5 Skull Base Craniotomies



10.1055/b-0039-169398

5 Skull Base Craniotomies



5.1 Frontal Craniotomies



5.1.1 Bifrontal

Torstein R. Meling and Marton König

The bifrontal approach was first described by Horsley and Cushing. A bifrontal craniotomy can be used for pathologies in the anterior skull base, those that extend into the nasal cavity, and those that extend back to the suprasellar region. This craniotomy allows a low approach to one or both frontal fossae. It is considered a traditional skull base approach used to target difficult tumors toward the front of the brain, based on the concept that it is safer to remove extra bone than to unnecessarily manipulate the brain. This approach is typically used for tumors that are not suitable for removal by minimally invasive approaches because of their anatomy, their possible pathology, or the goals of surgery (see ▶Fig. 5.1, ▶Fig. 5.2, ▶Fig. 5.3, ▶Fig. 5.4, ▶Fig. 5.5 ▶Fig. 5.6, ▶Fig. 5.7, ▶Fig. 5.8, ▶Fig. 5.9, ▶Fig. 5.10, ▶Fig. 5.11, ▶Fig. 5.12, ▶Fig. 5.13, and ▶Fig. 5.14).

Fig. 5.1 (a, b) Positioning. The patient is in a supine position with the body slightly elevated. The head is fixed in a three-pin headrest and elevated above the level of the heart. The head is extended 15° with no sideways rotation or flexion, so that gravity causes the frontal lobe to fall away from the anterior cranial fossa, allowing retractor-free intracranial dissection.
Fig. 5.2 (a, b) Shaving and draping. Once the patient has been correctly positioned, the hair is washed meticulously to reduce the risk of infection. The skin is minimally shaved, just behind the hairline. The skin incision line is infiltrated using 1% Xylocaine with 1:200,000 epinephrine. The hair is kept out of the surgical field—when necessary using hair ties and tape with underlying dressing (to avoid pulling of the patient’s hair during removal).
Fig. 5.3 (a,b) Anatomy overview. Important anatomical landmarks are: (1) the frontal sinus, (2) nasion, (3) glabella, (4) tragus, (5) superior sagittal sinus (SSS), (6) supraorbital artery, vein, and nerve, and (7) the superficial temporal artery (frontal and parietal branches).
Fig. 5.4 (a, b) Skin incision. Typically, a bicoronal skin incision running from ear to ear (also known as a Souttar incision) is made just behind the hairline using a No. 20 knife blade. Note that only the epidermis and dermis are incised, leaving the galea aponeurotica over the bone and the temporal muscles intact. The skin incision starts 1 cm in front of the tragus, slightly above the level of the zygomatic arch (at the approximate level of the orbital roof), and is continued behind the hairline to the contralateral side. This incision spares the branches of the facial nerve and the parietal branch of the superficial temporal artery. After meticulous hemostasis, Raney hemostatic clips are placed along the margins of the skin incision in order to secure the draping and obtain hemostasis.
Fig. 5.5 (a, b) Flap preparation. The skin flap is retracted anteriorly using four small fishhooks with elastic bands. Particular attention is paid to the dissection of the superficial and deep fascia of the temporalis muscle to prevent injury to the facial nerve and maintain the arterial vascularization of the temporal muscle. Incision of the temporalis muscle and fascia is not necessary, as a standard interfascial dissection over the temporal muscle is adequate for exposure. The galea and pericranium can now be harvested as a pedicled flap vascularized by the supratrochlear, supraorbital, and zygomaticotemporal arteries anteriorly. The posterior limit of the graft can be either at the level of the skin incision if a small graft is needed or all the way posteriorly to the superior nuchal line/lambda if a large pericranial flap is needed for reconstruction of large dural defects and/or defects in the anterior cranial fossa. In both cases, the lateral limit is along the superior temporal line. Great care should be taken to dissect the scalp in the subgaleal plane to avoid any defects in the flap as well as to avoid damaging the arterial blood supply to the pericranial flap. Sharp dissection is performed toward the superior orbital margins on both sides, paying particular attention to the supraorbital nerves. The nerve is dissected away, or freed, from its foramen.
Fig. 5.6 (a, b) Preparing the surgical field. The flap is covered with wet dressings to prevent tissue damage due to desiccation and thermal injury from the xenon microscope lamp.
