Cervical Spine Functional Anatomy and the Biomechanics of Injury Due to Compressive Loading

Normal Kinematics of the Upper Cervical Spine

The first cervical vertebra, the atlas, has often been labeled the cradle, because its articulation with the occiput of the skull provides a cradle for supporting the head (Figure 1).¹⁵ The atlas articulates with the occipital condyles, and its primary motions are flexion and extension. Normal flexion to hyperextension at the atlanto-occipital joint ranges from approximately 15° to 20°.^15,16 Rotation and lateral flexion between the occiput and atlas are not possible due to the depth of the atlantal sockets, in which the occipital condyles rest. Rotation to one side causes the contralateral occipital condyle to contact the anterior wall of its atlantal socket and the ipsilateral condyle to contact the posterior wall of its respective atlantal socket.¹⁵ Similarly, lateral flexion requires the contralateral occipital condyle to lift out of its socket, a movement that is restrained by the tight atlanto-occipital joint capsule.¹⁵

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Figure 1

Posterior view of C1 (atlas) and C2 (axis)

The weight of the head is transferred to the cervical spine through the lateral atlanto-axial articulations of C2, the axis (see Figure 1). The superiorly directed odontoid process extending from its body rests within a facet on the atlas that is created by the anterior arch and allows the atlas and head to rotate from side to side as one unit. The normal ranges of rotation of C1 on C2 are reported to be 50° to each side.¹⁶ However, results have varied, and this range has been noted to be 32° in cadavers,¹⁷ 75.2° through radiographic techniques,¹⁸ and 43° using computed tomography scanning.¹⁹ Nevertheless, this rotational ability of the atlanto-axial joint is possible due to the stabilizing function of 3 primary ligaments (transverse, alar, and apical), which act to hold the dens as a “fixed post” on which the atlas can rotate.²⁰

Rotation is also possible at C1 through C2 because, unlike the atlanto-occipital joint, the lateral superior and inferior articulating facets of the atlas and axis create a biconcave surface. The concavity of each articulating surface is due to the articular cartilage of the inferior and superior facets and is not visible on a radiograph.¹⁵ This characteristic allows for the anterior and posterior translation of the articular surfaces, and as the atlas continues to rotate, it settles into the axis as the superior articular process on each side slides down the anterior and posterior rims of the convex inferior surfaces. The biconvex nature of the atlanto-axial articulation means that cervical spine flexion and extension often create motion in the direction opposite that being experienced in the atlas.^13–15 Thus, when the cervical spine is flexing, the atlas extends, and when the cervical spine extends, the atlas flexes. This coupling motion is possible because the atlas is balanced on the concavity of the axis, and when the line of compression moves anterior to this balance point, as when the neck is extended, the atlas moves into flexion. The reverse follows as the cervical spine flexes, moving the line of compression posterior to the balance point and creating extension at the atlas. This coupling, or reversal of motion, is a unique characteristic of the spine, may be experienced at different levels, and will also be important in understanding mechanisms of injury (Figure 2).

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Figure 2

The biconvex nature of C1 and C2

A, Translation. B, Extension of C1 creating flexion in C2. C, Flexion of C1 creating extension in C2

Another feature of the atlanto-axial joint also found in other segments in the cervical region is that pure rotation of the atlas on the axis does not occur without a small degree of extension and lateral flexion and sometimes flexion.¹⁸ Again, the line of vertical forces being distributed through the occiput to the atlas as the head moves determines the amount of coupling motion in the atlas as it balances between the head and the axis.

Axial Loading in Football

The mechanism for injury that has received the most attention in athletics is axial loading. Only 13% of the 209 football-related injuries that resulted in permanent cervical quadriplegia between 1971 and 1975 resulted from hyperflexion (10%) and hyperextension (3%), whereas 52% were attributed to axial loading.³⁴ According to the National Center for Catastrophic Sport Injury Research, a total of 107 cases of permanent cord injuries in football occurred between 1977 and 1989, with most resulting from tackling.¹⁰ More recently, 15 cases of quadriplegia were reported in high school football between 1991 and 1993, with the principal cause attributed to axial compressive loading.³⁵

