limiting ambulation to 100 m. Imaging revealed Meyerding Grade I L4-L5 spondylolisthesis with dynamic instability and significant canal stenosis due to ossification of the ligamentum flavum, measuring 4.06 mm in maximal thickness. She underwent single-level Kambin's triangle lumbar interbody fusion at L4-L5. Postoperatively, rapid improvement was observed (visual analog scale low back pain: 7→1, leg pain: 8→0; Oswestry Disability Index: 52%→10% at 3 years). Follow-up computed tomography/magnetic resonance imaging demonstrated progressive regression of the ossification, from 4.06 mm to 3.2 mm at 1 year and 1.46 mm at 3 years (64.0%). This case suggests that Kambin's triangle lumbar interbody fusion with indirect decompression is an effective surgical option in lumbar ossification of the ligamentum flavum with segmental instability. The remarkable regression of the ossification suggests that spinal stabilization may suppress pathological mechanical stress, shifting bone remodeling toward resorption, and supports a mechanistic hypothesis. Further prospective studies are warranted to validate this pathophysiological mechanism.

Introduction

Ossification of the ligamentum flavum (OLF) is characterized by ectopic bone formation within the ligamentum flavum, a structure primarily composed of elastic and collagen fibers.¹^,²⁾ This pathology leads to spinal canal stenosis, contributing to compressive myelopathy or radiculopathy.¹^-³⁾ While OLF most frequently involves the thoracic spine, lumbar involvement-though less common-can manifest as radicular symptoms and neurogenic claudication.²^,³⁾

The pathogenesis of OLF is multifactorial, including genetic predisposition (predominantly among East Asian populations) and metabolic disturbances.²⁾ Of particular significance, mechanical stress has been implicated as a potent trigger, where abnormal tensile loading and micro-instability activate signaling cascades involving transforming growth factor-β (TGF-β), bone morphogenetic protein-2 (BMP-2), and mechanosensitive transcription factors such as Sox9, Runx2, Msx2, and AP-1.¹^,²^,⁴⁾ Recent findings implicate Piezo1 channels in transducing mechanical strain into intracellular osteogenic signaling.¹⁾

Conventionally, surgical treatment for symptomatic OLF relies on posterior direct decompression, including laminectomy and en bloc resection.²^,⁵⁾ Although effective for neural decompression, this approach carries a significant risk of serious complications, including dural adherence and tears, cerebrospinal fluid leakage, and iatrogenic instability.⁶^-⁸⁾ Managing such complications prolongs surgery and increases neurologic risk.⁶⁾

In recent years, indirect decompression via minimally invasive lumbar interbody fusion-including anterior lumbar interbody fusion (ALIF), oblique lumbar interbody fusion (OLIF), extreme lumbar interbody fusion (XLIF), and Kambin's triangle lumbar interbody fusion (KLIF)-has garnered increasing interest.⁹⁾ By restoring disc and foraminal height through cage insertion, indirect decompression employs ligamentotaxis to enlarge the canal and foraminal dimensions without posterior resection.⁹^,¹⁰⁾ Reports have demonstrated canal area increases of 55%-125% and foraminal height improvement of up to 21%.⁹⁾

Full-endoscopic trans-Kambin's lumbar interbody fusion (FE-KLIF), performed through Kambin's triangle, represents the least invasive lumbar interbody fusion technique, preserving posterior bony structures such as the facet joints.¹¹^,¹²⁾ However, reports of its application in OLF, particularly without direct decompression, remain scarce.

Here, we present a unique case involving lumbar OLF accompanied by degenerative spondylolisthesis. We describe the favorable long-term clinical and radiological outcomes of KLIF, which involves indirect decompression alone, and propose a novel mechanistic hypothesis for postoperative regression of the ossified lesion.

