C1 lateral mass screw placement via the posterior arch: a technique comparison and anatomic analysis.

Background Context: Instrumentation of C1 is becoming increasingly common. Starting points initially described for C1 lateral mass screws at the lateral mass/posterior arch junction are technically challenging. Recently, a number of techniques have evolved advocating varying starting points and trajectories. Despite being technically easier, there are new safety concerns. Insufficient evidence exists for optimal C1 lateral mass screw placement with starting points in the posterior arch.

Purpose: To determine anatomic variability of the C1 lateral mass and posterior ring and to compare safety and feasibility of C1 lateral mass screw placement techniques via the posterior arch.

Study Design: Descriptive anatomy for surgical technique.

Methods: One hundred thin-cut cervical spine computed tomography scans were acquired and formatted for virtual surgery. Four different described techniques were used for virtual placement of C1 lateral mass screws. Success was defined as avoidance of critical structures including the transverse foramen, vertebral groove, and spinal dura. Anatomic variability of the C1 vertebra and safe zones for screw placement were also clarified.

Results: Overall screw placement success for the four techniques was 50% (Resnick), 92% (Tan et al.), 58% (Ma et al.), and 85% (Christensen et al.). Average posterior arch height was 6.7±2.1 mm, and vertebral groove height 4.9±1.1 mm was the most limiting dimension to safe screw placement. A safe zone for screw placement was found in 100% of cases (200 screws), 17.0±1.1 mm from midline and a width of 12.6±1.7 mm. Posterior tubercle morphology was variable.

Conclusions: C1 lateral mass screws could be virtually placed bilaterally in each of 100 clinical cases without violating critical structures. However, none of the previously described approaches worked in every case because of significant anatomic variability. The vertical starting point was particularly critical, and vertebral groove height was the most limiting variable. Although a reliable safe zone could be found in every case, preoperative planning is essential to avoid critical structures.