The operation of trephining is made to include any method by which an opening is made in the uninjured cranium or by which an opening already existing is enlarged and made to subserve the surgeon’s purpose. Aside from the saws already alluded to, there are in use a variety of cutting bone forceps, rongeurs of various device, and a variety of chisels, which are to be used in connection with the mallet or hammer. In order to use any of the latter instruments to advantage the first attack should be made with a trephine of reasonable size, say 2 to 3 Cm. in diameter, after which forceps, chisel, or saw may be used. Straight saws also are of occasional usefulness. I do not favor the use of the chisel and mallet, feeling that the concussions resulting from blows of the hammer add to the shock of the operation. The common trephine is provided with a centre pin, which can be withdrawn after a shallow groove has been cut. To prevent slipping of the centre pin the point to which it is to be applied should be marked by cutting a nick with the point of a chisel.
The Gigli saw should be in every surgeon’s outfit. It consists of a piece of steel wire having a thread cut around and along it by a die, by which it is made as effective as a series of saw teeth. Two small trephine openings are made, and it is then passed into one and out of the other, the dura protected by depressing it, and the wire then handled as though it were a chain saw. It can thus be made to cut its way quickly through the bones of the skull.
Other aids in mechanical procedures are revolving small saws and the surgical engine.
Fig. 385
The Powell electric saw cutting a “trap-door” in the skull. (Illustrating the operation upon a cadaver.)
In the absence of a wound a flap of scalp is raised before applying the instrument. This flap is ordinarily of horseshoe shape, and should be made with its convexity pointing toward the occiput, as drainage is best afforded later by this arrangement. The old crucial incisions are now wellnigh abandoned. The pericranium is detached, after incision, with the periosteum elevator, and it should be turned up with its overlying scalp without completely separating it. The scalp flap can be held out of the way by temporarily sewing it to some other part of the scalp, every portion of which should be previously shaved closely and thoroughly scrubbed. The operator has his choice—to seize vessels as they bleed or to make the operation in large degree bloodless by applying an elastic tourniquet tightly around the scalp above the eyebrows and beneath the occiput, the ears preventing it from sliding. If the tourniquet be used the vessels will often bleed in an annoying way after the wound is closed. If the operation be performed for fracture of the skull, should there be an opening already made by the depression of fragments, it may not be necessary to use the trephine, but with suitable bone forceps fragments may be removed or detached. In this case, however, there are often sharp points of bone which will require removal by cutting bone forceps, for the surgeon should leave the margin of the bone opening comfortably round and smooth. Should there be no opening into which the point of an elevator or of bone forceps can be inserted, then one should be made; it is for this purpose that the trephine is mainly used in cases of fracture of the skull. It should now be applied upon a firm and undetached surface of bone, one which will bear the pressure necessary in the process of perforation. As used for this purpose it should be so applied that at least two-thirds of the circle cut by its teeth will be upon unbroken skull; the remaining segment of the circle may be over the fractured area. After it has begun to cut a distinct groove the centre pin should be withdrawn and the instrument maintained in its position during its work by a firm and steady hand, which will force it evenly through the bone and not exercise undue pressure. As the diploë is perforated the bone-dust becomes soft and bloody and the resistance is diminished. As the instrument sinks deeper the operator should frequently intermit its use, and determine his position by means of the irrigator and of the probe or other instrument. The nearer the inner surface is approached the more caution must be exercised, remembering that the bone is likely to be of unequal thickness. When the skull has been completely perforated at one or two points around the little circle the operator should introduce the point of an elevator and pry up the disk of bone, or by rocking the handle of the trephine he may be able to remove the button with that instrument. When the operation is performed in the ideal manner the dura is scarcely touched, certainly not raggedly injured by the teeth of the instrument ([Figs. 386], [387] and [388]).
Fig. 386
Construction of an osteoplastic flap; bone is exposed; first openings are made with a hand trephine or burr. (Marion.)