While all of us received training which prepared us to read static studies- and for us chiropractors, most especially the cervical spinal plain film series- none of us really ever had any experience with motion studies. Walter Pierce, one of the originators of the Pierce-Stillwagen technique, was using a videofluoroscope during his seminars in the late 1980’s when I was in school to demonstrate how much motion occurred when a P-A thrust was applied to the individual cervical segments, but the persistence of the phosphorescent screen obscured some of what was seen, and subtleties were difficult to pick up. But with the invention of the Visualizer 2000, the blurring was eliminated, and the motion is not only easy to watch, it can be stopped for measurement during any phase of the filming. To interpret the films, remember first that any comment that can be made about a static film can also be made about a motion film, so the DMX study is read for the same things for which you would read a static film.
So what do you look for? Let’s take a look at what can be seen on each of the views.
The Neutral Lateral Views (1)
- The nodding projection
The first thing you need to know is that this view isn’t done the way you would imagine it would be done, by dipping your head down and forward, as a human does when he/she nods. The cervical spine is correctly positioned as it would be for a static neutral lateral projection, with the shoulders squared and the chin lifted just enough so that the angle of the mandible does not overlie the cervical spine. Then, the patient is instructed to lift the chin as high as possible, and then bring it back to level, three times (all of the projections are done in sets of three because many times it is not until the third repetition when the aberrant movement occurs). Each view and its three repetitions should take about fifteen seconds.
With normal atlanto-occipital alignment, on the static projection, you should see a parallel line between the plane of the atlas and the plane of the occiput. According to White and Panjabi (Clinical Biomechanics of the Spine, 2nd Ed., 1990), normal range of motion in flexion-extension at the C0-C1 joint is 0-25 degrees. More recent sources (Radcliffe KE et al. In Vitro Biomechanics of the Craniocervical Junction- A Sequential Sectioning of Its Stabilizing Structures, The Spine Journal 15 (2015, 1618-1628) put the normal range of motion at 16 degrees.
Normal ADI (atlantodental interval) is 1-3 mm., but what is important about seeing this on a motion x-ray is that the ADI should not change with flexion or extension, in either the size of the gap or the relationship between the anterior portion of the atlas and the odontoid process, or dens. Drs. White and Panjabi found that the distance for adults was constant in full flexion and extension, with the maximum distance being 2.5 mm. (White and Panjabi, ibid, p. 94). It is not uncommon to see subtle changes in the ADI which are indicative of damage to the transverse ligament. Remember, it is the partial tears (sub-failure) of the ligaments which are difficult to demonstrate with the more common 5 mm slice MRI studies, but motion x-ray can highlight some of them. Take a look at the flexion and extension laterals on the study below, taking note of the appearance of the ADI at the beginning in neutral, and then at how it not only opens up with flexion but also loses the parallel relationship between the posterior surface of the anterior tubercle of the atlas and the anterior surface of the odontoid process (sign of “V”). This subfailure of the transverse ligament was later verified by MRI, in addition to a constellation of other atlanto-occipital findings, such as tearing of the dura mater and disruption of the myodural bridge.
Lateral tipping of the atlas is a common finding with flexion or extension of the cervical spine and it is the result of damage to the lower fibers of the alar ligaments. But when you see it, understand that it represents aberrant biomechanics of the spine due to the damage to the ligaments, and look for the ligamentous damage on some of the other views. Your bottom line? The atlas is not supposed to tip laterally with flexion and extension of the cervical spine.
Look for compression deformities of the anterior superior corners of the cervical vertebrae. After whiplash, they are very common.
