Operative treatments of the spine are becoming increasingly more common for the availability of a wide range of surgical and minimally invasive procedures. MR imaging allows for excellent evaluation of both normal and abnormal findings in the postoperative spine. This article provides the basic tools to evaluate complications after different operative procedures and offers an overview on the main topics a radiologist may encounter during his or her professional carrier.
Key points
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Imaging of the postoperative spine is a challenging task for the radiologist and requires a general knowledge of the surgical and the new minimally invasive procedures, and of the evolving spinal instrumentation.
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Thanks to its capabilities, MR imaging is crucial for the evaluation of patients with recurrent or new symptoms after surgery or minimally invasive techniques, including both early and late complications.
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Technical aspects have to be considered to reduce artifacts from metallic devices.
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For the correct interpretation of the postoperative spinal imaging, the radiologist must have detailed understanding of the initial pathologic condition, the surgical procedure performed, the clinical presentation of the patient, the time interval from the procedure to the imaging study, and the evolution of the expected postoperative changes.
Discussion of problem/clinical presentation
Neuroimaging following operative treatments of the spine, either by surgery or by minimally invasive procedures, depends on many factors, including cause of intervention, used technique, current symptoms, and time elapsed since procedure. Generally, postoperative neuroimaging is performed in patients with clinical symptoms (mostly pain with or without neurologic deficit), in which minor and major complications are to be excluded.
Postoperative complications may occur after both surgery and minimally invasive procedures. To understand the postoperative spinal neuroimaging, radiologists must know the operative and instrumentation options to explore the postprocedural complications.
Discussion of problem/clinical presentation
Neuroimaging following operative treatments of the spine, either by surgery or by minimally invasive procedures, depends on many factors, including cause of intervention, used technique, current symptoms, and time elapsed since procedure. Generally, postoperative neuroimaging is performed in patients with clinical symptoms (mostly pain with or without neurologic deficit), in which minor and major complications are to be excluded.
Postoperative complications may occur after both surgery and minimally invasive procedures. To understand the postoperative spinal neuroimaging, radiologists must know the operative and instrumentation options to explore the postprocedural complications.
Spinal treatment procedures in pills and new trends
Classically, spinal treatments can be categorized as follows:
- a.
Decompressive, performed to remove herniated disc material or to relieve a segment of spinal stenosis.
- b.
Spinal stabilization/fusion procedures, in cases of spinal instability from degenerative disc disease, spondylolisthesis, trauma, tumors, infections, and iatrogenic causes, such as prior surgery.
- c.
A combination of both.
Surgery
Spine surgery is used to treat diseases and injuries affecting the spinal column, including degenerative disorders, trauma, instability, deformities, infections, and tumors.
Surgical decompressive procedures include discectomy, laminotomy, laminectomy, and facetectomy. The term laminotomy refers to removal of only the inferior margin of the lamina and is often used in cases of microdiscectomy. In unilateral laminectomy, the entire lamina on one side of the spinous process is removed. Total or bilateral laminectomy involves removal of the lamina on both sides, plus the spinous process.
Surgical fusion procedures are often categorized based on the direction from which the spine is approached (anterior, posterior, lateral, caudal) as well as on their degree of invasiveness. Surgical approaches are presented in Table 1 .
| Type | Features |
|---|---|
| Cervical spine approaches | |
| Anterior cervical approaches | |
| Transoral-transpharyngeal approach | Access to anterior clivus, C1 and C2 |
| Anteromedial approach | Complications can be spinal cord and, rarely, vascular injury |
| Posterior cervical approaches | |
| Laminotomy, laminectomy, laminoplasty | Degenerative spondylosis, disc herniations Complications are vertebral artery injury, post-laminectomy kyphosis, and new cervical radiculopathy |
| Posterior fusion hardware | Posterior cervical fusion typically involves lateral mass screws from C3 to C6, with traditional pedicle screws being reserved for the larger C2 and C7 levels |
| Thoracic approaches | Relatively high incidence of neurologic injury |
| Posterior and posterolateral approach | Transpedicular, transfacet, and transforaminal |
| Costotransversectomy | — |
| Lateral extracavitary approach | — |
| Lumbar approaches | |
| Anterior lumbar approach | Performed when posterior decompression is not required; vascular complications are reported less than 5% |
| Posterior lumbar approach | |
| Standard open discectomy | Laminotomy or hemilaminectomy, resection of the ligamentum flavum and retraction of neural elements |
| Posterior lumbar interbody fusion (PLIF) | Involves bilateral laminectomies and partial facetectomy |
| Transforaminal lumbar interbody fusion | Variation of PLIF through the foramen |
| Posterolateral fusion | Alternative or supplement to PLIF: bone graft is placed laterally between the transverse processes |
| Total disc replacement | Alternative to spinal fusion. Used for discogenic pain without significant spondylolisthesis |
Endoscopic Surgery
Endoscopic operations are now considered standard for intraforaminal/extraforaminal disc herniations. The most common full endoscopic technique for patients with lumbar disc afflictions is the posterolateral transforaminal approach, but also full endoscopic interlaminar access was developed. Laser and bipolar radiofrequency current can be used. Basically, the transforaminal procedure has more limitations than the interlaminar, but it is less traumatic for the tissue.
