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Tip 141: True Incision Size
The incision size required to insert a circular instrument
of a known diameter can be determined by calculating the circumference
of the instrument, and dividing this by 2. For example, the diameter of
a 20-gauge needle is .9 mm. The circumference is 2.8 mm (3.14 x .9), and
the minimum incision size required for the insertion of a 20-gauge needle
is therefore l.4 mm.
In actual practice, however, it is virtually impossible
to insert a round, rigid instrument through the minimum incision size
as calculated above. The linear incision will not easily deform into a
perfect circle, and therefore needs to be slightly larger than calculated.
An incision for a 20-gauge needle therefore must be 1.5-1.6 mm.
A common misconception is that a 20-gauge needle with
a .9 mm diameter can be inserted through a .9 mm incision. This is a physical
impossibility.
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