Review paper
Biomechanics of the cervical spine Part 3: minor injuries

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Abstract

Minor injuries of the cervical spine are essentially defined as injuries that do not involve a fracture. Archetypical of minor cervical injury is the whiplash injury. Among other reasons, neck pain after whiplash has been controversial because critics do not credit that an injury to the neck can occur in a whiplash accident. In pursuit of the injury mechanism, bioengineers have used mathematical modelling, cadaver studies, and human volunteers to study the kinematics of the neck under the conditions of whiplash. Particularly illuminating have been cinephotographic and cineradiographic studies of cadavers and of normal volunteers. They demonstrate that externally, the head and neck do not exceed normal physiological limits. However, the cervical spine undergoes a sigmoid deformation very early after impact. During this deformation, lower cervical segments undergo posterior rotation around an abnormally high axis of rotation, resulting in abnormal separation of the anterior elements of the cervical spine, and impaction of the zygapophysial joints. The demonstration of a mechanism for injury of the zygapophysial joints complements postmortem studies that reveal lesions in these joints, and clinical studies that have demonstrated that zygapophysial joint pain is the single most common basis for chronic neck pain after injury.

Introduction

There is no universally accepted definition of what distinguishes major from minor injuries to the cervical spine. Extreme examples can establish the limits of a spectrum, such that a fracture-dislocation with spinal cord injury is obviously a major injury, whereas a sprained muscle is a minor injury. The difficulty that arises lies in establishing the boundary between major and minor injuries.

Effectively, and in clinical practice, the distinction lies in whether or not there has been a fracture or a dislocation. Fractures that constitute major injuries are fractures of the vertebral body, the pedicles, the odontoid process, and the ring of the atlas. Retrospectively, such fractures imply that a major external force was applied to the spine, as do dislocations. Prospectively, fractures and dislocations threaten the stability of the cervical spine and the integrity of its neural contents. These features render the injury serious in both a biomechanical and a clinical sense. Some fractures, however, can be minor, such as those in an articular process or across the anterior edge of a vertebral body. Because they do not threaten the stability of the cervical spine they constitute the “grey zone” between major and minor injuries.

Against this background, minor injuries of the cervical spine are essentially those in which fractures do not occur, or in which fractures are not readily apparent. By default, these injuries are usually classified as “soft-tissue injuries”, a term that implies that there has been no injury of bone and that if anything has been injured it must be one or more of the muscles or ligaments of the neck. The definition is no more specific than this because it is based essentially on the plain radiographic appearance of the neck. Since X-rays cannot demonstrate soft tissues, the definition is one of exclusion, i.e., if there is an injury but X-rays exclude a fracture or dislocation, the injury must be in the (invisible) soft tissue injuries of the neck.

Computerised tomography (CT) and magnetic resonance imaging (MRI) do not help in this regard. In the first instance, they are not routinely used to screen for fractures; plain radiographs remain the cardinal tool for that purpose. CT may be used to better define fractures already evident on plain films, or to search for occult fractures, but it does not resolve soft-tissue injuries. MRI has the capacity to resolve certain soft-tissues, but no correlations have been established between neck pain and any feature evident on MRI.

Several factors render soft-tissue injuries of the neck controversial. In the first instance, their very invisibility (under X-rays) is grounds for suspicion, e.g., if the injury cannot be demonstrated, perhaps there is no injury. Secondly, on the basis of extrapolation from soft-tissue injuries in the limbs, it is believed that soft-tissue injuries should heal rapidly, in a matter of days or weeks. Therefore, persistence of neck pain is incongruous with an archetypical soft-tissue injury and, in some circles, this is used as evidence that there was no injury and that the symptoms are manufactured or imagined. Compounding these clinical considerations is the influence of litigation. Soft-tissue injuries are often, if not most commonly, associated with insurance claims. The prospect of compensation and monetary gain confounds the clinical considerations. It seems to increase the likelihood that symptoms are manufactured or artificially prolonged in order to achieve gain.

Perhaps the best-known and most studied example of soft-tissue injury to the neck is whiplash. If nothing else it serves as the archetype for this type of injury. Fractures are typically not evident, yet the patient complains of symptoms, and a proportion of patients develop chronic symptoms that last well beyond the expected period in which soft-tissue injuries should have healed. Moreover, these injuries are subject to litigation. Consequently, whiplash injuries inherit all the suspicions that render soft-tissue injuries controversial. However, the more that whiplash has been studied, the more has scientific inquiry dispelled incorrect notions that caused this controversy.

