**2. Research background: Analysis of cervical spinal tolerances and injuries**

116 Injury and Skeletal Biomechanics

0.00

Motor Vehicle Accident

10.00

20.00

30.00

40.00

50.00

60.00

**Figure 1.** Frequency of Activities Causing Cervical Injuries, in Percentage [7]

making the likelihood of injury higher than it was in 1989.

**Table 1.** Sports Activities Causing Cervical Spinal Injuries

From Table 1, it can be seen that many common everyday activities offer the potential for serious injury. In diving, fractures and dislocations are the most commonly seen. Not only are the diver's form and function important, but depth of water, angle of entry and head velocity also prove crucial to injury severity. Degrees of participation within the various sports also play a role in the frequency of injury. Today, more athletes participate in football,

Canadian Causes of Injury US Causes of Injury

Occupational Domestic Sporting and

Recreational

Other

With motor vehicle accidents being the leading mechanism behind both spinal cord and vertebral fracture injuries, significant research has focused on improving the design and safety of automobiles [14]. Vertebral fractures can occur at any level to any degree, and can be caused by various types of loading. Figure 2 illustrates the frequency of injury to various levels of the cervical spine as well as the type of loading that causes that injury.

**Figure 2.** Frequency of Fractures per Level, Based on Type of Impact Loading

This data is also represented by use of the Abbreviated Injury Scale (AIS). Especially in the case of automobile and motorcycle accidents, the victim is usually injured because of some form of head contact with another object. Many factors contribute to the severity of the injury: position of the head and neck, the impact site, the nature of the impacted surface and the direction of the cervical spine loading. Determining the relationship between all of these variables is very complex and until recently has been based on frequency of occurrence data. Table 2 is a summary of the AIS scale characteristics. Figure 3 illustrates the amount of injuries that occurred in over 100 automobile accidents, in which a passenger attained a neck injury of some degree [25]. The degree of the injury is indicated by the AIS score.

With respect to frequency, the 5th and 6th vertebrae of the cervical spine see the most injury, and have the most critical injuries (AIS 5). This is the most significant score one can achieve and still survive, meaning that the ability to minimalize the chance of injury through improved vehicles and vehicle interior is crucial to assessing and preventing risk.


Cervical Spinal Injuries and Risk Assessment 119

A lower bound for the risk of injury of the neck under flexion-extension is based on the bending moment at the occipital condyle. For extension, noninjurious loading occurs at 35 ft-lbs (47.3 Nm), with ligamentous injury occurring above 42 ft-lbs (56.7 Nm). For flexion, pain can be felt at a load of 44 ft-lbs (59.4 Nm) and the risk of significant structural damage occurs around and above 140 ft-lbs (189 Nm). Bending moments of 24 Nm and resultant forces of 130 N can be felt without injury, but anything beyond that will most likely result in

This is the most researched type of loading, as a large amount of data can be found on compressive load analysis until failure is reached. The type of axial loading and the degree of constraint imposed on the contacting surfaces causes the results to vary among investigators. It has however been proven that compression-flexion and compression-extension injuries occur under smaller axial loads than pure compressive injuries. Bilateral facet dislocations can occur at loads of 1720 ± 1230 N, with flexion injuries occurring at approximately 2000 N. When dealing with pure compression, injuries have been reported under loads of 4810 ± 1290 N of loading. The average peak head and neck loads that can be reached before structural injury

Tension loading is not a commonly studied area of research. Past studies have indicated that the cervical spine has a tensile loading tolerance of 1135 N. With respect to automobile accidents, the average cranial accelerations are usually between 40 and 50 g [11]. This results in an estimated traction load of the Atlas (C1) of 1600 – 2000 N [3]. These types of loads produce disc damage, joint capsule tears and skull and vertebral fractures. The mean force at failure of intervertebral discs is 581 ± 220 N, but much still needs to be identified with respect to the amount of tensile force the vertebral bodies, and the entire cervical spine can

The estimated lower bounds of axial torsional tolerance are between 13.6 ± 4.5 Nm and 17.2 ± 5.1 Nm [13]. This amount of torque produces upper cervical spinal injuries. It has also been estimated that the cervical spine can withstand approximately 114 ± 6.3⁰ of rotation

Another area not too commonly researched is the amount of horizontal shear needed to produce cervical spinal injury. Most of this research is conducted to learn more about the

occurs are 5.9 ± 3.0 kN and 1.7 ± 0.57 kN, respectively [10, 14].

**2.1. Flexion-extension** 

injury [10, 12, 15].

**2.2. Compression** 

**2.3. Tension** 

withstand [19].

**2.4. Torsion** 

before injury occurs [6, 24].

**2.5. Horizontal shear** 

**Table 2.** AIS Scoring Details [12]

**Figure 3.** Automobile Injuries by Cervical Spinal Level and Their Associated AIS Scores

To assess the potential for injury, the loads that the cervical spine can withstand during various activities must first be understood. This can be broken down into the type of loading seen during the various activities. There are 5 main types of loading incurred by the cervical spine: Flexion-Extension, Compression, Tension, Torsion and Horizontal Shear. The main injury mechanisms are shown in Figure 4, while all are described in succeeding sections.

