Spondylolisthesis simply means the abnormal slip of one vertebra with respect to the vertebra immediately below. This is usually occurs forwards (anteriorly) of the upper vertebra, also known as anterolisthesis. Less common is a backwards (posterior) slip, also known as retrolisthesis. Much less common is a sideways (lateral) slip, called a laterolisthesis.
For most cases of isthmic spondylolisthesis (especially Grades I and II), treatment consists of temporary bed rest, restriction of the activities that caused the onset of symptoms, pain/anti-inflammatory medications, steroid-anesthetic injections, physical therapy and/or spinal bracing.
Spinal stenosis is due to congenitally short pedicles, or it may be acquired as a result of combinedfacet hypertrophy, degenerated bulging disk, and hypertrophy of the ligamentum flavum. Congenitalspinal stenosis can be idiopathic or associated with a developmental disorder, such as achondroplasia,hypochondroplasia, Morquio's mucopoly-saccharidosis, and Down's syndrome. Spondylolisthesis,trauma, and surgical fusion are other causes of spinal stenosis.
Juxtaarticular synovial cysts are associated with facet arthropathy, generally of fairly severedegree. They consist of a fibrous wall, often with a distinct synovial lining, and a cystic center thatmay or may not communicate with the facet joint. They are found most frequently at L4-5, the moremobile segment of the lumbar spine. Synovial cysts can compress the dorsal nerve roots and causeradicular symptoms.
When bulging disks, spondylosis, and ligamentum flavum hypertrophy progress to constrict thespinal canal and cord, a spinal stenosis develops. These changes are depicted on sagittal gradient-echo or T2-weighted images as hourglass narrowing of the thecal sac, usually involving multiple levelsin the mid- and lower cervical region. In patients with a congenitally borderline or narrow canal,relatively mild degenerative changes are sufficient to cause spinal stenosis. On T1-weighted scans,canal stenosis results in scalloping of the normally smooth dorsal and ventral margins of the cord. As learned from myelography, the degree of canal stenosis and cord scalloping shown on the imagesis greater when the neck is in a hyperextended position, due to buckling of the ligamentum flava. Imaging in a neutral position may show less severe stenosis. Nonetheless, the hyperextended viewillustrates what happens to the cord with acute hyperextension. The spinal cord is more susceptibleto traumatic injury in patients with spinal stenosis.
Lytic spondylolisthesis usually occurs at L5/S1 and normally presents in the teenage years or 20s. The classical example is the so-called fast (cricket) bowler’s “stress fracture”. It occurs due to repetitive stresses in the lumbar spine but it often appears with no obvious history of repetitive trauma.
Lytic spondylolisthesis may be treated with a though occasionally the lytic area can be repaired, thus avoiding a fusion between the two veretbrae.
The neural foramina are visualized on parasagittal images of the lumbar spine, and disk herniationcan be detected by obliteration of foraminal fat. Nevertheless, axial MR is better for visualizinglateral disk herniations. Lateral disks compress the nerve root within the foramen or just beyond itslateral margin distal to the nerve root sheath.
Degenerative spondylolisthesis usually occurs in older women, most often at L4/5, but can occur at other levels. This type of slip is due to degeneration of the pair of facet joints between the two affected vertebrae. It is virtually never worse than grade 1.
Isthmic spondylolisthesis can be the result of a genetic failure of bone formation in the spinal vertebrae; usually physical stresses to the spine then break down the weak or insufficiently formed vertebral components. Repeated heavy lifting, stooping, or twisting can cause small fractures to occur in the vertebral structure and lead to the slippage of one vertebra over another. Weightlifters, football players, and gymnasts often suffer from this disorder due to the considerable stress placed on their spines.
There are several methods used to “grade” the degree of slippage ranging from mild to most severe. Your surgeon will discuss with you the extent of your spondylolisthesis and how the severity indicates the type of treatment that is needed.
Symptomatic thoracic disks are uncommon, accounting for about 1% of alldisk herniations. The rib cage, small intervertebral disks, and coronalorientation of the facets joints all contribute to limited mobility of thethoracic spine, and consequently, a lower risk of disk herniation. The mostcommon level is T11-T12, where the spine is relatively less rigid. SagittalT2-weighted FSE sequences are excellent for displaying indentation of ventralthecal sac and impingement of the spinal cord by thoracic disks. Axial imageshelp delineate lateralization to either side. Disk morphology is similarto the cervical region. Calcification is more common in thoracic disk fragmentsand parent disks than in cervical or lumbar region.
In the evaluation of degenerative spine disease, multiple anatomic sites need to be imaged,including the intervertebral disk, spinal canal, spinal cord, nerve roots, neuroforamina, facet joints,and the soft tissues within and surrounding the spine. Many pulse sequences are available, andspecific protocols vary among different MR sites. There is general agreement that the spine needsto be imaged in at least two planes, and surface coils are used almost exclusively. In the cervical andthoracic regions a T2-weighted sequence is mandatory to assess damage to the spinal cord. Thinsections are required to visualize the neuroforamina, and pulse sequences must be tailored tocounteract CSF flow and physiologic motion. The imaging requirements for the lumbar spine are lessstrenuous because the anatomical parts are larger. Most protocols include a T1-weighted sequenceand some type of T2-weighted sequence to give a myelographic effect. Fast spin-echo (FSE)techniques allow enormous time savings, and if available, they have replaced conventional spin-echofor T2-weighted imaging of the spine. Three-dimensional gradient-echo (GRE) methods can achieveslice thicknesses less than one millimeter, an advantage for displaying cervical neuroforamina.
With the pars defect divides the vertebrainto an anterior part (vertebral body,pedicles, transverse processes, andsuperior articular facet) and aposterior part (inferior facet, laminae, and spinous process). The anterior part slips forward, leavingthe posterior part behind. As a result, the spinal canal elongates in its anteroposterior dimension, sothat spinal canal stenosis is uncommon with isthmic spondylolisthesis. Grade I spondylolisthesis isoften asymptomatic, but with progressive anterior subluxation, the intervertebral disk and theposterior-superior aspect of the vertebral body below encroach on the superior portion of the neuralforamen. The foramen is also elongated in a horizontal direction and may have a bilobedconfiguration. Exuberant fibrocartilage at the pars pseudarthrosis can further compromise the neuralforamen and cause nerve root compression.