6 Steps in Common Keyhole Approaches

6.1 Introduction

One of the most important features of keyhole surgery is that planning cases, at least initially, requires more mental effort than planning a large craniotomy, even though the actual execution of this plan is generally technically less demanding and simpler than performing a large craniotomy. To that end, our own practice of keyhole surgery relies heavily on simple, workhorse-type approaches, and stresses the use of the endoscope to visualize areas which previously were accessed by more demanding and time-consuming drilling of the skull base. This is intuitive: the fewer the number of steps in the approach, the fewer the opportunities for mistakes or complications, the simpler the repair, and the less time it takes to open and close. Furthermore, surgeons are generally at their best when performing approaches they use frequently. Thus by keeping it simple and familiar, working within a familiar set of workhorse approaches is likely to be ultimately more beneficial for the patient, compared with occasionally performing an unfamiliar approach in order to visualize one specific brain region.

This chapter provides technical descriptions of the keyhole approaches we commonly use, outlining the basic steps of these approaches and providing some tips for performing them well. It should be pointed out that the potential applications of these approaches are numerous, especially when the endoscope is used to its full potential. The remaining chapters in this book outline a few of these modifications, and discusses how and when to use them.

6.2 The Basic Convexity Keyhole

Fig. 6.1

This is by far the most commonly used keyhole craniotomy, and the basic concept and steps are familiar to all neurosurgeons; however, most neurosurgeons have not been formally trained in how to choose the best site for the craniotomy. The use of the basic convexity keyhole assumes that you are operating on pathology with minimal or no surface involvement, and thus are primarily approaching a deep target either transcortically or through a sulcus or fissure which will be exposed by this bone flap.

The basic convexity keyhole is essentially a 2 cm × 1.5 cm bone flap, underneath a short linear skin incision (approximately 3.5 cm). The type and size of this flap is similar to one many surgeons would use to address a loculated chronic subdural hematoma, or to perform an open brain biopsy. If you are not accustomed to performing major brain surgery through this size of craniotomy, it may seem odd at first. However, if you have planned correctly, this is more than enough room to access a deep target safely and easily using the keyhole techniques described in Chapters 2 Cognititive Principles of Planning Keyhole Approaches and 3 Technical Principles of Operating in Keyhole Craniotomies.

6.2.1 Basic Steps

The skin is incised down to the bone and the pericranium is widely elevated with a periosteal so that the opening is maximally used. A self-retaining retractor (and occasionally scalp hooks) is placed underneath the pericranium. After drilling a burr hole and stripping the dura, a bone flap is turned which maximally utilizes the incision. In order to ensure maximal use of the opening, an assistant usually needs to retract the skin outward so the bone flap can be made wide enough without injuring the skin with the drill. A common mistake is not maximally retracting in the corners of the incision, which in turn means that the bone flap is inadequate in these corners. Well placed scalp hooks in the corners can help make a small incision more square-shaped and maximize the utilization of a small opening.

Dural openings can be varied based on the anatomy; however, cruciate openings are usually easiest and, given the small opening, do not take much time to close. The dural leaflets are then retracted by placing a 4–0 stitch through them and hanging hemostats off the edge of the head. We never place tack-up sutures for these openings. This is unnecessary because symptomatic epidural hematomas are extremely rare with this approach (1 epidural in over 2500 keyhole craniotomies without tack-ups in our experience). Closure is simple and no different than with a bigger opening.

If surface involvement or previous surgeries force us to use a larger opening, we generally use similar techniques, with a larger linear incision and bone flap. For very large flaps, we will occasionally place tack-up sutures.

