剪刀如何工作的呢？

How do scissors work?

Prof. Dr. R. K. Mishra

                               
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Introduction:

Jean-Claude Margueron of Emar in Fourteenth Century BC invented Scissors [1].  Scissors are one of the oldest surgical instruments used by surgeons.  Scissors are used to perform many tasks in open surgical procedure but its use in minimal access surgery is restricted.  In minimal access surgery scissors require greater skill because in inexperienced hand it may cause unnecessary bleeding and damage to important structures.

Mechanism of cutting:

The scissors has three parts:

·        Blade

·        Fulcrum and

·        Handle

The cutting force of the scissors works on the law of lever.  Levers are one of the basic tools that were probably used in prehistoric times.  Levers were first described about 260 BC by the ancient Greek mathematician Archimedes (287-212 BC).

The force applied on the blade can be calculated by length of the handle and force applied on the grip of handle.  A pair of scissors is an example of two first class levers connected together at the joint known as fulcrum.

There are three type of lever:

                               
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Scissors works on the principle of class 1 lever (two class 1 levers).  In class 1 Lever, the pivot (fulcrum) is between the effort and the load.  The more the length of the handle or the fulcrum of the scissors, the less force of cutting will be required.  The laparoscopic scissors do not apply the exact law of lever because of the cylinder action of the long shaft, but the design of handle helps in the amplification of force by lever action.

                               
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D1+W1=D2+W2

Where D1 is length of the handle from fulcrum & D2 is the length of blade from the fulcrum.

W1 is force applied on the grip of the handle and W2 is the cutting force of the blade transmitted on the tissue.

Scissors function by the combination of:

1.      Gripping2.      Squeezing and3.      Tearing.

When the blades of scissors close, its sharp edges grind against each other and any tissue which comes between the blades of scissors will get cut.  The scissors-tissue interaction can be described in five stages: [2].

1.  Engagement

In the process of engagement the two blades of the scissors engage a piece of tissue to cut.  The amount of tissue engaged should not be more than the space between the jaw of blades otherwise the chance of slipping of tissue is more.  After engagement, the force applied on the handle of the scissors initiate cutting.

2.  Elastic deformation

This stage starts just after the engagement of tissue between the blades of the scissors.  In this process, the tissues between the two blades of scissor start deforming.  This stage is called elastic deformation, because if the force on the handle of scissors is removed than the tissue deformity will return to its normal state.

3.  Plastic deformation

Further force on the handle of scissors will cause the tissue between the blades to go into a plastic deformed state, which is irreversible.  After undergoing this state of tissue deformation, even if further process of cutting is stopped the impression on the tissue remains.

4.  Fracture

Further increased force on the fulcrum of scissors will result in the fracture of intercellular plane of the tissue.  This stage of cutting is peculiar to scissors because unlike the scalpel the site of tissue fracture is intercellular.

5.  Separation.

After the fracture the tissue separates along line of the blade of scissors, and then this whole process of cutting will continue on the engaged tissue.

Histology of the tissue after cutting:

Histological examination of the tissue after cutting with scissors shows that there is separation of tissue through intra cellular plane.  Microscopic examination shows serrated cut margin along the line of tissue separation.

Types of Laparoscopic Scissors:

                               
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Straight scissors

The blade of this scissor is straight and it is widely used as an instrument for mechanical dissection in laparoscopic surgery.

                               
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Curved Scissors

The blade of this scissors is slightly curved and this is the most widely used scissor in laparoscopic surgery.  The surgeon prefers this scissors because the curvature of the blade of this scissors abolishes the angle of Laparoscopic Instruments manipulation and better view through telescope.

                               
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Serrated scissors

The main advantage of this scissors is that the serrated edges prevent the tissue to slip out of the blades.  It is a useful instrument in cutting a slippery tissue or ligature.

                               
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Hook Scissors

The main advantage of this scissors is that, it encircles the structure before cutting: Tissue is held between its jaws and there is no chance of slipping.  The Hook scissor is especially useful for cutting secured duct or artery in laparoscopic surgery.  The cutting of nerve bundle in neurectomy becomes very easy with the help of this scissors.   Hook scissors is also helpful in partial cutting of cystic duct for intra-operative cholangiography.         

                               
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Micro tip scissors

The main advantage of this scissors is to cut the ducts partially for facilitating cannulation.  This scissor may be used for cutting the cystic duct for performing intra-operative Cholangiogram.  Exploration of small ducts like common bile duct is very helpful with micro scissors due to its fine small blades.  Fine micro scissors are also available in its curved form.

Scissors has following advantages:

1.    Inexpensive

2.    Safe in safe hand

3.    Operator determined précised action

4.    Closed blades, can work for blunt dissection and electrocautery

5.    Piercing tissue with closed blades and then opening helps in obtaining a good plane of dissection.

Scissors has following disadvantages:

1.    Non haemostatic

2.    Accidental chances of cutting small ducts and vessels

3.    Due to its pointed end, if overlooked there is chance of injury to viscera

4.    If used for electric coagulation its blades get blunt easily.

References:

[2] S.M.Shimi.  Dissection techniques in laparoscopic surgery: a review.  J.R.Coll.Surg. Edinb, 40, August 1995, 249-259