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Clinical Supplement: Head and Spine Trauma

The biomechanics of cervical spine injury and implications for injury prevention

WINKELSTEIN, BETH A.; MYERS, BARRY S.

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Medicine & Science in Sports & Exercise: July 1997 - Volume 29 - Issue 7 - p 246-255
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Abstract

Neck injury, and more generally neck pain resulting from neck loading, represents a broad class of complex clinical problems. These include non-head contact decelerations which commonly occur in motor vehicle accidents in which the torso is restrained and the neck is forced to stop the moving head. Also included are head contact neck injuries in which the head strikes a surface, and either the neck is forced to stop the moving torso or the neck is placed in tension by the head. Of the non-head contact injuries, whiplash syndrome is the most commonly observed. Catastrophic injuries, meaning either bony fracture or complete rupture of a cervical spinal ligament, can also result from non-head contact decelerations, though they are less common(21).

The volume of literature which has been devoted to the characterization of compressive cervical spinal impact injury is large (33). Despite this collection of writing, considerable confusion remains as to the basic mechanisms which result in catastrophic cervical spinal injuries. For example, it is commonly suggested that in order to produce a lower cervical flexion injury, the head must be flexed, causing the cervical spine to undergo flexion beyond a tolerable range and this results in neck injury(18). However, analysis of real-world injuries frequently results in paradoxical observations in which head motion is not consistent with the injury mechanism. For example, the radiograph shown inFigure 1 illustrates a lower cervical bilateral facet dislocation. Also shown are a posterior arch fracture of the atlas and a posterior arch fracture of the axis (a Hangman's fracture). In this case, a midsagittal laceration anterior to the head vertex, but posterior to the hairline, clearly defined the point of head impact. The radiograph illustrates an example of multiple noncontiguous spinal injuries in which the injury mechanisms differ (i.e., a lower cervical compression-flexion injury in association with an upper cervical compression-extension injury). This cannot be explained on the basis of a simple head motion and is not consistent with the site of head impact, which causes the head to move in extension.

Clearly, if we are to become more effective in preventing cervical spine injuries, a cogent understanding of how they occur must first be achieved. Fortunately, over the last 15 yr, a renewed interest in determining the basic biomechanics of neck injury has developed and has led to a substantive increase in our understanding of this problem. Therefore, the purpose of this review is to provide insight into how cervical spine injuries occur and to discuss a framework on which injury prevention strategies can be developed and refined. We will review the epidemiology of catastrophic cervical spinal injury, present a classification scheme of neck injuries, define the biomechanics of these injuries, and use this information to evaluate injury prevention strategies. The latter will hopefully be considered by medical personnel, coaches, and athletes in an attempt to reduce the incidence of cervical spine injuries in sports.

EPIDEMIOLOGY

Vertebral and spinal cord injuries have been reported to account for 6.5% of hospital admissions due to trauma, with injuries of the cervical spine contributing to 68% of all spinal injuries among fatalities and survivors(14). The total annual cost of spinal cord injuries has been estimated to be as high as 7.6 billion dollars (31). Although injuries to the cervical spine can result from a variety of activities, the literature suggests that automobile accidents, sports, falls, and dives are the most common causes of injury (33). A number of studies suggest that motor vehicle accidents account for between 52 and 68% of cervical spine injuries (33).

Cervical spine injuries commonly result from sports; however, incidence rates show considerable regional variation reflecting the degree of participation in the various activities. For example, surfing, snowmobiling, and rugby have been identified as the most common causes of cervical injury in sports in California, Canada, and New Zealand, respectively(12,43,48). Perhaps due to its visibility and high participation rate, football has been most recognized as a source of neck injuries in the United States and has been extensively studied(50-53). Investigations in the 1970s and early 1980s found that as many as 70% of nonfatal cervical spine injuries in football resulted from head impacts(2,50,51). This high incidence of cervical injury has been attributed by most to result from the development and use of improved head protection and the adoption of a tackling method using the head. With later awareness of these techniques, the subsequent use of“heads-up” tackling in 1976 has resulted in a drop in neck injuries in football. In addition, swimming and diving accidents have also been identified as a significant source of cervical spine injuries, with attention being placed on other sports-related activities involving cervical spine loading due to head impacts(1,15,29).