The combination of new operative accesses with technical advances now enables a full endoscopic procedure with visual control, which is considered equal to conventional operations when the indication criteria are heeded.
Indications for endoscopic surgery include the following:
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Sequestered or nonsequestered lumbar disc herniations, independent of localization;
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Recurrent disc herniations after conventional or full endoscopic operations;
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Lateral bony and ligamentary spinal canal stenoses;
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In selected cases, cysts of the zygapophyseal joint;
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In selected cases, positioning of implants in the intervertebral space;
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In selected cases, intervertebral debridement in spondylodiscitis.
Minimally Invasive Procedures: a Quick Look
In recent years, minimally invasive and/or less-invasive spine surgery and nonfusion devices have been developed. Minimally invasive techniques can reduce tissue damage and its consequences.
Vertebroplasty and balloon kyphoplasty may be carried out by vertebral interventional radiologists, orthopedists, and neurosurgeons.
Vertebroplasty describes a percutaneous procedure that introduces bone graft or acrylic cement (cementoplasty) to mechanically augment weakened vertebral bodies. Polymethylmethacrylate (PMMA) is the acrylic most commonly used as a bone filler in the treatment of pathologic and nonpathologic vertebral compression fractures.
In addition, cementoplasty can be used in combination with fusion techniques, because anchorage of pedicle screws can be improved with cement augmentation. Augmentation of the screw can be achieved using 2 different techniques: (1) cement insertion through cannulated pedicle screws with slots, or (2) vertebroplasty followed by insertion of the pedicle screw into the cement (either open or minimally invasive).
Balloon kyphoplasty describes a percutaneous procedure that introduces an expansive device within the vertebral body to create a cavity that can be filled by cement. The goal is to achieve reduction of the fracture without injuring the lateral margin of the vertebral body. Recently, many variants of balloon kyphoplasty are introduced. New titanium or polyetheretherketone implants can be used to obtain vertebral augmentation and relevant high restoration of vertebral body in combination with less amount of PMMA or biologic cement injection.
New Trends
Recent advances have led to a number of technical developments including the following:
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Spinal navigation
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Fluoroscopy
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Spinal implants
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Bone substitutes, stem cells, and growth factors
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Endoscopy
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Microscopy
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Neurophysiological monitoring
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Improved instruments and retractors
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High-frequency surgery.
Neuroradiological evaluation
Postoperative spinal imaging techniques include radiographs, computed tomography (CT), and magnetic resonance (MR) imaging. CT and MR imaging may be performed before and after contrast media injection. Generally, radiographs are not used in the diagnosis of early or late postoperative complications but only to check the positioning of the metallic implants.
Even though the role of CT in the postsurgical spine is marginal, it is useful to check correct positioning of metallic implants after instrumentation or fusion procedures, to show laminotomy/laminectomy defect, to evaluate postoperative spinal stenosis, and the result of the spinal stabilization, as well as acute hematoma or gas-filled collections.
Magnetic Resonance Imaging
The high spatial and contrast resolution of MR imaging allow for better evaluation of soft tissues, bone marrow, and intraspinal content. Thus, MR imaging is the modality of choice in cases in which postoperative complications are suspected. Patient positioning is reported in Box 1 .
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Supine position, if possible feet first, to diminish claustrophobia
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Patient as parallel as possible to the long axis of the magnet bore, to minimize inadvertent oblique positioning and to reduce the distorting effects of any underlying scoliosis
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Center of the coil(s) at the center of the region of interest, and in turn to the center of the magnet bore
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Lumbo-sacral spine: some authors propose to avoid a knee support, as this reduces the lumbar lordosis, and may lead to underestimation of the size and presence of disc herniation
There is no established protocol for the study of the postoperative spine with MR imaging. A routine protocol including sagittal fast-turbo spin echo (F-TSE) T2-weighted and T1-weighted and short-tau inversion-recovery (STIR) images, axial T1- weighted and T2-weighted, would suffice in most cases.
In the sagittal plane, T1-weighted and T2-weighted images offer complementary information. On T2-weighted images, normal intervertebral discs are bright. When degeneration, water loss, and collagen deposition occur, T2 relaxation time shortens and the discs gradually become darker (ie, low-signal degenerative or “black-disc” disease). Sagittal and axial T2-weighted images are also excellent for showing the spinal cord and the nerve roots of the cauda equina. Central spinal canal stenosis and impressions on the thecal sac are most easily recognized.
T1-weighted, STIR, or fat-suppressed T2-weighted images are sensitive to many bone-marrow diseases.
The normal epidural fat in the spine is bright on T1-weighted images and contrasts well with the dural sac and the adjacent normal or pathologic intervertebral disc. This is why axial T1-weighted images should be performed in the lumbar region. T1-weighted images are also excellent to differentiate between osteophytes and soft disc material.
