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Introduction
Thoracic ossification of the ligamentum flavum (TOLF or thoracic OLF) is one of the most common causes of thoracic myelopathy, especially in Asian populations. Progressive ossification of the ligamentum flavum gradually narrows the thoracic spinal canal, resulting in spinal cord compression, neurological deterioration, gait disturbance, sensory impairment, and lower extremity dysfunction.
Unlike cervical and lumbar degenerative diseases, thoracic spinal cord compression presents unique surgical challenges due to the limited space of the thoracic canal, poor tolerance of spinal cord manipulation, and the high risk of neurological complications.
Once neurological symptoms develop, surgical decompression remains the primary treatment option. Traditional posterior open decompression procedures, including laminectomy and laminoplasty, can effectively relieve spinal cord compression. However, extensive muscle dissection, removal of posterior elements, facet joint injury, and postoperative spinal instability remain major concerns.
In recent years, minimally invasive spinal surgery techniques have rapidly evolved. Unilateral biportal endoscopic (UBE) surgery has attracted increasing attention because it combines the advantages of endoscopic visualization with conventional spinal surgical techniques. Through a unilateral approach, UBE allows adequate decompression while minimizing damage to posterior supporting structures.
A recent clinical study by Lee et al. evaluated the radiological and clinical outcomes of UBE decompression for thoracic OLF. The study included 45 patients involving 55 thoracic segments, with an average follow-up period of 16 months. The researchers focused on quantitative imaging changes, neurological recovery, preservation of posterior structures, and postoperative complications.
The findings provide valuable evidence supporting UBE as a tissue-preserving surgical option for selected patients with thoracic OLF.
The ligamentum flavum is an elastic ligament located between adjacent laminae of the spine. Its primary function is maintaining posterior spinal stability while allowing spinal motion.
With aging and chronic mechanical stress, the ligamentum flavum may undergo degenerative changes, including hypertrophy, fibrosis, and progressive ossification. When this ossified ligament compresses the thoracic spinal cord, it is referred to as thoracic ossification of the ligamentum flavum (OLF).
Thoracic OLF commonly occurs in the lower thoracic spine, particularly at:
T9–T12 levels
Thoracolumbar junction
Multiple adjacent thoracic segments
The disease progression is usually slow, but once spinal cord compression becomes significant, neurological recovery may become limited. Therefore, early diagnosis and appropriate surgical intervention are critical.
The symptoms of thoracic OLF vary depending on the severity and duration of spinal cord compression.
Common clinical presentations include:
Lower extremity weakness
Walking difficulty
Spastic gait
Numbness or sensory disturbance
Lower limb pain
Bladder or bowel dysfunction in advanced cases
Because thoracic OLF develops gradually, early symptoms may be subtle and frequently misdiagnosed as lumbar disorders, peripheral neuropathy, or other neurological conditions.
Magnetic resonance imaging (MRI) and computed tomography (CT) play essential roles in diagnosis and surgical planning.
Posterior open surgery has historically been considered the standard treatment for symptomatic thoracic OLF. However, the procedure involves extensive exposure of the posterior spinal elements.
Potential disadvantages include:
Wide paraspinal muscle dissection may result in:
Increased postoperative pain
Muscle atrophy
Longer recovery time
Excessive removal of lamina and facet joints may compromise spinal stability.
Thoracic spine stability depends on the integrity of posterior elements. Damage to these structures may contribute to progressive kyphotic deformity, particularly in patients with pre-existing degeneration.
Severe OLF is frequently associated with dural ossification (DO). During open decompression, aggressive removal of fused ossified tissue may increase the risk of dural tears and cerebrospinal fluid leakage.
These limitations have encouraged surgeons to explore less invasive decompression strategies.
Unilateral biportal endoscopic surgery is an advanced minimally invasive spinal technique using two separate portals:
One viewing portal for the endoscope
One working portal for surgical instruments
Unlike traditional uniportal endoscopy, UBE provides independent visualization and instrument movement, similar to conventional open surgery but through smaller incisions.
