A circular stapler apparatus for stapling tissue, the apparatus comprising: (a) a staple head assembly operable to drive staples into tissue; (b) an anvil configured to proximally retract toward the staple head assembly, wherein the staple head assembly and anvil together are configured to at least sever or staple tissue based on a single rotary input; and (c) a shaft assembly, the shaft assembly comprising: (i) a plurality of joint segments, wherein each joint segment includes a distal portion and a proximal portion, wherein the distal portion of a first joint segment of the plurality of joint segments is configured to receive the proximal portion of a second joint segment of the plurality of joint segments, characterised by each joint segment comprising teeth in an interior wall surface of the distal portion and teeth on an exterior wall surface of the proximal portion, and by each joint segment comprising a resilient member, wherein in a first, uncompressed position the resilient member is configured to bias the first joint segment away from the second joint segment such that teeth in the interior wall surface of the distal portion of the first joint segment are spaced apart from the teeth on the exterior wall surface of the proximal portion of the second joint segment, such that the plurality of joint segments are pivotable relative to one another, and wherein in a second, compressed position the teeth in the interior wall surface of the distal portion of the first joint segment are configured to mesh with the teeth on the exterior wall surface of the proximal portion of the second joint segment, such that the plurality of joint segments are configured to lock against one another to prevent pivoting. (a) a staple head assembly operable to drive staples into tissue; (b) an anvil configured to proximally retract toward the staple head assembly, wherein the staple head assembly and anvil together are configured to at least sever or staple tissue based on a single rotary input; and (c) a shaft assembly, the shaft assembly comprising: (i) a plurality of joint segments, wherein each joint segment includes a distal portion and a proximal portion, wherein the distal portion of a first joint segment of the plurality of joint segments is configured to receive the proximal portion of a second joint segment of the plurality of joint segments, characterised by each joint segment comprising teeth in an interior wall surface of the distal portion and teeth on an exterior wall surface of the proximal portion, and by each joint segment comprising a resilient member, wherein in a first, uncompressed position the resilient member is configured to bias the first joint segment away from the second joint segment such that teeth in the interior wall surface of the distal portion of the first joint segment are spaced apart from the teeth on the exterior wall surface of the proximal portion of the second joint segment, such that the plurality of joint segments are pivotable relative to one another, and wherein in a second, compressed position the teeth in the interior wall surface of the distal portion of the first joint segment are configured to mesh with the teeth on the exterior wall surface of the proximal portion of the second joint segment, such that the plurality of joint segments are configured to lock against one another to prevent pivoting. (i) a plurality of joint segments, wherein each joint segment includes a distal portion and a proximal portion, wherein the distal portion of a first joint segment of the plurality of joint segments is configured to receive the proximal portion of a second joint segment of the plurality of joint segments, characterised by each joint segment comprising teeth in an interior wall surface of the distal portion and teeth on an exterior wall surface of the proximal portion, and by each joint segment comprising a resilient member, wherein in a first, uncompressed position the resilient member is configured to bias the first joint segment away from the second joint segment such that teeth in the interior wall surface of the distal portion of the first joint segment are spaced apart from the teeth on the exterior wall surface of the proximal portion of the second joint segment, such that the plurality of joint segments are pivotable relative to one another, and wherein in a second, compressed position the teeth in the interior wall surface of the distal portion of the first joint segment are configured to mesh with the teeth on the exterior wall surface of the proximal portion of the second joint segment, such that the plurality of joint segments are configured to lock against one another to prevent pivoting. wherein in a first, uncompressed position the resilient member is configured to bias the first joint segment away from the second joint segment such that teeth in the interior wall surface of the distal portion of the first joint segment are spaced apart from the teeth on the exterior wall surface of the proximal portion of the second joint segment, such that the plurality of joint segments are pivotable relative to one another, and wherein in a second, compressed position the teeth in the interior wall surface of the distal portion of the first joint segment are configured to mesh with the teeth on the exterior wall surface of the proximal portion of the second joint segment, such that the plurality of joint segments are configured to lock against one another to prevent pivoting.