Natural History of Odontoid Fracture

Cervical spine fractures involving the C2 vertebra, particularly the odontoid, are seen in up to 20% of cases. The majority of axis fractures affect the odontoid. These types of injuries are more prevalent in older adults due to falls from a standing height.

On the other hand, younger adults commonly experience odontoid fractures as a result of high-energy traumas like motor vehicle accidents or falls from significant heights, which often lead to neurological impairments. Additional injuries to the head, face, and lower cervical spine are frequently observed in these cases. In motor vehicle accidents, frontal impact is the primary cause of odontoid fractures.

The classification system developed by Anderson and D’Alonzo in 1974 is commonly utilized to categorize odontoid fractures. It comprises Type I, which refers to an oblique fracture occurring at the upper portion of the odontoid process, Type II, representing a fracture at the junction between the odontoid and vertebral body, and Type III, extending into the C2 body. Recently, Grauer et al. proposed a modified classification scheme aimed at better distinguishing between Type II and III fractures. Additionally, they introduced a more refined classification for Type II fractures based on factors such as fracture orientation, displacement, and comminution.

In 1979, Althoff conducted a thorough biomechanical study on odontoid fractures. The findings revealed that a mix of horizontal shear and vertical compression loading was necessary to consistently generate the three fracture types.

The primary cause of odontoid fractures was identified as sagittal impacts to specific regions of the head. However, due to certain limitations in the study, such as the absence of spinal load and motion measurements, the direct application of the results to clinical settings and the development of injury prevention systems are impeded.

Prior experimental studies have played a significant role in enhancing our understanding of odontoid fractures. For instance, Mouradian et al. successfully induced type II fractures by utilizing a simplified model, while Nightingale et al. observed type III fractures in the majority of their specimens through extension loading.

In another study by Doherty et al., it was demonstrated that type III fractures could be produced by applying posteriorly directed sagittal loads, while type II fractures resulted from a 45° deviation of the force vector from the sagittal plane combined with load application in the posterolateral direction.

Further biomechanical research is necessary to develop an acceleration-driven model that can provide detailed injury data, enabling informed decisions regarding ligamentous instability, reduction techniques, patient positioning, and specific stabilization methods for each fracture type. This research will contribute valuable insights for selecting appropriate treatment options, ranging from conservative approaches to nonconservative interventions.

A study in the literature investigated the mechanisms behind high-energy odontoid fractures. The results indicated that the primary cause of such fractures in 80% of the tested specimens was impact to the upper forehead in the midline. The study utilized a model comprising a cadaveric upper cervical spine specimen positioned between a surrogate head and a surrogate torso mass.

This model enabled the direct transfer of inertial loads to the upper cervical spine. The observations in the study included the occurrence of type II and high type III odontoid fractures. During traumatic events, the average peak loads and accelerations were observed prior to the peak motions, and the head exhibited maximum extension, translation, and compression between 62 and 68 milliseconds.

Motor vehicle crashes are the leading cause of odontoid fractures in younger adults. A clinical study involving 340 patients with axis fracture reported that 71% of the cases were attributed to motor vehicle crashes, followed by falls at 14% and diving at 4%.

Odontoid fracture was the most frequently observed injury, accounting for 58% of the cases. A pioneering study reported by literature, which included 49 patients with odontoid fracture, reported similar rates, with 71% caused by motor vehicle crashes, 20% by falls, and 8% by head impacts.

Another multicenter study reported by literature, involving 144 patients with an odontoid fracture found that 72% of the injuries were sustained in motor vehicle crashes. These findings are consistent with other clinical studies supporting the strong correlation between odontoid fractures and motor vehicle accidents.

As reported in the literature, odontoid fractures in older adults resulting from falls from standing height exhibit similar injury mechanics. Weakened bone, increased loads on the upper cervical spine, and a higher probability of head impact contribute to the higher frequency of these fractures in older individuals.

To validate these findings, future research should involve larger sample sizes and younger specimens. Additionally, exploring the relationship between subaxial cervical spine spondylosis, bone mineral density, and odontoid fractures is recommended. Despite its limitations, the study provides valuable insights into the mechanisms of high-energy odontoid fracture, as documented in the literature

A model described in the literature demonstrated notable strengths by effectively replicating odontoid fractures, ligamentous instability, and associated atlas injuries observed in clinical settings. Unlike previous models, this particular model successfully recreated the dynamic loads and accelerations experienced during real-life traumatic events like motor vehicle crashes, falls, or head impacts.

It accurately simulated the deceleration of the head, leading to load transfer, spinal compression, anterior shear force, and forward displacement of the axis in relation to the atlas. Moreover, the model captured the extensional rotation of the head and its upward translation relative to C3, closely resembling the dynamic responses witnessed in genuine trauma cases..

Attempts to reproduce odontoid fractures through impacts to the upper lateral side of the forehead or head have proven challenging, contrary to findings from previous studies. These studies utilized a model fixed at the C3 vertebra and subjected it to pendulum impacts, resulting in a 37% odontoid fracture rate among the tested specimens, some of which showed substantial ligament injuries or complete separation at the fracture site. In the study, the odontoid fracture rate was slightly lower but comparable at 31% among the specimens.

A specimen in a study reported in the literature displayed distinct head movements during trauma, characterized by significant translation and extension relative to the C3 vertebra. This specimen experienced an early high type III odontoid fracture, estimated to occur between 28 and 45 milliseconds, which aligns with findings from previous research. The fracture disrupted the stability of the odontoid ligaments, leading to hyperextension of the head. Notably, minimal upward translation was observed.

These biomechanical factors subsequently contributed to the trauma, involving compression of the posterior components and separation of the anterior components of the upper cervical spine, accompanied by shear load from the torso mass. Consequently, this compression resulted in bilateral fractures of the posterior arch of the atlas.

A biomechanical study reported in the literature explored type II and high type III odontoid fractures, along with associated soft tissue injuries and atlantal fractures. These findings have implications for future flexibility testing protocols to assess different stabilization techniques for the upper cervical spine.

The optimal treatment for upper cervical spine injuries, particularly type II odontoid fractures, remains a subject of clinical debate. Specimens with specific fracture orientations can be valuable for investigating the effectiveness of anterior odontoid screw fixation in providing postoperative stability while preserving natural motion.