Three-dimensional (3D)-TSE sequences have been suggested for MR imaging myelography. However, single-shot wide-slab T2-weighted sequences with a very long echo time have been proposed, making it possible to obtain a very short time imaging for single different views by running the sequence in different orientations, eliminating postprocessing.
Administration of a gadolinium-based contrast medium is particularly useful in patients with suspicious infection or previous discectomy, as discussed later. In gadolinium-enhanced T1-weighted images, fat-suppression techniques are used to increase the conspicuity of gadolinium-enhancement.
Metallic implants may create magnetic susceptibility artifacts. Metals that are not superparamagnetic, such as titanium, produce primarily radiofrequency artifacts, which are less marked, but may still obscure the neural foramina in the presence of pedicular screws. Sequences have been developed to reduce artifacts, but their use may necessitate increased image acquisition time and may result in image distortion. The technical aspects to be considered for artifact reduction are presented in Box 2 .
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Fast spin echo sequences are better than conventional spin echo (SE) sequences, and these latter are better than gradient echo sequences. The shortest echo time possible is recommended for metal artifact reduction in SE sequences.
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Short-tau inversion-recovery sequences should be used for fat suppression, as sequences based on selective fat-saturation pulses are associated with poor homogeneity.
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Increase bandwidth.
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Decrease voxel size.
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Adjust frequency-encoding direction: it should be parallel to the long axis of pedicle screw, as the artifact produced will be linear and parallel to the metal material.
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Use of chemical shift fat-suppression techniques.
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Multisequence imaging (MAVRIC, SEMAC).
Postoperative imaging after surgical discectomy/herniectomy: normal versus pathologic
Immediate postoperative imaging, in the first 6 to 8 postsurgical weeks, must be carefully evaluated considering changes in bone and soft tissues in relation to type and extent of surgery and time since the operation.
In early unenhanced images, postdiscectomy changes may mimic a residual disc herniation because of disruption of the annulus fibrosus and epidural tissue edema. Granulation tissue and/or fibrosis may physiologically present with mild epidural mass effect and homogeneous gadolinium-enhancement.
Epidural/peridural fibrosis consists of scar tissue that causes adherence of neural elements to other structures. Because scarring is part of the normal reparative mechanism of tissue after surgery, most patients with epidural fibrosis are asymptomatic. Fibrosis-induced pain may be due to irritation, compression, and traction of the fibrotic tissue on adjacent nerve structures.
The main differential diagnosis of epidural fibrosis is recurrent disc herniation.
Recurrent disc herniation is the most common complication after discectomy. It is defined as disc herniation at the level of prior surgery, and may be ipsilateral or contralateral to the previous herniation.
The reported incidence of recurrent disc herniation after lumbar discectomy varies between 3% and 18%. It is important to remember that residual disc herniation and epidural fibrosis are not certainly pathologic because extruded disc fragment can regress spontaneously mainly by phagocytosis and the minor dural sac deforming mass effect deriving from scar tissue usually diminishes within 6 months after intervention. However, deformity of the dural sac accompanying epidural scar is to be considered abnormal when observed 6 months or more after surgery.
Differential diagnosis between recurrent disc herniation and peridural fibrosis is important: notably, epidural scar does not benefit from reoperation.
The herniated disc tissues are usually isointense to the parent disc on T1-weighted images and isointense to hyperintense to the disc on T2-weighted images ( Fig. 1 ). After gadolinium administration, the disc material shows no enhancement; however, peripheral enhancement may be observed because of the granulation or dilated tissue of the adjacent epidural plexus.
If there is homogeneous diffuse enhancement, peridural fibrosis should be suspected; however, recurrent disc herniation may show a variable amount of enhancement in delayed scans. This is the result of gadolinium diffusion from adjacent vascularized granulation tissues into the capacious extracellular spaces of the relatively avascular disc and thus simulate peridural fibrosis. It is, therefore, mandatory to scan the patient as soon as possible after the administration of contrast.
Teaching points
Major differentiating criteria between recurrent disc herniation and epidural fibrosis include the following:
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Obliteration of the epidural fat by uniformly enhancing epidural fibrosis in the anterior, lateral, and posterior epidural space in epidural fibrosis (T1 high signal of normal epidural fat also contrasts well with postoperative epidural fibrosis, which is dark);
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Lack of early central gadolinium-enhancement in recurrent or residual disc herniation;
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Homogeneous gadolinium-enhancement pattern of scar tissue.
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Furthermore, slight deformation of the thecal sac and gadolinium-enhancement of nerve roots, defining aseptic radiculitis, are expected after surgery. They reflect transient sterile inflammation within the nerve root undergoing repair (see Fig. 1 ). However, differentiation between abnormal postoperative nerve roots enhancement and normal slight pial-root enhancement, remains challenging within 6 months after surgery, but in the late phase should be considered abnormal. In asymptomatic patients, also aseptic reactive vertebral endplates and posterior annulus enhancement can be seen between 6 and 18 months.
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