Herniated intervertebral disk

Clinical assessment

- Original disk not re- This may occur if a disk fragment is left in the inter-moved vertebral disk space, or if the wrong disk level was removed. Patients will continue to have the preoperative leg pain, due to continued mechanical compression and inflammation of the same nerve root. Patients will wake up from surgery complaining of the same preoperative pain, and will continue without ever being pain free. Patients will benefit from repeat surgery

- Recurrent disk at the Patients will develop a sudden onset of leg pain identi-same level cal to the preoperative pain, after a pain-free period of several months. An additional operation is indicated. In the case of recurrent disk at different level, patients will have a pain-free interval of more than six months, and suffer a sudden onset of leg and/or back pain. The neurological symptoms and the radiological findings, however, will be at a different level from the preoperative condition. Repeat surgery yields very good results

CT scan

- Without enhance- Recurrent disk material causes a nonspecific mass ef-ment fect, has a density of more than 90 HU, may show a gas or calcium collection and nodularity, does not conform to the margins of the thecal sac, and tends to have sharp margins. The majority of the disk material is centered at the intervertebral disk space

- With intravenous Herniated disk material does not enhance early on contrast after contrast administration. The disk material, however, enhances on the delayed CT scan images (e.g., 40 minutes after injection of the contrast material). Disks are typically seen as areas of decreased attenuation with a peripheral rim of enhancement, whereas epidural scar enhances homogeneously

MRI Within six weeks of surgery, the site of the operation shows a large amount of tissue disruption and edema (producing a mass effect on the anterior thecal wall) that is heterogenously isointense to muscle on T1-weighted images and increased on T2-weighted images. These disruptions heal within two to six months postoperatively. MRI may be used in the immediate postoperative period for a larger-scale view of the thecal SCA and epidural space, to exclude significant hemorrhage, pseudomeningocele, or disk space infection. Even using CT myelography, it is extremely difficult to distinguish between these entities on MRI, as they all appear as nonspecific extradural mass effects. Herniated disks show contiguity with the parent disk space (except for free fragments) and mass effect. Small protruding disks are low in signal intensity on T2-weighted images, whereas larger protruding, extruded, and free fragments can show a central high signal intensity on T2-weighted images. Recurrent herniations display a smooth polypoid configuration, with a hypointense rim outlining the high signal-intensity herniations, and this helps to distinguish the herniated material from the adjacent CSF on T2-weighted images

Fibrosis (scar tissue) Six weeks to six months after lumbar spinal surgery, there is a gradual replacement of the immediate postoperative changes by posterior scar tissue. Fibrosis can be extradural (the most common type) and intradural (arachnoiditis). Patients with arachnoiditis have a history of multiple lumbar spine operations, with pain-free intervals ranging between one and six months. They usually complain of both back and leg pain in varying degrees, and the neurological evaluation is inconclusive.

The diagnosis of scar tissue versus disk is extremely important. Surgery is not indicated for scar (epidural fibrosis), but may be beneficial if the disk can be diagnosed as a cause of the radiculopathy

Arachnoiditis The definitive studies for diagnosing arachnoiditis are:

- Myelography The myelographic findings of mild arachnoiditis are blunting of the caudal nerve root sleeves, segmental nerve root fusion, and small irregularities of the thecal sac margin. Multisegmental nerve root fusion, with root sleeve obliteration, intradural scarring, and locu-lation, is seen with moderate arachnoiditis. Severe adhesive arachnoiditis may cause a myelographic block

- Postmyelography CT CT scanning reveals nodular or cord-like intradural masses with moderate disease. Sometimes the nerve roots are annealed against the dura, and the thecal SCA appears empty or featureless

- MRI The MRI findings in arachnoiditis include intradural fi-

brosis, nerve root clumping, loculation and saccula-tion, root retraction, and adhesions

Epidural scar tissue

The best means of trying to identify epidural scar tissue are:

CT scan with and without enhancement (CT without contrast has been found correct 43 % of the time, while CT with contrast was correct 74% of the time in differentiating between scar tissue and disk material)

• Scar tissue causes retraction of the thecal sac to the surgical site, conforming to the thecal sac margin

• Linear strand-like densities occur within scar tissue

• The majority of the scar tissue is seen above or below the particular disk level

• Scar tissue shows attenuation of 75 HU or less, and shows contrast enhancement

Precontrast and postcontrast MRI has a 96% accuracy in differentiating between scar tissue and disk material

• Scar tissue enhances consistently immediately after injection on T1-weighted images. This enhancement occurs regardless of the time since surgery, even when surgery was over 20 years previously

• Scar tissue may occasionally show a mass effect, and should not be used as a major discriminator between epidural fibrosis and disk material

Lumbar spinal stenosis Cauda equina compression from central spinal stenosis results in neurogenic claudication, with bilateral leg pain that begins after walking a short distance. The pain is not well localized, and often is more of a dyses-thesia than true pain

- MRI with enhancement

Plain radiography The interpediculate distance increases from T12 to

L5. Interpediculate measurements of less than 16 mm at L4-5, or less than 20 mm at L5-1, and canal cross-sectional areas of less than 1.45 cm2 are considered abnormal

CT CT scanning shows bony encroachment onto the neural elements, and is especially useful in evaluating the lateral recesses and foramina. A cross-sectional area of less than 100 mm2 is abnormal

MRI Because soft tissue, such as the intervertebral disk and ligamentum flavum, contributes significantly to most cases of stenosis, MRI is useful. Sclerotic bone will have a low signal intensity on T1-weighted images and T2-weighted images, and is recognized by encroachment onto the epidural and foraminal fat. Osteophytes containing fatty marrow are recognized by their high signal intensity on T1-weighted images. Sagittal images are most useful in defining bony foraminal stenosis, or more generalized stenosis secondary to disk degeneration, with lost disk space height and rostrocaudal subluxation of the facets

Lumbar instability Instability of the lumbar spine causes pain on a mechanical basis in the multiple spine surgery patient. A coexisting spondylolisthesis, pseudoarthrosis, or an excessively wide bilateral laminectomy can cause spinal instability. These patients complain of back pain associated with activity (mechanical), and their physical examination may be negative. The diagnosis of lumbar spinal instability is based on plain radiographic features

Radiological elements Point value

Destruction or loss of function of the anterior elements 2

Destruction or loss of function of the posterior elements 2

Radiographic criteria 4 Flexion-extension radiographs

- Sagittal plane translation > 4.5 mm or 15% 2

- Sagittal plane rotation > 15°at L1-2, L2-3, and L3-4, 2 > 20% at L4-5, > 25% at L5-S1

Cauda equina damage 3

Dangerous loading anticipated 1

Instability is represented by a total score of 5 or more

CSF: cerebrospinal fluid; CT: computed tomography; HU: Hounsfleld unit; MRI: magnetic resonance Imaging; SCA: superior cerebellar artery.

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