The anatomical distribution of cervical injuries is not well understood because of the inherent bias in sources of data collection. Several studies have suggested differences in the distribution of fatal and nonfatal injuries. Understanding of injury frequency is complicated by the difficulty in diagnosing injuries in both survivors and fatalities, particularly at the craniocervical junction and in the upper cervical spine(13,27,29). Among studies of survivors, several investigations have shown that injury is most common between the C4 and C6 vertebrae (9,29,42,48,55). Incidence of C4-C5 injuries has been reported to be as high as 61% of the total cervical spinal cord injuries (9). An equally high incidence of injuries in survivors of swimming pool diving injuries has been reported for the C5-C6 motion segment (15). In contrast, upper cervical injuries have been thought to be less common among survivors, with an incidence of only 17% of cervical injuries (25). However, there are data to suggest that the C1-C2 joint is the most frequently fractured spinal segment in the cervical spine (27). Further, it has been suggested that the basis for these differences is related to age (46). Stratifying patients by age, it has been observed that the incidence of C2 fractures increases with age, accounting for 43% of spinal fractures in people over age 50. In subjects under age 50, injuries between C5 and C7 have been found to account for 66% of injuries, while C2 injuries accounted for only 19% of the injuries in this age group(46). Additionally, among fatal accident surveys and postmortem investigations the upper cervical spine has been identified as the most common site of injury. One study of both fatal and nonfatal injuries due to motor vehicle accidents has reported that C1 injuries were the most common with the levels of C5 through C7 having the next highest frequency of injury(14).

CLASSIFICATION

Classification of injuries provides a consistent and precise terminology on which both clinical and research studies may be founded and compared. It can also serve to provide insight into the mechanism of injury and thereby facilitate clinical decision making. Although numerous classification schemes exist for the description of cervical spinal fractures, little consensus has been reached(4,18,23,24,33,42,45,51,54). Methodologies for the development of these classifications have included speculation, case report summaries, retrospective reviews of radiographs, reconstruction of field injuries, review of videotapes of injury, and cadaveric biomechanical experimentation. In light of the diversity of sources for these data, it is of little surprise that attempts at finding agreement among them often fail.

In particular, many of these classifications are predicated on the assumption that movement of the head causing the spine to exceed its normal range of motion results in injury (23,24,42). For example, cervical flexion injuries are thought to occur in accidents in which the head flexes. In the same context, extension head motions have been assumed to be associated with extension injury mechanisms. Experimental studies have shown that this is not the case, however. Nusholtz et al. (1981, 1983) noted that head motion was not a good indicator of neck injury in whole cadaver drop tests (37,38). Bauze and Ardran (1978) and Myers et al. (1991) were able to produce bilateral facet dislocations, a compression-flexion injury, without any head flexion(6,32). Nightingale et al. (1996) also demonstrated that head motion was not correlated to neck injury in compression impacts(34). High-speed image analysis of the motions of the cervical spine following head impact showed that cervical spinal injury occurs between 2 and 20 ms, after head impact (Fig. 2). In contrast, the rotation of the head in either flexion or extension did not exceed 20° until 20-100 ms after impact; and head rotation of 90° did not occur until approximately 150 ms after head impact. It should be noted that analysis of conventional video does not actually capture the time at which injury occurs as each frame is taken 33 ms apart.

In contrast to the injury classification based on head motion approach, another mechanistic approach has been developed based on the forces and bending moments that act within the spine. First suggested by Roaf (1972) and Portnoy et al. (1972), this concept was popularized by White and Panjabi(1978) who coined the term the “major injuring vector,” or MIV(42,45,54). It has since given rise to a commonly used mechanistic classification scheme for lower cervical injuries suggested by Allen et al. (1982) (4). Mechanistically, each of the forces (compression, anteroposterior shear, and lateral shear) can be described by a single resultant force, F (Fig. 3). Bending moments, like flexion or extension, arise from the resultant force acting at a perpendicular distance from the spine. This distance is defined as the eccentricity. In cervical spinal injury, forces acting on a particular vertebra are always accompanied by some amount of bending moment. Thus, the bending moment describes where the resultant force is located. For example, a compression force in front of the spine is a compression force with flexion bending moment and is compression-flexion loading. Further, for a given force, different portions of the spine will not undergo the same amount of bending; and depending on the position of the spine and the location of the force, portions of the spine can be in compression-flexion while others are in compression-extension (Fig. 4)(34). Fortunately, several experimental studies have produced a variety of clinically observed injuries of the cervical spine and measured the force and the associated eccentricity(5,30,32,34,35,40,41). Assembling these data has produced a mechanistic classification based on the eccentricity of the resultant force acting at the site of injury(33). Specifically, when the compressive force acts behind the vertebral body (compression-extension), posterior element fractures are produced (Fig. 5). When the force acts through the middle of the vertebral body, compression fractures are produced. Burst fractures, wedge compression fractures, and facet dislocations are produced with increasing amounts of anterior eccentricity (compression with greater amounts of flexion bending moment). Generalizing this methodology to include tensile loading gives rise to a complete classification of neck injuries based on the force and its eccentricity for the injured motion segment(Table 1).