Case Presentation

A 68-year-old woman presented with advanced degenerative lumbar pathology, reporting a 2-year history of progressively worsening low back pain (visual analog scale [VAS]: 7) accompanied by bilateral radicular symptoms. During the 6 months preceding surgery, she developed severe neurogenic claudication, limiting her ambulatory capacity to approximately 100 m. Notably, her symptoms were predominantly provoked by motion and prolonged standing and were minimal or absent at rest. On initial neurological examination, no apparent motor weakness was observed. Preoperative functional assessment indicated severe disability, with an Oswestry Disability Index (ODI) of 52% and high-intensity leg pain (VAS: 8).

Preoperative radiological evaluation demonstrated Meyerding Grade I spondylolisthesis at L4-L5 with segmental instability, alongside marked central canal stenosis caused by extensive OLF, measuring up to 4.06 mm in anteroposterior thickness. On computed tomography (CT), the ossified lesion appeared as a continuous calcified mass arising from the ligamentum flavum and connected to the lamina, with secondary extension toward the bilateral facet joints, consistent with OLF (Figure 1A-D), and no ossified ligamentous lesions were identified at other spinal levels. No additional compressive pathology, such as disc herniation, was identified.

Figure 1

Preoperative CT and MRI findings.

(A) Sagittal CT scan demonstrating Meyerding Grade I L4-L5 degenerative spondylolisthesis accompanied by OLF (black arrow).

(B) Axial CT image showing hypertrophic OLF causing marked narrowing of the spinal canal (black arrows).

(D) Axial T2-weighted MRI showing significant canal compromise due to posterior compressive OLF.

CT: computed tomography; MRI: magnetic resonance imaging; OLF: ossification of the ligamentum flavum

The patient underwent FE-KLIF. A unilateral left-sided trans-Kambin's triangle approach was employed through a 12-15 mm skin incision following 4 percutaneous pedicle screws insertion at L4 and L5, allowing preservation of posterior spinal structures and minimizing paraspinal muscle trauma. Importantly, no direct decompression, such as laminectomy or excision of the ossified ligament, was performed. Neural decompression was achieved exclusively through indirect decompression via ligamentotaxis following the placement of an expandable titanium interbody cage to restore disc height and segmental stability (Figure 2A, B).

Figure 2

Intraoperative fluoroscopic views during full-endoscopic trans-Kambin’s triangle lumbar interbody fusion at L4-L5.

(A) Initial fluoroscopic image showing the pre-restoration disc space prior to cage placement. (B) Post-restoration image demonstrating increased disc height and achievement of indirect decompression following cage insertion.

The postoperative course was favorable, with rapid relief of radicular symptoms and resolution of neurogenic claudication. The patient regained independent ambulation by postoperative day 2. At the 3-year follow-up, clinical outcomes remained excellent, with complete resolution of leg pain (VAS reduced from 8 to 0) and sustained functional recovery (ODI improved from 52% to 10%).

Radiological follow-up confirmed durable structural correction, including a 56.9% increase in disc height (from 7.2 mm to 11.3 mm) and a 53.3% increase in spinal canal area (from 75 mm² to 115 mm²). Remarkably, serial CT/ magnetic resonance imaging (MRI) over the 3-year period demonstrated progressive regression of the ossified OLF mass, with the maximal thickness decreasing by 64.0% (from 4.06 mm to 1.46 mm)-an exceptionally rare phenomenon in lumbar OLF (Figure 3B, D).

Figure 3

Serial postoperative CT images showing marked regression of OLF over time.

(A, B) One-week postoperative sagittal and axial CT views showing residual OLF at the L4-L5 level (black arrows).

(C, D) Three-year follow-up CT demonstrating substantial disappearance of the OLF, with only minimal remnants remaining (black arrows).

