Do traumatic cervical disc ruptures occur in low velocity accidents?

Generally, disc changes are of degenerative origin and develop over a longer time span. Accidental incidents with isolated traumatic disc ruptures are rare events; however, occupants involved in low velocity accidents often claim a causal relationship between disc changes and accidents. In forensic casework the adequacy of the biomechanical load to cause traumatic disc rupture has to be assessed. Based on patient data we addressed the following issues: – Do traumatic cervical disc rupture occur in low velocity accidents? – Are there always accompanying injuries in neuroradiological imaging? – What are the main injury mechanisms? In this study we analyzed cases of traumatic disc ruptures without other severe injuries in patients who underwent surgical treatment between 2010 and 2021. The purpose was to identify cases with traumatic disc ruptures following low velocity scenarios. The sample contained 16 cases with the main diagnosis of traumatic cervical disc rupture. Falls caused traumatic disc rupture in 14 of the cases, while traffic accidents caused disc injury in 2 cases only. Radiological signs of acute trauma as well as sensorimotor deficits or pain were present in every case. Of the patients six did not sustain accompanying fractures. In the majority of cases a hyperextension mechanism was assumed, even though an exact reconstruction of the load was not possible in every case. This study renders traumatic genesis of a disc rupture highly unlikely in patients involved in low velocity scenarios. Medical imaging without accompanying injuries or signs of acute trauma, e.g., in terms of bleeding, bony or ligamentous lesions, supports a non-accidental cause of disc changes. Nevertheless, expert opinions require case by case evaluation taking account of patient-specific and case-specific conditions.


Introduction
Generally, disc prolapses and herniations are of degenerative origin with dehydration of the nucleus, formation of osteophytes, etc. [18]. In contrast, traumatic disc ruptures occur less frequently and are usually accompanied by other injuries. Occupants involved in low velocity motor vehicle accidents often claim for whiplash injuries. Especially in cases with prolonged posttraumatic symptoms and subsequent imaging with the diagnosis of cervical disc bulging, some claimants associate imag-ing findings with the accident sustained; however, there is a controversy on whether low velocity accidents can cause isolated cervical disc trauma or not. For example, experimental studies found disc annulus fiber strains and shear strains exceeding physiological levels even at low sled acceleration levels of 3.5 g [13]. According to the prevailing literature a traumatic cause of disc bulging or herniation only should come into question if the biomechanical load was appropriate, if there were no complaints prior to the accident and if the disc-related symptoms began immediately after trauma [10]. Concerning the appropriate biomechanical load, experimental research suggests that traumatic disc injuries are usually accompanied by fractures of the adjacent vertebrae [6]. Concerning injury mechanism, disc prolapses in a porcine model could be produced in repeated flexion of the axially compressed C3/C4 segment [15]. The experiments performed by Adams and Hutton resulted in lumbar disc ruptures following compression of the flexed lumbar segment [2]. On the other hand, several case reports deal with uncommon injury mechanisms of disc herniation, like cervical hyperextension [8] or whiplash kinematics [4].
Within this study, we analyzed cases of traumatic disc ruptures in patients without other severe injuries who underwent neurosurgery between 2010 and 2021. The purpose was to identify cases with traumatic disc ruptures following low velocity scenarios. We addressed the following main hypotheses: -Traumatic cervical disc injuries do not occur in low velocity accidents. -There are always accompanying injuries in neuroradiological imaging. -Most frequently, the injury mechanism is hyperflexion.
To give an impression of the issues forensic and biomechanical experts have to deal with, we first give an overview of real cases we were tasked with assessing in recent years.

Material and methods
For an overview of our expertise cases, we searched our database for interrogatories concerning accident causality of cervical disc protrusion or herniation. Cases without available quantification of the biomechanical load in terms of collision-induced change of velocity were excluded. The period analyzed was from 2009 to 2022. The data basis for neuroradiological and biomechanical assessment comprised clinical cases with the main diagnosis traumatic cervical disc rupture from 2010 to 2021. Cases were selected from the clinic's database containing data of all patients who received treatment in the neurosurgery department. That means that the main diagnosis of traumatic cervical disc rupture was made by the surgeon based on intraoperative findings. Based on the medical discharge report we were able to fill a database (FileMaker Pro 15.0) emphasizing the following issues: -Main cervical spine diagnosis, -Accompanying injuries of the cervical spine, -Accompanying injuries not concerning the cervical spine, -Anamnesis especially concerning the accident (occupant, pedestrian, fall from body height or less, fall from more than body height, fall from great height > 20 m), -Age and gender, -Degenerative changes.
A specialist in neuroradiology performed radiologic second look of the X-ray, computer tomography (CT) and magnetic resonance imaging (MRI). We focused on accompanying injuries of the cervical spine, e.g., in terms of involvement of the anterior (ALL) or posterior longitudinal ligament (PLL) and osseous lesions, as documented in the medical record and as seen in the second look. A reconstruction of the presumable injury mechanism differentiating hyperflexion, hyperextension, axial compression, lateral flexion and a combination of motion was done by considering the anamnesis, the blunt force trauma of the head as well as the accompanying injuries of the cervical spine. We introduced the variable head contact containing the head regions (frontal, temporal, etc.) with documented blunt injuries. If the case sheet contained information about a head contact but without documenting the injury or impact location, we wrote "yes, nfs" (not further specified). In cases without such injuries, we proposed presumable injury mechanisms derived from the accompanying (bony) cervical injuries.
We used FileMaker Pro 15 (Cupertino, CA, USA) and Microsoft Excel (Redmond, WA, USA) for data storage and analysis as well as IBM SPSS (Armonk, NY, USA) for descriptive statistical analysis.

