A surgical instrument (10) for removing a sample (50, 82) from a body of tissue (80) comprises a support structure (30) and a knife (32) arranged to cut into the body of tissue (80). The knife (32) has at least a first blade (36a), a second blade (36b) and a third blade (36c), and each blade has two cutting edges (42a, 44a 42b, 44b 42c, 44c) defining an apex. The knife (32) is movable between a first configuration in which the blades (36a, 36b, 36c) are substantially aligned along the longitudinal axis (X) of the support structure (30), and a second configuration in which the blades (36a, 36b, 36c) define the walls of a pyramidal enclosure for containing the sample (50, 82). In one embodiment, the instrument (10) further comprises a biasing member (52a, 52b, 52c) arranged to urge each blade (36a, 36b, 36c) towards the central axis (X) of the support structure (30). In another embodiment, the instrument (10) further comprises a plunger (72) disposed in, and being axially movable in a longitudinal direction along, the support structure (30). The plunger (72) is arranged to move the knife (32) between the first configuration and the second configuration.Un instrument chirurgical (10) pour prélever un échantillon (50, 82) dune masse tissulaire (80) comprend une structure de support (30) et un couteau (32) conçu pour découper la masse tissulaire (80). Le couteau (32) possède au moins une première lame (36a), une deuxième lame (36b) et une troisième lame (36c), chacune présentant deux bords tranchants (42a, 44a 42b, 44b 42c, 44c) formant un apex. Le couteau (32) est mobile entre une première configuration dans laquelle les lames (36a, 36b, 36c) sont sensiblement alignées le long de laxe longitudinal (X) de la structure de support (30), et une seconde configuration dans laquelle les lames (36a, 36b, 36c) forment les parois dun logement pyramidal destiné à contenir léchantillon (50, 82). Dans un mode réalisation, linstrument (10) comprend également un élément de sollicita
