Intrafacet Spacer Placement as a Mobility-Sparing Bailout Option in Atlantoaxial Fusion Construct Salvage

Background: Salvage revisions of atlantoaxial (AA) joint complex posterior segmental instrumented fusion constructs require careful individualized planning to prevent occipital extension. In this case report, we describe the use of bilateral intrafacet spacer placement as a mobility-sparing bailout option for the revision surgery. Case Report: A 64-year-old male with a history of diffuse idiopathic skeletal hyperostosis, extremely limited baseline cervical mobility, and prior AA posterior segmental instrumented fusion presented with increasing pain at his 6-month follow-up. Imaging showed fusion and hardware failures and dynamic instability. To prevent occipitocervical fixation, AA intra-articular fusion via a DTRAX spinal system (Providence Medical Technology, Inc) was used as an adjunct to a navigated C1 lateral mass and C2 pars screw posterior segmental instrumented fusion construct. The patient had an uneventful postoperative course and was discharged with resolution of symptoms. Three-month postoperative follow-up confirmed persistent resolution of symptoms and absence of complaints, along with successful arthrodesis on imaging. Conclusion: AA posterior segmental instrumented fusion revision is technically challenging, particularly when partial preservation of craniovertebral junction mobility is required. Bilateral intra-articular cages may be used as an adjunct to hardware revision in construct salvage when sturdy arthrodesis is desired without occipital extension and may represent a major potential strength of intra-articular cages.


CASE REPORT
A 64-year-old male with a history of diffuse idiopathic skeletal hyperostosis presented after a mechanical fall with worsening myelopathy.Cervical imaging demonstrated nontraumatic CVJ stenosis secondary to an anterior AA membrane pannus, continuous ossification of the anterior longitudinal ligament extending into the distal anterior atlanto-occipital membrane, and segmental ossification of the posterior longitudinal ligament with minimal involvement of the tectorial membrane (Figure 1).Given worsening symptoms, the patient was treated by C1 laminectomy and AA posterior segmental instrumented fusion with bilateral C1 lateral mass and C2 pars screws (Figure 2).
Following an uncomplicated discharge with resolution of myelopathy, the patient reported progressive CVJ pain at his 6-month follow-up.Dynamic imaging to evaluate arthrodesis showed fusion and hardware failures, with bilateral lateral mass screw fractures, pseudoarthrosis of the right pars screw, and movement of the C1 screw fracture segments (Figure 3).
The patient underwent hardware exploration and hardware removal via an open midline approach with the distal lateral mass screw segments retained within C1.The AA joint was exposed bilaterally using a modified Harms approach. 12The dorsal rami and venous plexuses of the C2 spinal nerves were found to be adherent and were transected.Intraoperative guidance was registered via surface matching of the C2 posterior elements to preoperative imaging using Brainlab Spine Navigation (Brainlab AG) to cannulate screw tract trajectories that avoided retained hardware.
Attention was turned to the AA facets, which were sharply incised, and the intra-articular space was prepared using a DTRAX spinal system (Providence Medical Technology, Inc).Final imaging (Figure 4) and transcortical motor evoked potentials were satisfactory.The patient had an uneventful

Noncontrast computed tomography (A) sagittal view of the craniovertebral junction at the plane of the left facet demonstrates a fractured C1 lateral mass screw (small arrows) and a C2 pars articularis screw with the tip in the region of the C2 transverse foramen. Failed arthrodesis is noted at the atlantoaxial articulation (block arrow), which was consistent contralaterally. Axial sequences at the (B) C1 and (C) C2 levels show bilateral fractured lateral mass screws (arrows) and pseudoarthrosis of the right pars screw (white arrowheads on medial aspect). Lateral (D) flexion and (E) extension dynamic plain films of the cervical spine show movement of the fractured lateral mass screws (arrows).
postoperative course and was discharged with symptom resolution.Three-month follow-up confirmed persistent resolution of symptoms, absence of complaints, and successful arthrodesis (Figure 5).

