A 43-year-old male presented with progressive numbness on right upper and lower extremities for 14 months. He had no special prior history. On neurological examination, it was found that there were bilateral hyperactive knee reflex and Achilles tendon reflex, bilateral positive Babinski’s sign amd Hoffmann’s sign, knee clonus, and ankle clonus. There was numbness but full motor strength (5/5) in his bilateral upper and lower extremities. The cervical X-ray image showed there was basilar invagination and C1-3 deformity. CT images demonstrated that there was amerism and deformity of C2-3, discontinuity of C7 lamina, C7 spinal canal dysraphism. MRI T2 images showed high signal intensity change in medulla oblongata (Figure 6).

The surgery was taken on August 12th, 2015. After general anesthesia, Mayfield tongs and patient’s position (prone) were placed to keep the head slightly flexed. A midline posterior skin incision and exposure was made from the occiput to C3. After removal of a piece of 3X3 cm bone from squamous occipital part, decompression area of the foramen magnum by drilling 6mm holes bilaterally, and removal of the residual posterior arch of C1 and spinous process and lamina of C3, we passed titanium cables under lamina of C2 and occipital bone. Tightened and secured the cables using temporary clips to increase the clivus-axial angle (CAA). Took a fluoroscopy image to make sure the reduction of C1-2 satisfied. Adjusted the Mayfield tongs to keep the head fully flexed. After we placed the patient tracker, robotic planning was carried out using TiRobot system. Virtual drill trajectories were planned on the 3-dimensional software (Figure 7).

Figure 7. Plan a trajectory intra-operatively

It could be clearly found in software that the right part of C2 was too seriously malformed to place a screw, and although left part of C1/2 lateral joint was also malformed (60°anteversion), it was able to find the only trajectory with the assist of the software. After planning step, we drilled a Kirschner wire along the C2 pars aiming toward the anterior arch of C1 using robotic guidance, then inserted a 4mm transarticular screw by the guidance of the K-wire. We found the placement of the screw was satisfied due to the intraoperative C-arm fluoroscopy scan. Then we loosed temporary clips on titanium cables, decorticated the posterior surface of occipital and C2 lamina using a burr to make the bed for bone graft. Two full-thickness rectangular bone grafts were taken from the iliac crest, and held in place by securing the cables. The surgery took 5 hours 30 minutes, and the time of screw placement was about 15 minutes. The blood loss was 800 ml.

There was no intraoperative complication. The patient had maintained full extremity strength and sensation. By postoperative CT images (5 days) (Figure 8), there was no perforation and loose of the screw.