Imaging for your spondylolisthesis and spondylolysis patients can be confusing due to different and controversial etiologies

Spondylolisthesis and spondylolysis are common entities that present to clinicians, and as such, are important conditions to consider. Spondylolysis refers to a “splitting” of the vertebra and relates to an interruption of the pars interarticularis, also known as a pars defect.

Spondylolisthesis refers to the slippage or displacement of one vertebral segment on the level below and most commonly, but not exclusively, affects the lumbar spine. Spondylolisthesis may occur in all planes—anterior, posterior, or lateral. The convention for describing the direction of the slip always refers to the level above relative to the level below, and is most commonly in the anterior direction,in clinical practice, in which case the term anterolisthesis is applied. There are a variety of etiologies behind the development of spondylolisthesis, including spondylolysis.

L5 is the most common level for a spondylolytic spondylolisthesis to occur. However, the remaining lumbar vertebra may be affected, as well as the cervical spine.¹ In the cervical spine, the most common level is C6, and in these patients about 50% have an associated posterior schisis defect (spina bifida occulta) at the same level.

Pars defects are estimated to be present in approximately 5% to 7% of the Caucasian population. An interesting phenomenon has been observed in the Eskimo population where up to 28% of these individuals have pars defect in the upper lumbar spine, which is unusual. The papoose used in this group has been attributed to the premature encouragement of the upright position, thus placing biomechanical stress on the pars region.

There is an association with the presence of a pars defect and spinal dysraphism, that of spina bifida occulta, which may not necessarily be at the level of involvement.^1,2 If a spina bifida occulta is observed, particular attention should be drawn to scrutinizing the pars region.

Much controversy has surrounded the etiology of a pars defect. These include congenital, stress fracture, trauma, and pathologic causes related to underlying bone disease including osteopetrosis. Explanations behind the development of spondylolysis rest with the addition of undue stress on the pars region, resulting in microfractures at the site of the pars.

The presence of a spondylolysis in any species other than humans is extremely uncommon, and therefore, the assumption of the upright posture with repeated hyperextension has been implicated in the development of a pars defect. However, the etiology has not been fully resolved.

With stress fractures in other regions of the body, the fracture heals with rest. However, pars defects do not behave in this manner, because there is repeated stress on the region in combination with the lack of immobilization to the region at the time of the injuries.

The most common age for the detection of a pars defect is between 10 and 15 years and especially in those individuals who have participated in a high volume of sports activity with repeated hyperextension, such as gymnastics, diving, and weightlifting.

One of the causes of spondylolisthesis is that of a pars defect, categorized in the isthmic group of the Wiltse classification of spondylolisthesis. Etiology of spondylolisthesis includes: dysplastic, isthmic, degenerative, traumatic, and pathologic causes. Of this group, the most commonly encountered categories are the isthmic group under 30 years of age and the degenerative group in the older population.

It is also important to understand the method behind measuring or grading the degree of slippage for determining the cause and in some cases, deciding the clinical management of the patient. Meyerding’s scale is used in this evaluation.

The level below that of the spondylolisthesis is determined and a line is drawn along the superior end-plate or in the case of the sacrum, along the sacral base. The line is then divided in to quarters and labelled from posterior to anterior from one to four. The position of the posterior vertebral body line (George’s line) is then determined of the level involved. A line is dropped from George’s line to intersect with that of one of the quarters and the grade of spondylolisthesis is assigned, from grade one to four or may also be referred to the percentage of slippage from 0% to 100% relative to the level below.

If the level above has slipped so far anteriorly that there is no intersection with the level below a grade 5 spondylolisthesis is described. This is also known as spondyloptosis, or dropping vertebra, and may be associated with difficulties with childbirth. To have a grade 2 or more spondylolisthesis, or more than a 25% slippage, there has to be a pars defect to allow this degree of translation to occur. If a grade 2 or more anterolisthesis is noted, then it can be assumed that a pars defect is present even though it may be hard at times to see it. In such a case, lumbar oblique views would not be necessary to confirm the presence of a pars defect.

Much discussion has centered on the anterior translation of a vertebra relative to the level below, this being the most common direction of movement. However, particularly in degenerative disc disease, posterior, and lateral translation may occur. Degenerative spondylolisthesis occurs most commonly at L4, to females more than 40 years of age, as opposed to that related to a pars defect, which occurs most commonly at L5.

The most important decision with respect to the degenerative phenomenon is whether there is instability of the segment. The best imaging tool to evaluate instability for the clinician is that of a dynamic study of the lumbar spine using distraction and compression to determine the extent of movement. If more than 4mm of translation is noted, the segment is described as unstable, likely to have a poor prognosis with conservative treatment, and to be symptomatic.

Previous flexion-extension studies have evaluated the degree of movement, but this method is now considered less useful and may be due to the influence of the paraspinal muscles.¹

Furthermore, recent evidence points to the basic plain-film study of the lumbar spine as being a better tool to assess the possibilities of instability compared to the flexion-extension views.³ Kinematic MRI studies have confirmed the lack of useful information gained with flexion-extension techniques.⁴ In the future, a similar study involving distraction-compression would further add to the use of dynamic radiographic techniques in current practice.

The PA (or AP) view is also helpful in the evaluation of a spondylolisthesis. The pars defect may be visible as an irregular radiolucency located just inferior to the pedicle. Depending on the degree of slippage anteriorly, there may be superimposition of the L5 vertebral body and transverse process over that of the sacral base, giving rise to the appearance of an inverted Napoleon’s hat sign.

If a spondylolysis occurs unilaterally, the additional biomechanical stress on the contralateral side may lead to a stress response and sclerosis about the unaffected pedicle—the observation called the Wilkinson’s syndrome, and may produce diagnostic dilemmas as to the etiology of a sclerotic pedicle.

Further imaging of patients with spondylolisthesis and spondylolysis has included the use of special imaging techniques such as MRI, CT, radio-nuclide and single photon emission computed tomography (SPECT).⁵ CP

Michelle Wessely, BSc (Chiropractic), DC, DACBR, FCC (UK/Radiology), is the Departmental Head of Radiology at the Institut Franco-European de Chiropratique, Paris, France. She recently completed the Radiology Research Fellowship at the Department of Osteoradiology, Veterans Affairs Healthcare Center, San Diego, under the direction of Donald Resnick, MD, professor of radiology. Wessely can be reached via email: michelle_wessely@hot mail.com.

References
1. Yochum T, Rowe L. Natural History of Spondylolysis and Spondylolisthesis. In: Essentials of Skeletal Radiology. 2nd ed. Baltimore: Williams and Wilkins; 1996:327–372.
2. Kumar R, Niall D, Walsh A, Khalilullah K, McCormack D. Spina bifida occulta in isthmis spondylolisthesis: a surgical trap. Eur Spine J. 2002;11(2):159–161.
3. Pitkanen MT, Manninen HI, Lindgren KA, Sihvonen TA, Airaksinen O, Soirnakallio S. Segmental lumbar spine instability at flexion-extension radiography can be predicted by conventional radiography. Clin Radiol. 2002;57(7):632–639.
4. McGregor AH, Anderton L, Gedroyc WM, Johnson J, Hughes SP. The use of interventional open MRI to assess the kinematics of the lumbar spine in patients with spondylolisthesis. Spine. 2002;27(14):1582–1586.
5. Van der Wall H, Storey G, Magnussen J, Frater CJ, Pattinson R, Pereira J, Stephen J. Distinguishing scintigraphic features of spondylolysis. J Pediatr Orthop. 2002;22(3):308–311.