The ligamentum flavum (LF) is a thick, segmental ligament that connects the laminae of adjacent vertebrae from the C2 vertebra to the S1 sacrum and lines most of the posterolateral aspect of the spinal canal [1,2]. It comprises approximately 80% elastic fiber and 20% collagen fiber [3]. The primary role of the LF is to preserve an erect posture and to ensure the straightening of the column following flexion [4]. A variety of pathophysiological changes may develop in the LF including hypertrophy, calcification, ossification, intraligamentous cysts and hematoma [5]. The incidence of hematomas in the LF (HLF) is extremely rare due to the limited supply of blood vessels in the LF. HLF can induce sudden radiculopathy by compressing the thecal sac or nerve root, mimicking symptoms similar to those in degenerative diseases such as intervertebral disc herniation or spinal stenosis. Although the exact etiology of HLF is not clearly understood, HLF is predominantly associated with mobile segments of the cervical and lumbar spine often linked to trauma or increased mechanical stress [6,7]. Here, the authors report a case of a patient who developed HLF after a long-level lumbar fusion procedure.

A 66-year-old male presented with a 5-year history of chronic low back pain and a progressively worsening left radiating leg pain. Magnetic resonance imaging (MRI) without contrast was performed and revealed multilevel degenerative lumbar spinal stenosis at the L2–3, L3–4, L4–5 levels and isthmic spondylolisthesis at the L5–S1 level (Figure 1). The patient underwent oblique lateral interbody fusion (OLIF) with percutaneous pedicle screw fixation from L2 to S1. Postoperative MRI confirmed effective spinal canal decompression and the patient reported significant improvements in low back pain and left-sided sciatica (Figure 1). However, at 9-month postsurgery, he experienced severe, recurrent left leg pain and weakness. A thorough neurological examination showed no urinary or fecal incontinence and a negative straight-leg-raising test; however, a marked decrease in motor weakness in the left ankle and great toe dorsiflexion was observed. A computed tomography scan demonstrated a well-fused state across all previous OLIF levels, with no hardware failure or adjacent segment disease (Figure 2). An MRI of the lumbar spine was performed, revealing a well-defined T1-iso to hypointense and heterogeneous T2-hyperintense extradural mass originating from the LF at the left L5–S1 level, displacing the thecal sac contralaterally and causing severe lateral recess stenosis (Figure 3). A uniportal full-endoscopic hemipartial laminectomy with flavectomy was performed at the left L5–S1 level, where a dark brown hematoma was discovered between the outer and inner layers of the LF (Figure 4). Following meticulous removal of the LF and hematoma, successful decompression of the thecal sac was achieved. Histopathological examination revealed myxoid degeneration with dystrophic calcification and pigment deposition, indicative of a hematoma, with no signs of malignancy, disc material, or infection. Subsequent MRI showed an effective decompression of the spinal canal and the patient's pain and weakness in the left leg improved remarkably, facilitating discharge within 2 days. IRB approval was obtained from the institutional review board of Korea University (KUIRB No. 2026AN0205), and written informed consent was waived due to the retrospective nature of the case report.

HLF is rare and was first documented in the English literature by Sweasey et al. [8] in 1992. The infrequency of HLF is due to the limited vascularity of the LF. Although trauma or mechanical stress are recognized as possible causes of HLF, cases such as ours that occur post spinal fusion without significant trauma propose a more complex etiology. Numerous reports of HLF involving the cervical, thoracic and lumbosacral parts of the spine have been published in the literature. A statistical analysis of previous case reports possesses several key characteristics: the patients were mostly aged older than 60 years old, the incidence rate was generally higher in males than in females, the lumbar region was the most frequently affected, minor trauma had been a predisposing factor and surprisingly, the majority of cases were from Asian countries [4,7]. Although, in our case, there was no record of trauma prior to the onset of symptoms, the patient was a relatively old (66 years of age) Asian male patient whose L5–S1 level was affected, consistent with the literature.

Various mechanisms have been proposed to be responsible for the pathogenesis of HLF. Minor trauma or trivial daily activities such as sneezing increase intra-abdominal and spinal epidural pressure that can exert excessive force on the irregularly dispersed capillaries in the LF [9,10]. As a result, the thin-walled blood vessels may rupture causing intraligamentous bleeding. Moreover, the elevated pressure is transferred more easily to the mobile sections of the cervical or lumbar spine whereas the thoracic region is barely affected due to the lack of motion [7]. Percutaneous interventions involving lumbar or epidural puncture may also lead to the accumulation of microtears in the LF making the ligament more prone to injuries [11]. Our patient denied any recent trauma to the abdomen or the back before the onset of symptoms; nonetheless, recalling details of minor trauma is difficult and trivial injuries may have occurred unknowingly while engaging in everyday activities.