Results
From 2010 to 2022 we had 6 cases where we were asked to evaluate the causality between the claimed disc herniation and the trauma, summarized in . Table 1. Only in one case (2011) MRI showed evidence of acute trauma in terms of hyperintensities of the anterior longitudinal ligament and of the cervical paravertebral musculature. The unbelted occupant was involved in a frontal crash with a change of velocity between 20 and 25 km/h. He suffered blunt force trauma to the left forehead and to the right upper arm.
Between 2010 and 2021 a total of 16 patients underwent a surgical intervention due to the main diagnosis of traumatic cervical disc rupture in the department of neurosurgery of the Jena University Hospital. . Table 2 summarizes patient-specific and trauma-specific information. Except in two cases of traffic accidents, falls were the cause of the traumatic disc ruptures. The age of the patients was between 51 and 89 years with a mean of 70.2 years and a standard deviation of 11.7 years. The column "head contact" contains the region where blunt injury was described. Relevant pre-existing degenerations are listed in the last column of . Table 2.
Accompanying injuries in the cervical spine as well as in other body regions are listed in . Table 3. The accompanying spinal injuries were diagnosed in terms of a second look of the MRI images that were available in all of the 16 cases. In 6 out of 16 patients there were no accompanying fractures in the cervical spine. In 13 out of 16 cases imaging showed signs of acute trauma in terms of edema, hematoma or signs of instability. Of the patients 5 did not sustain any injuries in other body regions, the other 11 patients did.
. Table 4 shows the neurological symptoms and pain the patients exhibited at hospital admission. Neurological symp-   toms could not be documented in six cases. Pain was described in six cases. In only one case there were neither neurological symptoms nor pain. According to the case sheet, sensorimotor deficits were not present; however, this patient sustained a pedestrian accident with severe injuries, so that pain might not have been documented explicitly. Based on the head contact parameter and/or accompanying cervical injuries we derived the presumed injury mechanism in terms of causative neck kinematics. In 10 cases an extension injury mechanism or at least a relevant extension load component can be assumed.