Invisibility is not evidence of absence. Rather, it is an indication that an inappropriate tool has been used to look for the injury. The literature is replete with studies that have shown small injuries to intervertebral discs, zygapophysial joints, and uncovertebral clefts, both in collagenous tissues and in cartilage and bones, that are plainly invisible on plain radiographs [1], [2], [3], [4], [5]. Radiography simply lacks the sensitivity to detect these injuries, and therefore, cannot be used to exclude or refute them. Normal radiographs do not mean that there has been no injury.

Extrapolation from the limbs about the nature of soft-tissue injuries and their period of healing is both inappropriate and false. One can expect a sprained muscle to heal and become asymptomatic within a few days. Experimental studies in animals indicate that lesions at the myotendinous junction repair within one week, and develop nearly normal tension by seven days after injury [6]. Recovery from contraction-induced injuries of muscles is usually complete within 30 days [7]. But the behaviour of muscle injuries is not a model for the behaviour of ligaments, capsules, joints, and intervertebral discs. Most sprained ankles recover within two weeks [8], [9], but some do not, and many patients are left with chronic symptoms [10] (even though compensation is not involved). Football injuries to knees, involving the menisci, do not all resolve; some can cause chronic pain and disability until surgery is undertaken to rectify the lesion. Small articular fractures, such as a Bennett's fracture of the thumb, can cause chronic disability. Intervertebral discs, like the menisci of the knee, are unlikely to heal spontaneously after injury, probably because of their relatively meagre blood supply. In essence, clinical experience abounds with examples of soft-tissue injuries to the limbs that do not summarily heal. Correctly used, therefore, extrapolation would predict that at least some injuries of the cervical spine would not heal. Whereas muscle sprains should resolve rapidly, some injuries to joints and discs may remain sources of chronic pain.

It is commonly believed that patients with neck pain exaggerate or perpetuate their symptoms for the purpose of financial gain [11], [12], [13], [14], but formal studies and reviews refute this conjecture [15], [16], [17], [18], [19], [20]. Although, in some instances, fraud does occur because of the monetary compensation that is available, the majority of patients behave as if they do have an injury, not as if they are malingering. Their symptoms persist despite and regardless of settlement of compensation claims [17], [18], [19].

At the root of all this controversy is the nature of the injury and how it might arise. Some physicians have difficulty crediting that a seemingly minor incident can inflict an injury that is not detectable radiologically, and which produces lasting and disabling symptoms. Although how and why symptoms persist is another matter, biomechanics research over the last 40 years has at least provided insights into how and where the injuries occur.

Section snippets

Modes of investigation

Investigators have used a variety of means in an effort to determine the biomechanics of injuries that might occur in whiplash. They have used mathematical models, finite-element models, physical models, animal models, and experiments using human cadavers.

Mathematical modelling involves programming into a computer, facsimiles of the cervical vertebra and equations that represent the forces exerted on these vertebrae by their ligaments, discs, and muscles. The objectives of mathematical

Historical perspective

A seminal study of whiplash mechanics was published by Severy et al. in 1955 [30]. They used human volunteers in two rear-impact tests at 13 and 15 kph, respectively. Accelerometers applied to the heads of the volunteers in the target vehicles recorded peak accelerations of 5 and 3 g, respectively. Most critically, these experiments demonstrated the phasing of acceleration of different components of the target vehicle and the volunteer (Fig. 1). Peak acceleration of the vehicle preceded that of

Modern studies

Modern studies of the biomechanics of whiplash have used high-speed photography and high-speed cineradiography to determine the kinematics of the cervical spine as a whole and of individual segments, both in cadavers and in human volunteers. The results have been illuminating.

Using high-speed photography, Geigl et al. [48] studied six cadavers and 25 human volunteers undergoing rear-end impacts between 6 and 15 kph. They recorded the pattern of motion of the head and neck, and the timing and

Discussion

The critical revision brought about by modern research into whiplash is that it is not a cantilever movement that is injurious; i.e., it is not an extension–flexion movement of the head, as was commonly believed previously. Rather, within less than 150 ms after impact, the cervical spine is compressed. During this period the cervical spine buckles; upper cervical segments are flexed while lower segments extend around abnormally located axes of rotation. It is during this period of compression

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