Innovative One-Hole Split Endoscopy for Posterior Atlantoaxial Lateral Mass Joint Fusion: Technical Details and Preliminary Clinical Application

Surgical Indications

Radiological assessment provides the objective basis for the diagnosis of AAI/D, with emphasis on anatomical integrity and joint congruency.1,14 Anterior dislocation is the most prevalent form, defined as an atlantodental interval (ADI) greater than 3 mm in adults and 5 mm in children. In contrast, posterior or rotational dislocations may present with normal ADI but abnormal congruency of the lateral mass joints.15,16 Surgical intervention is indicated in symptomatic patients to prevent neurological decline.17 Preoperative evaluation should exclude tumors, infections, acute fractures, congenital craniovertebral anomalies, and prior atlantoaxial surgery. Essential imaging includes dynamic cervical radiographs to assess alignment and reducibility, computed tomography (CT) with 3-dimensional reconstruction for deformity and dislocation severity, CT angiography for vertebral artery course, and magnetic resonance imaging for neurological pathology.18–20

Anatomic Layers and Safety Corridors

Preoperative mastery of the regional anatomic layers, together with precise delineation of the safe corridor, is critical to surgical safety and procedural reproducibility. The surface projection of the C1/C2 lateral mass joint lies within the paramedian suboccipital region, approximately 1.5 to 2.0 cm lateral to the midline. From superficial to deep, the approach proceeds through the skin/superficial fascia, trapezius, splenius capitis, semispinalis capitis, deep fat layer, and obliquus capitis inferior, which directly overlies the C1 posterior arch, the C2 lamina, and the lateral mass joint (Figure 1A). The longitudinal fiber orientation in this region facilitates blunt dissection. The occipital nerve typically courses in the deep fat layer and ascends deep to the semispinalis capitis. The C2 nerve root and its venous plexus are situated at the cranial margin of the lateral mass joint capsule, while the vertebral artery ascends posterolateral to the joint (Figure 1B).

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Figure 1

Schematic diagram of the anatomic layers for OSE-PALF. (A) In the paramedian suboccipital region, sequential dissection of the trapezius (blue triangle), splenius capitis (black triangle), semispinalis capitis (green triangle), and the deep fat layer exposes the deeper obliquus capitis inferior (yellow triangle) and rectus capitis posterior major (red triangle). The C1 to C2 lateral mass joint (red arrow) typically lies anterior to the obliquus capitis inferior, with the C1 posterior arch cranially and the C2 lamina caudally. The C2 nerve root and venous plexus course through the interval between the obliquus capitis inferior and the lateral mass joint capsule (yellow arrow). (B) After cranial retraction of the obliquus capitis inferior together with the C2 nerve root and surrounding venous plexus, the capsule and articular space of the C1 to C2 lateral mass joint are exposed. OSE-PALF, one-hole split endoscopy for posterior atlantoaxial lateral mass joint fusion.

The endoscopic dissection should be strictly confined: medially and cranially, the C2 laminar isthmus and the C1 posterior arch serve as bony boundaries to spare the suboccipital nerve and the muscles; laterally, the lateral edge of the facet defines a danger boundary to guard against vertebral artery injury. Guided by the layered anatomy, establishing the working corridor through fluoroscopic localization, channel dilation, and subperiosteal dissection can enhance the safety and accuracy of endoscopic navigation.

Positioning and Traction

To facilitate intraoperative localization, position the patient prone on a Jackson table with the head secured in a Mayfield headrest. Administer cranial traction under general anesthesia and maintain it throughout the procedure. Adjust the traction weight based on the degree of atlantoaxial joint reduction, not exceeding one-fifth of the patient’s body weight.

Surgical Approach and Corridor Creation

Prior to endoscopic navigation, localize the C1/C2 segment under C-arm fluoroscopic guidance and accurately position a localizing needle at the C2 laminar isthmus (Figure 2A). Along the posterior cervical midline, make a 2.5- to 3.0-cm longitudinal skin incision, then expose the trapezius-splenius capitis plane approximately 1.5 cm lateral to the midline. Along the direction of the muscle fibers, separate the trapezius, splenius capitis, and semispinalis capitis muscles using a blunt-tipped guiding rod. Stop advancing once the guide rod contacts the C2 lamina and recheck the placement under fluoroscopy (Figure 2B). Establish the working channel with soft-tissue dilators, insert the endoscopic system and instrument arm, and then initiate low-pressure continuous irrigation to maintain visual clarity.

