Lumbar Spine: Disc Disease and Stenosis

Imaging Protocols

To achieve a high degree of accuracy, it is imperative that the proper imaging protocol be observed. Thin-section axial images (4 or 5 mm) should be obtained from the midbody of L-3 to the midbody of S-1. Angling of the plane of imaging to be parallel to the discs is not necessary ( Fig. 11.2 ), and contiguous images without skip areas are considered mandatory ( Fig. 11.3 ). Even though sagittal images will be obtained, a host of entities are more easily identified on the axial images than on the sagittal. These include migrated free disc fragments, spondylolysis (pars breaks), conjoined nerve roots, the facets, the neuroforamen, the lateral recesses, and intraspinal synovial cysts. We looked at 103 consecutive spine MRI studies in which we obtained our standard axial and sagittal T1- and T2-weighted sequences; the axial cuts were contiguous, stacked images without gaps or angling. In addition, we obtained axial images that were angled parallel to the discs and covered only the disc spaces, thereby leaving a gap between one level and the next ( Fig. 11.2 ). The angled axial images were combined with the sagittal images and evaluated for free disc fragments and pars breaks, and then compared to the original interpretations, which had the stacked axials and the sagittals. Our standard imaging protocol found 15 cases of spondylolysis and 8 free disc fragments. The angled axial protocol only showed 8 cases of spondylolysis and 3 free fragments, even though the sagittal images were available. We missed three-quarters of the free fragments! These patients would likely have all had failed surgery with the angled axial protocol. There’s no reason I’ve ever heard of for angling the axial cuts and leaving gaps, yet about a third of all the consult spine cases I look at use this protocol. I think it’s the number one cause of missed diagnoses in spine imaging.

Both T1- (or proton density) weighted and T2- (or T2∗-) weighted images should be obtained in both the sagittal and the axial planes. Coronal images have not been shown to add additional information and are not typically included.

Disc Disease

Terminology plays a large role in how radiologists describe disc bulges or protrusions. Since the advent of CT in the 1970s, disc bulges have been described by their morphology. A broad-based disc bulge was said to be a bulging annulus fibrosus, while a focal disc bulge was called a herniated nucleus pulposus (HNP) ( Fig. 11.4 ). This is no longer the accepted terminology.

Although no universally accepted classification of disc disease is present, most agree on something like the following: a broad-based disc bulge is called a bulge; a focal bulge is called a protrusion; a piece of disc that has migrated from the parent disc is a free fragment or sequestration. The term HNP is no longer considered acceptable. More significantly, most surgeons do not care what name is applied to a disc bulge—they do not treat a bulging annulus any differently than a protrusion. They treat the patient’s symptoms and have to decide if the disc bulge is responsible for those symptoms. It has been shown in multiple studies that from 40% to 50% of asymptomatic people have disc bulges or protrusions; hence, just seeing a disc bulge on CT or MR imaging does not mean it is clinically significant.

One of the most widely used classifications has the terms protrusion , extrusion , and extruded as the basis for describing the type of disc bulge present. I have seen this terminology cause misadventures for patients because it was either used incorrectly by the radiologist or not understood by the surgeon. In this classification, a protrusion is a focal disc bulge with a wide neck or base ( Fig. 11.5A ), an extrusion is a focal bulge with a narrow neck or base ( Fig. 11.5B ), and an extruded disc is a free disc fragment ( Fig. 11.5C ). Surgeons don’t treat a patient with a protrusion any differently than one with an extrusion—it’s an artificial distinction based solely on the neck width of the bulge. If that’s all it were, so what, but it’s much more significant because of the term extruded , which is a free fragment. Missing a free disc fragment is one of the leading causes of failed back surgery, and identifying it will guide the surgeon to search for it and remove it. I have seen several cases in which the terms extrusion and extruded were misused or not understood, and patients had free fragments left behind (the surgeon didn’t realize the term extruded meant a free fragment), and in others, the surgeon searched for a free fragment when there was none because the term extruded was incorrectly applied. Since there is no clinical or surgical difference between having a protrusion or an extrusion, and surgical mistakes can occur when this terminology is employed, I do not let my residents and fellows use these terms.

