Microtome Knife

 Planoconcave Profile Knife:

It is plane on one surface and has a hollow ground on other side. This knife is usually

recommended for celloidin or paraffin was embedding blocks. It is less rigid than wedge

profile knife because its base is weaker due to concavity. When plane surface of the knife is

used to sharpen the edge, knife back is required while it is not required when edge is

sharpened from concave side. Concave side is helpful in picking slightly curved sections in

rocking microtome.

Biconcave Profile Knife:

Its both surfaces are hollow ground. It is used for softer tissue blocks like paraffin wax

blocks. Its edge is weakly supported and is not vibration free while sectioning. Knife back

is not required during sharpening of this knife. The popular Heiffer knife has a biconcave

profile. This knife with its distinctive integral handle was designed for use with the

Cambridge rocking microtome.

Tool Edge Profile Knife:

This knife is plane on both surfaces with a steep cutting edge. It is used for cutting extra

hard materials such as un-decalcified bone. With the exception of Heiffer knife, most

blades have separable handles fitted toward one side.

Disposable Knives:

Nowadays disposable blades which eliminate the use of sharpening are now in common

use. These can be fitted in disposable blade holders and use for sectioning of a number of

blocks depending on the degree of hardness.

When such a blade becomes blunt, it can be replaced by a new one. The blade holder

permits easy loading and replacement when loaded. The holder is placed on microtome in

usual way. Use of disposable blades saves a worker from the fatigue of sharpening and

moreover, thin, fine sections are easy to obtain.

Sharpening of Microtome Knives:

A microtome knife requires sharpening whenever its cutting edge becomes blunt or

damaged. The process of sharpening is divided into two stages, honing and stropping, and

every one of these tasks might be performed either the hard way or through programmed blade 

sharpening machines.

• Honing is the grinding of metal from the knife edge with an abrasive substance until all
• scratches have been eliminated, and the edge is sharp and straight.

• Stropping is cleaning or polishing the knife edge on a softer material, usually leather͘.

Manual Honing

The honing is the grinding of metal from the knife edge with an abrasive substance until all

scratches have been eliminated and the edge is sharp and straight. The hone is a rectangular block

of regular or manufactured stone, evaluated coarse, medium or fine as indicated by the level of its

abrasiveness. It is also called oil stone because oil is used as a lubricant. A widely used

regular stone of mid-range grade is the Belgian yellow stone, which gives great outcomes at

reasonable speed. Other stones in common use are:

Arkanas which is natural stone of clear white to pale yellow colour. It is less abrasive than

Belgian and slower in action.

Aloxite which is a series of composite stones ranging in abrasiveness from coarse to

superfine.

Procedure:

The knife is placed at one end of the hone and is pushed diagonally forward in such a way

that cutting edge is leading. At the end of stroke, the knife is turned over on its back and

with cutting edge leading again (with back trailing), it is pulled back along the hone towards

the operator. Tension on the blade should be only adequate to keep in touch

with the surface of the hone. The quantity of strokes required relies upon the state of

the knife, but honing is complete when all the major nicks have been removed.

Stropping

After honing, if edge is seen under a low-power microscope the edge will be seen fine but

regularly serrated. These serrations are due to the abrasiveness of stone and are removed by

the polishing process termed stropping.

Stopping is done in a manner similar to honing from toe to heal but unlike honing, back is

leading and edge is trailing. Strops may be either flexible or rigid. Good quality leather such

as horsehide is recommended due to its durability and effectiveness.

Automatic Sharpening Machines

These machines offer tremendous saving in time and relatively inexperienced personnel can

produce well sharpened knives with a uniform level. This machine is provided with a glass

or copper plate and is incorporated with rotary and oscillatory movements. In this machine

a main spindle is provided having the knife holder. The plate is lapped with an abrasive

material and knife is fixed in the holder in such a way that only edge machine is switched

on, the plate moves beneath the knife edge and it is sharpened. After a specific interval of

time, the spindle rotates and the knife turns over at its back and then sharpening of the other

edge of the knife takes place. The machine is provided with a digital pad where all the

knobs for controlling speed of movement and time of sharpening etc. are given. 

Knife Parts:

a) Foot b) Edge c) Base d) Heel

Knife Back:

It is U-shaped piece placed at base of knife so that knife is lifted and only edge is rubbed on

a bone.

Knife Angles

There are three types of angles that can be observed using a microtone knife.

a. Slant Angle

b. Clearance Angle

c. Bevel Angle

Slant Angle

In case of base sledge microtome the knife is horizontally placed and the angle produced is

called slant angle. Thus resistance is decreased and tough tissue material can be sectioned

easily. The cutting surface is always different from main surface.

Clearance Angle

The surface of blocks should be parallel to cutting facet of bevel and not to main surface of

knife. Therefore, the knife is set at a tilt on the microtome to allow a clearance angle

between the cutting facet and block surface. Freedom points somewhere in the range of 1 and 6 degrees have

proved to the most satisfactory. Biconcave knife requires a smaller clearance angle than

wedge-type knives.

Bevel Angle

The angle between two cutting facets of microtone knife is called bevel angle.