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Figure 2

Schematic illustration of surgical approach and fluoroscopic localization for OSE-PALF. (A) With C-arm fluoroscopy, the C1 to C2 segment and the C2 laminar isthmus (yellow triangle) are localized. With reference to the C1 to C2 level, a 2.5- to 3.0-cm longitudinal skin incision (red line) is made along the posterior cervical midline. (B) Insert a blunt-tipped guide rod to expand the soft tissue corridor, then securely dock it on the bony surface of the C2 lamina under fluoroscopic guidance. OSE-PALF, one-hole split endoscopy for posterior atlantoaxial lateral mass joint fusion.

Endoscopically, expose the C2 laminar isthmus cortex to ensure reliable localization. Perform subperiosteal dissection along the C2 isthmus toward the cranial side to reveal the predetermined bony boundaries and prevent unnecessary dissection. Mobilize the suboccipital muscles cranially together with the C2 nerve root and venous plexus with a blunt nerve dissector, avoiding prolonged sharp retraction or compression. For venous plexus bleeding, perform brief, low-power, punctate bipolar coagulation; for cortical oozing, apply bone wax or hemostatic materials.

Atlantoaxial Lateral Mass Joint Fusion

Intraoperative real-time fluoroscopy combined with endoscopic navigation ensures precise localization and anatomical delineation (Figure 3A). Excise the joint capsule with electrocautery, then use the high-speed spherical burr to debride hypertrophic bone and optimize joint space exposure (Figure 3B–D). Abrade the articular surfaces to expose the subchondral bone with a 2-mm spherical burr. Keep the burr continuously visible to avoid excessive depth and injury to the retropharyngeal soft tissues (Figure 3E).

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Figure 3

Intraoperative images of OSE-PALF. (A) Precise localization of the lateral mass joint under C-arm fluoroscopic guidance. (B) Clear endoscopic visualization of the C2 isthmus and joint capsule. (C) Lateral mass joint visible after retraction of the C2 nerve root and joint capsule. (D) Close-up view showing the joint space and C1 lateral mass facet. (E) Endoscopic grinding of the lateral mass articular surface with a spherical burr. (F) Insert allograft cancellous bone into the joint space using a graft rod. (G) Direct visualization of successful lateral mass joint graft fusion. (H) Confirmation of the implant position and atlantoaxial reduction with intraoperative fluoroscopy. OSE-PALF, one-hole split endoscopy for posterior atlantoaxial lateral mass joint fusion.

After exposing the fresh bony surface, graft the allogeneic cancellous bone granules (size about 2–5 mm) into the joint space using a bone-graft cannula system (Figure 3F). Hold the cannula flush against the joint, thereby preventing graft spillage into blind areas and shielding the C2 nerve root and venous plexus. Furthermore, monitor the graft volume to promote joint fusion and avoid excessive distraction (Figure 3G). In most cases, unilateral lateral mass joint fusion is sufficient to maintain atlantoaxial stability; if necessary, bilateral fusion can be performed to enhance biomechanical symmetry. The Supplementary Video demonstrates the key steps of the fusion procedure.

Atlantoaxial Screw Fixation

Subsequently, we perform atlantoaxial internal fixation through the original midline incision. Establish bilateral subfascial working corridors: the fusion side through the preexisting arthrodesis channel, and the contralateral side with a minimally invasive intermuscular approach. On the fusion side, under endoscopic guidance, set the screw entry point at the C2 lateral mass center and insert a pedicle screw. For the C1, set the entry point at the junction of the posterior arch and the lateral mass, then insert a lateral mass screw. On the contralateral side, bluntly separate the trapezius, splenius capitis, and semispinalis capitis muscles, then retract the obliquus capitis inferior and rectus capitis posterior to expose the screw entry points. Insert the C1 lateral mass and C2 pedicle screws in the same manner. During screw placement, apply real-time C-arm fluoroscopy to evaluate the insertion angle and depth, ensuring accurate trajectory and preventing neurovascular injury.

