Pathophysiologic Evidence of Injury

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CHAPTER 53 Pathophysiologic Evidence of Injury

Imaging is the conventional approach to finding evidence of injury. When major trauma to the neck results in fractures or dislocations, these are typically evident on plain radiographs of the neck.¹ Computed tomography (CT) can provide more detailed reconstructions of the injury. Magnetic resonance imaging (MRI) can reveal soft tissue components or consequences of the injury.

Minor injuries to the neck do not result in major fractures or dislocations. Therefore, such injuries are not evident on imaging.² Indeed, conventional imaging is typically normal. This lack of evidence of injury on imaging is used by some commentators to infer that there is no injury. That is false. The lack of evidence only reflects the limited resolution on conventional imaging. Plain radiographs or CT demonstrate only lesions in bone; they do not show soft tissue injuries. MRI demonstrates soft tissues, but it will not necessarily reveal small lesions. Normal imaging, therefore, does not exclude the presence of lesions that are beyond the resolution of the imaging technique used.

Meanwhile, two lines of evidence have shown that lesions can and do occur, after minor injuries to the neck. Postmortem studies have shown the types of lesions that can and might occur. Clinical studies have shown the sites at which these lesions must be present.

POSTMORTEM STUDIES

Independent studies, in countries as respectively remote as Sweden ³ and Australia,⁴^–⁶ have provided similar conclusions, which have now been systematically reviewed.⁷ Both sets of studies harvested the cervical spines of victims of fatal motor vehicle accidents. The Australian studies included victims who survived for various periods after the accident. The pathology of major head injury and suboccipital injuries was ignored. Instead, the studies focused on other lesions, which were not the cause of death, but which indicated what could happen to the cervical spine, in a motor vehicle accident.

The same spectrum of injuries was consistently observed (Fig. 53.1). They included tears of the anterior anulus fibrosus, avulsion of the anterior anulus, contusions of the posterior anulus, tears or avulsions of the capsules of the zygapophyseal joints, contusions of the meniscoids of these joints, intra-articular hemorrhage, fractures of the articular cartilage, and subchondral or greater fractures of the articular pillars. Conspicuously, virtually none of these lesions was evident on plain radiographs of the cervical spine, taken postmortem, even when read retrospectively with the knowledge that a lesion was present.³^–⁶

Fig. 53.1 The possible lesions after minor injuries to the cervical spine.

These lesions are consistent with the known biomechanics of whiplash injury.² Anterior separation of the vertebral bodies could cause tears or avulsions of the anterior anulus fibrosus. Posterior impaction of the zygapophyseal joints could cause articular and subarticular fractures, or contusions of the intra-articular meniscoids. Excessive separation of the zygapophyseal joints, in extension or in flexion,⁸ could cause tears of the joint capsules.

The nature of these lesions is such that they should not be evident on conventional imaging. Injuries to fibrous connective tissues will not be seen on plain radiographs or CT. They cannot be seen on conventional MRI. In the future, perhaps high-resolution MRI or spectroscopic MRI might be able to resolve them, but studies in this regard have not yet been completed.

PATHOPHYSIOLOGY

The nature of connective tissue injuries is that they are likely to be painful. Capsular tears, small fractures, and intra-articular hemorrhage are all causes of pain elsewhere in the musculoskeletal system. There is no reason why they should not be sources of pain if and when they occur in the neck.

For these lesions to be symptomatic the structures in which they occur have to be innervated. In the cervical spine, the anuli fibrosi of the intervertebral discs are innervated, and the zygapophyseal joints are innervated. The discs receive their innervation from the vertebral nerve, the sympathetic trunks, and the sinuvertebral nerves.⁹ The zygapophyseal joints are innervated by the medial branches of the cervical dorsal rami.¹⁰

Because they are innervated, these structures can become sources of pain, even though the lesions that cause that pain are effectively invisible. It is the pain that implies the presence of the injury; and the site of injury can be determined by pinpointing the source of pain.

CLINICAL STUDIES

Pinpointing a source of pain requires a diagnostic test that can be applied in patients, who cooperate by reporting if their accustomed pain is uniquely provoked by the test or is relieved by it. The available procedures differ according to the target structure. Intervertebral discs can be tested by discography. Zygapophyseal joints can be tested with medial branch blocks.

Discography

Cervical discography is a procedure designed to determine if a particular intervertebral disc is painful or not.¹¹ It involves introducing into the nucleus of the disc a needle, which is used to stress the disc with an injection of contrast medium. The test is deemed to be positive if the patient’s pain is reproduced, but provided also that stressing adjacent discs does not reproduce the pain. Furthermore, in order to be valid, cervical discography must reproduce the patient’s pain to a clinically significant extent. The operational criteria require an intensity of evoked pain of 7 or more on a 10-point scale.¹²

Cervical discography has been described extensively in the surgical literature as a means of determining at which segments arthrodesis should be carried out for the treatment of neck pain.¹³^–¹⁶ Other information, however, is scarce. No studies have shown what the prevalence is of discogenic neck pain, as diagnosed by discography. No studies have shown that cervical fusion for neck pain is particularly successful. None has shown that discography leads to better outcomes. Meanwhile, two studies have warned of the capricious validity of cervical discography.

It is uncommon for a single cervical disc to be painful alone. More often, two, three, or more discs appear symptomatic.¹⁷ Indeed, the more segments that are investigated, the more discs emerge as painful. Consequently, cervical discography is not complete unless every cervical disc is studied.