Fig. 5.7 (a, b) Craniotomy configuration and landmarks. Important anatomical bony landmarks such as the nasion, glabella, supraorbital fissure, frontotemporal suture, stephanion, and bregma must be exposed, and serve as major orientation points. In addition to such landmarks, neuronavigation assists in exact placement of the craniotomy. If the approach is above the frontal sinus, the craniotomy is more kidney-shaped; if the craniotomy needs to be low and to cross the sinus, it is more superelliptic.
Fig. 5.8 (a, b) Craniotomy outline. A bifrontal craniotomy is made using a high-speed electric craniotome. The bone flap is usually 8–10 cm wide (coronal plane) and 3 cm deep (sagittal plane). The superior margin of the craniotomy is marked by the posterior burr hole placed over the SSS. The lateral borders of the craniotomy are approximately at the midpupillary line. The inferior margin of the craniotomy is planned according to the intracranial target. Using the drill and the navigation pointer, the craniotomy outline is engraved into the bone.
Fig. 5.9 (a, b) Craniotomy. A single 5 × 10 mm oval burr hole is made 6 cm behind the glabella using a high-speed drill with a sharp 4-mm drill bit. It is situated directly over the SSS and oriented transversely to avoid burr hole defects in the forehead. The craniotomy is performed away from the SSS. If the frontal sinus can be avoided, care should be taken to prevent inadvertently entering it, by using a neuronavigation system or taking into account the position of the lateral border of the frontal sinus as measured on preoperative imaging (CT scan and/or MRI).
Fig. 5.10 (a, b) Bone drilling. If the frontal sinus can be spared, the tabula interna and frontal bone crest are removed using a high-speed drill with a sharp 4-mm drill bit. If the approach needs to traverse the frontal sinuses (e.g., for a transbasal approach, for a low angle of attack, or in patients with large frontal sinuses), the frontal sinuses may be cranialized by complete removal of the posterior wall of the frontal sinus and eversion of the mucosa down to the frontonasal duct. Opening of the frontal sinuses, either inadvertently or deliberately, may increase the risk of postoperative meningitis.
Fig. 5.11 (a, b) Dura opening. The dura is opened in a slightly curved form transversely in the midline. Surgicel is placed extradurally to support hemostasis. Tacking sutures in the dura are attached to skin or previously drilled holes in the frontal bone to gently retract the dura and expose the frontal brain surface. The SSS may be divided basally, by electrocoagulation, next to the crista galli, near the orbital roof, with little risk. The falx is cut until its deep edge.
Fig. 5.12 (a–c) Intracranial dissection and targets. Further dissection beginning with the olfactory tracts is performed under the microscope. We try to preserve the olfactory tracts by careful, sharp dissection and by covering them with fibrin sealant. The tracts might be sacrificed if necessary, either unilaterally or bilaterally, allowing separation of the dura from the cribriform plate and the crista galli. Frontal craniotomies provide surgical access to the frontal lobe(s) of the brain (e.g., for intrinsic brain tumors), to the third ventricle via lamina terminalis (e.g., craniopharyngiomas or brainstem cavernomas), to the parasellar region (e.g., tuberculum sella or planum sphenoidale meningiomas), to the anterior skull base (e.g., olfactory groove meningiomas), to the ethmoid region (e.g., transbasal resection of ethmoidal tumors), and for repair of ethmoidal cerebrospinal fluid (CSF) fistulas.
Fig. 5.13 (a, b) Closure. Meticulous hemostasis is performed both intra- and extradurally. The dura is closed using a PDS 5–0 running suture. A sheath of TachoSil is placed over the dura mater to reduce the risk of CSF leaks, if deemed necessary. The bone flap is refixed and plates are used to cover large and cosmetically relevant bone gaps.
Fig. 5.14 (a, b) Galea and skin closure. Care should be taken to level bony structures so that they are flush with the surrounding cranium during the replacement of the bone flap. Any remaining irregularities and larger gaps are filled using hydroxyapatite cement. The galea is closed with a running Monocryl 3–0 suture. The skin is closed using skin staples. The wound is dressed using Steri-Strips.