The main advantages include:
High-definition magnified visualization
Improved surgical maneuverability
Reduced muscle injury
Preservation of posterior structures
The general principles of UBE decompression include:
Establishing unilateral access points
Performing laminotomy on the approach side
Removing hypertrophic or ossified ligament tissue
Performing contralateral undercutting decompression
Preserving facet joints and posterior stabilizing structures whenever possible
The unilateral approach allows surgeons to decompress both sides of the spinal canal while minimizing bilateral muscle disruption.
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Accurate classification of thoracic OLF is essential for surgical planning because different ossification patterns are associated with different levels of spinal cord compression and operative difficulty.
The Sato classification system categorizes thoracic OLF into five major types based on axial CT findings:
Fig. 1 Sato classification of thoracic OLF on axial CT: (A) lateral (localized), (B) extended (medial extension), (C) enlarged (large volume), (D) fused (midline confluence of bilateral lesions), (E) tuberous (nodular, with severe compression).
The ossified ligament is limited to one side of the spinal canal.
Characteristics:
Relatively localized compression
Usually easier decompression
Lower risk of extensive dural adhesion
The ossification extends medially toward the center of the spinal canal.
Characteristics:
Greater spinal canal occupation
Increased compression severity
Requires careful decompression strategy
The ossified ligament becomes significantly enlarged and occupies a large portion of the spinal canal.
Characteristics:
Severe canal narrowing
Higher possibility of neurological impairment
Increased surgical complexity
Bilateral ossified ligaments merge across the midline.
Characteristics:
Extensive posterior compression
Frequently associated with dural ossification
Higher risk of dural injury during surgery
The ossified lesion forms a nodular mass projecting into the spinal canal.
Characteristics:
Severe focal compression
Strong adhesion with dura may occur
Requires meticulous surgical handling
Fig. 2 Imaging signs of dural ossification (DO): (A) Tram track sign — separation of ossified dura from OLF, resembling parallel tram tracks; (B) Bridge sign — bony bridge connecting bilateral OLF via ossified dura; (C) Comma sign — anterolateral extension of ossified dura with fusion to OLF, forming a comma shape.
Preoperative imaging evaluation is essential for:
Determining the extent of ossification
Evaluating spinal cord compression
Predicting surgical difficulty
Selecting appropriate decompression strategies
CT is considered the most accurate modality for evaluating:
Ossification morphology
Bone density
Fusion pattern
Dural ossification signs
MRI provides information regarding:
Spinal cord compression
Intramedullary signal changes
Degree of neurological injury
Dural ossification is one of the most important factors affecting surgical risk.
Several CT-based signs have been described:
The ossified dura separates from the ligamentum flavum, creating a double-line appearance similar to railway tracks.
Bilateral ossified lesions connect through ossified dura, forming a bridge-like structure.
The ossified dura extends anterolaterally, creating a comma-shaped appearance.
Recognition of these imaging signs helps surgeons anticipate possible dural adhesion and reduce intraoperative complications.
Unilateral biportal endoscopic (UBE) decompression is an advanced minimally invasive spine surgery technique that combines the advantages of conventional microscopic surgery and endoscopic visualization.
Unlike traditional open thoracic laminectomy, UBE uses two independent portals, allowing surgeons to achieve sufficient decompression while preserving important posterior spinal structures.
The main surgical objectives of UBE treatment for thoracic ossification of the ligamentum flavum include:
Adequate spinal cord decompression
Preservation of facet joints and posterior tension structures
Reduction of paraspinal muscle injury
Prevention of postoperative spinal instability
Patients are placed in the prone position under general anesthesia.
After confirming the surgical level with fluoroscopy, two small skin incisions are created:
Viewing portal: used for endoscopic visualization and continuous irrigation
Working portal: used for surgical instruments, drilling, and decompression procedures
This separated camera-and-instrument system provides a wider surgical field compared with traditional uniportal endoscopy.
The magnified endoscopic view allows surgeons to clearly identify:
Lamina
Facet joint
Hypertrophic ligamentum flavum
Dural surface
Compressed spinal cord structures
The procedure usually begins with partial laminotomy on the approach side.
The surgeon carefully removes the hypertrophic or ossified ligamentum flavum while protecting the dura and neural structures.
Compared with bilateral open exposure, UBE minimizes unnecessary removal of posterior elements.