Axial loading occurs when the head and neck are flexed to approximately 30°, as in a head-first tackle. In this position, the normal lordotic curve disappears, which removes the energy-absorbing elastic component of the region. When contact is applied to the crown of the head or helmet in the football player, the cervical spine experiences a compressive load from the torso. As the padding provided by the helmet reaches its absorptive limits, the head then reverses direction, resulting in an increased compressive load as the cervical spine is compressed between the head and torso. When this compressive force exceeds the spine's absorption capabilities, soft and hard tissue components fail. Compressive load limits of the cervical vertebrae have been calculated at 3340 to 4450 N.^25–27 Interestingly, the upper compressive load limits of the cervical spine have been reached in less than 11 milliseconds during simulated impacts using a speed and mass equivalent to approximately half those of a conditioned athlete.³⁶ Moreover, CSI occurs when the compressive loads on the cervical spine are increasing at an increasing rate, demonstrating that during an axial load, the potential for serious neck injury exists independent of neck strength.³⁶

Too frequently, these injuries are brought about by a conscious effort to spear, or use the crown of the head as the initial point of contact during a tackle. Despite rule changes, equipment improvements, and an overall decrease in the number of catastrophic injuries that result from these significant epidemiologic findings, spearing appears to be as prevalent now as it was before.^37–40

Cervical Spine Buckling

During axial loading in the cervical spine, compressive forces result in a transient deformation, or buckling effect.²⁵ This buckling produces large angulations within the cervical spine as a means of releasing the additional strain energy that has been produced from the vertical loading and is the causative factor of injury (Figure 4).^24,25,27

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Figure 4

Buckling effect in the cervical column under axial load

Nightingale et al^24,25,27 assessed the dynamic responses of the cervical spine to axial loading using high-speed video and cadaver specimens mounted (inverted) to a drop-track apparatus. Buckling occurred in 1 of 2 distinct orders.^24–26 First-order buckling resulted in extension of the upper cervical spine through C5 and flexion through T1. Second-order buckling created flexion of C1 through C3, extension in C4 and C5, and flexion in C6 through T1. The levels at which the spine reverses its motion are where pivot points have been created, as in the abrupt reversal of curve described earlier.¹⁴ Buckling and injury were produced in the specimens within 2 to 31 milliseconds after impact, well before observable movement in the head and neck complex was noted (20 to 100 milliseconds).^25,26 In essence, a cervical vertebra can almost immediately be placed in an extreme position of hyperflexion or hyperextension during buckling, even though the head and neck complex is not yet experiencing observable motion. This fact is critical for the athletic trainer to understand: motion at the head is not a reliable indicator of the motion at various levels in the spine; therefore, the potential for injury should not be underestimated.

Although investigating human injury mechanisms with cadavers is valuable, a limitation lies in the inability to account for the contribution of forces and joint stability from soft tissues. Even though injury occurs before the approximate time of reflexive muscular contraction (60 milliseconds),⁴¹ some degree of muscle activity and stiffness can be assumed to be present in the athlete's cervical column at the moment of impact. Furthermore, to our knowledge, most of the research performed on vertical and axial-loading biomechanics have involved load applied to a cadaver specimen in a vertical direction.^{25,26,42–46} Few instances in sport place the athlete in a pure upright or inverted position when receiving an axial load. More often, the player purposely lowers the head or falls, and the head and trunk travel in a parallel or oblique direction relative to the ground. The effect of gravity, coupled with the static stabilization of posterior muscles in the upper back and cervical region in these situations, could affect the buckling mode and direction of resultant force vectors. Researchers to date have not attempted to simulate these internal and external forces in a nonvertical position and their effect on impending injury, and they have not assessed the effect of adding mass and size to the head, such as when wearing a helmet. Further research in this area specific to the functional demands and injury mechanisms in sports is necessary.

Head Orientation and Padded Surfaces

Nightingale et al^24–26 assessed dynamic response in cadavers to axial impacts by adjusting the contact-surface orientation so the head received impact anterior to the vertex of the head (15° and 30°), at the vertex (0° or midpoint), or posterior to the vertex of the head (15°). The greatest extent of injury was when the impact was at the vertex or anterior to the vertex of the head, with no injuries to the cervical spine with an impact posterior to the vertex.^25,26 For the anterior and vertex impact conditions, only 2 of 6 specimens did not experience injury, whereas 2 specimens experienced injuries at multiple levels.²⁶ The authors correlated injury risk to impact location. Specifically, when the head experienced an impact directly at or anterior to the vertex of the head, the neck-loading vector responsible for moving the head out of the path of the torso was too small and could not accelerate the head out of the way before injury occurs. However, the further the impact point is in the posterior direction, the easier it is for the cervical spine to flex and move the head out of the way of the increased force of the torso.²⁶

Nightingale et al^24–26 also investigated impact into padded surfaces. Padded surfaces within helmets are used globally in sport and act as protective covering for the extremities and objects such as goalposts. Yet the relationship of this padding to the kinematics of the cervical spine has not been established. Investigators, in fact, have failed to demonstrate that padding acts to protect the cervical spine, and it has even been shown to increase the likelihood of major injury to the cervical spine.^24–26