**Figure 4.** Injury Mechanisms of the Cervical Spine

### **2.1. Flexion-extension**

118 Injury and Skeletal Biomechanics

**Table 2.** AIS Scoring Details [12]

**Figure 3.** Automobile Injuries by Cervical Spinal Level and Their Associated AIS Scores

8

1

1 1

1 1

1

4 4

4

C1 C2 C3 C4 C5 C6 C7

2

5

11

15

10

9

17

4

3

2

AIS 2 AIS 3 AIS 4 AIS 5 AIS 6

**Figure 4.** Injury Mechanisms of the Cervical Spine

To assess the potential for injury, the loads that the cervical spine can withstand during various activities must first be understood. This can be broken down into the type of loading seen during the various activities. There are 5 main types of loading incurred by the cervical spine: Flexion-Extension, Compression, Tension, Torsion and Horizontal Shear. The main injury mechanisms are shown in Figure 4, while all are described in succeeding sections.

**AIS Score Injury**

1 Minor 2 Moderate 3 Serious 4 Severe 5 Critical 6 Unsurvivable A lower bound for the risk of injury of the neck under flexion-extension is based on the bending moment at the occipital condyle. For extension, noninjurious loading occurs at 35 ft-lbs (47.3 Nm), with ligamentous injury occurring above 42 ft-lbs (56.7 Nm). For flexion, pain can be felt at a load of 44 ft-lbs (59.4 Nm) and the risk of significant structural damage occurs around and above 140 ft-lbs (189 Nm). Bending moments of 24 Nm and resultant forces of 130 N can be felt without injury, but anything beyond that will most likely result in injury [10, 12, 15].

### **2.2. Compression**

This is the most researched type of loading, as a large amount of data can be found on compressive load analysis until failure is reached. The type of axial loading and the degree of constraint imposed on the contacting surfaces causes the results to vary among investigators. It has however been proven that compression-flexion and compression-extension injuries occur under smaller axial loads than pure compressive injuries. Bilateral facet dislocations can occur at loads of 1720 ± 1230 N, with flexion injuries occurring at approximately 2000 N. When dealing with pure compression, injuries have been reported under loads of 4810 ± 1290 N of loading. The average peak head and neck loads that can be reached before structural injury occurs are 5.9 ± 3.0 kN and 1.7 ± 0.57 kN, respectively [10, 14].

### **2.3. Tension**

Tension loading is not a commonly studied area of research. Past studies have indicated that the cervical spine has a tensile loading tolerance of 1135 N. With respect to automobile accidents, the average cranial accelerations are usually between 40 and 50 g [11]. This results in an estimated traction load of the Atlas (C1) of 1600 – 2000 N [3]. These types of loads produce disc damage, joint capsule tears and skull and vertebral fractures. The mean force at failure of intervertebral discs is 581 ± 220 N, but much still needs to be identified with respect to the amount of tensile force the vertebral bodies, and the entire cervical spine can withstand [19].

### **2.4. Torsion**

The estimated lower bounds of axial torsional tolerance are between 13.6 ± 4.5 Nm and 17.2 ± 5.1 Nm [13]. This amount of torque produces upper cervical spinal injuries. It has also been estimated that the cervical spine can withstand approximately 114 ± 6.3⁰ of rotation before injury occurs [6, 24].

### **2.5. Horizontal shear**

Another area not too commonly researched is the amount of horizontal shear needed to produce cervical spinal injury. Most of this research is conducted to learn more about the mechanisms that cause occipito-atlantoaxial injuries. Transverse ligament rupture has been seen to occur at a load of 824 N, with anterior shear of the atlas [5]. Odontoid fractures reportedly occur at 1510 ± 420 N of shear force [4]. Additional, higher tolerances have been reported up to 5500 ± 2500 N when the shear force is applied at the chest [2].

Cervical Spinal Injuries and Risk Assessment 121

3 Yr Old 6 Yr Old Small Adult Mid Size Adult Large Adult

3 Yr Old 6 Yr Old Small Adult Mid Size Adult Large Adult

Anything above each individual curve in Figure 5 indicates the potential for significant neck injury due to tension loading, while anything below indicates that significant neck injury due to loading is highly unlikely. Compression and shear data was analyzed and compiled in a similar fashion, to develop the curves in Figures 6 and 7, respectively. Anything above each individual curve indicates the potential for significant neck injury due to compression loading, while anything below means that significant neck injury due to loading is highly

> **Axial Neck Compression Loading (N) as a Function of Time Load Applied (ms)**

unlikely.

**Figure 6.** Compression Loading on the Neck [13]

0 5 10 15 20 25 30 35

**Shear Neck Loading (N) as a Function of Time Load Applied (ms)**

> Above the curve means Potential for Significant Injury due to Shear Force is Likely, but Below the Curve it is Unlikely

0 10 20 30 40 50 60

0

1000

2000

3000

4000

5000

6000

**Figure 7.** Shear Neck Loading Data [13]