Cervical discography can be false-positive. The movement that discography induces can stress the zygapophyseal joints of the same segment. If these, rather than the disc, are the source of pain, the discography appears to reproduce the patient’s pain, but falsely so.¹⁸ The disc is not the source of pain; the zygapophyseal joints are. For this reason, some 25% of positive discograms are false-positive (Table 53.1).

Table 53.1 A correlation table between responses to provocation discography and zygapophyseal joint blocks in patients who had both procedures at the segments indicated

At present, cervical discography is the only means by which discogenic pain can be tested. For this reason, it is attractive to some practitioners. However, because of the limitations to its validity, and the lack of any prevalence data, cervical discography has not yet evolved to be a validated means of providing inferential evidence of injury to the neck.

Furthermore, if the injury is a partial tear of the anterior anulus, discography will not necessarily reveal it. The physical basis of cervical discography is to stress it by raising its intranuclear pressure. However, cervical discs typically decompress as contrast medium escapes through uncovertebral clefts. Under this condition, the induced stresses achieved, before decompression, may not be great enough to stress significantly a tear in the outer, anterior anulus. To diagnose anterior annular tears requires a different procedure.

Anulus blocks

No-one has yet developed a technique whereby symptomatic tears of the anterior anulus of a cervical disc might be diagnosed. In principle, such a procedure could involve accessing the tear with a needle, which could be used either or both to visualize the tear with an injection of contrast medium, and to anesthetize it selectively with a tiny volume of local anesthetic. Were such a procedure to be developed it would provide obtainable evidence of a symptomatic tear of the anulus. The procedure could be used in individual patients, to establish a diagnosis not possible by other means; it could be used to test if tears that might be apparent on high-resolution MRI are symptomatic or not. But until such a test, or a better one, is developed, evidence of symptomatic anterior tears will remain elusive.

Medial branch blocks

In contrast to cervical discography, the science of cervical medial branch blocks is highly developed, and has been extensively tested for validity and utility. These blocks test if a particular zygapophyseal joint is painful or not. They are based on the principle that if the joint is painful then anesthetizing the medial branches that innervate it should relieve that pain (Fig. 53.2).

Fig. 53.2 A lateral radiograph of the cervical spine showing a needle in position for a C5 medial branch block.

The face validity of cervical medial branch blocks has been established.¹⁹ When small volumes of local anesthetic are used (0.3 mL) to anesthetize a given nerve, the injectate does not spread to adjacent structures that might be alternative sources of pain. It does not spread to adjacent medial branches; it does not spread to the spinal nerve; it does not indiscriminately anesthetize the posterior neck muscles. The injectate pools around the target nerve, and spreads only in a planar fashion along the cleavage plane between multifidus and semispinalis capitis.

The construct validity of cervical medial branch blocks has been established in several ways. Single diagnostic blocks are not valid. They can be associated with a false-positive rate of up to 27%.²⁰ To be valid, blocks must be controlled in each and every case that they are used. These controls can be placebo controls or comparative blocks. Comparative blocks have been validated using statistical methods,²¹ and by comparing them with placebo-controlled blocks.²² Furthermore, their diagnostic validity has been confirmed by successful subsequent treatment. Denervating the painful joint by radiofrequency neurotomy provides complete and lasting relief of the pain.²³^–²⁵

Implications

When positive, controlled medial branch blocks implicate a genuine source of pain in the joint targeted. The criterion for a positive block is complete relief of pain. It is not partial relief or 50% relief. The pain must be completely relieved.

Cervical medial branch blocks, performed under controlled conditions, have provided the strongest available pathophysiologic evidence of injury to the neck. Multiple and independent studies have established how often the zygapophyseal joints are the source of chronic neck pain.

Among patients with neck pain after whiplash, two separate studies from the same unit have established the prevalence of cervical zygapophyseal joint pain to be 54%²⁶ and 60%.²⁷ A third study from the same unit established the prevalence of zygapophyseal joint pain in drivers injured in high-speed motor vehicle accidents to be 75%.²⁸ An unrelated study, from a pain clinic serving a general population, found the prevalence of zygapophyseal joint pain to be at least 36%.²⁹ This figure constituted a worst-case analysis. Patients who did not return for control blocks were not treated as positive. Another study from a pain clinic reported a prevalence of 60%.³⁰

The confidence intervals of these various studies overlap (Table 53.2). Collectively, they paint a consistent picture. Zygapophyseal joint pain accounts for some 60% of patients with chronic neck pain after injury. The pattern of involvement is diverse. Single joints may be painful, or both joints of a particular segment may be painful. Joints at other, adjacent or remote segments may be painful. The more common patterns are one or both joints painful at C5–6, or C2–3; and joints painful at C5–6 and C2–3. Less often, consecutive joints, at C5–6 and C6–7, are painful.

Table 53.2 The prevalence and 95% confidence intervals of cervical zygapophyseal joint pain amongst patients with chronic neck pain

Study	Prevalence	95% Confidence Intervals

Barnsley²⁶	54%
Lord²⁷	60%
Speldewinde²⁹	36%
Manchikanti³⁰	60%
Gibson²⁸	75%

The prevalence and distribution of cervical zygapophyseal joint pain correlate with two other, and independent, lines of evidence. The postmortem data point to injuries of the zygapophyseal joints that should be painful.³^–⁶ The biomechanics data reveal that the mechanism of whiplash could cause injury to the zygapophyseal joints, of the nature found at postmortem.² Furthermore, the biomechanics data implicate C5–6 and C2–3 as the most likely segments to be involved. Consequently, the postmortem data, the biomechanics data, and the clinical data, all point to the same conclusion. This constitutes convergent validity for the concept that cervical zygapophyseal joints can be injured and do became painful.³¹