C2–C3 Anterior Cervical Discectomy and Fusion (ACDF) for Hangman's Fractures with C2 Posterior Dislocation: A Modi ed Classi cation of Hangman's Fractures and Technical Notes

Yufei Chen Air Force Medical Center of PLA Guannan Luan Chinese Academy of Medical Sciences & Peking Union Medical College Xiaojie Li Air Force General Hospital PLA Hongxing Zhang Air Force General Hospital PLA Jingyuan Li Air Force General Hospital PLA Ye Peng Air Force General Hospital PLA Shuang Ma sanmenxia yellow river hospital Songlin Li Air Force General Hospital PLA Jing Xue Air Force General Hospital PLA Junjie Du (  dujunjie205@hotmail.com ) Air Force Medical Center of PLA


Introduction
Hangman's fracture, also known as "traumatic spondylolisthesis of the axis (TSA)", is de ned as bilateral fracture of the pars interarticularis of the C2 vertebra that causes traumatic spondylolisthesis of C2 [1]. Hangman's fracture accounts for about 5% of all cervical spine fractures and 20% of all axis fractures [2].
The overwhelming majority of hangman's fractures cause anterior dislocation of C2. Hangman's fracture with posterior dislocation of C2 is extremely rare, only one paediatric case was reported in 2018 to date [3]. The classi cation system of hangman's fractures developed by Effendi [4] in 1981 and later modi ed by Levine and Edwards [5] in 1985 is most widely accepted nowadays. The management guideline for hangman's fractures is mainly based on the Levine and Edwards modi ed Effendi classi cation system. Hangman's fracture with C2 posterior dislocation do not t into any of the subtypes of Levine and Edwards modi ed Effendi classi cation system and the management of hangman's fractures with C2 posterior dislocation is different from C2 anterior dislocation. Given that there is no case of adult hangman's fracture with C2 posterior dislocation reported, scarcely any experiences of management of this situation can be learned. We reported the rst adult cases of Hangman's fracture with posterior dislocation of C2 here, proposed a new subtype of hangman's fractures and discussed management technical notes for the new subtype to avoid pitfalls.

Ethics Statement
The institutional Ethics Review Board of Air Force medical center of PLA approved the study. Moreover, we obtained written informed consent for the publication of this case by the patient.

Case description
A 31-year-old man presented neck pain and decreased muscle strength of upper limbs after fell into a 50cm deep roadside ditch when riding a motorcycle was admitted in our institution on 29 th June 2016.
The pain localized in the back side of neck, visual analogue scale (VAS) of neck pain is 8, without radiating pain.
Physical examination revealed muscle strength of the bilateral upper extremities was grade 4 and slight numbness of bilateral upper extremities, no dysmyotonia or sphincter dysfunction were revealed. Laboratory tests revealed nothing abnormal. Neck Disability Index (NDI) of the patient before surgery was 0.88, Frankel scale of the patient before surgery was D, and Japanese Orthopaedic Association (JOA) score[6] of the patient before surgery was 16.
Radiograph and computed tomography (CT) on admission showed fractures through both pars of C2 and C2 posterior dislocation. The injury pattern did not t any of the classic patterns based on Levine classi cation, but similar with a Levine classi cation type hangman's fracture [5] (Fig.1 A and B). A teardrop fracture of anterior-inferior side of C2 vertebral body is observed ( Fig.1 A and B, arrowhead). Magnetic resonance imaging (MRI) on admission showed high T2-weighted signal intensity of cervical spinal cord and compression of cervical spinal cord by posterior dislocation of C2 vertebral body and ruptured C2-C3 intervertebral disc ( Fig.1 C and D). After 5 days of skull traction through a Gardner-Wells tongue with 5 kg weight before operation, the dislocation aggravated ( Fig.1 E).
A C2-3 anterior cervical discectomy and fusion (ACDF) was performed using a standard anterior Smith-Robinson approach. After general anesthesia, the patient was placed in supine position. A 5cm long incision standard anterior horizontal incision, about 1cm below the right mandible, parallel to the mandible was made along the inner edge of sternocleidomastoid muscle toward the thyroid cartilage. The tissue was separated layer by layer to the front of cervical 2-3 intervertebral disc, the submandibular gland and supralaryngeal nerve were carefully protected. After con rming that the operation segment is correct by C-arm, posterior dislocation of C2 vertebral body was found to be severe, and the reduction was di cult (Fig 2 A). The teardrop fracture fragment was removed, the C2-3 disc was removed completely, and the bone graft bed was fully prepared. A screw tap for limbs fracture (diameter: 4.5mm) was employed to drill into C2 vertebral body for traction and reduction under close nerve monitoring (Note that the position for titanium plate on C2 is reserved . Fig 2 B). A hook-shaped nerve dissector (Fig 2 C) was employed to hook the posterior-inferior edge of C2 vertebral body and pull anteriorly during the traction and reduction process to avoid C2 vertebral body sudden rebound and damage the cervical spinal cord. After reduction, the autogenous bone grafts harvested from the anterior inferior edge of C2 vertebral body and the posterior superior edge of C3 vertebral body were lled into a cage and implanted into C2-3 intervertebral space. The teardrop fracture fragment was put back to its original place (Fig 2 E, arrow). A titanium plate and 4 screws were used for internal xation. Fracture reduction and xation were con rmed by intraoperative C-arm imaging before closing the wound (Fig 2 D and E). Closure of the wound was performed in layers with the use of a wound drain which was removed after 24 hours.
Postoperative cervical immobilization was accomplished by a Philadelphia collar for 12 weeks after operation.