The Neutral Lateral Views (2)
- Lateral flexion and extension
In chiropractic school, we were taught that the biomechanics of the cervical spine were simple, in that when the neck is flexed, the spinal segments, starting with the occiput, would slide forward first (translation, a completely linear movement) until it reached the limits of movement prescribed by the ligaments, and then tip forward (an angular movement) on its anterior inferior corner, until it reached the limits of movement, and then the next segment would do the same, and the next, and the next, with all the movement occurring in a smooth, coordinated fashion. We were taught that the amount of normal translation was 2-3 mm, and it took at least 3.5 mm for a segment to be considered to be unstable. However, it turns out that these parameters were never based on sound research, but were in fact only radiological opinion. In the October 1987 issue of the The American Chiropractor, an authority no less than Arthur Croft, DC, presented the following in his article, The Anterior Subluxation: A Subtle Manifestation of Soft Tissue Damage: “They (Panjabi and White) suggested that 2.7 mm (3.5 mm on lateral x-ray) was the upper limits of normal translation movement. Green et al. have stated that 1-3 mm of movement should be considered subluxation and that 3.5 mm or greater is only seen in frank dislocation, fracture or pseudosubluxation. Others have suggested 1-2 mm as indicating subluxation. Scher studied normal cervical spine x-rays and found none that traveled more than 1 mm in translation. He stated that the 3.5 mm criteria as suggested by White and Panjabi is seldom seen clinically.” In other words, there was a lot of disagreement about exactly what normal translation should be. With the advances in computer technology and the complete digitization of x-rays in the 21st century, advanced and sophisticated computerized measurement systems have been applied to the mensurations of the human body found on both plain film and motion x-ray, and a radically different picture of what’s normal appears. Lin RM et al (Characteristics of Sagittal Vertebral Alignment in Flexion Determined by Dynamic Radiographs of the Cervical Spine, Spine Vol. 26(3), February 1, 2001, pp.256-261) established that the normal amount of cervical translation found with flexion or extension is a mere 0.6 mm. 0.6 mm is not detectable by the human eye, so any translational movement seen on a motion study is always going to be abnormal. Validation of the mensuration software products has been done on numerous occasions, in which the error factor has been found to be less than 5%, or about .7 mm. for a vertebra with 15 mm. of depth (Frobin et al. Sagittal Plane Segmental Motion of the Cervical Spine: A New Precision Measurement Protocol and Normal Motion Data of Healthy Adults. Clin Biomech (Bristol, Avon), 2002 Jan;17(1):21-31). What this means is that this is very accurate measurement.
And for the record, just how much is 3.5 mm., considered to be normal translation by many to this day? Pull two quarters out of your pocket and hold them sandwiched together, and look at them on edge. That’s what 3.5 mm. looks like.
We were also taught in chiropractic school to recognize what a healthy cervical disc space looked like. We were taught that it was supposed to be slightly wider to the anterior and slightly more narrow to the posterior in the neutral posture, and that the appearance of the discs could reverse with flexion and become more accentuated with extension. The article by Lin also establishes that between full flexion and full extension, normal change in the appearance of the disc is less than 7 degrees, and that the angle between the adjacent end plates sandwiching a disc should never go beyond parallel (or 0 degrees).
So, in summary, what this means is that the posterior vertebral line (George’s Line) should always remain intact whatever position the cervical spine is in, and that the cervical spine is best described as a rigid bar with some elasticity. In chiropractic school, we were taught that the cervical spine was a very flexible column of bones which got its shape on the neutral lateral projection based on what the surrounding muscles were doing. After injury, with spasm, we were taught that it was common for the cervical curve to lose its normal lordosis and straighten out or even become reversed. What we were not taught was how extensive the damage to the ligamentous support system could be, and how far reaching the effects could be, as far as they applied to the degenerative process.
The instability shows up in one of two ways, or sometimes, it shows up in both directions. With flexion, when there is damage to the posterior longitudinal ligament, the body of one cervical vertebra will translate forward visibly on the vertebral body below, or, with extension, the translation will occur to the posterior. Translation to the posterior is indicative of damage to the anterior longitudinal ligament. A tearing of the posterior longitudinal ligament may also allow the posterior disc space to widen visibly with flexion, and a tearing of the anterior longitudinal ligament may allow the anterior disc to widen visibly. With the computer software program, lines can be applied to the superior end plates of adjacent vertebra and the disc angle can be measured, but oftentimes, this not necessary because the two end plates are obviously beyond the parallel state.
