Optimal Entry Point and Trajectory for Anterior C1 Lateral Mass Screw

Atlas (C1) lateral mass screw placement using the intersection between lateral mass and inferomedial edge of the posterior arch: a cadaveric study

PURPOSE

To compare the Atlas (C1) lateral mass screw placement between screw trajectories of 0° and 15° medial angulation while using the intersection between lateral mass and inferomedial edge of the posterior arch.

METHODS

Forty-eight Atlas lateral masses were prepared and divided into 2 groups: Group 1; screws inserted at 3 mm lateral to the reference point with screw trajectory of 0° angulation(N = 24) and Group 2; those inserted with screw trajectory of 15° medial angulation(N = 24). We evaluated the atlas anatomy, screw purchase and the presence of any breaches using CT scan.

RESULTS

The radiographic parameters for Groups 1 and 2 were found statistically different (p-value < 0.05): bilateral intraosseous screw lengths (17.92 ± 1.47 mm. vs. 20.71 ± 2.4 mm.), bilateral screw length (29.92 ± 1.72 mm. vs. 33.13 ± 1.78 mm.), left screw medial angulation (x°) (0.67° ± 0.78° vs.14.17° ± 3.51°), right screw medial angulation (y°) (0.83° ± 1.03° vs.14.25° ± 2.53°) and bilateral screw medial angulation (0.75° ± 0.9° vs. 14.21° ± 2.99°). Twenty-two screws (91.67%) using the 0° medial angulation and nineteen screws (79.17%) using the 15° medial angulation had no cortical violations (Grade 0). However, two screws (8.33%) with 0° medial angulation and five screws (20.83%) with 15° medial angulation had breach less than 2 mm (Grade 1). There were no screws with breach between 2 and 4 mm (Grade 2) or greater than 4 mm. (Grade 3).

CONCLUSION

A starting point of 3-mm lateral to the intersection between lateral mass and inferomedial edge of the Atlas posterior arch can be safely and effectively used to insert C1 lateral mass using both 0° and 15° medial angulation.

Three-dimensional anatomic analysis and navigation templates for C1 pedicle screw placement perpendicular to the coronal plane: a retrospective study

OBJECTIVE

To explore the corridor for atlas pedicle screw placement perpendicular to the coronal plane and to develop patient-specific navigation templates for pedicle screw placement.

METHODS

This study is a retrospective analysis. A total of 71 patients (41 males and 30 females) were involved in this study from June 2016 to June 2018, and they were divided into a template group (39 patients) and a conventional group (32 patients). The CT-based 3D reconstruction models were analyzed in the template group. From the perspective of the 3D atlas in a coronal view, the pedicle corridor was obtained. Taking the center of the tangential circle of the pedicle as the entry point, we simulated screw placement perpendicular to the coronal plane. Then, the individual navigation template was designed and used in surgery. In the conventional group, free-hand pedicle screw placement was performed perpendicular to the coronal plane. The diameter of the maximum tangential circle and screw length of the pedicle corridor were measured in the template group. The surgical time, radiation exposure time, screw grading criteria and so on were evaluated and compared between the two groups after surgery.

RESULTS

;The diameter of the maximum tangential circle and the distance between the entry point and posterior tubercle were significantly greater in males than in females. The operation was successfully completed in all patients, without aggravation of nerve injury, and the follow-up was 12-20 months, with an average of 15.6 months. A total of 78 screws were inserted in the template group, and 64 screws were inserted in the conventional group. The surgical times in the template group and conventional group were 76.47±24.44 min and 125.63±36.41 min, respectively. The radiation exposure times in the template group and conventional group were 3.51±1.77 and 10.15±4.95, respectively, and there was a significant difference between the two groups. In the template group, the deviation in the screw entry point and screw angle were 1.92±1.67 mm and 2.08±1.98°, respectively. The medial angle deviation between the left and right sides was 2.71±1.88° in the template group, which was significantly less than the deviation of 3.76±2.22° in the conventional group.

CONCLUSION

A pedicle screw trajectory perpendicular to the coronal plane can be quickly obtained based on the perspective of 3D models. The technique of screw placement perpendicular to the coronal plane assisted by navigation templates has a shorter operative time, lessintraoperative fluoroscopy and a higher safety of screw placement than traditional surgery.

Finding the "Sweet Spot" for C2 Root Transection in C1 Lateral Mass Exposure

BACKGROUND

Atlantoaxial fusion often requires C2 nerve transection for complete C1 lateral mass exposure. Nerve transection is made ideally at the preganglionic segment proximal to the dorsal root ganglion to minimize the risk of postoperative dysesthesias. If the nerve is transected too proximally, cerebrospinal fluid leak may be encountered by violation of the dura and arachnoid where the sensory and motor nerve rootlets exit the subarachnoid space. In this study we aimed to quantify the length of the C2 nerve preganglionic segment using cadaveric specimens and develop a method for reliable intraoperative localization for sectioning during C1-2 arthrodesis.

METHODS

Using microsurgical techniques, 16 C2 nerves from 8 frozen and injected cadaveric cervical spine specimens were dissected. Two key measurements were taken to establish a reliable method of preganglionic segment identification. The "sweet spot" for nerve transection was based on the approximate location of the midpoint of the preganglionic segment.

RESULTS

The final determination of the ideal spot for C2 nerve transection using these calculations was 3 mm lateral to the medial border of the lateral mass.

CONCLUSIONS

This anatomic study found remarkable consistency in the preganglionic segment length. The medial border of the lateral mass appeared to be a consistently reliable landmark for identification of the preganglionic segment of the C2 nerve root. By using relationships between known anatomic structures intraoperatively, safety of atlantoaxial fixation can be optimized to maximize complication avoidance and satisfactory patient outcomes.

Optimal area of lateral mass mini-screws implanted in plated cervical laminoplasty: a radiography anatomy study

PURPOSE

Lateral mass mini-screws used in plated cervical laminoplasty might penetrate into facet joints. The objective is to observe this complication incidence and to identify the optimal areas for 5- and 7-mm-long mini-screws to implant on lateral mass.

METHODS

47 patients who underwent plated cervical laminoplasty were included. The optimal area for mini-screws implanting was set according to pre-operative 3D CT reconstruction data. Then, each posterior-lateral mass surface was divided into three regions: 7-mm region, 5-mm region, and dangerous area. The mini-screw implanted region was recorded. Post-operative CT images were used to identify whether the mini-screws penetrated into facet joints.

RESULTS

235 mini-plates and 470 lateral mass mini-screws were used in the study. 117 (24.9%) mini-screws penetrated 88 (37.4%) facet joints. The 5-mm-long mini-screw optimal area occupied the upper 72, 65, 65, 64, and 65 % area of the posterior-lateral mass surface for C3-7, while the 7-mm-long mini-screw optimal area encompassed the upper 54, 39, 40, 33, and 32 %. Only 7-mm-long mini-screws were used to fix the plate to the lateral mass. 4 of 240 mini-screws in 7-mm region, 67 of the 179 mini-screws in 5-mm region, and 46 of the 51 mini-screws in dangerous region penetrated into the facet joint. The differences in the rate of facet joint penetration related to region were statistically significant (P < 0.001).

CONCLUSIONS

The facet joint destruction by mini-screws was not a rare complication in plated cervical laminoplasty. The optimal areas we proposed may help guide the mini-screw implantation positions.