Unstable Upper Cervical Spine Injury with Concomitant Bilateral Ponticulus Posticus: A Case Report

Abstract

Ponticulus posticus (PP), also known as arcuate foramen, is an anatomical variation of the atlas (C1), which is an ossification of the posterior atlanto-occipital membrane and through which the V3 segment of the vertebral artery (VA) runs. Placement of a polyaxial screw in the C1 lateral mass is considered risky in those with PP because of possible risk for VA injury. We report a case with bilateral PP in which posterior fixation for unstable upper cervical spine injury was performed. The patient, a 70-year-old man, sustained a fall-induced unstable upper cervical spine injury (traumatic C2 spondylolisthesis with unilateral dislocation of the C3-4 facet joint). Following initial conservative treatment, a posterior fixation surgery was performed in the subacute phase. Preoperative images revealed the presence of bilateral PP. Therefore, an original plan to place the screws in the C1 lateral mass was discarded, and instead, placement of the laminar hook on one side and placement of the screw into the C1 posterior arch on the other side was performed. His postoperative course was uneventful, and he was transferred to a rehabilitation hospital 27 days after the injury. In cases of upper cervical spine instability with concomitant PP, it is essential to consider the fixation method based on the extent of the injury and the course of the VA on a case-by-case basis.

Keywords

upper cervical spine injury - vertebral artery - ponticulus posticus - lateral mass screw - arcuate foramen

Introduction

Ponticulus posticus (PP), also known as arcuate foramen, is an anatomical variation of the atlas, characterized by ossification of the posterior atlanto-occipital membrane that connects the vertebral artery (VA) groove to the occipital bone.[1] [2] [3] [4] [5] [6] This variation partially or completely encases the VA at the section where it traverses the VA groove (V3 segment) in a tunnel-like structure.[1] [2] [3] [4] [5] [6] When a posterior fixation surgery involving the atlas is required, the presence of PP incites a safety concern for possible iatrogenic VA injury during screw placement into the C1 lateral mass. Here, we report a case of unstable upper cervical spine injury with bilateral PP for which a posterior fixation surgery became necessary. A simple posterior fixation technique to eliminate the risk of iatrogenic VA injury in those who have been found to have PP is presented.

Case Presentation

A 70-year-old man with a medical history of lacunar infarction, for which clopidogrel had been prescribed, fell from a ladder of 2.5 m height to the ground, and was brought to our emergency department by an ambulance. He was fully alert and oriented. All of his vital signs were within normal limits and had normal respiratory pattern. However, he complained of intense neck pain. Neurological examinations revealed paresis of the extremities, with manual muscle testing (MMT) scores of 3/5 in the right upper extremity, 2/5 in the left upper extremity, and 4/5 in both lower extremities. No sensory impairments, sensory dissociation, or rectal/bladder dysfunction were noted.

Imaging Studies

A cervical spine computed tomography (CT) revealed bilateral fractures of the axis pars interarticularis (Hangman's fracture, Effendi type II), right-sided C3-4 facet subluxation, C3 vertebral body teardrop fracture, and C7 spinous process fracture ([Fig. 1A–E]). A cervical spine T2-weighted magnetic resonance imaging (MRI) demonstrated a hyperintense intramedullary signal at the level of the C3 vertebra ([Fig. 2A], arrowhead), leading to the diagnosis of cervical cord injury at the C3 level together with multiple spinous ligament injuries ([Fig. 2A], asterisks). The injury mechanism was speculated to be hyperflexion trauma to the head caused by the fall from height. Based on the neurological and imaging findings, the patient was diagnosed as having an incomplete cervical cord injury (American Spinal Injury Association Impairment grade C). The fall-induced cervical spine instability (type II Hangman fracture and unilateral C3-4 facet subluxation) together with preexisting cervical spinal canal stenosis might have resulted in the incomplete cervical cord injury. The horizontal (sagittal) displacement of the axis vertebra was relatively mild; however, instability due to the unilateral facet subluxation at C3-4 was highly likely ([Fig. 1A–D]).

Fig. 1 (A) A parasagittal computed tomography (CT) image showing a fracture of the right C2 pars interarticularis (PI) (arrowhead) and C3-4 facet subluxation (asterisk). (B) A sagittal image showing a fracture of the C3 vertebra (arrow) and a C7 spinous process fracture (circle). (C) A parasagittal image showing a fracture of the left C2 PI (arrowheads). (D) A coronal image showing a fracture of the right C2 PI (arrowhead) and C3-4 facet subluxation (asterisk). (E) An axial image showing the bilateral facture of the C2 PI.

Fig. 2 (A) A cervical spine magnetic resonance imaging (MRI) (T2-weighted image, sagittal image) shortly after injury showing a high-intensity signal at the C3 level of the cervical cord (arrowhead) together with multiple ligament injuries (asterisks). (B) A cervical spine MRI obtained 182 days postinjury showing disappearance of the high signal density.

