ガイド付き歯内療法におけるスリーブ材料、高さ、およびドリル表面の変化：ビッカース硬度および走査型電子顕微鏡評価（パイロットスタディ）

Guided endodontic access uses a CBCT-derived template with a guide sleeve to direct the drill for precise canal location, particularly in calcified teeth. This technique is more conservative, time-efficient, and accurate than conventional methods. Guide sleeves are fabricated from various materials (e.g., zirconia ceramic, cobalt-chromium alloy, titanium alloy, or 3D-printed resin), but their hardness and durability under drilling stress are not well characterized. However, guiding long, narrow drills through sleeves may generate excess heat due to friction between the surfaces, potentially causing tool wear. Studies in implant dentistry show significant drill wear with repeated use, but the effects of sleeve material and sleeve height on endodontic drill wear remain uncharacterized. This study used Scanning Electron Microscopy (SEM) to examine drill surface changes after guided Minimally Invasive Endodontic Access (MIEA) with different sleeve materials (resin, zirconia [Y-TZP], Ti-6Al-4 V, CoCr alloy) and heights. This study evaluated Sleeve Material, Height, and Drill Surface Changes in Guided Endodontics through the Vickers Hardness Test and Scanning Electron Microscope.

In vitro, 30 guide sleeves (10 sizes: 3 mm, 5 mm, 7 mm; 4 different materials) fit a 1.5 mm carbide bur with limited clearance. Sleeve materials tested in the current study are dental photopolymer resin, Y-TZP, Co-Cr, and Ti-6Al-4 V. Before and after drilling, the sleeve hardness was measured through Vickers Hardness Test (HV0.2). Hardness variations (ΔVHN) were determined. A one-way ANOVA was used to assess hardness among materials and sleeve heights, with significance at p < 0.05. Additionally, three teeth were drilled under irrigation by one group of standardized high-speed carbide drills per guide. Cleaning and imaging of drill tips and flutes were done using a SEM (1000x magnification). Two examiners rated wear as Grade 0 (no wear), Grade 1 (minor blunting or edge rounding), or Grade 2 (severe chipping or edge loss). Five drills showed a noticeable thermal discoloration ("burn marks") and were included in the analysis. This study is a pilot study or an early feasibility study. The data will be analysed using a Descriptive approach.

While zirconia sleeves had the highest hardness rating (mean ~ 1392 HV pre-drilling; ~1389 HV post-drilling), resin sleeves had the lowest (~ 6 to 7 HV both pre- and post-drilling), while CoCr and Ti-alloy sleeves had intermediate hardness (~ 315 and ~ 405 HV). ANOVA showed significant differences in material type (p < 0.001), but no significant effect of sleeve height on hardness (p > 0.8). Due to modest hardness variations, drilling cycles do not strain-harden or soften the sleeve. Four drills (30%) exhibited Grade 1 wear. Six drills (70%) had Grade 2 wear. Medium- and short-resin sleeves showed Grade 1 wear, whereas Zirconia, Titanium, and CoCr showed Grade 2 wear. Short sleeves produced Grade 1 wear, while medium/long sleeves frequently caused Grade 2 wear. Five drills with burn marks were classified as Grade 2. Overall, Grade 2 wear was observed in 60% of drills, and Grade 1 in 40%.

Sleeve hardness was primarily determined by the material type, with sleeve height having no significant effect. Zirconia guide sleeves were harder than CoCr, Ti, and resin, and may also offer greater wear resistance and durability for guided endodontic template sleeves. All tested materials maintained their hardness after simulated guided access drilling. Guided minimally invasive endodontic access drill wear was considerable with static sleeves. The majority of the drills experienced thermal damage and were classified as Grade 2 surface damage. Drill abrasion was notably increased when using resin, zirconia, and cobalt-chromium (CoCr) sleeves, attributed to their greater hardness, wear resistance, and sleeve height. While resin sleeves may help reduce wear during minimally invasive guided endodontics, the absence of an effective root canal irrigation system limits cooling efficiency. Clinicians must be aware that more rigid guides and sleeve systems may demand additional measures, such as optimized irrigation or intermittent drilling, to avoid thermal and mechanical damage. Proper selection of guide sleeves and implementation of effective cooling mechanisms are essential to the success of guided endodontic procedures.