Background <p>The Y-incision technique enables implantation of larger prosthetic valves by facilitating effective aortic annular enlargement. Although this approach improves hemodynamic outcomes, rare and clinically significant complications may occur and remain insufficiently described.</p> Case presentation <p>A patient undergoing surgical aortic valve replacement with concomitant Y-incision annular enlargement developed early postoperative severe central mitral regurgitation. Standard transthoracic and transesophageal echocardiography demonstrated abnormal aorto-mitral geometry and annular distortion. Reoperation revealed deformation of the mitral annulus adjacent to the patch implantation site. Based on intraoperative findings, imaging data, and schematic analysis, we hypothesized that turbulent flow within the pouch-like neo–aortomitral curtain combined with patch sutures placed near the mitral annulus contributed to annular distortion and subsequent mitral valve incompetence.</p> Conclusion <p>Severe mitral regurgitation represents a rare but clinically important complication of the Y-incision technique. Patient-specific aorto-mitral anatomy, including annular distance and aorto-mitral angle, may play a critical role in this mechanism. Careful preoperative anatomical assessment and meticulous intraoperative patch positioning are essential to minimize this risk and improve procedural safety.</p>