To avoid denervation of the suboccipital muscles, adopt intermuscular blunt dissection rather than fiber transection. Enter the suboccipital region through the trapezius-splenius capitis interval and preserve deep fat to protect the greater occipital nerve. Restrict dissection in the suboccipital triangle to prevent injury to the suboccipital nerve and vertebral artery.

Finally, remove the cranial traction device, insert prebent titanium rods, apply compression to the bilateral screw-rod system, and then complete fixation (Figure 3H). Before closure, perform adequate hemostasis and clear residual bone debris; place a drainage tube on the fusion side if needed.

Case Presentation and Evaluation

A 12-year-old boy with diagnosed AAI and os odontoideum presented with neck pain, impaired grip, and gait instability. With disease progression, he developed urinary difficulty, limb weakness, and ambulatory impairment, without respiratory distress or urinary incontinence. Neurological examination revealed sensory abnormalities below the shoulders, increased muscle tone, hyperreflexia, and positive pathological signs. Baseline scores: visual analog scale, 5; Japanese Orthopedic Association, 6; Neck Disability Index, 57%.

Cervical radiographs showed anterosuperior displacement of the C2 body, with osseous separation from the odontoid process. The ADI remained normal, and C3 to C7 morphology and alignment were preserved (Figure 4A and B). Dynamic radiographs showed marked anterior translation of the atlas anterior arch relative to the C2 body during flexion. In extension, the translation largely resolved, with a mild posterior shift (Figure 4C and D). CT and angiography demonstrated the dens remaining below McRae’s line and anterolateral displacement of the C1 lateral mass relative to the C2, with no high-riding, ectopia, or tortuosity of the vertebral artery (Figure 4F–J). Magnetic resonance imaging revealed craniovertebral junction compression with intramedullary signal change and without Chiari malformation or syringomyelia (Figure 4E).

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Figure 4

Preoperative cervical images of the pediatric patient with atlantoaxial instability. (A and B) X-ray images show anterior-superior displacement of C2 vertebra and odontoid separation. (C and D) Dynamic x-ray images indicate the significant instability of atlantoaxial joint. (E) Magnetic resonance image reveals compression at the craniovertebral junction with intramedullary signal changes. (F–H) Computed tomography scans show the os odontoideum, not extending beyond McRae’s line, with normal atlantodental interval. (I and J) Three-dimensional reconstructions indicate malalignment of the lateral mass joint, with no abnormalities of the vertebral artery.

Clinical Efficacy

The patient underwent the OSE-PALF, supported by a perioperative management protocol (see online Supplementary Material). The operation proceeded smoothly without major complications, with 4 axial screws (3.5 mm diameter, 20 mm length) inserted, an operative time of approximately 120 minutes, and an estimated blood loss of 30 mL.

Postoperatively, the child exhibited marked improvement in somatosensory function, and limb strength recovered to grade 4+ with independent ambulation. No surgery-related complications such as dysphagia or dyspnea occurred. Imaging confirmed atlantoaxial joint reduction, well-positioned instrumentation, and satisfactory graft fusion (Figure 5A–D). Postoperative scores: visual analog scale, 2; Japanese Orthopedic Association, 13 (improvement rate, 63.6%); Neck Disability Index, 25.7%.

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Figure 5

Postoperative and follow-up computed tomography images of OSE-PALF. Multiplanar images demonstrate that precise screw placement (A and B), atlantoaxial reduction achieved with no spinal canal stenosis (C), and the bone graft within joint space (D). Three-month follow-up showed stabilization of the internal fixation device (E), no spinal cord compression is observed (F), and formation of the continuous bony bridge visible within the joint space (G and H). OSE-PALF, one-hole split endoscopy for posterior atlantoaxial lateral mass joint fusion.

Follow-up and Outcomes

At the 6-month follow-up, the child’s pain symptoms had completely resolved, with restoration of independent ambulation. Bladder function had returned to normal, although mild somatic sensory deficits persisted. CT with 3-dimensional reconstruction demonstrated stable atlantoaxial instrumentation without screw loosening or breakage (Figure 5E–F). Continuous osseous bridging across the atlantoaxial lateral mass joint space indicated early fusion (Figure 5G–H). No signs of osteolysis or bone resorption were observed at the fusion site. The patient wore a cervical orthosis for 3 months, and no complications occurred.