Clinical Data
Our study was a retrospective analysis of 38 consecutively treated patients with unstable C1 burst (Jefferson) fractures from May of 2010 to September of 2015. There were 10 males and 8 females with the average age of 51.3 years (range, 23-63 y) in the Rapid Prototyping Template (RP) group and 13 males and 7 females with the average age of 53.5 years (range, 29-69 y) in free-hand (FH) group. All included patients underwent radiological evaluation preoperatively with plain cervical spine radiographs, computed tomography (CT), and magnetic resonance imaging (MRI). The mechanism of injury for the patients in this study were: motor vehicle accident in 15 cases and fall from height in 23 cases. All patients presented with neck pain, stiffness, and decreased range of motion without neurological injury. Radiographic studies confirmed that all included cases had unilateral anterior and posterior arch fracture (Semi-ring fracture, Landells type II), and all the patients had fractures and avulsion at the attachment site of transverse ligament (Dickman type II). The preoperative ADI, LMD and VAS were recorded. Our study was approved by the Regional Ethics committee of Ningbo No.6 Hospital.
Preoperative care
Initial stability and alignment was obtained with 2-3kg of inline traction for all cases. Similarly, all patients received routine perioperative antibiotics (cefuroxime 1.5 g) and were positioned in slight hyperextension with a Mayfield head positioner. After exposure, patients were randomly selected to have their C1 pedicle screws placed either free-hand (FH group) or with the help of a personalized rapid prototyping template (RP group). The three-dimension models of atlas were reconstructed by Mimics software and the ideal trajectory for C1 pedicle screws was planned to minimize risk based upon the individuals corresponding anatomical structure (Figure 1). Then drill guide template was materialized in a rapid prototyping machine and sterilized by low-temperature plasma.
Surgical Technique
After induction of general endotracheal anesthesia, patients were positioned prone in a Mayfield head holder as detailed above. An experienced spinal surgeon performed all cases in both groups. A midline skin incision 4-6 cm in length was made beneath the external occipital protuberance. Fascia and ligaments were dissected along the midline to reveal the posterior arch of C1; subsequently, the junction of the lower edge of C1 posterior arch and lateral mass were exposed. During the operation, the C1-C2 venous plexus was protected as much as possible. For patients in the RP cohort, the personalized rapid prototyping template was placed onto the posterior arch using the lock-and-key principle. After the pedicle screw tracts were drilled, the corridors were marked with a blunt k-wire to allow removal of the template. The paths were then tapped and pedicle screws were placed (Figure 2). Intraoperative fluoroscopy was used for surveillance to confirm the correct placement of all screws. The surgical method previously decribed by Hu et al [3] for direct C1 pedicle screw fixation of unstable atlas fractures was used in free hand group.
Assessment of Accuracy of Pedicle Screw Placement
After pedicle screws were placed into the pilot hole that was created with the customized drill template, postoperative CT scans were obtained to compare the accuracy of placement between the traditional FH group and RP group. The screw placement was examined in both the axial and sagittal plane of the CT scan and screw position was graded into four groups: screw is completely within bone (grade 0), less than 50% of the screw diameter is outside of the pedicle (grade 1), greater than 50% of the screw diameter is outside of the pedicle (grade 2), the entire diameter of the screw is outside of the pedicle (grade 3).
Postoperative treatment and follow-up
All patients were immediately mobilized and treated with external immobilization via hard collar for 8-12 weeks. Antibiotics were continued for 2 days. AP and lateral radiographs were taken at 3, 60, 90 and 180 days postoperatively. CT scans were taken at 3, 90, 180 days postoperatively. An independent radiologist reviewed the placement of the pedicle screws and assessed for pedicle breach. Thin-slice (1 mm) CT scans and 3-dimensional reconstruction were obtained to assess accuracy of screw placement and reduction (Figure 3) and bone healing (Figure 4). Dynamic flexion and extension radiographs were taken at 180 days postoperatively to detect any flaws in fracture consolidation (Figure 5). The operative time and blood loss were recorded at the time of surgery. Postoperative radiographic measurement of the Atlanto-Dens Interval (ADI) and the Lateral Mass Displacement (LMD) as well as the VAS pain score were documented at the 180 days post-operative visit.
Statistical analysis
Statistical analyses were performed using SPSS Statistical program, (version 17.0; SPSS Inc, Chicago, IL, USA). An independent sample t test was used in the comparison of operation time and introperative blood loss between the traditional free-hand group and RP group. A two independent-sample t test was used to compare the preoperative ADI, LMD and VAS to the postoperative ADI, LMD and VAS. Chi-square test was applied to assess the comparison of the screw accuracy between the free-hand group and the RP group and the chi-square correction test and fisher’s exact probability test was used when the data was not suitable for chi-square test. Values are expressed as ranges, and mean ± SD (standard deviation) as appropriate. Statistical significance of the morphometric data was determined by the use of a Student’s t test at a 95% level of significance. A P value of < 0.05 was considered to be significant.