The OLIF technique offers numerous advantages over the posterior lumbar interbody fusion (PLIF) as the OLIF provides indirect neural decompression whilst preserving the posterior anatomical structures including the LF. Lee et al. [12] conducted a prospective study and discovered that the LF underwent remodeling following OLIF; the thickness and area of the LF were reduced due to the stretching and atrophic changes induced by OLIF. Other indirect neural decompression techniques such as the anterior lumbar interbody fusion and the extreme lateral lumbar interbody fusion have also been demonstrated to significantly reduce the LF size [13,14]. After fusion, it seems that the stability of the spine reduces mechanical stress inducing atrophy and remodeling of the LF [15]. This raises the question of how intraligamentous bleeding can occur after fusion, as in our case. Nevertheless, such rare phenomenon has been described twice previously in the English literature. Haghnegahdar et al. [16] reported a case of a 29-year-old male who developed HLF at the level of C4 vertebra after posterior cervical spine fixation and fusion for a C2–3 left-sided locked facet and C3–4 subluxation. Kim et al. [17] reported a 67-year-old male who presented with HLF at the L2–3 level after a long-level posterior fusion at L3–S1. Both reports have shown that HLF can occur after spinal fusion, similar to our case. Although no definite mechanism for the relationship between HLF and spinal fusion has been postulated, one possible explanation is that the continuous biomechanical stress and increased passive motion of the adjacent segments may mediate the development of HLF after spinal fusion. To date, there are promising research showing that lumbosacral fixation leads to compensatory increase in load and mobility of the adjacent segments due to the loss of motion at the fused level [18-23]. Li et al. [24] exhibited that multilevel interbody fusion aggravated adjacent segment degeneration and increased stress concentration in the lower adjacent segment. Mu et al. [25] conducted a biomechanical study on goat lumbar spine and noted that multisegment fixation progressively increased the stress and range of motion on the facet joints of adjacent segments. As the adjacent segments in a long-level spinal fusion are subjected to constant biomechanical stress, this may precipitate fibrotic changes on the LF by releasing inflammatory cytokines such as transforming growth factor-beta, matrix metalloproteinases, microRNAs and angiogenesis-related proteins including vascular endothelial growth factor (VEGF) and interleukin-6 [4,26]. These molecules play a critical role in the pathogenesis of LF degeneration that may induce the rupture of microvessels in LF [4,26,27]. Hur et al. [28] exhibited that mechanical stress in the lumbar spine initiates an inflammatory response that promotes VEGF-mediated angiogenesis in the LF. Immunohistochemical evaluation of degenerative LF also revealed marked angiogenesis [29,30]. Thus, biomechanical stress and neovascularization may contribute to intraligamentous bleeding providing a plausible explanation of the pathogenesis of HLF after a long-level lumbar fusion [31].

For detecting HLF, MRI is the imaging modality of choice to assess the intensity and stage of the hematoma [8,10,32]. Signal intensity of the hematoma gradually changes as hemoglobin evolves into 4 different forms over time: oxyhemoglobin, deoxyhemoglobin, methemoglobin and hemosiderin. In the first 3 days, oxyhemoglobin degrades to deoxyhemoglobin presenting isointense T1- and hypointense T2-weighted images. In a typical subacute HLF, intracellular methemoglobin creates hyperintense T1- and hypointense T2-weighted images, and as extracellular methemoglobin accumulates over time both hyperintense T1- and T2-weighted images are produced [27]. Consecutive MRI scans improve the diagnostic process due to the time-dependent signal intensity of the hematoma [33]. HLF must be distinguished from other relatively common diseases such as synovial cyst, ganglion, epidural abscess, and neoplasm, and the administration of the contrast agent gadolinium-DTPA may be useful in the differential diagnosis [32-34]. Histopathological assessment of the surgical specimen confirms the diagnosis of HLF. Surgical excision is indicated in the presence of intractable pain and neurological deficit [35]. In the present case, a uniportal full-endoscopic hemipartial laminectomy with flavectomy was performed due to severe L5 radiculopathy and motor weakness.

To the best of the authors’ knowledge, our report is the first case of HLF following OLIF in the English literature. The uniqueness of our case lies in 2 key points: (1) The difficulty of diagnosing HLF due to its rare occurrence and misleading symptoms; in the differential diagnosis of lumbar radiculopathy after spinal fusion surgery, intraligamentous hemorrhage must be taken into consideration, especially in middle-aged male Asian patients; (2) A uniportal full-endoscopic spine surgery was used and successful resection of the hematoma was achieved with favorable outcomes demonstrating the effective use of uniportal endoscope as a revision option for postoperative HLF complications following OLIF. The exact pathogenesis of the development of HLF after a long-level lumbar fusion warrants further study.

The OLIF provides numerous advantages over PLIF including reduced damage to posterior paravertebral muscles, posterior ligamentous complex and the lamina. However, despite the benefits of preserving posterior anatomical structures, surgeons must remain alert to potential complications involving the posterior column such as the LF. This case demonstrates that rare complications such as HLF can occur after spinal fusion at any period postsurgery, emphasizing the importance of considering the possibility of HLF in patients with unexpected neurological deficits following OLIF. Mechanical stress and angiogenesis are thought to induce the development of intraligamentous bleeding and future directions should focus on the pathogenesis of HLF after a long-level fusion