Important surgical principles include:
Avoid excessive facet joint resection
Preserve contralateral posterior structures
Perform adequate decompression under direct visualization
Maintain spinal stability whenever possible
One of the key advantages of UBE is that bilateral spinal canal decompression can be achieved through a unilateral approach.
After completing ipsilateral decompression, the endoscope and instruments are angled toward the opposite side to perform under-cutting decompression.
This technique allows:
Removal of contralateral compressive lesions
Preservation of contralateral facet joints
Reduced muscle stripping
Smaller surgical exposure
The ability to achieve bilateral decompression while maintaining posterior stability is one of the major reasons why UBE has gained popularity in thoracic spinal surgery.
The study by Lee et al. [1] investigated the radiological and clinical outcomes of unilateral biportal endoscopic decompression for thoracic ossification of the ligamentum flavum.
This retrospective study included patients who underwent UBE decompression for symptomatic thoracic OLF.
The study included:
45 patients
55 thoracic OLF segments
Mean follow-up period: 16 months
Patients were evaluated before and after surgery using clinical assessments and imaging examinations.
Preoperative and postoperative CT and MRI examinations were performed to evaluate structural changes.
The measured radiological parameters included:
DSCA was measured on axial MRI images to evaluate spinal canal expansion after decompression.
An increased DSCA indicates improved space available for the spinal cord.
Fig. 3 Measurement parameters: (A–B) DSCA delineation on axial T2WI; (C–D) Facet joint length (red arrows) and area (yellow ROI) on axial CT; (E) Segmental kyphosis by Cobb method on standing lateral radiograph; (F) Intramedullary hyperintensity (red arrows) on axial T2WI.
CT-based measurements were used to evaluate preservation of posterior stabilizing structures.
The researchers analyzed:
Ipsilateral facet preservation
Contralateral facet preservation
This parameter was important because excessive facet removal may contribute to postoperative instability.
Standing lateral radiographs were used to evaluate postoperative alignment changes.
The segmental kyphotic angle was measured using the Cobb method to determine whether decompression caused progressive deformity.
Neurological and functional outcomes were evaluated using standardized clinical scoring systems:
The mJOA score was used to assess neurological function, including:
Lower extremity motor function
Sensory function
Bladder function
Higher scores indicate better neurological status.
The Nurick grading system was used to evaluate walking ability and functional disability caused by thoracic myelopathy.
Pain severity was evaluated using VAS scores, including:
Back pain
Lower extremity pain
The study demonstrated that UBE decompression achieved effective spinal canal enlargement while maintaining posterior structural preservation.
The quantitative imaging results provide objective evidence supporting the minimally invasive characteristics of UBE surgery.
One of the most important radiological findings was significant enlargement of the dural sac after UBE decompression.
The mean DSCA increased from:
70.4 ± 20.4 mm² before surgery
to:
119.8 ± 31.0 mm² after surgery
The average expansion rate reached:
75.0%
with a range from:
3.8% to 200.2%
This significant increase indicates that UBE can provide sufficient decompression of the thoracic spinal cord despite the limited surgical exposure.
The findings suggest that complete removal of all ossified lesions may not always be necessary, as functional decompression can be achieved by restoring adequate space around neural structures.
Preservation of posterior spinal elements is considered one of the major advantages of minimally invasive decompression.
Postoperative CT evaluation showed:
Ipsilateral facet joint preservation: 64.2%
Contralateral facet joint preservation: 79.4%
The significantly higher preservation rate on the contralateral side demonstrates the structural protection achieved through unilateral access.
Maintaining facet integrity may help reduce:
Postoperative instability
Progressive deformity
Need for additional fusion surgery
Compared with traditional open bilateral decompression, UBE provides a more tissue-preserving surgical pathway.
Postoperative spinal alignment remained relatively stable after UBE decompression.
The segmental kyphotic angle changed from:
5.4° preoperatively
to:
6.9° postoperatively
The average increase was only:
1.5°
Only three segments (5.5%) showed progression greater than 5°.
One patient required secondary fusion because of delayed instability. However, this patient had several risk factors:
Fused-type OLF
Associated dural ossification
Limited facet preservation
These results suggest that in appropriately selected patients, UBE decompression can effectively relieve spinal cord compression while maintaining postoperative spinal stability.