Using the same methods of cadaver preparation and impact platform orientation, the researchers covered the rigid surface with a padded material.^25,26 Impact forces at the head decreased, as would be expected, but the resultant neck forces increased, causing more severe injuries than with nonpadded impacts, regardless of impact orientation.^25,26 The greater impulse, due to the padding, allows for impact forces to be experienced over a greater time, and in most scenarios in sport, this is desirable. However, with regard to the cervical spine, the longer the head is placed in axial compression, the longer the time injurious forces are applied to the neck. Therefore, the padding creates a “pocket” for the head during impact. This pocketing causes the head to become depressed into the padded foam surface, which acts to “hold on” to the head.²⁶ Hence, a pocketing event increases risk of injury to the cervical spine due to the increased time of exposure to compressive force.^25–27

Impacts to the head in contact sports are often into a surface with some degree of padding. This can include padding over equipment, a pad worn by an opponent, the soft tissue of an opponent, compressive playing surfaces, or padding within the helmet itself. These situations may all present some degree of pocketing, making it much more difficult for the spine to escape an axial load. Although the presence of padding provides a risk for CSI, its use is necessary to protect against the more commonly occurring blunt trauma encountered in sport that involves the skull or other anatomical structures. Athletic trainers should consider the type of impact surface involved in a suspected CSI and, according to the findings of research, should appreciate the potential severity of injury due to pocketing and impact orientation.

Whiplash

Although more common in motor vehicle crashes,⁴⁷ whiplash injury can also take place in contact sports.³² An example of a whiplash-type injury in football occurs when the quarterback is sacked from behind, particularly if the quarterback is not expecting to be hit. Yet, contrary to what is generally considered to be an injury caused by extreme range of motion in the head or neck,³² some cervical injuries from whiplash are due to compressive forces.^48–50 The difference in the mechanism between whiplash and the typical axial load is that the compressive force is superiorly directed from below the cervical spine,⁴⁹ as opposed to an inferiorly directed force from axial load.

The combined efforts of several researchers using human subjects,⁵¹ cadaver specimens,^{43–45,48,52} and mathematical modeling⁵³ have provided a detailed timing of events in the cervical spine during whiplash to explain the compressive mechanism. The first joints and segments of the body to experience movement during a whiplash are the hips, back, and trunk.⁵¹ These structures of the body not only move forward after a rear-end collision but upward as well. This upward thrust of the trunk compresses the cervical spine.^48,50,51 Coupled with the forward displacement of the trunk, this combination of motions causes the head to revolve backward into extension, creating tension and buckling where the lower cervical segment extends and the upper cervical segment flexes.⁵¹ Peak extension of the neck during whiplash is reported to be 45°, which is not beyond the neck's normal physiologic limits and, hence, is not the actual cause of injury.^48,49,54 Rather, the injurious mechanisms (compression, forward trunk displacement, and buckling) act together to abnormally rotate individual vertebrae into extension. As a vertebra experiences this rotation, the anterior components are separated and the posterior components, particularly the facet joints, experience extreme compression.^48,50,51 Minor fractures of the inferior facet joints have been discovered in cadaver specimens⁵⁰ and during autopsy in cadavers after motor vehicle crashes.⁵⁵ Therefore, injury in the vertebral region is not necessarily due to shearing or excessive cervical spine hyperextension. Rather, an individual vertebra positioned between the flexed and extended regions of a buckling spine extends around an abnormally high axis of rotation, which forces it into the superior facet joints of the subjacent vertebra (Figure 5).^{48–50,52,54,56,57}

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Figure 5

Forward displacement of the trunk and compression and upward rotation of the cervical spine and head

In describing the events that occur during whiplash, most researchers have studied subjects sitting in car seats with headrests.^{48,49,51,52,58} However, whiplash-type injury in sport other than competitive auto racing is more likely to occur when the athlete is in an upright position without a head restraint. The upward thrust of the torso may not be as great in an upright position as in a seated position, possibly minimizing the buckling and compressive effect. However, abnormal rotations in the vertebrae may still take place, particularly because there is no posterior restraint to the head and neck.

If initial radiographs do not reveal cervical fractures after whiplash, typically the diagnosis is generalized to soft tissue injury, and no further diagnostic techniques are pursued.⁵⁴ However, threats to cervical column integrity are still possible from whiplash and may not be clearly identified with any radiographic technique, which may lead to an underestimation of recovery time. Research investigating the true mechanisms and resultant injury of whiplash specific to sport is warranted.