Heterotopic ossification refers to the appearance of bone tissue in areas where bone formation should not normally occur. This phenomenon is usually seen in soft tissues, which are regions that do not typically contain bone, such as muscle, ligament, tendon, or almost any site of connective tissue. Heterotopic ossification is a common complication of spinal cord injury (Cao et al., 2023). Ossification of the ligamentum flavum (OLF) is a type of ectopic ossification disease of the spinal ligaments, characterized by the transformation of the fibrous tissue within the ligament into bony tissue. OLF can occur in the cervical, thoracic, and lumbar regions of the spine, with the thoracic region, particularly the lower thoracic segment (T9∼T12), being the most common site of occurrence (Guo et al., 2010; Lang et al., 2013; Hur et al., 2009).

Thoracic ossification of the ligamentum flavum (TOLF) is characterized by ectopic ossification of the ligamentum flavum in the thoracic spine and is considered the main cause of thoracic spinal stenosis and spinal cord disease. The onset of TOLF is insidious, and there are no obvious symptoms when the spinal cord is not compressed in the early stage. However, slow and progressive compression of the thoracic spinal cord nerves can cause serious consequences such as sensorimotor disorders, bowel and bladder dysfunction, and even paraplegia (Liang et al., 2019). However, underlying mechanisms of TOLF are not well understood.

There are two forms of bone formation: intramembranous osteogenesis and endochondral osteogenesis. Intramembranous osteogenesis involves the differentiation of mesenchymal cells into an embryonic connective tissue membrane, which then forms bone within the membrane. This process occurs in the craniofacial and clavicle regions (Compston et al., 2019); Osteogenesis within cartilage is characterized by the process of pre-formed cartilage serving as a template and being replaced by bone, which is much more complex than intramembranous ossification. Most bones, such as limbs, trunk, and skull base, occur in this way (St-Jacques et al., 1999). Osteoblasts differentiate from mesenchymal stem cells and undergo multiple stages controlled by different transcription factors. Ihh is an essential transcription factor for endochondral osteogenesis and does not participate in intramembranous osteogenesis (St-Jacques et al., 1999). It is indispensable for the differentiation of mesenchymal cells into osteoblasts; The transcription factor Runx2 is involved in the regulation of endochondral and intramembranous osteogenesis, and cells expressing Runx2 can differentiate into osteoblasts or chondrocytes (Komori et al., 1997); Osteoblast specific transcription factor Osterix (Osx) is an essential transcription factor for bone formation and osteoblast differentiation (Nakashima et al., 2002). Embryos of OSX gene deficient mice have no bone formation and cartilage formation is not affected. Our research group has identified some important downstream target genes of OSX, including VDR, SATB2, VEGF, DKK1, SOST, etc., confirming the importance of OSX in bone formation (Zhang, 2012). OSX also regulates the osteoblast marker gene bone sialoprotein directly (Yang et al., 2016).

On the contrary, osteoporosis is the most common metabolic bone disease characterized by reduced bone mass, microstructural changes in bone tissue and decreased bone strength. Osteoporosis is caused by the functional imbalance between osteoblasts and osteoclasts. At present, most osteoporosis treatment drugs aim to inhibit bone resorption, and there is no drug that can specifically stimulate new bone formation directly. Elucidating the gene regulatory molecular mechanisms of bone formation is of great significance for guiding the development of new drugs for osteoporosis to specifically promote bone formation. There is no bone formation or ossification without Osx. Genome wide association analysis studies have confirmed that OSX is associated with osteoporosis (Timpson et al., 2009). Although OSX is associated with the phenotype of osteoporosis, further research is needed to determine the molecular mechanisms of OSX in bone formation, including exploring upstream regulatory factors of Osx. The treatment of osteoporosis urgently requires new drugs for bone formation. Thoracic ossification of the ligamentum flavum is a unique bone disease. Due to unknown pathological reasons, TOLF stimulates the formation of new bone in the ligament flavum, which might provide a great reference for searching for upstream regulatory factors of Osx.

Imaging examination is the most commonly used method for the localization and qualitative diagnosis of TOLF (Hirabayashi, 2017). Thoracic vertebrae lateral X-ray film is the basic initial screening method for TOLF, but due to the influence of the occlusion of structures such as the shoulder or liver, the qualitative and positional diagnostic role of X-ray for TOLF is limited, and its clinical application value is low. At present, CT and MRI play important roles in the imaging diagnosis of TOLF, which can respectively display the morphology, position, ossification, and nerve compression of TOLF in the axial and sagittal positions. Among them, CT can better reflect the degree of ossification, while MRI can better reflect the spinal nerve compression. The combination of the two is conducive to the precise diagnosis of TOLF (Takahashi et al., 2018; Yu et al., 2013).