Checklist




  • Use neuronavigation planning to familiarize yourself with your trajectory relative to the frontal sinus anatomy. Cave: very lateral extensions.



  • Before skin incision, make sure the head is retroflexed enough to enable the frontal lobe to fall away from the anterior cranial fossa and allow for retractor-free intracranial dissection.



  • Make the bicoronal skin incision > 1 cm behind the hairline, starting just anterior to the tragus. Note that only the epidermis and dermis are incised, leaving the pericranium over the bone and the temporal muscles intact.



  • Continue to dissect the skin flap in the loose areolar skin layer in a posterior-to-anterior fashion all the way to the superior orbital rim area, preserving the pericranium on the bone and preserving the supraorbital arteries anteriorly.



  • When performing the interfascial dissection laterally, stay directly on the superficial layer of the deep temporal fascia at all times to avoid injury to the temporal branch of the facial nerve.



  • Harvest the pericranial flap from the bone by making a horizontal cut posteriorly, at the lambda if a large flap is needed or at the bregma if a smaller flap is sufficient, and along the linea temporalis superior laterally.



  • Stay subperiosteally at all times and harvest the flap in a posterior-to-anterior fashion all the way to the superior orbital rim. Cover the pericranial flap in wet gauze. Cave: subperiosteal dissection of margo orbitalis superior can injure the supraorbital nerve if not properly detached from its canal/groove.



  • Use neuronavigation to identify the borders of the frontal sinus.



  • When a low trajectory is needed and the frontal sinuses are being cranialized, make a small, oval burr hole directly on the superior sagittal sinus, 5 cm posterior to the glabella.



  • Make the first cut C-shaped, starting from the burr hole and running to the apex of the margo orbitalis superior, which defines the lateral extension.



  • Make the last cut straight and run it as close to the superior orbital rim as possible in a lateral-to-medial direction.



  • Repeat the same two steps on the contralateral side.



  • Cranialize the frontal sinuses by removing the mucosa and posterior wall completely and drilling down all trabeculae, while leaving the tabula externa intact for cosmesis.



  • Toward the frontonasal ducts, the residual mucosa is gently scraped off the tabula interna in a superior-to-inferior direction to preserve its blood supply, and is used to plug the duct.



  • When a higher trajectory is needed and the frontal sinuses can be preserved, perform the craniotomy 3 cm further posteriorly and make the straight, basal cuts superior to the frontal sinus.



  • Open the dura with a straight, horizontal incision on either side of the superior sagittal sinus 1 cm above the horizontal bone cuts, and retract the dura anteriorly. Use bipolar electrocoagulation or hemo-clips to ligate the superior sagittal sinus and make additional cuts to connect the two durotomies.



  • Continue to open the falx cerebri toward the anterior cranial base and use bipolar electrocoagulation to ligate the inferior sagittal sinus. Cave: do not drift off too far posteriorly.



  • Close the dura with a running monofilament 6–0 suture. Make sure that it is watertight, as any CSF leak here might cause rhinorrhea and/or infections. “Stress-test” your closure by injecting water subdurally via a venous catheter and close the puncture site.



  • Fix the bone flap with 1-mm titanium mini-plates. Take care to level the bone flap flush with the surrounding cranium so as to avoid any irregularities, and use hydroxyapatite cement to fill any larger gaps.