Although almost all cervical injuries are well described by this classification scheme, a limitation of this approach is that a particular injury may have more than one mechanism and therefore be assigned to more than one class. In the context of clinical or epidemiological studies, in which the forces acting on the spine may be unknown, a scheme with multiple categories for a given injury may be confusing. However, by adding pure bending groups to the classification, flexion and extension, this problem is resolved, and a unique mapping of injuries to classes is established (Table 2).

In order to understand how multiple, noncontiguous injuries of apparently differing mechanisms occur(7,26,34,36,37,47), like the case shown in Figure 1, it is necessary to understand buckling. Buckling defines a mechanical instability in which a structure deforming primarily in compression suddenly changes its deformation to a pattern of primarily bending with compression (11). An example of a buckling event occurs when a long ruler is compressed and the ruler suddenly forms a bow shape. First suggested by Torg et al. (1979) in a study of athletic injuries, the cervical spine has been shown to buckle in experimental studies(32,34,35,50-53). While buckling is a mechanical instability, it is not actually an injury. For example, returning to the ruler analogy, if the force is removed from the ruler, it returns to its original straight position, unbroken. In contrast, if the force is increased, the ruler ultimately breaks, and the failure is consistent with the deformation pattern of a compression-bending mechanism. The same is true of the cervical spine. Thus, while buckling is not injury, in part because it precedes the injury, the deformation caused by buckling does indeed explain the local injury mechanism. In the same context, buckling of the cervical spine causes a complex shape to develop, with concurrent regions of pure compression, compression-flexion, and compression-extension(Fig. 4). Therefore, while Nightingale et al. (1996) showed that head motion was not able to explain injury, they also observed that the cervical injuries produced were explained by considering the shape of the buckled spine (34). Returning to the case study, this mechanical foundation of local force, moments, and buckling explains an otherwise paradoxical situation. Prior to the injury, as the force was increasing the spine buckled (Fig. 4). The upper cervical posterior element and Hangman's fractures resulted from compression-extension because the force was behind the vertebral bodies. In addition, because of the complex shape of the spine, the same force also acted in front of the lower cervical spine, causing a compression-flexion, bilateral facet dislocation. The head motion, which was one of extension, occurred after the cervical spinal injury had occurred and was unrelated to the mechanisms of injuries.

CERVICAL INJURY BIOMECHANICS

When evaluating injuries it is often suggested that the degree of bony comminution is a measure of the energy required to produce the fracture. Accordingly, complex cervical injuries are often considered to be the result of high-speed collisions. Unfortunately, this is not always the case. Reconstruction of 67 diving injuries by McEl-haney et al. (1979) revealed that head impact velocities as low as 3.1 m/s (10 ft/s) were adequate to produce neck injury (29). This is equivalent to falling from a height of only 0.5 m (19 in). Therefore, an average individual diving from a standing position or running into a stationary object has sufficient energy to produce a cervical fracture. In this context, the vast majority of injuries are impact injuries. Slow (quasistatic) injuries, in which the body is compressed between two objects, are uncommon and are often reportable case studies (49). The mechanism of cervical spinal injury results from the cervical spine's being called upon to stop the mass of the moving torso. Biomechanical experiments support this conclusion and have also shown that neck injuries can be produced with only a small percentage of the total body weight, approximately 16 kg, following the head and neck in an impact with a velocity of 3.1 m/s (34,35). In this regard, the cervical spine is fractured by only the momentum of the upper torso, and not that of the lower torso or the extremities.

