Neurosurgery notes/Spine/Special diseases of the spine/Ankylosing diseases/OLF

OLF

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Ankylosing diseases

General

  • Ossification of the Ligamentum Flavum (OLF)
  • Historically, OLF and OPLL have been referred to as "Japanese disease" due to fewer reported cases in Western countries, though this difference in prevalence might be due to variations in recognition by researchers.

Definition

  • OLF is a neurological disease where the ligamentum flavum undergoes degenerativechronic ossification, leading to compression of the spinal cord, nerve roots, conus medullaris, and cauda equina.

Epidemiology

  • Often combined with OPLL at cervical and/or thoracic levels.
    • 48.7% of 254 patients with cervical OPLL were combined with thoracic OLF.
    • 64.6% of patients with cervical OPLL had OLF, mainly in the thoracic spine.
    • 34% of cervical OPLL patients also had thoracic OLF in a study of 1,500 Japanese individuals.
  • Prevalence
    • In Japanese populations,
      • 4.5% of outpatient X-ray films.
    • 12% - 36% of CT scans
      • 15% in men and 7.7% in women
    • Another CT study of 3,013 Japanese patients reported a prevalence of thoracic OLF of 36%.
    • The prevalence rate of thoracic OLF was 16.9% in 2,134 Korean individuals.
  • Peak age:
    • Almost all in 50s in Japanese populations.

Pathophysiology

  • OLF usually occurs at the lower thoracic level.
    • The most common levels for OLF are T10/T11 or T11/T12.
    • At the lower thoracic level with kyphotic spinal alignment, strong traction force from rotational flexion movement of the thoracolumbar spine is believed to influence the ossification mechanism.
      • In contrast, cervical and lumbar spines (lordotic alignment) tend to form calcification without trabecular structure, where traction force is not thought to be strong.
  • Ligamentum flavum
    • Located within the spinal canal posterolaterally
    • Connecting two adjacent laminae
    • Composed of 80% elastic fiber and 20% collagen fiber.
  • OLF is considered a presentation of degenerative changes of the entire spinal column.
  • The cause is unclear, but two factors are conjectured:
    • Systematic factors
      • Heredity
        • Genetic studies suggest a role of genes like collagen 11A2 and collagen 6A1, as well as growth and transcription factors such as cartilage-derived morphogenetic protein-1, promyelocytic leukemia zinc finger, and tumor necrosis factor-alpha-stimulated gene 6 in OPLL formation.
        • Runt-related transcription factor 2s may be responsible for ectopic bone formation in spinal ligaments like OLF and OPLL in Chinese populations.
      • Abnormal carbohydrate/calcium metabolism
      • Abnormal gender hormone secretion
      • Ligament degeneration
    • Local factors
      • Mechanical stress to the enthesis, especially at the capsular portion → Activates chondrocytes to produce type-II collagen →Type-II collagen transforms into type-I collagen during endochondral ossification → forming bone within the ligament.

Clinical Presentation

  • Patients often experience local symptoms due to
    • Local symptoms
      • Examples
        • Back pain
        • Stiffness of back muscle
        • Pain on movement
        • Constriction-like discomfort from intercostal nerve irritation.
        • Due to general degenerative changes of the spinal column (vertebral body, intervertebral disc, facet joint, ligaments, paravertebral muscles).
  • Among causes of thoracic myelopathy requiring surgery, OLF accounts for 52%, OPLL for 12%, disc herniation for 11%, and combined OLF and OPLL for 9%.
  • Systemic neurological symptoms arise from compression of the spinal cord and spinal nerves:
    • Myelopathy
      • Example
        • Numbness, pain, and muscle weakness in the lower extremities.
          • Spastic or flaccid paralysis in the lower extremities.
        • Gait disturbance (e.g., spastic gait).
        • Bladder-bowel disturbance.
      • Due to OLF is primarily caused by a static compression mechanism, as vertebral movement in the thoracic spine is limited due to its rigid connection with the thoracic cage.
    • Conus medullaris syndrome
      • Supra-conus medullaris syndrome:
        • affects spinal cord segments L4-S2 (at ~T12 vertebral level).
        • Characterised by muscle atrophy and weakness below the knee, drop foot, and sensory disturbance below the knee to the perianal area, usually without root pain or bladder-bowel disturbance.
      • Conus medullaris syndrome:
        • affects spinal cord segments lower than S3 (at ~L1 vertebral level).
        • Characteristically, there is saddle-shaped sensory disturbance around the perianal and severe early bladder-bowel disturbance, without muscle weakness or abnormal deep tendon reflexes in genuine cases without root effect.
    • Cauda equina syndrome
  • Clinical examination
    • Determining the pathognomonic level is difficult at the thoracic spine.
    • Deep tendon reflexes
      • are considered the single reliable index;
      • dominantly higher reflexes in the lower extremities compared to upper extremities strongly suggest thoracic myelopathy, necessitating broad imaging.

