A cannula comprising: a head portion (304, 602) that defines a proximal opening sized to receive one or more surgical instruments (312, 622); and an inner tube (308, 604), having an elongated longitudinal dimension, that depends from and is rigidly fastened to the head portion (304, 602) and that defines a distal opening, wherein the inner tube (308, 604) includes an inner wall that defines an elongated conduit between the proximal opening (316, 610) and the distal opening, wherein the inner wall has a transverse dimension sized to receive the one or more instruments inserted in through the proximal opening (316, 610) and extending through the conduit to the distal opening, and wherein the inner tube (308, 604) includes an outer wall; characterized in that the cannula further comprises: an overtube (310, 606), having an elongated longitudinal dimension, that is coaxially aligned with the inner tube (308, 604) and that extends about a portion of the inner tube (308, 604) and that depends from and is rigidly fastened to the head portion (304, 602) and that includes an inner wall that is spaced apart from the outer wall of the inner tube (308, 604), wherein the overtube (310, 606) includes an outer wall; and one or more sensors disposed to provide an indication of at least one of a force and a moment applied to the outer wall of the overtube (310, 606) in a direction generally transverse to the longitudinal dimension of the overtube (310, 606). a head portion (304, 602) that defines a proximal opening sized to receive one or more surgical instruments (312, 622); and an inner tube (308, 604), having an elongated longitudinal dimension, that depends from and is rigidly fastened to the head portion (304, 602) and that defines a distal opening, wherein the inner tube (308, 604) includes an inner wall that defines an elongated conduit between the proximal opening (316, 610) and the distal opening, wherein the inner wall has a transverse dimension sized to receive the one or more instruments inserted in through the proximal opening (316, 610) and extending through the conduit to the distal opening, and wherein the inner tube (308, 604) includes an outer wall; characterized in that the cannula further comprises: an overtube (310, 606), having an elongated longitudinal dimension, that is coaxially aligned with the inner tube (308, 604) and that extends about a portion of the inner tube (308, 604) and that depends from and is rigidly fastened to the head portion (304, 602) and that includes an inner wall that is spaced apart from the outer wall of the inner tube (308, 604), wherein the overtube (310, 606) includes an outer wall; and one or more sensors disposed to provide an indication of at least one of a force and a moment applied to the outer wall of the overtube (310, 606) in a direction generally transverse to the longitudinal dimension of the overtube (310, 606). an overtube (310, 606), having an elongated longitudinal dimension, that is coaxially aligned with the inner tube (308, 604) and that extends about a portion of the inner tube (308, 604) and that depends from and is rigidly fastened to the head portion (304, 602) and that includes an inner wall that is spaced apart from the outer wall of the inner tube (308, 604), wherein the overtube (310, 606) includes an outer wall; and one or more sensors disposed to provide an indication of at least one of a force and a moment applied to the outer wall of the overtube (310, 606) in a direction generally transverse to the longitudinal dimension of the overtube (310, 606).