Results
VAS of neck pain reduced to 2 immediately after operation and reduce to 0 at two weeks after operation. The patient was allowed to move freely with a Philadelphia collar one day after operation. At one week after operation, radiograph showed that internal xation was good, and the reduction was satisfactory ( Fig. 3A and B). At 6 months after operation, CT ( Fig. 3C and D) and dynamic radiograph ( Fig. 3E and F) showed bony fusion of C2-3 and both pars was achieved. MRI showed the T2-weighted signal hyperintensity of cervical spinal cord before surgery disappeared (Fig. 3G). NDI of the patient at 6 months after operation was 0.022, Frankel scale of the patient at 6 months after operation was E, and JOA score of the patient at 6 months after operation was 17.

Discussion
Haughton rstly described bilateral fractures and dislocation of the axis pars interarticularis in human subjects executed by hanging in 1866 [7]. Wood-Jones reported the bony anatomy of individuals executed by hanging with the knot in the submental position in 1913[8]. Schneider used the term "hangman's fracture" to describe this fracture pattern caused by tra c accidents in 1965 [9]. Different from its original injury mechanism resulted from a submental knot position causing a hyperextension distraction injury, modern hangman's fracture is mainly resulted from falls and tra c accidents causing a hyperextension and compression injury [10]. Based on mechanism of injury and morphology of fracture, several classi cation systems for hangman's fracture have been proposed. The classi cation system developed by Effendi [4] and later modi ed by Levine and Edwards [5] is most widely accepted. According to their classi cation system, type I fractures are described as non-angulated and displacement < 3 mm result from a hyperextension-axial loading force; type fractures are described as signi cant angulation (> 11 degrees) and displacement (> 3 mm) result from a combined hyperextension-axial loading force, with an additional anterior exion and compression load; type a fractures are described as severer angulation with minimal displacement result from a exion-distraction force; type fractures are described as severe angulation and displacement with facet dislocation result from a exion-compression force. Our case did not t into any of the four subtypes but bore close characteristics to type II fracture. Different from the typical C2 anterior displacement in most hangman's fractures which result in enlargement of spinal canal, posterior displacement of C2 in our patient resulted in compression of spinal cord and stenosis of spinal canal.
A teardrop fracture of C2 vertebral body is observed in the current case. Cervical tear drop fracture is divided into two types based on the mechanism of injury: exion and extension. Extension teardrop fractures of C2 are caused by avulsion fracture of the bottom of the anteroinferior side of C2 vertebral body by anterior longitudinal ligament during hyperextension injury. Although tear drop fracture of C2 vertebral body usually manifest as a hyperextension injury [11,12], the current case is more consistent with radiological features of a exion teardrop fractures: backward displacement of the fractured body, posterior displacement of the upper column of the divided cervical spine, and kyphotic deformity of the cervical spine at the level of injury [13,14]. Different from the current case, extension teardrop fractures of C2 are usually described in elderly patients with osteoporosis and with no associated neurological de cit, for which surgery is rarely needed [11,15]. Consistent with the current case, exion teardrop fractures of C2 are usually described in younger patients with high-energy trauma and with high potential for cervical instability and neurological impairment, for which surgery is usually needed [14]. Considering the cause of injury, features of radiological nding and clinical symptoms of the patient, injury mechanism of the current case is speculated as a exion-compression force.
To our knowledge, hangman's fracture with C2 posterior dislocation in adult is rstly reported here.
Ibebuike et al reported an 8 years old male sustained a bilateral C2 pars fracture with C2 posterior subluxation on C3 case in 2018 [3]. The paediatric case is similar to but different from our case. First, the paediatric case presented with signi cant angulation and minimal displacement, bore close characteristics to Levine and Edwards type a fracture; while our case presented with signi cant angulation and sever displacement, bore close characteristics to Levine and Edwards type fracture. Second, the paediatric case showed separation of the C2-C3 intervertebral disc from the C3 vertebrae through the cartilaginous layer without any disruption of the C2-C3 intervertebral disc and displaced C2-C3 disc protruded into the intraspinal canal causing cord compression, while in our case the C2-C3 disc was ruptured and the cervical spinal cord was compressed by posterior dislocation of C2 vertebral body and ruptured C2-C3 intervertebral disc. Third, our case showed a tear-drop fracture while the paediatric case didn't. Forth, a exion-distraction injury mechanism was inferred in the paediatric case while a exion-compression injury mechanism was inferred in our case. These differences may be due to the differences in the anatomical structure of the spine between adults and children. The wedge shaped anteroinferior edge of paediatric developing C2 vertebral body is different from adult which may have facilitated the posterior slippage of C2 on C3 [16,17]. Interestingly, the fracture of C2 anteroinferior edge in our case (teardrop morphology) also made the C2 vertebral body of the adult become a wedge-shaped anteroinferior edge like that of a paediatric C2 vertebral body. The combination between intervertebral disc and endplate in children is loose and intervertebral disc has not yet degenerated in children, this may have led to the C2-C3 disc was intact and totally prolapsed in the paediatric case.
