Temporal Bone Dissection Gauri S Belsare
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Temporal Bone Dissection LaboratoryChapter 1

The Temporal Bone Dissection Laboratory requires certain important equipments and instruments. The laboratory should appear like an operation theater.
 
 
Table
It should be of comfortable height and accessible to the instructor for the supervision. The table should be akin to that of an operation theater table.
 
Chair
It should be revolving and adjustable. One can adjust its height according to the individual's convenience.
 
Microscope
Operating microscope is an essential instrument for the temporal bone dissection laboratory. It should be of a standard making and its proper usage and maintenance is very important. The dissector should be familiar with various operational controls. Proper fixing of the microscope to the table is to be ensured.
 
Drill
Each table should have a good drill, a handpiece and various sizes and types of burr bits like cutting burrs, polishing burrs and the diamond burrs. The dissector should be familiar with the parts and should be able to assemble them. The drill should be used with the right hand and suction tip with the left hand—a practice especially very important for delicate control.
 
Bone holder
The temporal bone holder is designed in such a way that it allows three point fixation of temporal bone. For proper angulation, an adjusting wheel at the base of the temporal bone holder is available. Get familiar with fixing the bone in the bone holder. The three point fixation of the temporal bone in the temporal bone holder gives stability during the dissection and the adjusting wheel allows proper angulation simulating the head position in live patient.2
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Figures 1.1A and B: Bone holder
The drain in the trough can be connected to small tubing which will not allow water to get collected in the bowl, making it convenient for the dissector (Figs 1.1A and B).
 
Suction apparatus
It is very important for temporal bone dissection laboratory. It sucks the dust, irrigating fluid and other debris during the dissection. The suction cannula is held with left hand during the dissection (Fig. 1.2).3
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Figure 1.2: Suction apparatus
 
Other instruments
The following instruments are provided for the dissection of the temporal bone (Fig. 1.3).
  • Instrument trays
  • Suction irrigator cannulae
  • Various sizes of suction cannulae with adaptor
  • Sickle knife
  • Bone curette
  • Alligator and cup forceps and microscissors
  • Scalpel and BP handle
  • Straight pick and right angle picks
  • Aural speculum
  • Periosteal elevator
  • Scissors
  • Tooth forceps.
 
Bones
Temporal bones are provided for the dissection. The bones are soaked in water, two days prior to the dissection to reduce the smell of the formalin. Most of the exercises can be performed on dry bones though certain exercises like stapedectomy, facial nerve decompression, etc. are better practiced in fresh bones.4
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Figure 1.3: Instruments:
  • Periosteal elevtors
  • BP handle and scalpel
  • Scissor
  • Tooth forceps
  • Sickle knife
  • Bone curette
  • Straight pick and right angle picks
  • Alligator and cup forceps and microscissors
  • Handpieces and burrs
  • Big and small suction cannulae with adaptor
  • Syringe.
The soft tissue needs to be cleaned before starting the dissection. The side of the bone should be identified and it should be fixed in the holder as it will be in a live patient.
 
Irrigation system
The tap water can be directly used with tubing connected to the water supply. Suction irrigator cannula is attached to this tubing during initial gross drilling. Saline bottle with intravenous set and needle attached to it, is useful for irrigation when we switch over to suction adapter with micro-suction cannulae, during the deeper dissection.
 
The microscope
The initial part of the dissection is done with head light. The purpose of the microscope is to magnify the operating field and provide sufficient illumination while working in the depth.5
 
Parts of the microscope (Fig. 1.4)
The operating microscope consists of three parts
  1. Optical system or optics;
  2. Lighting or illumination;
  3. Stand.
Perfect optics and good illumination are the basic requisites for the microscope.
 