The obvious inability of the cervical spine to stop the moving torso at a very low velocity would suggest that the probability of injury in any head impact is great. This is not the case, however, as cervical injuries are actually an uncommon consequence of head impact (22). Neck injury risk depends on a number of factors, including the constraint of motions which allow escape from the torso and the orientation of the impact surface. Interestingly, while some have suggested that preflexion of the spine, which removes its natural lordosis, is a requirement for neck injury, several investigations have produced cervical fractures in cadavers with unconstrained, naturally curved cervical spines(3,34-38,51,52,57). The infrequency of cervical spinal injury following head impact can probably be best explained by the remarkable flexibility of the neck. Roaf (1960) first pointed out and Myers et al. (1991) later quantified that the cervical spine can be flexed through an angle in excess of 96° without injury(32,44). As a result, in most head impacts, the head and neck are able to bend out of the path of the moving torso, the torso then contacts the impact surface and stops without loading the neck or causing injury. Therefore, the addition of a constraint, as might occur when the head pockets into an impact surface, decreases the ability of the head and neck to escape out of the way of the torso and can increase the risk for neck injury. Hodgson and Thomas (1980) suggested that constraining the motion of the atlantoaxial joint increases the likelihood of injury(19). Yoganandan et al. (1986) also suggested that increasing constraint in impact also increased the number of injuries produced in impacts to cadavers (56). Myers et al. (1991) demonstrated that without constraint, neck injury could not be produced when forces were applied slowly (32). However, addition of constraint increased the stiffness of the neck, decreased the ability of the neck to move out of the path of the torso, and resulted in neck injuries(32).

Impact experiments by Nightingale et al. (1996) supported the hypothesis of increased injury risk with increased head constraint(34). The study also showed that the mass of the head could act as a constraint in a rapid impact injury as compared to a slowly applied compressive force. That is, in order to bend out of the path of the torso, the neck must accelerate the head out of the way. Because this must occur in less than the 20 ms required to produce injury, large head accelerations and, therefore, large spinal forces can be generated resulting in injury even in cases of head impacts to lubricated Teflon-coated steel (a surface without any constraint). From this biomechanical experience, we have concluded that constraints to head motion increase the probability of injury; however, in impact injury additional constraint of the head by the impact surface (pocketing) is not required for injury.

The experiments by Nightingale et al. also reveal that the point of head impact has a profound effect on the risk for neck injury and explains why very similar impacts can have dramatically different consequences(34,35). By allowing the head to strike the impact surface at various angles, these authors showed that impacts in which the head is already moving in the escape direction, and in which the cervical spinal force acts immediately to push the head in this same direction, are at statistically significantly lower risk for injury (Fig. 6). Thus, impacts which are more perpendicular to the cervical spine place it at greater risk for injury than those in which the spine's orientation is less perpendicular to the impact surface(36-38). Recognizing that the neutrally positioned cervical spine has a flexion rotation of approximately 25° from horizontal at T1 (34)(Fig. 4), it is not surprising that impacts to the head vertex and slightly anterior (15°) to the head vertex have a higher frequency and severity of cervical spinal injuries than impacts to the posterior region of the head(36). Similarly, if the impact surface orientation is sufficiently far forward that the face hits the contact surface, the probability of neck injury is also decreased. Clinical studies support this finding, including the absence of facial injuries among divers with neck injuries and the success of “heads-up” football in reducing cervical injuries (29,53). Thus, a change in orientation of the impact surface of as little as 15° can mean the difference between catastrophic neck injury and no neck injury at all(36).

INJURY PREVENTION

Biomechanical and epidemiological studies provide a guide for prevention strategies for mitigating neck injuries. These strategies include decreasing the incidence of blows to the head, shifting the angles at which the head is impacted, using appropriate padding and helmets, as well as equipment that transfers forces directly from the impact surface to the shoulders. Each of these strategies shall now be considered.

The single most important tenet of cervical spinal injury prevention is the avoidance of head impact. Because the head mass is small compared to the torso mass, considerably fewer situations exist in which there is sufficient energy to cause injury when the neck stops the moving head as compared to those in which the neck is forced to stop the torso (21,39). In contrast, even low-velocity impacts to the head in which the neck must stop the torso have sufficient energy to allow the torso to break the neck. Efforts to decrease the risks of head impacts, particularly in sports, recreational activities, and diving, can only serve to reduce the incidence of these catastrophic injuries. Examples include the prevention of shallow water dives and the prevention of head-first sliding, either on water slides or with backyard toys. Equally important is the recognition that impacts to the top, or slightly forward from the top of the head, pose the greatest risk for neck injury when compared with impacts to the back of the head and to the face. Efforts to reduce the incidence of these dangerous impacts may also result in reductions in cervical spinal injury. One of the most successful applications of these principles is evident in the football experience.“Heads-up” football mitigates neck injury through both of these mechanisms (52,53). By keeping the head up, the frequency of intentional (spearing) and accidental head impacts delivered to the top of the head is reduced. Additionally, those impacts that do occur are sufficiently far forward on the head and face to allow the head and neck to escape injury by moving into extension.

