LAUSR

BACKGROUND Basilar invagination (BI) is a developmental anomaly and commonly presents with neurologic findings. The incidence of BI associated with other osseous anomalies of the craniovertebral junction is high, including incomplete ring of C1 with spreading of the lateral masses, atlanto-occipital assimilation, hypoplasia of the atlas, basiocciput hypoplasia, and occipital condylar hypoplasia. However, BI combined with C1 prolapsing into the foramen magnum (FM) is an extremely rare condition. CASE DESCRIPTION A previously healthy 6-year-old girl (case1) presented with extremities numbness and left limb weakness over a period of 3 months. Radiograph and computed tomographic (CT) scanning demonstrated basilar invagination with C1 and dens upward into the FM, C2-3 congenital fusion. Magnetic resonance (MR) imaging showed ventral brainstem and medulla compression and the medulla-cervical angle was about 100°. The patient underwent transoral anterior decompression, reduction and fusion by TARP surgery. The symptoms of extremities numbness and limb weakness were all alleviated after surgery. Postoperative MRI showed that the medulla-cervical angle was improved from 100°to 143°. An 11-year-old boy (case2) presented with a 2-months history of limbs numbness and weakness. The computed tomographic (CT) scanning and MRI demonstrated BI and compression of spinal cord, with the craniospinal angle of only 63°. The 3D printed model showed that the anterior arch and lateral of C1 was 90 degrees flipping and vertically upward prolapsing into the FM together with the dens, the width of atlas was greater than the maximum diameter of the FM, which resulted in "structural incarceration". The patient received posterior occipito-cervical fixation and fusion surgery with hyperextension skull traction. Postoperative CT scan revealed the craniospinal angle increased to 102°. CONCLUSIONS We present 2 rare cases of BI combined with C1 prolapsing into the FM. We adopted different surgical strategies with satisfying outcome for these patients. We deem that the treatment of unique BI should be individualized according to the different image characteristics. The image-based modern rapid prototyping and 3D printed techniques can provide invaluable information in pre-surgical planning for complex craniovertebral junction anomalies.