DISCUSSION
4][35][36][37]43 In the United States between 1993 and 2014, primary C1-C2 arthrodesis surgeries increased 111% annually, while between 2006-2014, revision surgeries at the same level decreased by only 6% per year, with a resultant 57% proportional increase in hospital cost secondary to revisions.10 Recent years have also seen multiple reports of alternative techniques aimed at optimizing AA posterior segmental instrumented fusion constructs, 38,42  despite a reported 95% to 100% arthrodesis rate in most series.[32][33][34][35][36][37][38][39][40][42][43][44] The authors speculate that the high rate of revision surgeries required in ostensibly successful fusions may be associated with ambiguity in assessment parameters and terminology.][36][37]43 Many authors define stability on dynamic plain films as a surrogate for arthrodesis success, assuming perceived gross stability and bony fusion to be synonymous.[33][34][35][36][37][38][39][40][42][43][44][45] Regardless, a 2022 report that used dynamic stability to report fusion outcomes described fusion failure in up to 20% of patients following Harms technique AA posterior segmental instrumented fusion, which is likely to be a more accurate estimate of successful arthrodesis compared to studies boasting 100% success rates.41 Therapeutic arthrodesis manipulates bone healing responses following cortical breach or articular destruction, with hardware accelerating ossification and maintaining stability during proliferation of inflammatory mesenchymal derivatives during the acute phase.27,28 The added rigidity is critical in revision procedures, where fibrous scar tissue has already blanketed the area, creating a microenvironment that does not promote fusion and distorts normal anatomy.29 Intrafacet cage systems have been described to supplement both posterior and anterior constructs, with commercial systems approved as an adjunct to posterior segmental instrumented fusion constructs for subaxial cervical spondylosis.[15][16][17][18][19][20][21][22][23][24][25][26] Additional anecdotal application has been described with good short-term outcomes during occipitocervical revision and naïve AA posterior segmental instrumented fusion constructs in both pediatric and adult populations.13,19 Rationales for concurrent intrafacet constructs involve observed segmental biomechanical rigidity comparable to subaxial interbody fusion, with continued joint distraction and ligamentous tension.19,25,26 Increased rigidity along previously mobile joints also raises force magnitudes placed on adjacent articulations, increasing susceptibility to adjacent segment disease, which has been reported secondary to intrafacet spacer placement.25 The AA joint complex is known to compensate for subaxial spine spondylosis, risking adjacent segment disease in both AA and subaxial posterior segmental instrumented fusion constructs.42,45 In such cases, consideration of occipitocervical extension to increase fixation points and dissipate strain has been advocated.12,42,45 The authors speculate that our patient developed his presenting disease from such processes, the extended lever of the autofused cervical spine inducing continuous CVJ strain, ongoing inflammation, retro-odontoid pannus formation, and a loss of joint integrity.The baseline lack of mobility in this case increased our desire to fortify the AA construct rigidity, preserving the atlanto-occipital joint to prevent depriving the patient of virtually all CVJ mobility.As such, adjunctive intrafacet spacers were considered as a possible solution to increase segmental rigidity in a difficult anatomic region under increased stress with iatrogenically limited potential fixation points.Furthermore, while the AA joint complex is easily located in naïve patients, it was anatomically distorted in our patient, with a difficult postsurgical haptic and visual environment.
AA joint complex revisions characteristically have fewer potential fixation points compared to first-time procedures in the setting of bone resection and resorption, often necessitating extension. 13,14Hardware fracture increases complexity, limiting potential screw trajectories and accentuating the restraints of regional neurovascular anatomy, with increased vertebral artery injury risk being associated with alternative screw trajectories. 30Vertebral artery integrity is also a major consideration in primary fusions for AA joint complex trauma, which account for most primary and iatrogenic spinal vascular injuries. 12,42,45The supplemented use of intrafacet spacers, especially in revision or trauma surgeries with displaced or resected fixation options and increased vertebral injury risk, could provide a potentially quality of lifeimproving alternative, avoiding occipitocervical posterior segmental instrumented fusion constructs or less efficacious fallback constructs. 31,32e suggest that major indications of intrafacet fusion include AA posterior segmental instrumented fusion revisions; AA joint complex trauma cases with a high risk of vertebral artery injury; and AA cervicothoracic constructs where increased proximal fixation and the risk of vertebral artery injury can preclude the need for occipitocervical extension, risking additional morbidity. 13,19,22While the literature describing the use of intrafacet spacers in first-time AA posterior segmental instrumented fusion reports good fusion rates and outcomes, 13,18,26 evidence of successful fusion in revision cases remains limited, and our conclusion is anecdotal despite the impressive lateral AA arthrodesis observed in our patient at 3 months.We note that the resolution of pain in our case, a significant source of morbidity for the patient, could have been at least partially influenced by the bilateral C2 neurectomies.4][25] The observed symptom improvement was noted on a short-term follow-up period of 3 months, and continued monitoring is necessary to confirm durability of treatment.

CONCLUSION
Revision of the AA joint complex segment is technically and strategically challenging, particularly in scenarios with an increased need to avoid occipitocervical extension and preserve a degree of CVJ mobility.We describe the successful novel application of bilateral intra-articular cages as an adjunct to AA posterior segmental instrumented fusion construct salvage in a patient with hardware failure and a known history of diffuse idiopathic skeletal hyperostosis.We suggest that revision procedures are a major potential strength of intra-articular arthrodesis, with the likelihood of further application to trauma cases with risk of injury to the vertebral artery and to proximal AA joint complex fixation of longsegment constructs.

Figure 1 .
Figure 1.Noncontrast T2-weighted (A) sagittal and (B) axial magnetic resonance imaging of the cervical spine obtained on initial presentation shows significant canal stenosis (block arrow) at the craniovertebral junction secondary to significant ventral soft tissue compression and tectorial membrane buckling (arrow).(C) Noncontrast sagittal computed tomography of the cervical spine shows diffuse idiopathic skeletal hyperostosis with continuous anterior (arrows) and segmental posterior (block arrow) ossified longitudinal ligaments extending into the anterior atlanto-occipital and tectorial membranes.
Bilateral 4-mm DTRAX Cervical Cage-B intrafacet implants (Providence Medical Technology, Inc) were packed with demineralized bone matrix (Johnson & Johnson) and inserted under fluoroscopy.The surrounding surfaces were decorticated, and the construct was completed with rods adjoined to freehand bilateral lateral mass screws (3.5 × 30 mm, Johnson & Johnson) and pars screws (3.5 × 18 mm, Johnson & Johnson) placed within the prepared tracts.

Figure 2 .
Figure 2. (A) Lateral and (B) anteroposterior plain films of the cervical spine show the initial construct of bilateral lateral mass and pars screws across C1 and C2.

Figure 4 .Figure 5 .
Figure 4. Postoperative plain films of the craniovertebral junction.(A) Lateral view shows successful placement of revision hardware across C1 and C2 with bilateral lateral mass and pars screws (arrows), along with intrafacet implants at the lateral atlantoaxial joints (block arrow).(B) Open mouth anteroposterior odontoid view shows standard posterolateral construct combined with bilateral intrafacet C1-C2 spacer placement (arrows).
article meets the Accreditation Council for Graduate Medical Education and the American Board of Medical Specialties Maintenance of Certification competencies for Patient Care and Medical Knowledge.©2024 by the author(s); licensee Ochsner Journal, Ochsner Clinic Foundation, New Orleans, LA.This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license(creativecommons.org/licenses/by/4.0/legalcode)that permits unrestricted use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.