Initial Treatment

Given the marked stenosis at the C3 level and mechanical instability, a posterior fixation from the atlas to C4, spanning the C2-3 vertebral body and C3-4 facet joint combined with decompression, was deemed necessary. Following hospitalization, no deterioration in neurological or general condition was observed. Furthermore, because of antiplatelet therapy for lacunar stroke, it was deemed unsafe to proceed with surgery in the hyperacute phase. Therefore, conservative management, including maintaining cervical spine immobilization and ensuring hemodynamic stability to maintain spinal blood flow, was continued during the acute phase. Because of the use of clopidogrel, addition of anticoagulation was withheld and intermittent pneumatic compression devices were used to prevent deep venous thrombosis.

Surgery

Posterior fixation surgery was carried out on the ninth day postinjury. Initially, the plan had been to place lateral mass screws on both sides of the atlas and C3-4, connected with titanium rods. However, preoperative imaging revealed the presence of bilateral PP ([Fig. 3A–C]), making lateral mass screw placement in the atlas a high-risk procedure. Besides, the right VA injury at the transverse foramen was noted ([Fig. 3D]). Due to bilateral axis pars interarticularis fractures, the use of C1-2 facet joint screws (Magerl method) was deemed unsafe. Therefore, the initial plan was modified to use a laminar hook on one side of the C1 posterior arch and a short polyaxial screw placed into the other side. Subsequently, lateral mass screws were placed onto the C3 and C4 bilaterally, followed by C3 laminectomy. Finally, the hook and screws were connected bilaterally with titanium rods ([Fig. 4A]).

Fig. 3 A spine computed tomography (CT) angiography showing the presence of bilateral ponticulus posticus (PP). (A) A parasagittal view, right side. (B) A parasagittal view, left side. (C) A coronal view showing PP in both sides of the C1 posterior arch (arrowheads). (D) Irregularity of the vessel wall showing the presence of right vertebral artery (VA) injury (star).

Fig. 4 (A) A lateral cervical spine X-ray showing the implant system. (B, C) Postoperative cervical spine computed tomography (CT) obtained 165 days postinjury showing bony fusion. (B) A parasagittal image.(C) A sagittal image.

Postoperative Course

After the fixation surgery, the patient participated in a vigorous rehabilitation program, and he was transferred to a rehabilitation hospital 27 days after the injury. On day 161 postinjury, the patient was discharged home from the rehabilitation hospital. At that time, he was able to walk without a cane or brace with mild paralysis remaining (MMT 3/5) in the left upper extremity. Cervical spine CT on day 165 postinjury showed good bony fusion ([Fig. 4B] and [C]), with no displacement or loosening of the screws or hooks. Cervical spine MRI on day 182 postinjury revealed the disappearance of the hyperintense signal ([Fig. 2B]).

Discussion

The presence of the PP becomes a concern for surgeons when it becomes necessary to place screws in the C1 lateral mass. To date, three approaches for screw placement into the C1 lateral mass have been reported, including the original method by Harm-Goel.[7] [8] [9] When PP is present, there is a substantial risk of VA injury with screw placement, regardless of those three approaches. Therefore, it has generally been recommended to avoid screw placement in the C1 lateral mass when PP is present.[5] [6] We could find only a few case reports after literature search regarding the attempt to place C1 lateral mass screws in patients with the upper cervical spine instability with concomitant PP. Hattori et al reported successful placement of C1 lateral mass screw from the caudal side of the C2 nerve root in a nontraumatic case with unilateral PP, using both surgical navigation system and O-arm.[10] The technique may be worth consideration in institutions where those expensive medical resources are available. In the present case, a fixation system was constructed by placing a laminar hook on the posterior arch of the atlas on one side and a screw in the posterior arch on the other side ([Fig. 4A]). If laminar hooks had been placed bilaterally on the C1 posterior arch, there was concern about potential dislodgement of the instruments during neck hyperextension, and therefore, the use of laminar hook had been limited to one side. A cervical spine CT at day 165 postinjury showed bony fusion ([Fig. 4B] and [C]), indicating that the objective of the surgery had been achieved. Another alternative fixation method reported in the literature was the use of sublaminar wiring.[11]

Depending on whether the atlas alone or lower vertebral bodies are also involved in the injury, fixation method and segments to be fixated may vary. If there is no damage to the axis and the course of the VA at that level is not high-riding, fixation using the atlanto-axial joint screw (Magerl method) may have an advantage of avoiding VA injury even in cases with PP. The presence of bilateral axis pars interarticularis fractures, however, led to the decision not to use the Magerl method in the present case. Therefore, it may be necessary to individually tailor the fixation approach based the degree of injury and the surgeon's skill level in cases with concomitant PP.

Conclusion

In cases of upper cervical spine instability with concomitant PP, it is essential to consider the fixation method based on the extent of the injury and the course of the VA on a case-by-case basis. While combination of laminar hooks and C1 posterior arch screws has limitations in terms of fixation strength, it offers safety and may be considered as one alternative to lateral mass screw fixation.

Conflict of Interest

None declared.

Note

This report was approved by the patient himself and was conducted in accordance with the Declaration of Helsinki.

Authors’ Contributions

M.F. contributed to the writing of the manuscript, S.W. was responsible for data collection, and J.I. provided supervision throughout the study.

Patients' Consent

The authors have been granted permission for publication from the patient and his family.

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Publication History

Article published online:
31 March 2025