6.3 Eyebrow (Supraorbital) Craniotomy

This is a great and versatile approach for a number of reasons. Firstly, it is simple and quick to perform (once you have some experience). It achieves a frontal exposure with minimal damage to the temporalis muscle and minimal risk to the frontalis branch of the facial nerve (which is inferolateral to this approach). Most importantly, it provides access to many important structures, such as the optic apparatus, the carotid artery, the anterior communicating artery complex, the hypothalamus, etc. It also provides the very desirable anterior-to-posterior trajectory, which many surgeons would normally achieve by removing the orbital rim. If necessary, the proximal sylvian fissure can be split to provide room much faster and easier than the full sylvian split required in an anterolateral approach. With the endoscope, this approach allows access to the interpeduncular cistern, without having to open the cavernous sinus, retract the mesial temporal lobe, or perform any other complex maneuvers otherwise required to access this deep area. The endoscope can also be used to provide some access to the olfactory groove, the interhemispheric fissure, the third ventricle, and the middle fossa, provided the involvement of these regions is not extensive. It can also be used to access the inferior frontal lobe, and in this way can provide a keyhole looking down the long axis of some inferior frontal pathology. Finally, the cosmetic and functional outcomes of this approach are excellent, if performed correctly (Fig. 6.2). Thus, while not appropriate for all patients, the eyebrow approach is one of our first choices for many situations.

**Fig. 6.2** For those concerned about cosmetic outcomes with the eyebrow approach, this collage illustrates the cosmetic outcomes we have obtained using this approach.

6.3.1 Basic Steps

Fig. 6.3, Fig. 6.4, Fig. 6.5, Fig. 6.6

**Fig. 6.3** **a–j** Steps of the eyebrow approach. **(a)** Schematic demonstrating the relative locations of the incision and bone flap in this approach. **(b)** The head is positioned so that the malar eminence is the highest point on the field; this requires some contralateral head rotation. The head is pinned behind both ears so that the clamp does not impair the use of the keyhole. **(c)** The head is extended to encourage the frontal lobe to fall away from the orbital roof with gravity. **(d)** The incision is made in the brow and extends from slightly medial to the supraorbital notch to the edge of the brow laterally, and when the eyebrow is thick the incision is preferably closer to the upper border. **(e)** The soft tissue work aims to expose the lateral orbital rim. In this image, the cut edge of the frontalis muscle is seen, as is the pericranial flap which is cut underneath the frontal side of the skin incision and elevated anteriorly with a small corner of the temporalis muscle. The soft tissue work needs to expose the lateral orbit until the frontozygomatic suture is identified, and until the edge of the superior orbital rim is palpable; **(f)** demonstrates the completed soft tissue dissection. The frontal skin is retracted with scalp hooks, and the pericranial patch is sutured inferiorly to protect it during the bone work.

**Fig. 6.3** **Fig. 6.3 continued (g)** A single bur hole is made beneath the temporalis at the keyhole. **(h)** Following the creation of a bone flap which is as close to the frontal floor as possible, frontal dura is stripped off the orbital roof in preparation to flatten the orbital roof prominences. **(i)** These prominences should be drilled until completely flush with the sphenoid wing as this will provide more room, and make cerebrospinal fluid (CSF) access easier during the initial approach; **(j)** demonstrates an adequately flattened orbital roof. Note that the inner table of the frontal bone is also drilled away to improve the line of sight to the skull base.

**Fig. 6.4** **a–e** Steps of the eyebrow approach. **(a)** The skin incision is kept within the eyebrow hair. If it is necessary to extend it more laterally, the incidence of frontalis palsy will increase. **(b)** After incising the skin, the frontalis muscle is cut with monopolar cautery and then undermined to elevate it off the underlying pericranium (patient 2). **(c)** This is then cut under the frontal skin (leaving a cuff to sew to at the end of the case), then reflected anteriorly as shown (patient 3). Image **(d)** demonstrates this approach after drilling the orbital roof (patient 4), and **(e)** demonstrates the dural opening (patient 5). It is also important to tack the frontal dura anteriorly during the closure, typically tacking it to the pericranium.

