The area of bone to be removed depends on the age of the patient; in the adult it is about half an inch square, having as its boundaries the zygomatic ridge above and Henle’s spine in front.
The bone should be removed by short decided taps of the mallet on the gouge or chisel, held in contact with the bone in a sloping direction (Fig. 217). This precaution is specially indicated whilst in the act of removing the bone from above downwards and from behind forwards, in order to prevent injury to the middle fossa, which may be low lying, or the lateral sinus, which may project abnormally far forward (Fig. 218).
To permit of better control over the instrument, the hand holding it may rest lightly against the patient’s head, which is now covered with a sterilized towel. This control should always be sufficient to prevent the chisel or gouge being driven unexpectedly too far inwards, an accident which may easily happen if, by chance, there is a sudden diminished resistance to the stroke owing to unexpected softening of the bone or the inadvertent exposure of the dura mater. It is this accidental slipping of the instrument which is often responsible for injury to the lateral sinus or the facial nerve. With regard to choice of instruments, I prefer the gouge, as it is safer than the chisel, owing to it having rounded edges.
On removal of the superficial part of the cortex, the mastoid process may be found to be sclerosed, or to consist of small or large cells filled with granulations or purulent secretion.
(a) If the bone be sclerosed. The operation may be extremely difficult, as the antrum is frequently of small size and very deeply placed. As the tympanic cavity must not be interfered with, it is not permissible to insert the seeker along the auditory canal into the attic in order to determine the position of the aditus. The only guides, therefore, are the anatomical landmarks.
The best method is to chisel away the bone close to and parallel to the upper posterior margin of the external meatus. In chiselling along the upper wall of the opening, the gouge, instead of being directed downwards, as was the case in removal of the outer portion of the cortex, is now directed inwards and at the same time slightly upwards and forwards. In enlarging the lower part of the opening, the bone is chiselled away obliquely inwards and upwards. The strokes of the gouge are made alternately from above and below, so that gradually a funnel-shaped opening is formed, having its point directed towards the aditus.
Anteriorly, the bone is removed as close to the posterior wall of the auditory canal as possible, including the suprameatal spine. Above, the line of chiselling must not extend beyond the zygomatic ridge, whilst below sufficient bone should be removed towards the tip of the mastoid process to permit of inspection of the deeper parts of the wound.
From time to time the operator makes use of the seeker (Fig. 219). This is a blunt-pointed probe whose tip is bent at right angles to its shaft. With it any opening is probed carefully to see whether it is merely a mastoid cell, or dura mater covering the outer wall of the lateral sinus, or the middle cranial fossa, or if indeed it is the antrum itself. The chief mistake is to work too low down. If the antrum be small it may be missed, and the bone may be chiselled away too deeply in endeavouring to discover it and the facial nerve or the external semicircular canal injured. It is wiser, therefore, to work high even if the dura mater of the middle fossa is exposed by doing so. This should not lead to any harmful result provided the dura mater is not injured.