MR imaging has a high degree of accuracy in delineating disc protrusions and showing if neural tissue is impressed ( Fig. 11.6 ). MR imaging can also show if annular fibers of the disc are disrupted ( Fig. 11.7 )—a so-called HIZ (high-intensity zone). Although CT cannot diagnose annular tears, clinicians do not currently treat them surgically. The annulus is innervated by the sinuvertebral nerve, which goes to the dorsal root ganglion and can mimic a focal disc protrusion at that level. They can cause back pain and even sciatica (buttock and leg pain), but typically resolve with conservative management.

Free Fragments

A type of disc abnormality that is critical to diagnose is the free fragment or sequestration. Missed free fragments are one of the most common causes of failed back surgery. The preoperative diagnosis of a free fragment means the surgeon needs to explore more cephalad or caudally during the surgery in order to remove the free fragment. As free fragments can be very difficult to diagnose clinically, imaging is critical in the evaluation of the spine for any patient contemplating surgery. At times, it can be difficult to be absolutely certain as to whether or not a disc that lies above or below the disc space is still attached to the parent disc or is really “free.” So long as disc material is above or below the level of the disc space, it really does not matter if it is attached or not. The key element is recognizing that disc material is present away from the level of the disc space (caudally or cranially) so the surgeon will be aware that he may have to increase his exposure to find and account for the additional disc material, whether it is attached to the parent disc or not.

Free fragments are diagnosed on MR imaging by noting disc material cephalad or caudal to the disc space ( Fig. 11.8 ). Free fragments may migrate either cranially or caudally, with no apparent preference.

Axial images often show the free fragment more conspicuously than the sagittal images ( Fig. 11.9 ); therefore contiguous axial images without large skip areas or gaps are imperative in order to not miss free fragments.

A conjoined root, which is a normal variant of two roots exiting the thecal sac together or in an anomalous manner (seen in 1% to 3% of the population) ( Fig. 11.10 ), or a Tarlov cyst, a normal variant in which a nerve root sleeve is dilated, can have a similar appearance to a free fragment but can almost always be differentiated from disc material by their signal staying isointense to the thecal sac on both T1 and T2 sequences. It is critical to identify a conjoined root or a Tarlov cyst and not confuse them for a free fragment. A surgeon will often change his procedure and certainly his amount of surgical exploration if he thinks there is a free fragment present. Many surgeons have inadvertently damaged conjoined or anomalous nerve roots, thinking they were free fragments. Obviously, a free fragment should be removed, and a conjoined nerve root should be left alone—the imaging study is where that difference should be ascertained, not during surgery.

Lateral Discs

Discs will occasionally protrude in a lateral direction, causing the nerve root that has already exited the central canal to be stretched ( Fig. 11.11 ). Although not common (less than 5% of cases), these are frequently overlooked and are known to be a source of failed back surgery. Since they affect the previously exited root, they can clinically mimic symptoms of a disc protrusion from one level more cephalad ( Fig. 11.12 ). For example, in a patient with multilevel disc disease and symptoms referable to the L3-4 disc, the disc protrusion is usually a posterior L3-4 bulge that impresses the L4 nerve root. However, a lateral disc at L4-5 could impress the L4 nerve root and cause the same symptoms. If not noticed, surgery could be performed at the L3-4 disc—the wrong level. Unfortunately, I have seen this on several occasions. Also, it is important to notify the surgeon that the disc is lateral to the neuroforamen, as a standard surgical approach through the lamina might not allow removal of a lateral disc.

Lateral discs are best identified on axial images. Sagittal images will often show a lateral disc occluding a neuroforamen, but many times, a lateral disc will not extend into the foramen, and the sagittal images will appear normal.