Optical system
It consists of three parts which control the distance between lens and object and the magnification.
The parts are:
  1. Binocular assembly
  2. Magnification changer and
  3. Objective lens.
Binocular assembly: The eyepieces are fitted in ocular tubes. There are various eyepieces available with the magnification—10X, 12.5X, 16X, 20X. 20X eyepieces give maximum magnification but the field of vision is small.
Generally12.5X lenses are used. These eyepieces have diopter scale of −5 to +5 and this can be adjusted according to the refractive error of the dissector.
Magnification changer: The knobs are arranged at the side of the head of the microscope. The magnification varies from 4 to 25. For routine ear work, 6 magnification is used. For finer work, 16 magnification is used.
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Figure 1.4: Microscope
1—Optical systems; 2—Lighting; 3—Stand
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For identification of structures, 25 magnification is occasionally used. Magnification above 25 is rarely used in ear surgery.
Objectives lens: The focal length of a lens is the distance between the object and the lens. The objective lens is fitted at the bottom of the head. It can be easily screwed and unscrewed.
For ear surgery, 200 mm focal length lens is used. For nose and paranasal sinuses surgery, 300 mm lens is used. We can get 250 mm focal length lens for ear surgery as well as transmastoid skull base approaches which need more space for instrumentation. For laryngeal surgery, 400 mm focal length lens is used.
 
Lighting and illumination
Adequate illumination is necessary for good microscopic work but lighting should not be too bright otherwise it will give glaring effect of the operating field. For illumination
  1. Halogen lamp and
  2. Fiberoptic light source is used.
 
Stand
The microscope should run easily on the stand. It is fixed on the stand tightly with knobs provided. The arms and their control knobs are so arranged on the stand that, the head of the microscope can be tilted in any direction at any level, convenient to the operator.
 
Proper usage of microscope
After knowing all the parts of microscope, the surgeon should know the proper usage of it. For that, the following points should be practiced.
  • The inter pupillary distance of ocular tubes is adjusted to get a single image according to the individual convenience.
  • Adjusting the eyepieces: Dissector with astigmatism should wear the glass, and emmetropic persons should set the dioptric scale at ‘o’ and ammetropic scale persons should adjust the dioptric scale according to their refractive error.
  • Focusing the microscope: The microscope should be focused on the field of vision properly. For that, the dissector should practice first using letter from a printed paper as the field by adjusting the height of the microscope and adjusting the fine focus control.
  • Adjusting the fine focus control should be practiced thoroughly.
 
Drill
For proper temporal bone work, the drill and the burrs have become the standard tools.7
Drilling enables the operator adequate and accurate removal of bone and also allows proper exposure and elimination of disease.
This system consists of three parts:
  1. Motor with power supply
  2. Foot control
  3. Handpieces and burrs.
 
Motors
These are available in the form of:
  1. Hanging type: These motors usually work with 12,000 to 20,000 rpm.
  2. Micromotors: Which are more frequently used these days. They are small and handy and work with 30,000 to 40,000 rpm (Fig. 1.5).
 
Handpieces
Two types are available:
One is straight and the other is contrangular. Contrangular handpiece is useful to work in the depth, where straight handpiece cannot access, e.g. near IAM. For footplate work skeeter drills are available. It has thin handpiece and the burrs are also thin and long.
It is possible to incorporate continuous irrigation in the handpiece itself (Fig. 1.6).
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Figure 1.5: Micromotor drill
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Figure 1.6: Skeeter
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Burrs
There are three varieties:
  1. Cutting
  2. Polishing
  3. Diamond.
Each variety is available in various sizes from 0.6 mm to 8 mm size. The cutting edges of these burrs are made up of tungsten carbide. The shape of the burrs is usually round, oval or conical (flame). Even perforating burrs are also available. Standard burrs are made up of stainless steel.
High speed burrs are made up of tungsten carbide.
Cutting burrs are used for most of the bone work. With cutting burrs, bone work will be faster. Polishing burrs are used at the end of operation for smoothening the cavity. Diamond burrs are used near the important structures like fallopian canal, dural plate and sinus plate. Diamond burr is also used for hemostasis (Fig. 1.7).
Whatever type diamond burr is used for hemostasis in whatever speed, heat is produced and this may cause damage to the facial nerve or inner ear. Also, bone dust will be produced during the drilling. To prevent heating and clogging, constant irrigation with suitable fluid and suction is essential.
In live operations, Ringer lactate is used. However, normal tap water can be used during dissection.
Burrs and handpieces are sterilized by formalin vapor.
The handpieces and burrs should be cleaned thoroughly and lubricated with three-in-one oil.
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Figure 1.7: Straight and angled handpieces, and cutting and diamond burrs