A surgical instrument comprising: a handle assembly; a body extending distally from the handle assembly, the body defining a first longitudinal axis; a tool assembly pivotably supported on a distal end of the body, the tool assembly defining a second longitudinal axis, the tool assembly being pivotable between a non-articulated position in which the first longitudinal axis is aligned with the second longitudinal axis and at least one articulated position in which the second longitudinal axis is at an angle to the first longitudinal axis; and an articulation mechanism including a receptacle positioned adjacent the handle assembly, a main shaft having a base portion, the main shaft being rotatably supported within the receptacle, a lower clutch fixedly positioned within the receptacle, the lower clutch having a serrated portion including a plurality of serrations and being positioned about the main shaft, an upper clutch slidably positioned about the main shaft, the upper clutch being rotatably fixed to the main shaft such that rotation of the main shaft affects rotation of the upper clutch, the upper clutch including at least one projection positioned to engage the serrations of the lower clutch to releasably retain the main shaft at a rotatably fixed position, and an articulation link having a proximal end operatively connected to the base portion of the main shaft and a distal end operatively connected to the tool assembly, wherein the main shaft is rotatable to move the articulation link to affect movement of the tool assembly between the non-articulated position and the at least one articulated position; wherein a radial angle between a first pair of adjacent serrations (450a, 450b) is defined as a first angle (α1), a radial angle between a second pair of adjacent serrations (450b, 450c) is defined as a second angle (α 2), and characterised by a radial angle between a third pair of adjacent serrations (450d, 450e) is defined as a third angle (α 3), and wherein the first angle, the second angle, and the third angle are each different from one another having a common center and wherein the tool assembly is articulated by the same distance for the closer spaced serrations as for the serrations with greater spacing. a handle assembly; a body extending distally from the handle assembly, the body defining a first longitudinal axis; a tool assembly pivotably supported on a distal end of the body, the tool assembly defining a second longitudinal axis, the tool assembly being pivotable between a non-articulated position in which the first longitudinal axis is aligned with the second longitudinal axis and at least one articulated position in which the second longitudinal axis is at an angle to the first longitudinal axis; and an articulation mechanism including a receptacle positioned adjacent the handle assembly, a main shaft having a base portion, the main shaft being rotatably supported within the receptacle, a lower clutch fixedly positioned within the receptacle, the lower clutch having a serrated portion including a plurality of serrations and being positioned about the main shaft, an upper clutch slidably positioned about the main shaft, the upper clutch being rotatably fixed to the main shaft such that rotation of the main shaft affects rotation of the upper clutch, the upper clutch including at least one projection positioned to engage the serrations of the lower clutch to releasably retain the main shaft at a rotatably fixed position, and an articulation link having a proximal end operatively connected to the base portion of the main shaft and a distal end operatively connected to the tool assembly, wherein the main shaft is rotatable to move the articulation link to affect movement of the tool assembly between the non-articulated position and the at least one articulated position; wherein a radial angle between a first pair of adjacent serrations (450a, 450b) is defined as a first angle (α1), a radial angle between a second pair of adjacent serrations (450b, 450c) is defined as a second angle (α 2), and characterised by a radial angle between a third pair of adjacent serrations (450d, 450e) is defined as a third angle (α 3), and wherein the first angle, the second angle, and the third angle are each different from one another having a common center and wherein the tool assembly is articulated by the same distance for the closer spaced serrations as for the serrations with greater spacing.