Occipital Bone Erosion Following C1–2 Posterior Fixation: A Comparative Per-Screw Analysis of Tan versus Harms-Goel Techniques

Objective: To investigate the relationship between C1 screw trajectory and occipital bone erosion in C1-2 posterior fixation. Methods: This retrospective cohort study analyzed 27 patients (54 screws) who underwent C1-2 posterior fixation between March 2018 and March 2023 at a single institution by multiple surgeons. Screws were classified by trajectory: Tan technique (n=39) or Harms-Goel (HG) technique (n=15). Primary outcome was occipital bone erosion; the secondary outcome was breach of inner cortical layer. Per-screw analysis was performed using generalized estimating equations to account for bilateral screw clustering. Results: Mean follow-up was 48.3 (range, 24-84) months. Occipital bone erosion occurred in 51.3% (20 of 39) of Tan screws versus 6.7% (1 of 15) of HG screws (p<0.001). Breach of inner cortical layer occurred exclusively with the Tan technique (10.3% vs. 0%, p=0.302). In mixed Tan+HG cases (n=3), erosion occurred only on the Tan side (2 of 3 screws) with no erosion on HG side (0 of 3 screws). C1 upper line transgression was a critical risk factor; no erosion occurred in screws that did not exceed this anatomical landmark (adjusted relative risk [RR], 6.82; 95% confidence interval [CI], 2.41-19.31). Additional risk factors included height O-C1 ≤4.5 mm (RR, 3.81; 95% CI, 1.51-6.28) and height O-C1 extension ≤1 mm (RR, 4.86; 95% CI, 2.05-11.53). No patients required reoperation for erosion-related symptoms during follow-up. Conclusion: Screw trajectory is the primary determinant of occipital bone erosion following C1-2 fixation. The HG technique demonstrated significantly lower erosion rates (6.7% vs. 51.3%). When anatomically feasible, HG technique may be considered to reduce erosion risk.