A screw-anchored stencil for responsive neurostimulation craniectomy: a randomized crossover bench study

Background The responsive neurostimulation (RNS) pulse generator is implanted in a craniectomy that must match its ferrule. The outline is traced around a hand-held template that moves freely on the convex skull. A poor outline is discovered only when the ferrule fails to fit, generally after craniectomy. We designed a 3D-printed stencil that is fixed during marking by one screw placed in the bone to be removed. Methods In a randomized crossover bench study, eight surgical residents each performed four craniectomies on 3D-printed skulls, two marked with the stencil and two with the template. The primary outcome was first-pass fit (flush seating of the ferrule at the first attempt). Secondary outcomes were the number of attempts, operative time, craniectomy size, and operator confidence. Exact within-operator randomization tests were used. Results The ferrule seated at the first attempt in 16 of 16 stencil and 4 of 16 template craniectomies (100% vs 25%; p < 0.001). With the stencil, operative time was shorter (99.7 vs 136.9 s; p < 0.001), craniectomies were smaller (22.5 vs 23.8 cm2; p = 0.03), and confidence was higher (6.6 vs 4.1 of 10; p < 0.001). Every operator needed fewer attempts, was faster, and made a smaller craniectomy with the stencil, regardless of seniority. Conclusions In a bench model, a marking stencil anchored to bone that is about to be removed gave first-pass fit of the RNS ferrule in every craniectomy, without enlarging the craniectomy or leaving a screw hole in the skull.