There are numerous TOLF classification systems clinically established based on ossification morphology, distribution, spinal cord compression degree, and the relationship with surrounding tissues. Among them, Sato classification (lateral type, extensive type, enlarged type, fusion type, nodular type) based on CT (Sato et al., 1998) and Kuh classification (beak type, round type) based on MRI (Kuh et al., 2006) are more commonly used clinically. Other scholars' classifications such as Chen classification (focal type, skip type, continuous type) and Muthukumar classification (Tram track sign, Comma sign) (Muthukumar, 2009) with ossification of the dural sac have expounded on the types of TOLF from different angles and dimensions, with certain clinical value in reflecting ossification mechanisms, assessing disease risks, guiding treatment plans, or judging prognosis. These classification systems are of high accuracy in diagnosis and reliability in classification. Table 1 summarizes various classification systems that have been reported currently.

The ligamentum flavum is located at the posterolateral side of the vertebral canal of the spine, connecting two adjacent vertebral plates. It is divided into two parts: the capsular and the interlaminar portions. It consists of 80% elastic fibers and 20% collagen fibers (Yong-Hing et al., 1976). Ossification of the ligamentum flavum is more likely a process of endochondral ossification (Yayama et al., 2007; Nishikawa et al., 2023). The pathological changes initiate from the dural surface, characterized by early disruption of fiber arrangement, degeneration and degradation of elastic fibers, along with proliferation, swelling, and mucoid degeneration of collagen fibers. Meanwhile, local fibroblasts undergo chondrogenic differentiation into fibrochondrocytes, secreting chondromatrix to form cartilage. Subsequently, the cartilage undergoes calcification, with neovascularization invading the cartilage, which induces local mesenchymal cells to differentiate into osteoblasts. These osteoblasts then secrete bone matrix to form mature bone. Ossified ligamentum flavum typically exhibits four transitional zones from superficial to deep: ligament zone, chondroid zone, calcified cartilage zone, and ossified zone.

Based on previous studies, TOLF mainly occurs in Asian populations in China, Japan, South Korea and other countries. The prevalence of TOLF is 3.8%–63.9% in Chinese (Guo et al., 2010; Lang et al., 2013; Liang et al., 2019), 3.6%–36% in Japanese (Fujimori et al., 2016; Mori et al., 2013; Ohtsuka et al., 1986), and 16.9%–21.8% in Koreans (Kim et al., 2018; Moon et al., 2015). Meanwhile, TOLF is more common in the elderly population. One report has shown that the average age of onset is 61 years old. When Asians are over 65 years old, the incidence can be up to 20% (Aizawa et al., 2006). However, there are few reports on the incidence of TOLF in European, American and African populations (Pascal-Moussellard et al., 2005; Ricciardi et al., 2021). Due to the limitation of the number of studies and sample sizes, it is difficult to compare the prevalence of TOLF among Asian, European and American populations.

TOLF is a static compressive disease with insidious onset, and its clinical symptoms may progress as ossification of ligamentum flavum extends. Furthermore, TOLF frequently coexists with other spinal diseases, such as intervertebral disc herniation or ossification of the posterior longitudinal ligament. The lesion distribution can occur from a single segment to the entire thoracic spine, and it may lead to complex and diverse clinical manifestations that are prone to misdiagnosis and missed diagnosis (Yamada et al., 2021). Once TOLF compresses the thoracic spinal cord, it causes severe damage to motor neurological function, and the clinical symptoms are often severe, with an extremely high rate of paralysis (Feng et al., 2015).

Surgical intervention is widely recognized as the only effective method currently available for the treatment of TOLF. The aim of surgical intervention is to remove the ossified segments and provide adequate decompression. Posterior decompression surgery is the most common surgical approach, and patients often experience significant improvement in symptoms after the procedure. Various surgical techniques have been reported, including open-door laminectomy, en bloc laminectomy, hemilaminectomy, and fenestration, each with different decompression ranges (Kuh et al., 2006). A consensus has generally emerged that the resection should include the laminae, the medial 1/3 to 1/2 of the facet joints, the thickened and ossified ligamentum flavum, as well as one or two layers above and below the affected segments. In TOLF patients with dural ossification, the ossified dura mater and OLF are resected en bloc with preserving arachnoid (Hirabayashi, 2017).

