Surgical apparatus and blade elements are provided for performing a tissue cutting process to cleave a tissue portion from a main tissue section. The apparatus comprises a reference member (1) having a cavity (3) for holding a tissue portion subject to the tissue cutting process and a blade guiding surface (2), a blade element (100, 200, 370, 470) adapted for slidable engagement with the reference member, such that the blade moves along the guiding surface of the reference member to perform the cutting process, and a biasing mechanism (80, 82, 91, 92, 102, 202) for moving the blade along the guide surface such that a pressure-relieving space is formed between the blade and the tissue section upon cleavage to avoid deviation of the cutting edge from a desired path. The blade elements can have a contact ridge (107) for guiding the blade element along the predetermined path. In some embodiments, the blade element can include a flexible plate member (101) having first and second opposite sides, a cutting edge (103) formed at a first longitudinal end portion of the plate member, and a manipulating block (102) mounted at a second longitudinal end portion of the plate member (101) for applying a biasing force to the plate member to maintain contact between the blade element and the reference member.Linvention porte sur un appareil chirurgical et sur des lames pour cliver une partie de tissu. Lappareil comprend une référence (1) ayant une cavité (3) pour maintenir en place la partie de tissu a couper, une surface de guidage de lame (2), une lame (100, 200, 370, 470) conçue pour se déplacer sur la référence et un mécanisme de sollicitation (80, 82, 91, 92, 102, 202) pour déplacer la lame le long de la surface de guidage de sorte à former un espace de décharge de pression entre la lame et la section de tissu pour éviter une déviation du bord de coupe à partir dun trajet désiré. Les lames ont une arête de contact (107) pour guider la lame le long du trajet prédéterminé. La lam