Concerning that this unique subtype of hangman fracture is rare but do exist and cannot be catalogued using current classi cation schemes, we suggest that this unique subtype of hangman fracture be de ned as type IIb: type II hangman's fracture with C2 posterior dislocation. Its injury mechanism is speculated as a exion-compression force. The Levine and Edwards modi ed Effendi classi cation system of hangman's fractures was developed in 1980's while the rst paediatric case of hangman's fracture with C2 posterior dislocation was reported in 2018 and the rst adult case of hangman's fracture with C2 posterior dislocation was reported here. We can infer that the situation of C2 posterior dislocation was not taken into account when Effendi, Levine and Edwards proposed their classi cation system of hangman's fractures. C2 vertebral body is always anterior dislocation in other type of hangman's fractures which enlarges the spinal canal, so spinal cord injury is rare in hangman's fractures with an incidence of 6.5-10.3% [2,18,19]. Different from other type of hangman's fractures, C2 vertebral body posterior displaced and C2-C3 intervertebral disc ruptured in type IIb hangman's fractures, resulting in spinal canal stenosis and cord compression. Both reported cases showed symptoms of spinal cord injury (incidence: 100%).
The guidelines for the management of hangman's fractures are mainly based on the Levine and Edwards classi cation nowadays. The fusion rate of non-operative treatment for hangman's fractures is 100% in type I, 60% in type , 45% in type a and 35% in type III, respectively [20]. Non-operative treatment with immobilization either with cervical collar or halo-vest is choice for stable hangman's fractures or for the patients with surgical contraindications [21][22][23]. Surgical stabilization techniques through anterior, posterior, and combined anteroposterior approaches have been reported for treating unstable hangman's fractures [1,[24][25][26][27][28][29]. Surgical options for unstable hangman's fractures through anterior approaches rely on a C2-C3 ACDF while posterior approaches mainly rely on a C2-C3 posterior cervical decompression and fusion (PCDF) with C2 pedicle and C3 lateral mass screws [1]. Both an anterior and a posterior approach result in a high rate of fusion [22], but C2-C3 ACDF seems to be superior to posterior approach as it is less invasive, simpler procedure, fewer complications, direct access to the main pathology and effectively restore cervical lordosis [21,[30][31][32][33].
C2-C3 ACDF is obviously more suitable for type IIb hangman's fractures than posterior approach for the following reasons: First, instead of C2 anterior dislocation which results in widening the canal in most hangman's fractures, there is a posterior dislocation of C2 vertebral body and rupture of C2-C3 intervertebral disc in type IIb hangman's fractures, which results in spinal canal stenosis and spinal cord compression. C2-C3 ACDF can directly decompression, reduction and effectively restore cervical lordosis. Second, concerning that bilateral pars of C2 was completely ruptured and displaced in type IIb hangman's fractures due to severe exion-compression force, C2 pedicle screws are di cult to be implanted and the holding force of screws are poor. Third, there is cervical spinal canal stenosis and spinal cord compression in type IIb hangman's fractures, spinal cord injury can be aggravated by positioning for posterior surgery. Forth, C2-C3 intervertebral discs are totally ruptured, bilateral pars of C2 are completely ruptured, and C2 bodies are severely displaced posteriorly in type IIb hangman's fractures, the stability of anterior cervical column is so poor that posterior xation alone may be di cult to maintain spinal stability.
There are pitfalls in C2-C3 ACDF for type IIb hangman's fractures. First, traction before surgery is not recommended for type IIb hangman's fractures.   Radiological imaging during surgery A) Lateral cervical spine radiograph by intraoperative C-arm showed posterior dislocation of C2 vertebral body severe, and the reduction was di cult. B) A screw tap for limbs fracture was employed to drill into C2 vertebral body for traction and reduction under close nerve monitoring. Arrow: the position for titanium plate on C2 is reserved. C) A hook-shaped nerve dissector was employed to hook the posterior edge of cervical 2 vertebral body during the traction and reduction process to avoid C2 vertebral body sudden rebound and damage the cervical spinal cord. D) and E) Fracture reduction and xation were con rmed by intraoperative C-arm imaging before closing the wound. The teardrop fracture fragment was put back to its original place (arrow).