In recent years, with significant advancements in surgical techniques, clinicians have obtained abundant and precise preoperative information through CT and MRI imaging, and intraoperative imaging modalities such as navigation systems have been introduced, making decompression surgery safer (Hirabayashi, 2017). Some scholars have explored the use of endoscopic techniques for the resection of TOLF. Yang et al. performed an endoscopic full-spine surgery, utilizing matched ultrasonic osteotomy to directly remove single-level TOLF, achieving satisfactory surgical outcomes (Yang et al., 2021). Bagga et al. conducted a retrospective cohort study and found that compared to conventional high-speed burr resection of TOLF, the use of ultrasonic osteotomy significantly shortened the operative time, reduced dural tears, and increased the safety and efficacy of decompression surgery (Bagga et al., 2023). Liu et al. utilized percutaneous endoscopic thoracic decompression (PETD) to treat multi-level TOLF, also achieving favorable results with symptom relief (Liu et al., 2023). This demonstrates the significant advantages of spinal endoscopic techniques in TOLF treatment. However, endoscopic thoracic surgery poses technical challenges and requires experienced surgeons, as it may be accompanied by serious complications (Park et al., 2023).

Every surgical procedure carries its unique set of complications. Common surgical complications associated with TOLF include spinal cord or nerve root injury, dura mater injury with cerebrospinal fluid (CSF) leakage, epidural hematoma formation, postoperative spinal instability, and even kyphosis. Among these, dura mater injury and CSF leakage are the most prevalent (Osman et al., 2018), primarily resulting from severe ossification causing compression of the dura mater, leading to its adhesion and even ossification. Zhang et al. (2023) reported in their study of 266 TOLF patients that the incidence of dura mater injury combined with CSF leakage was 32%, causing discomfort to patients. Through a series of comprehensive treatments such as intraoperative dura mater repair, postoperative prone positioning, and continuous drainage, 91.8% of patients with CSF leakage or dural tears were successfully managed. The most severe complication of TOLF surgery is postoperative paraplegia caused by spinal cord injury. In the study by Li et al. (2016), four patients experienced late postoperative neurological deterioration or persistent worsening paralysis, leading to catastrophic outcomes. Therefore, early diagnosis and treatment are crucial for TOLF patients to achieve better surgical outcomes and minimize complications.

Currently, the pathogenesis of TOLF remains unclear. The occurrence of OLF has been reported to be associated with various factors such as mechanical factors, genetic factors, endocrine, and trace elements, and none of these single factors can fully explain its pathogenesis (Kuh et al., 2006). Here we summarize the related research on TOLF in recent years, aiming to provide new insights for the diagnosis and treatment of TOLF.

Although there have been relevant reports on TOLF, its pathogenesis is still in the exploratory stage. The relevant factors mentioned above, such as mechanical stress, genetic susceptibility genes, endocrine and trace element metabolism abnormalities, are currently unable to clearly explain the pathogenesis of TOLF. Due to the fact that genetic and molecular changes often occur earlier than imaging pathological manifestations, how to effectively prevent and treat TOLF is still an unresolved medical challenge. The in-depth study of the molecular mechanism of TOLF will provide clues for exploring its etiology and early diagnosis. The differential expression protein mass spectrometry obtained through proteomics technology in the early stage suggests that inflammatory factors may be involved in the occurrence of TOLF. The study found that TNF-α regulates cell proliferation through cyclin D1 and c-Myc, and induces osteoblast differentiation through OSX to participate in TOLF, revealing the involvement of TNF-α in TOLF and its possible mechanism. Proteomics approach enables a comprehensive understanding of the pathological pathogenesis of TOLF at the protein level. On the other hand, the later research on gene expression profiles based on RNA sequencing shows that angiopoietin-2 is involved in the occurrence of TOLF (Yang et al., 2018b). This study also shows that angiopoietin-2 promotes the gene expression of OSX and is an upstream factor of Osx. In summary, TOLF may be a disease involving multiple factors. Considering the involvement and mechanisms of various factors will help to gain a more comprehensive and in-depth understanding of the pathogenesis of TOLF. On the other hand, conducting research on the pathogenic mechanism of TOLF may provide new ideas for finding upstream regulatory factors of Osx and further developing novel drugs to promote new bone formation to treat osteoporosis.