When you receive a digital copy of your study from me, stop the film in full flexion, and then take a business card and use it as a straight edge to evaluate the subtle anterolistheses in the cervical spine. Place the edge of the business card along the posterior border of each cervical vertebra, working from the bottom to the top. If the posterior inferior corner of the vertebra above your straight edge does not touch the straight edge, then there is a tear in the posterior longitudinal ligament at that level. When I do a study for you, one of the things I do routinely is measure the A-P width of the superior vertebral end plates at each level, and then the amount of displacement wherever applicable, and compute the percentage of translation it represents. According to researchers (Daffner et al. A New Classification for Cervical Vertebral Injuries: Influence of CT. Skeletal Radiology 2000; 29: 125-32), who did a study based on their analysis of 1052 separate cervical spine injuries, a spinal injury should be classified as “major” if any of the following radiographic/CT findings were present:
- More than 2 mm of displacement of vertebrae in any plane;
- Wide vertebral body in any plane;
- Wide interspinous-interlaminar space;
- Wide facet joints;
- Wide disc space;
- Vertebral burst fracture;
- Locked perched facets;
- Dens fractures;
- Occipital fractures;
- Excess widening of the interspinous or interlaminar space of three contiguous levels;
- Excess widening of the interpedicular distance between two contiguous levels;
- Unilateral or bilateral lateral C1-C2 offset in frontal view;
- Anterolisthesis or retrolisthesis with flexion/extension. No clinical significance if the spinolaminar line remains intact during flexion/extension (this only applies to Swischuk’s line);
- Facet joint widening primarily in flexion.
By the first bullet point alone, instability can be established in just about every motion x-ray I produce. This article is extremely important because it points out so many ways in which the instability can be seen, as opposed to only two ways with the AMA Guides to Evaluation of Permanent Impairment.
Another thing I have learned to look for is called early anterolisthesis/retrolisthesis. When we use our necks, we usually function in the first 50% of our range of cervical motion. Just pay attention to yourself while you move around and you will see that that is true, and that only rarely do you do things which require you to use your full range of motion (such as backing your car out of a parking spot). “Early” anterolisthesis means that the anterolisthesis occurs early in the ROM (during the first 50%), and then many times either completely corrects itself or reduces the slippage dramatically by the end of the ROM. Naturally, this would never be seen on a stress x-rays, as the measurements are done at the end of the ROM, and is considered to be evidence of even greater instability (Shippel AH and Robinson GK, Radiological and Magnetic Resonance Imaging of Cervical Spine Instability: a Case Report. J Manipulative Physiol Ther 1987: 10:316-22) .
The interspinous ligaments fill in the gap between the spinous processes and limit the amount of forward flexion of the cervical spine. They are commonly torn in both rear and front end collisions, and very often the tears show up at multiple levels. The rule of the thumb for diagnosing a torn interspinous ligament is to take the measurement of the anterior-posterior depth of the superior end plate of the C4 vertebra. Then measure, during full flexion, the distance between two adjacent inferior and superior spinolaminar lines. From C3-C7, if the distance between the spinolaminar lines is greater than 50% of the width of the C4 superior endplate, then there is damage to the interspinous ligament, and by extrapolation, the supraspinous ligament. For C2-C3, use 30%. That’s the easiest rule to use, but there is another: there is interspinous ligament tearing whenever the widening is greater than 30% relative to an adjacent level (40% between C1-C2 and C2-C3). (Eubanks AC et al. Reference Data for Assessing Widening Between Spinous Processes in the Cervical Spine and the Responsiveness of These Measures to Detecting Abnormalities.Spine 2010 March; 10(3):230-237). The study has two parts involving measurements- one involving live, asymptomatic patients and one with cadavers. The ligamentous posterior structures of the cadaver’s cervical spines were resected sequentially from posterior to anterior, and abnormal spreading of the spinous processes did not occur upon full flexion until the resection was made all the way to the posterior annular fibers of the intervertebral disc. Think about that! And, surprisingly, the amount of pre-existing degenerative change present “was not a significant factor in interspinous distances or displacements.”