Impact surface padding and helmets have frequently been identified as both sources of cervical injury and means of mitigating neck injury. To understand the effect of a helmet, we must understand the dynamics of cervical spine injury (Fig. 7)(36). During an unhelmeted impact to the vertex of the head, the head hits the impact surface and generates a large force, over 8 kN, which stops the head in only a few milliseconds (Fig. 7A). Over these few milliseconds, the neck has virtually no force acting on it. A few milliseconds later, the neck is compressed between the stopped head and the still moving torso, and cervical injury occurs. Addition of a helmet, in the case of this experiment a thick compliant pad, reduces the head impact force dramatically to under 4 kN(Fig. 7B). However, as in the unhelmeted case, the neck force does not develop until after the head has stopped. These results show that a helmet clearly protects the head. In the tests shown, the Head Injury Criterion, a measure of injury risk, was reduced from 1360 to 197. Also illustrated is that using a pad which is both thicker and more compliant than that which can be used in a realistic helmet does not protect the neck because the neck is injured after the head has been stopped.

Because of the obvious benefits in reduction of head injury risk, and the high frequency of head injuries as compared to cervical spinal injuries, padding is often appropriately added to impact surfaces. However, the addition of very compliant pads to the impact surface may increase the risk of injury to the cervical spine (36). The mechanism by which this increase in risk occurs is more involved than just the mechanism of surface pocketing and injury risk due to constraint. In impacts with a compliant surface, the head pockets into the pad and its motion parallel to the impact surface is opposed. In other words, the deformed padding applies forces to the head which oppose the escape of the head and neck. Additionally, padding, by virtue of a change in stiffness and not by pocketing, also delays the escape of the head from the impact surface. As a result, the head's motion in the escape direction is delayed and diminished. Nightingale et al. (1997) have shown that very compliant pads significantly increase the frequency and severity of neck injuries when compared with impacts against rigid surfaces(36). This finding is particularly clear in the cases where the head is impacted posterior to the vertex. For this situation, the head and neck are directed into flexion by the rigid impact surface(Fig. 6). In the padded impacts, the deformed padding applies forces directed posteriorly and increases the time the head spends in the pad. As a result, the cervical spine is exposed to a significantly larger torso impulse and suffers injuries. It should be recognized that the materials used in these experiments were considerably more compliant than those used currently in automotive and other safety environments. Thus, the significance of this pocketing and stiffness mechanism in potentiating real-world injuries has not been demonstrated. However, it should also be apparent that the addition of surface padding should not be considered as a method of protecting the neck from injury.

These observations create a challenge to the injury prevention community, which includes medical personnel involved with the care of athletes. Pads clearly protect heads, and head injuries are more frequent than neck injuries. Further, although often less severe than catastrophic neck injuries, head injuries result in significantly greater social costs than neck injuries(8,17). The most important conclusion is to use pads only in those situations in which head injuries occur, recognizing that head injuries require larger impact velocities than cervical spinal injuries(3,29). It should also be a goal to design head protection systems to minimize the constraints to head motion. However, we must consider the caveat that head injury is a larger social problem and that most protective systems already in place likely provide very little constraint to head motion. At the research level, continued efforts to evaluate the significance of constraint in producing neck injury and to optimize pads to provide head protection while minimizing the risk for neck injury are needed.

An additional group of devices has been developed which connect a helmet directly to the shoulders. Used in aerospace and automotive racing, these devices limit the ability of the head and neck to flex and extend(16,20). In these environments, both neck fatigue and catastrophic cervical spinal injury occur in flexion and extension without head impact or compression loading (21,42). In contrast, compression neck injuries may be less frequent. Thus, despite the constraints imposed by these devices and their tendency to align the head, neck, and torso, they are likely an effective means for reducing cervical injuries in these specialized environments. More recently, a similar group of devices has been patented for use in sports like hockey and football in which the helmet is extended down to the shoulders or the shoulder pads. Unlike the aerospace and automotive devices, these devices are hypothesized to prevent compression cervical spinal injuries. These force-distributing systems rely on the transfer of force around the neck directly to the torso. Mechanically, the proportion of force managed by the neck and by the device is a function of the stiffness of the neck as compared to the stiffness of the device in series with the shoulders and shoulder padding. However, the constrained head-cervical spine system has an average stiffness of between 40 and 200 kN/m in compression and requires approximately 1.4-1.8 cm of displacement for catastrophic failure (32,41). Unfortunately, the stiffness of the device-shoulder complex and, therefore, the feasibility and design requirements of this injury prevention strategy have yet to be determined. Further, the extent to which these devices constrain motion has also not been evaluated. Indeed, other than an estimate of the required deployment time of an airbag system (10), no feasibility studies on these types of devices have been published.