CT: computed tomography; OLF: ossification of the ligamentum flavum

Discussion

Effectiveness of indirect decompression in OLF with segmental instability

The primary objective of surgery in this case was to address the segmental instability resulting from degenerative Grade I spondylolisthesis. Spinal fusion remains the gold-standard treatment for instability and symptomatic lumbar stenosis.¹³⁾ KLIF achieved successful clinical outcomes by effectively stabilizing the motion segment and increasing the canal area via indirect decompression.¹⁰^,¹³⁾ Mechanistically, indirect decompression operates through the restoration of disc height (56.9%), generating posterior tension across ligamentous structures (ligamentotaxis), thereby enlarging the central canal and neural foramina (53.3%).⁹⁾ These results support the hypothesis that, in selected OLF cases where instability is the primary cause of symptoms, decompression can be achieved without direct resection of posterior structures. Moreover, because KLIF is a facet-preserving technique, it avoids the iatrogenic destabilization often associated with traditional laminectomy or facetectomy.¹¹^,¹³^,¹⁴⁾ This suggests that in cases where both OLF and dynamic instability coexist, fusion-based strategies may provide symptomatic relief while reducing surgical morbidity.

Regression of OLF: A novel finding and mechanistic hypothesis

Although histopathological confirmation was not obtained, the cumulative radiological evidence strongly supported the diagnosis of OLF rather than calcium pyrophosphate dihydrate deposition disease. The lesion demonstrated continuous ossification directly connected to the lamina with extension into the bilateral facet joints on CT, lacked surrounding inflammatory edema on MRI, and was confined to a single spinal level (L4-L5). In addition, no chondrocalcinosis or calcific lesions were identified in peripheral joints outside the spine. These features are characteristic of OLF, whereas spinal calcium pyrophosphate dihydrate typically presents as discontinuous, well-circumscribed nodular calcifications, often involving multiple levels and sometimes accompanied by inflammatory changes during acute phases.¹⁵⁾ The most remarkable feature of this case was the significant radiological regression of OLF over a three-year period (Figure 3A-D). Spontaneous or postoperative reduction of lumbar OLF is exceedingly rare.⁷⁾ This finding suggests that mechanical stabilization of the spine may induce a biological environment that favors resorption of heterotopic ossification, analogous to observations in cervical ossification of the posterior longitudinal ligament, where posterior fusion significantly suppresses the progression of ossification in comparison with motion-preserving procedures.⁷^,¹⁶⁾ We propose that spinal stabilization not only restores mechanical stability but also may trigger a biological process that contributes to the reduction of OLF. First, OLF can be viewed as a mechanobiological condition, as it is closely linked to abnormal mechanical stress, especially tensile strain caused by spinal instability. These mechanical forces activate bone-forming pathways involving TGF-β, BMP-2, Sox9, Runx2, and mechanosensitive ion channels such as Piezo1.¹^,²^,¹⁶⁾ Second, solid fusion achieved through the KLIF procedure eliminates abnormal micromotion at the L4-L5 segment. With the removal of this mechanical irritation, the ligamentum flavum is no longer exposed to signals that promote ossification.⁷^,⁸^,¹⁰^,¹⁴⁾ Third, according to Wolff's law, in the absence of ongoing mechanical stress, the ossified tissue becomes subject to normal bone remodeling.⁷⁾ Over time, the balance shifts toward bone resorption rather than bone formation, leading to a gradual decrease in the size of the ossified mass over 3 years in the present case (Figure 3A-D). These observations suggest that spinal fusion should be viewed not only as a structural procedure, but also as a biological intervention that may influence the disease process of ectopic ossification. Postoperative imaging demonstrated regression of the OLF with no substantial additional enlargement of the spinal canal. Although the patient's symptomatic improvement was primarily attributed to stabilization surgery, regression of OLF may have helped maintain the effectiveness of neural decompression, thereby supporting long-term neurological stability.