In addition to significant radiological improvement, unilateral biportal endoscopic (UBE) decompression demonstrated favorable neurological and functional outcomes in patients with thoracic ossification of the ligamentum flavum (OLF).
The study evaluated postoperative recovery using multiple clinical assessment tools, including the modified Japanese Orthopaedic Association (mJOA) score, Nurick grade, and Visual Analog Scale (VAS).
Overall, patients showed significant improvement in neurological function, walking ability, and pain relief after UBE decompression.
The modified Japanese Orthopaedic Association (mJOA) score is widely used to evaluate neurological impairment caused by thoracic myelopathy.
In this study, the mean mJOA score improved significantly:
Preoperative mJOA score:
7.7 ±
Postoperative mJOA score:
9.7 ±
The calculated neurological recovery rate reached:
71.2%
This improvement indicates that UBE decompression can effectively restore spinal cord function by relieving chronic compression caused by ossified ligamentum flavum.
The neurological improvement observed after surgery is particularly meaningful because thoracic OLF often develops gradually, leading to chronic spinal cord compression and potential irreversible neurological damage.
The results suggest that timely surgical decompression may provide substantial functional recovery in appropriately selected patients.
The Nurick grading system evaluates disability caused by spinal cord compression, particularly focusing on gait disturbance and walking ability.
Following UBE decompression, patients demonstrated significant functional improvement.
The mean Nurick grade improved from:
2.3 before surgery
to:
0.6 after surgery
This improvement reflects enhanced lower extremity function and improved daily mobility after spinal cord decompression.
The findings support the effectiveness of UBE not only in radiological decompression but also in meaningful neurological recovery.
Pain outcomes were assessed using the Visual Analog Scale (VAS).
Significant reductions were observed in both back pain and lower extremity pain.
The mean back pain VAS score decreased from:
6.2 preoperatively
to:
2.4 postoperatively
The mean lower extremity pain VAS score decreased from:
4.7 preoperatively
to:
2.1 postoperatively
Both improvements were statistically significant (p < 0.001).
These findings demonstrate that UBE decompression provides effective symptom relief while minimizing surgical trauma associated with conventional posterior approaches.
Complete removal of ossified ligamentum flavum is traditionally considered the goal of decompression surgery.
However, in thoracic OLF cases, especially those associated with severe dural adhesion or dural ossification, aggressive removal may increase the risk of complications such as dural tears and cerebrospinal fluid leakage.
The concept of the floating strategy has therefore gained increasing attention.
Instead of completely removing the ossified lesion, surgeons may intentionally leave a thin layer of OLF attached to the dura while creating sufficient space for spinal cord decompression.
In this study:
Residual OLF was observed in 18.2% of segments
All residual lesions occurred in patients treated with the intentional floating strategy
Interestingly, these patients achieved greater DSCA expansion:
DSCA expansion rate:
Floating strategy group: 102.7%
Complete removal group: 68.9%
Despite the presence of residual ossification, neurological recovery showed no significant difference.
This suggests that:
Adequate functional decompression may be more important than complete radiological removal of OLF.
The floating technique may provide several advantages:
Reduced risk of dural injury
Lower incidence of cerebrospinal fluid leakage
Safer management of severe OLF with dural adhesion
Preservation of neurological structures
However, appropriate patient selection remains essential.
Floating strategies should be considered particularly in patients with:
Severe OLF adhesion
Dural ossification signs on CT
High risk of dural tear
Although thoracic OLF surgery is technically demanding, UBE decompression demonstrated an acceptable complication profile in this study.
The reported complications included:
Three cases of dural tears occurred.
All patients with dural tears had:
Severe OLF
Associated dural ossification
These findings highlight that dural ossification remains one of the strongest predictors of intraoperative complications.
Careful preoperative CT evaluation is therefore essential.
One patient developed postoperative hematoma.
The hematoma was successfully treated with endoscopic evacuation.
This demonstrates that UBE techniques can also provide advantages in managing certain postoperative complications due to improved visualization.
