The height for screw index (HSI) predicts the development of C2 nerve dysfunction associated with C1 lateral mass screw fixation for atlantoaxial instability.

Purpose: To propose a new measure, the height for screw index (HSI), as a predictor of C2 nerve dysfunction in patients who have received posterior C1 lateral mass screw (C1LMS) fixation for atlantoaxial instability and to examine whether the HSI scores correlated with the development of C2 nerve dysfunction through retrospective analysis of 104 C1LMS inserted in 52 patients with atlantoaxial instability.

Methods: The medical records of patients who underwent C1LMS fixation were retrospectively reviewed. C1LMS, 3.5 mm in diameter, was inserted for atlantoaxial stabilization. The sagittal plane of the planned C1LMS trajectory was reconstructed from CT images. The HSI was defined as the difference in height between C2 ganglion and its corresponding foramen. C2 nerve function was assessed using a validated visual analog scale questionnaire. Each foramen receiving C1LMS was considered as a single unit and patients were categorized to group 1, HSI ≥4.0 mm; group 2, HSI <4.0 mm.

Results: The mean HSI score was 4.7 ± 0.8 mm (range 3.1-6.5 mm) with 85 (81.7 %) units in group 1, and 19 (18.3 %) units in group 2. Fourteen (13.5 %, 14/104) units developed C2 nerve dysfunction. C2 nerve dysfunction was reported in 4 units in group 1, and 10 units in group 2, respectively. The percentage of C2 nerve dysfunction was significantly higher in group 2 than that in group 1 (P < 0.001, Pearson Chi-square test).

Conclusions: The HSI score correlates with the development of C2 nerve dysfunction in patients receiving C1LMS fixation for atlantoaxial instability and may be a useful predictor of C2 nerve dysfunction.