New Research, the AMA Guides, the Cervical Spine DRE-IV and Ligamentous Subfailure

The 5^th edition of the AMA Guides to the Evaluation of Permanent Impairment is used in workers' compensation and personal injury cases to make determinations about a patient's level of impairment.² These physician-derived determinations form a critical foundation for the valuation of claims and subsequent awards. When the 5^th edition was first released, it contained some rather serious errors pertaining to the determination of impairment based on spinal instability. They later released an errata sheet to remedy this error. But the framers of the cervical spine section in the Guides may not have fully understood the methodologies they were recommending. Or perhaps they simply did not anticipate the potential for confusion that would ensue. In any event, great confusion now exists and I often have argued these issues with radiologists, orthopedic surgeons and chiropractors. This article will disentangle the misconception, I hope, once and for all.

Let's begin with an interesting new study by Centeno, et al., including their abstract and my thoughts.¹

Abstract

BACKGROUND: Clinical and cadaveric studies have implicated that sub-failure cervical instability likely occurs in a subset of whiplash injury patients. Cadaveric studies have suggested that female specimens suffer from more ligamentous stretch injury than males when exposed to simulated rear- end crash vectors. However, these findings have never been tested in an in vivo clinical setting.

DESIGN: A prospective evaluation of total cervical translation on sagittal flexion radiographs versus impact vector in a late whiplash population.

OBJECTIVES: To determine if total cervical translation on radiographs is correlated with impact crash vector.

METHODS: Consecutive late whiplash patients in a subspecialty pain clinic setting were sent for radiographs using a strict stress flexion-extension protocol. Information concerning crash vector and damage was recorded. Vertebral translation was read by a blinded reader and recorded.

RESULTS: Males did not significantly differ in total translation in flexion-extension radiographs when involved in sagittal plane crashes (n=75) compared to coronal plane crashes (n=10). In a front-end collision, males (n=16) and females (n=26) did not differ in total translation in flexion-extension radiographs. In a rear-end collision, females (mean translation 4.61 mm, n=103) did differ significantly in total translation in flexion-extension radiographs from their male counterparts (mean translation 3.29, n=48) (P<0.001).

CONCLUSION: This investigation suggests that specific crash vectors lead to particular patterns of radiographic translation in female subjects. A realization that sub-failure cervical instability occurs in whiplash may help design more effective treatments.

My Thoughts

This paper provides some very interesting information concerning abnormal, cervical spine translational, intervertebral motion in a population of late (i.e., chronic) whiplash patients recruited from a pain clinic in Colorado. The authors aptly noted a practical limitation mentioned earlier by Dvorak, et al., concerning the false negative error rate when looking for instability on standard flexion/extension radiographs of the cervical spine.³ Namely, that hypermobility (or instability) often is masked by muscular protective mechanisms which are at play during the active range of motion (ROM).³ Dvorak, et al., found that after applying additional external loading force on the head and neck (i.e., switching from the usual active ROM to a passive ROM), 63 percent more instances of intervertebral hypermobility and 28 percent less hypomobility. The clinical ramifications of this are both obvious and enormous. In an attempt to circumvent this problem, patients in the Centeno, et al., study were pre-medicated with oral diazepam. Overpressure (i.e., bending force) was then performed to replicate the work of Dvorak, et al. The cervical intervertebral translation in flexion and extension was then summed to get total translation for each segment.

Centeno, et al., also cited the seminal work of White, et al.,⁴ as reporting that the intervertebral translation of 2.7 mm (3.5 mm, considering radiographic magnification at 72-inch tube distance) and beyond was indicative of instability.

The findings in the present study were expressed in total translation. Separate anterior and posterior translation values were not provided. Note that there also is an angulation method recommended by White, et al. (which also is incorporated into the 5^th edition of the Guides), but it was not evaluated in the study by Centeno, et al. The authors acknowledged limitations included the lack of a control group. Another problem is the lack of normative data, which would at least have been partially solved by using a control group. Additionally, there is a potential problem with external validity, in that the authors used a technique to replicate that of Dvorak, et al. (passive ROM and overpressure in flexion and extension). This is not used in routine radiography, and might even be dangerous in some situations. The authors' findings might not be representative of clinical findings obtained using unmedicated, standard radiographic procedures in which active ROM is used to position patients in flexion and extension. Nevertheless, they are interesting and important findings. Perhaps we should use passive ROM when obtaining radiographs.

