3

1. Skin incision (Fig. 3.1): Position the incision so that it is hidden by the auricle. Placement of the incision in the post-auricular sulcus is uncomfortable, particularly for individuals who wear glasses. Thus it should be placed 0.5–1.0 cm posterior to the sulcus. The incision begins superiorly within a few millimeters of the attachment of the auricle, and ends inferiorly at the level of the meatal floor.

Fig. 3.1 Placement of the post-auricular incision.

2. Fascial dissection (Fig. 3.2): The scalpel blade should then be oriented at 45° to the horizontal and the incision deepened as the auricle is pulled laterally. Deep to the posterior auricular muscle a layer of areolar tissue is encountered superficial to the temporalis fascia. Place a finger in the external auditory canal (EAC) meatus while pulling the auricle laterally, and dissect in this plane staying superior to the meatus. Continue dissecting in this plane anteriorly until in front of the meatus and then inferiorly to your finger without violating the meatal skin.

3. Periosteal cuts (Fig. 3.3): The first periosteal cut, along the temporal line, should be made directly adjacent and tangential to the superior aspect of the meatal skin, extending from just anterior to the finger to the posterior edge of the incision. A second periosteal cut should be made from the posterior end of the first to the mastoid tip. This cut should spare the facial nerve as it exits the stylomastoid foramen anterior to the mastoid tip.

Fig. 3.2 Fascial dissection relative to the external auditory canal (EAC) is aided by placement of the finger within the meatus.

Fig. 3.3 Placement of periosteal cuts relative to the EAC.

4. Periosteal elevation (Fig. 3.4): The periosteum is elevated including the temporalis muscle exposing the temporal line as far anteriorly as the root of zygoma, and the spine of Henle at the superior-posterior edge of the bony EAC.

Cortical Landmarks

Remove soft tissue and stabilize the temporal bone in the bone-holder. Orient in the surgical position. Examine the surface anatomy and consider the relationship to deeper structures (Figs. 3.5, 3.6).

Fig. 3.4 Elevation of the periosteum to the spine of Henle.

The spine of Henle is the lateral limit of the posterior EAC cortex (Fig. 3.5). Macewan’s triangle and associated cribriform area located just posterior to the spine of Henle marks the approximate location of the antrum. The temporal line which is continuous with the zygoma, estimates the middle fossa floor and superior limit of the mastoidectomy (Fig. 3.6). The lateral sinus is the posterior limit but lacks a surface landmark. Note the convention used in this book to describe orientation and relative anatomic position.

Fig. 3.5 Surface landmarks of the antrum.

Fig. 3.6 Surface landmarks indicating approximate limits of the mastoid dissection.

Drill three shallow grooves in the mastoid cortex delineating the limits of dissection (Fig. 3.7). Beginning at the 12 o’clock position above the EAC, one groove is parallel to and 1 cm above the temporal line. The second curves inferiorly parallel to the border of the EAC, from the zygomatic root to the mastoid tip. A third (posterior) groove completes the tri angle and overlies the lateral sinus.

Fig. 3.7 Initial cortical bone cuts parallel to anticipated limits of dissection.

Fig. 3.8 Remove intervening cortical bone, opening into mastoid air cells.

Superior Boundary: Delineate Mastoid Tegmen

Remove the mastoid cortex between the three shallow grooves and begin the exenteration of the mastoid air cells by finding the tegmen, the upper margin of dissection and cortical lining of the middle fossa floor (Fig. 3.8).

Long anterior-to-posterior strokes with the side of a 5 mm or larger burr, at the superior margin of the dissection, will gradually reveal the dense inner cortex of the middle fossa. This cortical plane is easily found superior to the temporal line where it forms the lateral wall of the middle fossa. Expose the tegmen fully at least 1 cm above the temporal line. Thin this cortical bone until translucent from the zygoma to the sinodural angle. This reveals the middle fossa dura along the full anterior-posterior extent (Fig. 3.9).

Fig. 3.9 Dissection of superior margin in search of inner cortical lining of middle fossa.

Fig. 3.10 (A,B) Location of inner middle fossa cortex relative to temporal line.

Once identified superiorly, the inner cortex of the middle fossa can be followed inferiorly and sculpted as it bows medially and becomes the mastoid tegmen. As this occurs you will simultaneously achieve the desired saucerization (Fig. 3.10A,B).

Note, if the superior cortical cut is placed too low, the trough can miss the cortex of the middle fossa floor (Fig. 3.11A,B).

Widen the existing trough in a cephalad direction. The inner cortical plane can be identified more easily here and followed into the mastoid where it becomes the tegmen. At the same time, the expanding superior rim of the mastoid cortex maximizes your “saucerization” of the cavity (Fig. 3.12A–D).

This task is the first of several sculpting procedures that requires careful “reading” of the bone as the drilled surface is transformed from pock-marked aerated bone (Fig. 3.13A,B), to the dense cortical layer that encases all structures of concern (Fig, 3.13C), to the increasing revelation of the target structure as this cortical layer is thinned. Clues to thin tegmental bone include the visualization of dural vasculature and the appreciation of a high-pitched sound to the rotating burr. Sculpt the entire inner cortex of the middle fossa floor leaving an intact but translucent layer of cortex overlying the dura. The changing appearance of the drilled surface is an indication of diminishing bone thickness and should prompt appropriate alterations in burr path and pressure. Areas that are translucent should not be over-drilled. Recognize dural exposure through defects in the cortical plate and avoid further drilling and dural injury.

Fig. 3.11(A,B) A cortical cut at the temporal line may be too low to access the middle fossa floor.

Fig. 3.12 (A–D) Identify the inner middle fossa cortex along its lateral wall, above the temporal line, and follow into the mastoid where it becomes the tegmen.

Fig. 3.13 (A–C) Changes in texture and translucency of bone guide the tegmental dissection.

Your ability to accomplish this task will pay dividends in the contracted mastoid in which a sagging tegmen may limit access to the antrum and deeper structures. Every milli meter of additional space will be needed as your dissection proceeds medially.

Sculpting of the cortical lining of the posterior EAC should start with a razor sharp lateral cortical edge that encompasses the spine of Henle. Lower the plane of aerated bone to the deepest level at which the tegmen is clearly defined.

Use the anterior surface of the burr (not its tip) to remove bone at the anterior edge of dissection up to the EAC cortex (Fig. 3.14A,B). Sharpen the lateral edge of the EAC wall using long strokes from left to right while viewing it on-edge. An adequately thinned EAC cortex is translucent enough to reveal the canal skin or outlines of an instrument placed subcutaneously within the lumen.

Use the appropriately sized cutting burr to extend sculpting of both the tegmen and EAC cortex anteriorly toward the root of zygoma (Fig. 3.14C,D).

Fig. 3.14 (A,B) Sharpen lateral cortex of the EAC. (C,D) Sculpt EAC and tegmen toward root of zygoma.

Remove air cells overlying the proximal portion of the sinus leaving a thin cortical layer. Use long strokes directed from the sino-dural junction superoposteriorly toward the region below the EAC antero-inferiorly (Fig. 3.15).

Observe the transformation of surface features from irregular aerated to dense cortical bone, the increasingly dark appearance of the sinus as the cortex is thinned and listen for the high-pitched sound generated by the burr on the thinned bone (Fig. 3.16A,B,C).

Fig. 3.15 Drill along anticipated path of sigmoid sinus.