One patient developed delayed postoperative instability requiring additional fusion surgery.
This patient had several risk factors:
Fused-type OLF
Dural ossification
Limited facet preservation
The result emphasizes that although UBE preserves spinal structures, not all patients are suitable candidates for decompression alone.
No postoperative infection was reported in this study.
The minimal soft tissue disruption associated with UBE may contribute to reduced wound-related complications.
Traditional open posterior decompression remains an effective treatment for thoracic OLF; however, it may involve extensive muscle dissection, greater bone removal, and increased risk of postoperative instability.
UBE offers several potential advantages.
Because UBE requires only small portals rather than a large posterior exposure, it minimizes:
Paraspinal muscle stripping
Soft tissue injury
Postoperative pain
The unilateral approach allows preservation of:
Contralateral facet joints
Posterior ligamentous structures
Normal spinal anatomy
This may reduce the risk of postoperative deformity.
The endoscopic magnified view allows surgeons to identify:
Ossified ligament
Dural surface
Neural structures
This may improve surgical precision, especially in complex OLF cases.
By preserving posterior structures, UBE may reduce the need for additional stabilization procedures in appropriately selected patients.
However, patients with severe deformity, instability, or extensive fused OLF may still require fusion-based procedures.
Although UBE provides promising results, careful patient selection is essential.
Ideal candidates may include patients with:
Symptomatic thoracic myelopathy caused by OLF
Localized or moderate OLF compression
No significant preoperative instability
No severe thoracic kyphotic deformity
Preserved posterior spinal structures
UBE decompression should be carefully considered in patients with:
Extensive fused-type OLF
Severe dural ossification
Advanced spinal deformity
Significant instability
Multilevel complex lesions requiring reconstruction
In these situations, conventional decompression with fusion may provide better long-term stability.
Although the results of UBE treatment for thoracic OLF are encouraging, several limitations should be considered.
The current evidence mainly comes from retrospective case series.
Without randomized controlled trials, it remains difficult to directly compare UBE with conventional open decompression.
The mean follow-up period in this study was approximately 16 months.
Long-term outcomes regarding:
Spinal stability
Kyphotic progression
Recurrence
Fusion requirement
require further investigation.
Patients with advanced dural ossification and complex fused or nodular OLF patterns were relatively limited.
Therefore, the applicability of UBE in these challenging cases remains uncertain.
Future studies should include:
Larger patient populations
Longer follow-up periods
Comparative studies with open surgery
Patient-reported outcome measurements
to further define the role of UBE in thoracic OLF treatment.
The treatment strategy for thoracic ossification of the ligamentum flavum (OLF) has gradually evolved from extensive open decompression toward more precise, tissue-preserving surgical techniques.
Traditional posterior decompression remains an effective treatment option; however, concerns regarding muscle damage, facet joint removal, postoperative instability, and the potential need for additional fusion have encouraged the development of minimally invasive approaches.
Unilateral biportal endoscopic (UBE) decompression represents an important advancement in this transition.
The current evidence demonstrates that UBE can achieve:
Significant spinal canal expansion
Effective neurological recovery
Preservation of posterior spinal structures
Limited postoperative alignment changes
These findings support UBE as a promising minimally invasive option for selected patients with thoracic OLF.
The future direction of thoracic OLF surgery is likely to focus on balancing three major goals:
Adequate neural decompression
Maximum preservation of spinal stability
Reduction of surgical trauma
Rather than pursuing complete removal of every ossified lesion, future surgical concepts may increasingly emphasize functional decompression and preservation of normal anatomy.
The floating strategy used in severe OLF cases represents this changing philosophy — achieving neurological improvement while avoiding unnecessary manipulation of adherent dural structures.
Future developments may include:
Artificial intelligence-assisted imaging analysis
Three-dimensional CT reconstruction
Improved endoscopic navigation systems
Real-time intraoperative visualization technology
These technologies may help surgeons better predict:
Dural ossification risk
Surgical difficulty
Optimal decompression range
Need for fusion procedures
Although current studies demonstrate encouraging outcomes, further research is required.