Investigation

Radiology

  • X-ray films:
    • OLF at middle and lower thoracic levels can be seen laterally, but upper thoracic OLF is obscured by shoulder overlap.
  • CT:
    • Sato OLF classification is based on CT images
    • OLF with and without Dural ossification (Zhao 2023)
        • Red arrows: ossified ligamentum flavum
        • Yellow arrow: ossified dura.
        • Incidence of dural ossification in patients with thoracic OLF who underwent surgical decompression was approximately 35%.
        • Risk of dural ossification (DO) increased in
          • Sato classification:
            • Approximately 60% of cases with fused and tuberous OLF types are combined with ossification of the dura mater.
            • One study found that patients classified with the tuberous type had a significantly higher rate of DO (86%) compared to other OLF types (9%).
            • Due to severe spinal stenosis and increased local compression stress caused by tuberous OLF are major contributors to the development of DO.
          • Larger Supine Local Kyphosis Angle (LKA) (> 9°):
            • Measured using the traditional Cobb method, as the angle between the upper endplate of the second proximal adjacent vertebra and the lower endplate of the second caudal adjacent vertebra on CT
            • Large LKA may promote of DO through:
                1. Increase local mechanical stress on both the ligamentum flavum and the dura mater → promoting osteogenesis.
                1. Larke LKA → Backward migration of the spinal cord, → pushes dura mater and the ossified ligamentum flavum together → Promoting the transfer of osteogenic biological factors from the ossified ligamentum flavum to the dura mater.
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  • Magnetic Resonance Imaging (MRI):
    • Aim to identify the most pathognomonic OLF when multiple levels are affected.
    • There is not always a correlation between the degree of spinal cord compression seen on imaging and the severity of neurological findings; severe compression can exist with minimal or no symptoms.

Sato classification

  • Based on axial CT done at the middle level of the facet joint.
  • Categorised the configuration of OLF into five types:
      • Lateral type (A)
        • Bilateral ossifications are not fused at the centre of the lamina.
        • ossification of only the capsular portion of the ligamentum flavum.
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      • Extended type
        • Bilateral ossifications are not fused at the centre of the lamina.
        • extended ossification of the interlaminar portion.
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      • Enlarged type:
        • Bilateral ossifications are not fused at the centre of the lamina.
        • anteromedial thickening and enlargement of ossification
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      • Fused type
        • Bilateral ossifications are fused at the centre.
        • Without anterior growth of the fused ossified mass
        • Often spread to multiple levels and are combined with ossification of the dura mater in approximately 60% of cases.
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      • Tuberous type:
        • Bilateral ossifications are fused at the centre.
        • Anterior growth of the fused mass of ossification.
        • Often spread to multiple levels and are combined with ossification of the dura mater in approximately 60% of cases.
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Natural History

  • OLF is a chronically progressive compression disease that may worsen as ossification extends.
  • Clinical courses are variable;
    • some patients have no symptoms despite large OLF on imaging, while others experience progressively worsening neurological findings and symptoms.

Management

Conservative Treatment:

  • Not effective because OLF’s thoracic compression is mainly static, and dynamic mechanisms are less influenced by external orthoses like corsets.
  • Medications:
    • Nonsteroidal anti-inflammatory drugs (NSAIDs), muscle relaxants, vitamin B12, and palliative drugs for severe neuralgic pain in the lower extremities.
  • Physical therapy:
    • Recommended for back pain and stiffness.
    • Hot therapy to improve local blood circulation and walking are effective.
  • Do not perform:
    • Dynamic moving therapy of the vertebral column (e.g., massage, stretching by others) as it may add stress to the lesion.

Surgical Treatment:

  • IOM
    • Spinal cord monitoring aims to precisely identify the pathognomonic level.
    • However, reliability issues exist concerning the accuracy of wave interpretation and discrepancies between electrophysiological and imaging findings.
    • Examples include discrepancies between the most prominent OLF level in images and the level of positive waves in monitoring, and even false negatives where monitoring showed recovery but the patient developed complete motor paralysis post-surgery.
  • Indication:
    • Ineffective conservative treatment
    • Neurological deficit
      • Severe spastic gait
      • Severe muscle weakness of the lower extremities
      • Bladder-bowel disturbance,
    • If there is combination of OLF and OPLL at the same level
  • Early decompression surgery is advised due to OLF's progressive nature as a static compression disease.
  • Procedure:
    • Resection of the ossification and affected lamina via a posterior approach.
    • Options
      • Open-door type laminectomy,
      • En bloc laminectomy
      • Fenestration
      • Hemilaminectomy.
    • If the dura mater is ossified and adhered to the OLF, both the ossified dura mater and OLF are resected en bloc, preserving the arachnoid.
    • Prophylactic treatment for opening the dura mater from normal caudal and cranial sides is suggested for fused and prominent OLF types where dural ossification is common.
    • Postoperative Stability:
      • Despite wide resection of posterior skeletal structures, postoperative instability is uncommon, making additional instrumentation surgery generally unnecessary.
  • Surgical Results and Complications
    • Numbness of the lower extremities and spastic gait are known to hardly improve and often persist even after surgery.
    • Outcomes are often assessed using the Japanese Orthopaedic Association (JOA) score for myelopathy.
      • Tanaka et al. reported good results in 9 of 16 patients (2 excellent, 7 good), with better outcomes for OLF without OPLL.
      • Zhong et al. reported 9 excellent and 8 good results in 22 patients, with a mean Hirabayashi recovery rate of 65.49%.
    • Complications
      • Dural tear (5 patients in one study)
      • CSF leakage
      • Immediate postoperative neurological deterioration
      • Epidural hematoma
      • Wound infection
    • Risk factors for poor surgical outcomes
      • Disease duration
      • Combined ossification of the dura mater
      • Discontinuous type ossification of the DISH.
      • Preoperative severity of myelopathy