The Oblique Views
We were all taught in chiropractic school that the purpose of the oblique views was to visualize the intervertebral foramina (IVF) and the facet joints. The intervertebral foramina are supposed to be oval to figure-8 shaped, with the top half of the hole made by the vertebra above and the bottom half of the hole by the vertebra below. With movement, such as the rotations or lateral flexion, the shape of the holes would change, becoming either larger or smaller. Spinal nerves exit through the intervertebral foramina, and depending what level of the spine you are talking about, the spinal nerve takes up from 1/6th to 1/8th of the space. Then, there is a vein and an artery, and the rest of the space is packed in fat, so the spinal nerve is well protected- that is, until your patient gets whiplashed, and the facet capsular ligaments are torn, which results in excess gapping at the facet joints with motion, and increased foraminal encroachment when the holes become smaller.
The DMX protocol calls for posterior obliques, as anterior oblique projections tend to cause the mandible to overlie the cervical spine during the flexion phase. The patient is positioned so that there is a clear view of the IVF’s and the facet joints, and then with the cervical spine in a position of rotation away from the side visualized, the cervical spine is alternately flexed and extended. With flexion, the main focus is on the facet joints and any unusual gapping (diastasis) which may occur between the inferior and superior articular processes. This may appear as increased sliding between the two bony surfaces in comparison with adjacent levels, or the two bony surfaces may lose their parallel appearance. On the extension phase, with any “looseness” in the facet joints, foraminal encroachment will be amplified.
When you look at the spine on the APLC, remember that the facet joints are now referred to as the articular pillars, and remember that the cervical spine is supposed to be a rigid bar with some elasticity, because the first portion of the APLC is done with the spine taken alternately into right lateral flexion and then left lateral flexion. Tearing of the capsular ligaments is indicated when gapping is seen in the articular pillars. Normally, no gapping should be visible.
The second phase of the APLC involves maximal cervical rotation to the right and to the left. The focus on this view is the relationship between the odontoid process and the angle of the jaw. With normal range of motion, the angle of the mandible should directly overlie the odontoid process. But when the alar (check) ligaments are torn, it is common to see the angle of the jaw go past the limit of the odontoid process. And this is seen often in cervical spines with a measured deficiency in the rotations.
Probably the most dramatic presentation of ligamentous damage is seen on the AP Open Mouth view. The DMX machine, despite the fact that it uses only 2 MA to produce the study, produces a near hospital quality picture which adequately shows the anatomical relationship between C1 and C2. In the neutral position, you are looking at the alignment between C1 and C2 which would be present whenever a recumbent cervical MRI is done. The alignment between C1 and C2 is maintained by a very strong ligamentous support system which includes the upper and lower fibers of the alar ligaments, the joint capsules of the lateral atlantoaxial joints, the accessory ligaments, and the transverse ligament. When the cervical spine is laterally flexed to the right or to the left, the ligaments are supposed to maintain this alignment. The lateral edges of the lateral masses of C1 should be in perfect alignment with the lateral masses of C2; think of it as being just like George’s Line on the lateral projections. If the alar and accessory ligaments have been damaged, the lateral masses of C1 will translate to the side of the tipping of the head, and the para-odontoid spaces, which should be equal bilaterally and stay that way, will change size with the motion. When you see big changes in the para-odontoid spaces, you know that the atlas is slipping around on top of the axis like a loose washer, and the entire area is very unstable.
MRI can detect the tears in the alar and accessory ligaments, and much more, such as tearing in the dura mater, herniation of the cerebellar tonsils, and disruption of the myodural bridge, but the study has to be done as a thin slice proton density weighted study in the upright position, and in three positions (neutral, flexion, extension), and currently, there are no facilities in the Tucson area which can adequately perform these studies.