Future investigations should focus on:
Prospective multicenter studies
Long-term follow-up data
Direct comparison between UBE and open surgery
Cost-effectiveness analysis
Patient-reported quality-of-life outcomes
With increasing clinical experience, UBE may become an important component of minimally invasive thoracic spine surgery.
Both UBE decompression and traditional open surgery can effectively treat thoracic OLF. However, they differ significantly in surgical philosophy and tissue impact.
Parameter | UBE Decompression | Conventional Open Decompression |
|---|---|---|
Surgical approach | Unilateral minimally invasive portals | Wide posterior exposure |
Muscle injury | Reduced | Greater paraspinal muscle dissection |
Visualization | Magnified endoscopic view | Direct open visualization |
Facet preservation | Higher potential preservation | Greater risk of facet removal |
Postoperative pain | Usually reduced | Usually greater |
Recovery time | Potentially faster | Longer rehabilitation |
Learning curve | Requires advanced endoscopic skills | Familiar conventional technique |
Severe deformity/instability | Limited indications | More suitable when fusion is required |
The major advantages of UBE include:
Small portals reduce soft tissue disruption and may contribute to:
Less postoperative pain
Lower muscle injury
Faster recovery
Because UBE avoids unnecessary bilateral posterior element removal, it may decrease the risk of:
Segmental instability
Progressive kyphosis
Secondary fusion surgery
UBE is particularly attractive for patients with:
Localized thoracic OLF
Preserved spinal alignment
No significant instability
Moderate neurological compression
Despite these advantages, UBE is not a universal replacement for open surgery.
Open decompression with fusion may remain preferable for patients with:
Severe kyphotic deformity
Extensive multilevel fused OLF
Significant instability
Complex reconstruction requirements
Therefore, surgical selection should be individualized based on imaging findings, neurological status, and spinal stability.
Thoracic ossification of the ligamentum flavum is a condition in which the ligamentum flavum gradually becomes hardened and replaced by bone-like tissue.
As the ossified ligament enlarges, it can compress the thoracic spinal cord and cause:
Lower extremity weakness
Numbness
Gait disturbance
Myelopathy symptoms
Not every patient requires surgery.
Patients without neurological symptoms may be monitored with regular clinical and imaging follow-up.
However, when spinal cord compression causes progressive neurological symptoms, surgical decompression is generally recommended.
Potential advantages of UBE include:
Smaller surgical incision
Reduced muscle damage
Better preservation of posterior spinal structures
Effective spinal cord decompression
Lower risk of postoperative instability in selected patients
Complete removal is not always necessary.
In patients with severe adhesion between OLF and dura, intentional floating of residual ossified tissue may provide adequate decompression while reducing the risk of dural injury.
The goal of surgery is neurological recovery, not necessarily complete radiological removal.
UBE may not be suitable for patients with:
Severe spinal deformity
Significant instability
Extensive fused OLF
Complex reconstruction requirements
These patients may require traditional decompression combined with spinal fusion.
UBE procedures require specialized minimally invasive spine instruments, including:
Endoscopic visualization systems
Working cannulas
Kerrison punches
High-speed burrs
Graspers
Bipolar coagulation instruments
The quality and precision of surgical instruments directly influence safety and surgical efficiency.
Unilateral biportal endoscopic decompression represents an important advancement in the surgical treatment of thoracic ossification of the ligamentum flavum.
Current clinical and radiological evidence demonstrates that UBE can achieve:
Significant expansion of the dural sac cross-sectional area
Effective neurological improvement
Meaningful pain reduction
High preservation of posterior spinal structures
Limited postoperative kyphotic progression
The study by Lee et al. provides valuable quantitative evidence supporting UBE as a tissue-preserving alternative for carefully selected thoracic OLF patients.
However, UBE should not be considered a universal replacement for open decompression. Patient selection remains critical, especially in cases involving severe dural ossification, spinal deformity, or instability.
Future research with larger multicenter studies and longer follow-up will further clarify the long-term role of UBE in minimally invasive thoracic spine surgery.
Lee SW, Jung J, Son SK, et al.
Radiologic and clinical outcomes of unilateral biportal endoscopic decompression for thoracic ossification of the ligamentum flavum.
European Spine Journal. 2026.
https://doi.org/10.1007/s00586-026-10138-5
