Sheath for surgical instrument

A method of equalizing a pressure differential between a body cavity and a surgical apparatus comprising a shaft having an interior lumen extending from a proximal end to a distal end of the shaft, an internal seal within the shaft to fluidically divide the lumen into a proximal portion and a distal portion, a wrist structure coupled to the distal end of the shaft, and an end effector coupled to the wrist structure, the method comprising:

• installing a sheath to surround the wrist structure with a first sheath portion and to surround at least part of a length of the shaft with a second sheath portion, the first sheath portion being made of a first material having a porosity allowing insufflation gas to permeate through the first sheath portion, and the second sheath portion being made of a second material different than the first material, the interior lumen of the shaft having an initial pressure equal to an ambient pressure outside the body cavity;
• inserting at least the end effector, the wrist structure, and the distal portion of the shaft including the sheath into the body cavity;
• insufflating the body cavity to a surgical insufflation gas pressure higher than the initial pressure;
• introducing the insufflation gas from the body cavity into the distal portion of the interior lumen of the shaft via permeation of the insufflation gas from the body cavity through the first sheath portion, the internal seal preventing the insufflation gas introduced to the distal portion of the interior lumen of the shaft from flowing to a proximal portion of the interior lumen of the shaft, the proximal portion of the interior lumen of the shaft being at a pressure lower than the insufflation gas pressure while at least the portion of the shaft is in the body cavity,
• wherein the introducing of the insufflation gas into the distal portion of the interior lumen of the shaft while preventing the insufflation gas from flowing to the proximal portion of the interior lumen increases a pressure of the distal portion of the interior lumen of the shaft so as to equalize a pressure differential between the body cavity and the distal portion of the interior lumen of the shaft.