人工歯用キャップフリーインプラント修復物の5年間の臨床成績：機械的検証

Background: The use of prosthetic caps in screw-retained implant restorations aims to enhance passivity and protect abutment threads; however, these components may increase prosthetic volume and impair esthetics. Advances in high-strength zirconia have raised the question of whether such caps remain necessary. Methods: A retrospective clinical analysis was conducted on 20 partial screw-retained zirconia restorations comparing cases fabricated with and without a prosthetic cap. All restorations were followed for 3-5 years. Clinical outcomes included screw stability, marginal adaptation, esthetics (VAS), hygiene access, and biological response. A supplementary mechanical verification was performed on four standardized zirconia crowns fabricated through digital and conventional impression workflows to qualitatively assess their behavior under 30 N·cm torque and compressive loading above 1200 MPa. Results: Throughout follow-up, no mechanical or biological complications were recorded in either group. One restoration with a cap required screw re-tightening, while none failed in the cap-free group. Radiographic analysis showed smaller mean marginal gaps in cap-free restorations (0.183 mm) compared to those with caps (0.289 mm; p < 0.01). Esthetic satisfaction scores were higher in the cap-free group (VAS = 9.3 ± 0.1 vs. 8.2 ± 0.1; p < 0.001). Mechanical verification confirmed that all zirconia crowns tolerated torque and compressive loads without visible fracture or deformation. Conclusions: Within the study limitations, cap-free screw-retained zirconia restorations exhibited excellent 5-year clinical stability, improved esthetics, and better hygiene access compared with capped designs. The small-scale mechanical verification supported the clinical findings, indicating that cap omission does not compromise mechanical performance when accurate fit and digital workflow precision are ensured.