Advantages of KLIF and surgical strategy considerations

FE-KLIF represents the least invasive form of lumbar interbody fusion currently available, with the procedure performed entirely endoscopically through the trans-Kambin's triangle corridor.¹⁴⁾ This approach preserves the posterior structures, including the facet joints, and requires only a 12-15 mm skin incision, causing minimal paraspinal muscle trauma compared with conventional minimally invasive transforaminal lumbar interbody fusion or posterior lumbar interbody fusion techniques.¹⁰^,¹⁴^,¹⁷⁾ In addition to its minimal invasiveness, FE-KLIF offers several notable advantages. First, it effectively avoids major complications commonly associated with anterior or oblique approaches. It is considered the safest among lateral-based fusion techniques, as it eliminates the risk of injury to major vessels, the ureter, and the abdominal viscera-complications occasionally encountered with ALIF, OLIF, and XLIF.¹¹^,¹⁴⁾ Second, the procedure carries a markedly lower risk of postoperative complications. Because the surgery is performed outside the epidural space and does not require extensive bone resection, the likelihood of postoperative hematoma formation or dural tears is significantly reduced. Several clinical studies have reported an extremely low incidence of surgical site infection following FE- KLIF.¹³^,¹⁴⁾ Third, the operation ensures excellent visualization and precision while maintaining soft-tissue integrity, enabling rapid postoperative recovery and early ambulation.¹¹^,¹²⁾ However, one important technical challenge in FE-KLIF is the proximity of the exiting nerve root within Kambin's triangle, posing a potential risk of exiting nerve root injury.¹¹^-¹³⁾ To ensure procedural safety, intraoperative neurophysiologic monitoring is recommended, and adequate disc space-ideally at least 12 mm between the facet joint and the exiting nerve root-is required for safe cage insertion. With appropriate preoperative planning and strict adherence to these safety parameters, FE-KLIF achieves both effective stabilization and indirect decompression, while minimizing the risks inherent to direct posterior decompression.

In this case, FE-KLIF simultaneously addressed instability and stenosis while avoiding those inherent risks. Ideal candidates for indirect decompression may include patients with dynamic symptoms and instability-driven pathology, rather than fixed compressive lesions.

Limitations

The principal limitation of this report is its single-case design (Level V evidence), which significantly limits the external validity of the findings. Additionally, the regressed lesion was not histologically evaluated; therefore, it remains unclear whether the observed reduction reflects true osteolysis or a transition toward fibrous, non-ossified tissue. Consequently, the proposed biomodulatory mechanism must be interpreted with caution and regarded as hypothetical. Future investigations with larger cohorts, extended follow-up, and, when feasible, tissue-level analysis will be essential to confirm these observations and elucidate the underlying biological processes. Furthermore, the successful treatment in this case may be limited to a very narrow indication, as it relied on the coexistence of segmental instability and only moderate OLF-related stenosis. Therefore, this indirect-only strategy should not be generalized to patients with severe, fixed neurological compression or OLF without underlying instability, in whom direct decompression may still be necessary.

Conclusion

This case demonstrated that in lumbar OLF with symptomatic degenerative spondylolisthesis, indirect decompression and stabilization via FE-KLIF-without direct resection of the ossified mass-can yield excellent clinical and functional outcomes (ODI 10% at 3 years). The most distinctive finding was the progressive reduction of the OLF mass (64.0%) over 3 years, suggesting that spinal fusion may initiate beneficial bone remodeling within the ectopic ossification.

We hypothesize that mechanical stabilization through FE-KLIF eliminated pathological stress, shifting local bone metabolism toward resorption via a biomodulatory mechanism. As FE-KLIF is a minimally invasive, facet-preserving technique with a low risk of complication, it may represent a viable treatment strategy for selected OLF patients.

Further prospective research is required to verify this biological phenomenon and reconsider the role of fusion not only as a mechanical correction but as a potential disease-modifying therapy.

Acknowledgments

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

Conflicts of Interest Disclosure

All authors have no conflict of interest.

Informed Consent

Written informed consent was obtained from the patient for the publication of this case report.

Availability of Data and Materials

The data supporting the findings of this case report are not publicly available due to privacy and ethical restrictions, but are available from the corresponding author upon reasonable request.

References

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