**Fig. 6.5** **a–f** Steps of the eyebrow approach. This sequence shows **(a)** the skin incision, **(b)** the pericranial flap, **(c)** the craniotomy, and **(d)** flattening of the orbital roof. The bone should be replaced so that it is approximated with the superior skull edge **(e)**, and the bony gap is hidden under the eyebrow **(f).**

**Fig. 6.6** **a–n** Steps of the eyebrow approach. **(a)** The head position. **(b)** Preparation of the field includes infiltration with lidocaine and epinephrine, and a temporary suture tarsorrhaphy on the ipsilateral eye. **(c)** The pericranial flap has been separated from the frontalis muscle above with scissors. **(d)** Cutting of the pericranial flap is performed with the monopolar on the frontal side, leaving a cuff to sew to during the closure. **(e)** The pericranium is reflected anteriorly. **(f)** The relative size of the bone flap.

**Fig. 6.6 continued (g)** Extradural dissection of the orbital roof. **(h)** Drilling of the orbital roof. **(i)** The finished product. **(j)** The dural opening with a piece of Telfa™ wound dressing placed over the brain to protect it during the initial approach to the CSF spaces. **(l)** the craniotomy is replaced after dural closure and tackup sutures are placed. The tackup sutures should pull the basal dura anteriorly by sewing it to the underside of the base of the pericranial flap. This helps to obliterate the dead space created by dural elevator used to drill the orbital roof. **(k)** The frontal dural tack-ups and some Surgicel placed over the dura. **(m)** The bone flap “shifted” superiorly to hide the gap under the brow. **(n)** The pericranial patch is then sutured back to its cuff, which is located beneath the frontal skin, the frontalis is closed with interrupted Vicryl stitches, and the skin is closed with a subcuticular nylon stitch.

**Fig. 6.6 continued (m)**The bone flap “shifted” superiorly to hide the gap under the brow. **(n)** The pericranial patch is then sutured back to its cuff, which is located beneath the frontal skin, the frontalis is closed with interrupted Vicryl stitches, and the skin is closed with a subcuticular nylon stitch.

The patient is positioned with the head extended to allow the frontal lobe to fall away from the orbital roof and slight rotation to the contralateral side to allow the frontal lobe to separate from a “fixed” temporal lobe. An ipsilateral temporary tarsorrhaphy is performed to protect the cornea during the skin preparation and from accidental drying out during the case. The skin incision should be in the eyebrow, closer to the upper border of the hair-bearing skin, and extend from just medial to the supraorbital notch to the lateral edge of the brow. After carefully transecting the frontalis muscle with cautery, the frontal pericranium is undermined frontally and an incision is made in the pericranium on the frontal side, leaving a cuff which will be closed at the end of the case. There is usually no need to dissect medially toward the supraorbital notch or foramen to free the nerve and minimize the tension on it. Additionally, it is generally unnecessary to perform osteotomies to completely mobilize the nerve, as you do not need to take the opening this medial. The pericranium is then elevated in the direction of the orbit until the lip of the orbital rim is exposed. A small portion of the temporalis overlying the keyhole is elevated as part of this pericranial flap, but minimization of temporalis dissection is one of the principle benefits of the eyebrow approach. The subperiosteal dissection should continue down the lateral orbit until the frontozygomatic suture is encountered, and the beginning of the orbital rim should be palpated. This flap is then secured and retracted anteriorly with sutures, and the frontal scalp is retracted superiorly with scalp hooks. It is unnecessary to retract the skin on the orbital side as this will be retracted downward with the pericranium. A single bur hole is made at the keyhole under the small portion of temporalis which has been elevated. The frontal sinus is identified using preoperative imaging or intraoperative frameless stereotaxy, and all attempts are made to avoid entering it unless there is no choice but to do so. A bone flap is then turned to maximally utilize the frontal extent of the exposure, turning anteriorly on a plane just lateral to the supraorbital nerve and frontal sinus, and then running as flush as possible with the orbital roof.

It is critical at this point that the dura is dissected from the orbital roof and that the inner table of the inferior frontal bony edge and ridges of the orbit be drilled as flat as possible. This maneuver gains several millimeters of room, and provides a flat access to the suprasellar cisterns to drain cerebrospinal fluid (CSF) with only minimal brain retraction. The dura is then opened in a curvilinear fashion and reflected downward.

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