CONCLUSION

Studies of the biomechanics of cervical spinal injury have shown that these injuries occur very quickly following head impact. In addition, the mechanism of injury is not related to the head motion but is related to the local forces and deformations which act on the vertebrae at the time of injury. These studies also show that the cervical spine is injured when it is forced to stop the moving torso and that the neck can suffer catastrophic injury at low velocities with only a portion of the weight of the torso following the neck. In contrast, cervical spinal injury potential is decreased when the head and neck are unconstrained and able to move out of the path of the torso. Cervical spinal injury potential is also decreased when the impact surface is not perpendicular to the neck. Because this is among the most important variables, both clinical and biomechanical investigations suggest that contacts to the face and back of the head have reduced cervical spinal injury potential compared to impacts to the vertex and slightly forward of the vertex of the head. Coaching and training efforts, such as those adopted in“heads-up” football, therefore represent a mainstay of cervical spinal injury prevention.

Studies demonstrate that while helmets protect the head effectively, they do not protect the neck because head impact and cervical spinal injury occur at different times. They also show that highly compliant pads may have the potential to increase cervical spinal injury risk. Accordingly, pads should only be used in those settings in which head injuries occur and should be designed to minimize the constraints imposed on the head. Additionally, although a number of new devices have been proposed to prevent cervical spinal injuries in sports, they add constraint to the neck, and their utility in preventing neck injury has yet to be demonstrated. Finally, additional investigations that examine the relationship between head and neck injuries and make use of materials currently used in automotive, sporting, and other safety industries are recommended.

F1-7
Figure 1.-Lateral radiograph illustrating a head impact neck injury in which a C6-C7 bilateral facet dislocation and C1 and C2 posterior element fractures are seen. These injuries illustrate the apparently paradoxical observation of upper cervical compression-extension injuries with concurrent lower cervical compression-flexion injuries.
F2-7
Figure 2.-Two high-speed images of a head impact into a padded surface illustrating the time at which injury occurred, 18 ms after head impact, and the time required for large head motions. 90 ms. In this case, the neck injury is the result of lower cervical compression-flexion. The head motion, extension, occurs after neck injury and is unrelated to the injury mechanism.
F3-7
Figure 3.-Drawing of a vertebra illustrating the simultaneous occurrence of all of the sagittal plane forces including a compression force, C; a shear force, S; and a bending moment, M. These forces can be replaced by an equivalent resultant force system; F is the vector sum of C and S and acts at an eccentric distance from the vertebra, e, such that M = Fe. Thus, the position of the force describes the amount of bending moment acting on the spine.
F4-7
Figure 4.-Drawing of the spine showing the normal curvature with T1 oriented approximately 25° below horizontal (A) and the curvature after buckling deformation (B). The complex shape of the buckled spine and the location of the resultant force, F, show how compressive flexion (CF) injuries at one level can occur simultaneously with compression (VC) and compressive extension (CE) injuries at another level. Distractive-extension (DE) and distractive flexion (DF) injuries can also occur.
F5-7
Figure 5.-A series of drawings illustrating the relationship between the location of the resultant force, F, and the type of injury produced. As the resultant force moves from behind the spine to in front of the spine the mechanism changes from compression-extension to pure compression to compression-flexion.
T1-7
T2-7
F6-7
Figure 6.-Drawing of head impact posterior to the head vertex (A) and anterior to the head vertex (B) showing that in impacts posterior to the vertex, part of the head velocity, the impact surface force, and the neck force all act to push the head into flexion and forward translation. In contrast, impacts that are more perpendicular to the spine (B) are less able to move out of the way because the head velocity, the impact surface force, and the neck force are less well aligned.
F7-7
Figure 7.-Force-time histories for a head impact into a rigid surface (A) and a padded surface (B). In both cases, the head hits the impact surface, generates large forces, and stops. A short time later the neck is injured. These data show that padding can protect the head by lowering the peak head force but does not provide neck protection.

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    Keywords:

    SPINE; INJURY; PREVENTION; CERVICAL; BIOMECHANICS

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