Perhaps conveniently for me, the study unintentionally underscored a very common misunderstanding that has had very far-reaching effects. The original research by White, et al.,⁴ has been incorporated into the 5^th edition of the Guides.² Specifically, if there is 3.5 mm or more of horizontal translation in the cervical spine, using radiographs taken at 72 inches, one can assign a DRE-IV rating which carries a 25 percent to 28 percent whole-person impairment rating. When I first looked at the 5^th edition of the Guides, I noticed that the figure describing the measuring process was wrong. I later found the publisher had issued errata that corrected this error. I should also point out that the framers of the Guides borrowed some aspects of the White, et al., instability assessment (e.g., the 3.5-mm translation and 11-degree angulation parts) but disregarded other aspects of it. But that is the subject of another article.

As time went on, I realized that even this 3.5-mm translation issue was a source of great confusion. Many radiologists and IME doctors were measuring the total translation (i.e., the sum of anterior and posterior translationand assigning a DRE when this summed motion reached or exceeded 3.5 mm. This is an incorrect application of the White, et al., research. It evolves from an incorrect, but apparently prevalent, interpretation of the White, et al., original study. The Guides are ambiguous on the matter. I have been strongly urging anyone who draws this conclusion to read the original paper by White, et al. Most recently, this issue has come up in the AMA Guides Newsletter (March/April 2007, in an article by Patrick Luers). He interprets the original White, et al., study to imply that total cervical translation is 7 mm (3.5 mm in flexion plus 3.5 mm in extension). So let us review that original White, et al., paper now because this is an important issue.

The Seminal Research

White, et al., working with cadavers, reported that the majority of ligaments had to be transected for the segment to fail completely. With some ligaments cut and just before complete segmental failure, the maximum displacement recorded was 4.89 mm. With all ligaments intact, however, the displacement never exceeded 2.67 mm, and the angular displacement never exceeded 10.7 degrees. Thus, they noted, "Only modest displacement occurred even when the spinewas about to fail."

Now, here is the definitive wording in this report that should dispel any further misinterpretation.

Instability, they wrote, exists when one finds: "(1) either all of the anterior elements or all of the posterior elements are destroyed or unable to function; (2) more than 3.5 mm horizontal displacement of one vertebra in relation to an adjacent vertebra, anteriorly or posteriorly, measured on resting lateral or flexion/extension roentgenograms of the spine; or (3) more than 11 degrees of rotational distance to that of either adjacent vertebra, measured on resting lateral or flexion/extension roentgenograms of the spine."

We should not lose sight of the fact that segmental instability, as White, et al., clearly demonstrated, is not a very sensitive proxy for injury. The relationship between segmental damage and translation is not at all linear. They needed to cut nearly all ligaments in order to really see much translation. Moreover, they applied a shear load on the segment, which was equivalent to 25 percent of the body weight. It is difficult to conceive of this in terms of the total load that would have to be applied to the human head during flexion. However, once again, external validity is a big potential problem, as is the problem with their methodology of sectioning ligaments sequentially -a lesion or combination of lesions that simply do not have a clinical counterpart. The AMA's choice of using this 3.5 mm translation was surprising and highly questionable in my opinion, particularly because I have seen patients with 3.5 mm of translation who were not very symptomatic. However, the Guides do note that one must not base ratings on films alone.

Anterior or posterior translation of one cervical segment over another of 3.5 mm or more can be in accord with a DRE-IV. To add 2 mm of forward translation from the flexion film to 2.5 mm of rearward translation from the extension film, and conclude that this 4.5 mm of total translation is in accord with the DRE-IV, is clearly incorrect.

Finally, let me state for the record that I am very hesitant to apply the White, et al., findings to real clinical situations for several reasons. Their study used cadaveric materials and applied forces which were non-representative. They also made it clear that ligamentous integrity is not linearly related to translation. In other words, it is not a sensitive indicator. Finally, the instability that results from certain forms of trauma such as whiplash produces a subcatastrophic ligamentous failure (subfailure) which the Yale biomechanics research lab of Panjabi has more recently described in numerous publications. This condition, in which the structural integrity is preserved while the functional integrity is degraded, is not replicated by simply sectioning ligaments. Clinical determinations of instability may be aided by plain radiographs, but these cannot rule out instability in every case. A spine can be clinically and biomechanically unstable and still not translate more than 3.5 mmin either direction. Conversely, some stable spines will exceed this translation.

References

1. Centeno CJ, Elkins W, Freeman M, et al. Total cervical translation as a function of impact vector as measured by flexion-extension radiography. Pain Physician, 2007;10:667-1.
2. Cocchiarella L, Andersson GBJ. Guides to the Evaluation of Permanent Impairment, 5^th ed. Chicago: AMA Press, 2001.
3. Dvorak J, Froehlich D, Penning L, et al. Functional radiographic diagnosis of the cervical spine: flexion/extension. Spine, 1988;13:748-55.
4. White AA, Johnson RM, Panjabi MM, et al. Biomechanical analysis of clinical stability in the cervical spine. Clin Orthop Rel Res, 1975;109:85-95.