Discussion
Over a time span of 12 years we found 16 cases with the main diagnosis cervical traumatic disc rupture. While cervical spine injuries occur between 2 and 3% of blunt trauma patients [7], the small number of cases shows that accidents causing traumatic cervical disc ruptures without other severe injuries are very rare events. This is particularly true for isolated cervical disc ruptures without cervical fractures or dislocations [5]. In our sample the leading causes of traumatic disc rupture were falls of older persons partly with pre-existing degenerative changes. Lowery et al. [11] found a bimodal distribution of cervical spine injuries with 2 peaks from 15 to 45 years and from 65 to 85 years. Due to the selection criterion main diagnosis of traumatic cervical disc rupture without other severe injuries used in this work, we did not receive cases of younger polytraumatized patients after severe traffic accident.
. Figure 1 shows CT and MRI images of patient 12 with pre-existing DISH syndrome and traumatic disc rupture C4/C5. Due to anterolateral bridging and ankylo-  [17].
In the current study traumatic disc ruptures involved the lower cervical spine segments C4-C7. This finding is in accordance with the data given in [3]. Cervical arthroplasty for traumatic disc rupture most often affected the levels C4/C5 and C5/C6. The same holds for degenerative changes with "punctum maximum" in the lower spine levels. In asymptomatic subjects the highest frequencies of spine degenerations were found in descending order in C5/C6, C6/C7 and C4/C5 [12].
Accompanying injuries in terms of osseous spinal lesions or soft tissue involvement were present in every case, although only 10 patients sustained accompanying fractures. Duma et al. performed dynamic axial compression tests using human cervical spine samples where traumatic disc injuries occurred only in combination with severe vertebral body fractures [6]. They concluded: ,"This is consistent with the findings of previous researchers who have reported that intervertebral disc failures do not occur due to single acute loading events without associated severe bony fractures." Also, according to Sane et al. [14] traumatic disc injury without osseous involvement should be a rare condition. Based on our results, this statement seems not to be true in every case, especially for older patients and for scenarios with more complex injury mechanisms com-pared to pure axial compression. There are also case reports referring to isolated traumatic spinal disc ruptures in younger patients. According to [5] a 29-year-old male as well as a 40-year-old male suffered C5/C6 disc rupture following a fall during a football game and due to contact violence with witnessed trampling, respectively; however, without any signs of acute soft tissue or osseous injury traumatic genesis of disc rupture remains questionable. With respect to soft tissue injuries, Davis et al. [4] emphasized that MRI findings in terms of ligamentous abnormalities are best seen within 2 weeks of injury and that paravertebral abnormalities could be resolved after 4 weeks.
Except for one patient with severe accompanying injuries, all patients showed symptoms in terms of typical disc-associated neurological deficits or pain. Neurological deficits werenotpresentinsix cases. Accordingly, Schwarze et al. [16] stated that traumatic disc ruptures can be responsible for a diverse spectrum of neurological deficits and pain beginning immediately after the trauma. Nevertheless, a slightly delayed onset of neurological symptoms does not contradict a traumatic genesis if the patient complains pain (bridge symptom) between the incident and the symptom onset [16]. Only in six cases, was pain explicitly documented; however, falls or other accidents with cervical injuries are usually associated with pain. Thus, we suppose documentation bias.
. Figure 2 shows imaging of patient 10 with an isolated disc rupture C6/C7 and pre-existing degeneration in C5/C6 and C3/C4. According to the case sheet, the 76year-old male sustained a fall from a staircase 2 days prior to hospitalization. Initially, he reported pain but without neurological symptoms. This case provides evidence that not every patient with traumatic disc rupture immediately exhibits distinct neurological deficits.
In five cases, a head contact was neither mentioned in the anamnesis nor were any head injuries documented. The traumatic events in the five cases were falls from bodyheight, falls from heights greater than body height, falls down staircases and a car accident with airbag deployment. From a biomechanical perspective, falls as well as car accidents with airbag deployment often involve head contacts. Thus, in these cases a head contact at least seems to be possible.
In the majority of our cases, we assumed extension and not flexion as the main injury mechanism, although there is uncertainty concerning the exact neck kinematics causing disc rupture. In many cases, we assume a more complex injury mechanism with different load vectors. Nevertheless, disc ruptures following hyperextension are also reported in the literature [1]. An MRI study [9] showed in 20 of 21 patients a b prevertebral hematoma following cervical spine hyperextension. In our sample, 10 patients were identified with prevertebral hematoma and ALL involvement as typical hyperextension findings. Several limitations of our study have to be kept in mind. Statistical analysis stratified by age, accident type, degree of degeneration etc. would be of great interest, however, our sample is far too small. There were only two patients after traffic accidents who underwent neurosurgery due to traumatic disc rupture. We assume that trauma surgeons instead of neurosurgeons could have treated some patients not captured by our filter: however, patients treated by trauma surgeons most probably suffered high energy trauma with other severe main injuries. Those cases were not in the focus of our study.
In summary it can be stated that low velocity accidents still have to be assessed as an inadequate trauma to cause traumatic disc rupture in the cervical spine. In falls of older patients, traumatic disc rupture without accompanying osseous lesions is not uncommon; however, every patient in our sample showed signs of acute trauma in MRI. Inadequate trauma and MRI without signs of acute trauma indicate a non-traumatic cause of the disc lesion; however, it might be that subthreshold biomechanical loads can lead to an aggravation of pre-existing degenerative disc disease. An MRI study of asymptomatic subjects [12] showed that asymptomatic disc changes are not uncommon and that the prevalence increases with age.
In five of our forensic expertise cases there were neither radiological signs of acute trauma nor any noncervical accompanying injuries, for example indicating head contact. From a biomechanical point of view, we concluded that the s-shaped deflection of the cervical spine in low velocity accidents without hyperflexion or hyperextension does not represent an adequate trauma. In the case from 2011, we found a higher change of velocity with blunt injury of the forehead and radiologic signs of acute trauma and relevant degenerative changes in an adjacent segment. In this case, we affirmed causation.
Besides the aforementioned experimental study [13] claiming a potential injury risk in low velocity rear impacts there is as far as we know only one case report with two patients suffering traumatic disc injury in a rear end collision [19]. Both occupants required neurosurgery. According to this report, there was a kind of locking between the head and the integrated seat and head restraint with flexion and axial compression of the cervical spine. Referring to this case report it should be clear that every expertise needs case by case evaluation taking account of patient-specific and case-specific conditions.

Conclusion
-Our study renders traumatic genesis of a disc rupture highly unlikely in patients involved in low velocity scenarios. -Medical imaging without accompanying injuries or signs of acute trauma, e.g., in terms of bleeding, bony or ligamentous lesions, supports nonaccidental cause of disc changes. -In the majority, a hyperextension and not a hyperflexion mechanism was assumed in our cases. -Forensic and biomechanical causality assessment needs close collaboration with radiologists and technical experts. -Expert opinions require case by case evaluation taking account of patientspecific and case-specific conditions.