5.2 Frontotemporal Craniotomies



5.2.1 Facial Nerve Anatomy and Protection

Andreas Raabe and Peter A. Winkler

The branches of the facial nerve are at risk of mechanical or thermal injury when the skin–galea flap is separated from the temporalis muscle inferior to the superior temporal line (STL). In the lower half of the temporal region, between the STL and the zygomatic arch, the easy plane of dissection during elevation of the scalp (loose connective tissue = LCT) merges into the fibrous fatty connective tissue and becomes difficult to separate because fibrous adhesions bind the scalp to the superficial layer of the temporalis fascia. When dissection and elevation of the scalp flap is continued in this area, the facial nerve may be injured. Due to the presence of fatty tissue, this part of the LCT is also called the suprafascial fat pad (sFP). The size of the fat deposit in this subgaleal–suprafascial plane varies between individuals (▶Fig. 5.15).

Fig. 5.15 Frontotemporal tissue layers. dFP: deep fat pad; FM: frontalis muscle; iFP: interfascial fat pad; LCT: loose connective tissue; MM: masseter muscle; sFP: suprafascial fat pad; STL: superior temporal line; TF: temporalis fascia; VII: facial nerve; ZA: zygomatic arch.

It is in this layer, between the STL and the zygoma, that the facial nerve is at highest risk of injury. Therefore, the galea should not be separated from the layers below in the lower half between the STL and the zygomatic arch (see ▶Fig. 5.16, ▶Fig. 5.17, ▶Fig. 5.18, and ▶Fig. 5.19).

Fig. 5.16 The three temporal fat pads and their significance. There are three fat pads in relation to the layers of the temporalis fascia: 1. Interfascial (iFP): between the superior and the deep layers of the temporal fat pad lies the interfascial fat pad. There is no facial nerve running in this layer. This is the first fat pad that becomes visible through the superficial temporalis fascia when the scalp flap is turned anteriorly and separated from the temporalis muscle. 2. Suprafascial (sFP): this is the middle part of the LCT plane but it is composed of fatty and fibrous tissue, as described above. It covers the lower half of the distance between the STL and the zygomatic arch, but varies in size and extension. It is through this layer that the facial nerve passes. 3. Deep fat pad (dFP): this is located below the deep layer of the temporalis fascia.
Fig. 5.17 Anatomical course of the temporal branches of the facial nerve and landmarks. There are typically two to four branches of the facial nerve crossing the zygomatic arch with a course that runs anteriorly and superiorly to innervate the orbicularis oculi, frontalis, and corrugator muscles. Below the level of the zygoma, these branches are located in the LCT layer. After crossing the zygomatic arch superficially, they enter the fibrous and fatty sFP. At the sFP, they enter the galeal or fascial layer of the skin and travel to their muscles. The following landmarks help to avoid injury of the facial nerve: 1. The appearance of the yellowish interfascial fat pad (iFP) during lifting of the scalp flap indicates that the critical suprafascial fat pad (sFP) is close. 2. Subgaleal adhesions: the easy subgaleal plane ends when adhesions connect the galea to the superficial temporal fascia, indicating the sFP. 3. STA: the appearance of the frontal branch of the STA within the galea when elevating the flap.
Fig. 5.18 Metric measurements for safe zones. Metric measurements for safe distances may also help to avoid the facial nerve. Safe zones are: 1. At the level of the zygomatic arch: up to 1.5 cm anteriorly to the tragus. 2. At the level of the orbit: from the lateral cantus 2 cm laterally, parallel to the lid line. 3. At the level of the STL: from the middle of the eyebrow less than 1 cm above the eyebrow. 4. Posterior or superior to the danger zone corridor (see below). Danger zones are: 1. At the level of the zygomatic arch: anteriorly to the safe 1.5 cm zone. 2. At the level of the orbit: a corridor of 3 cm posterior to the safe zone. 3. At the level of the STL: a corridor of 3 cm posterosuperior to the safe zone.
Fig. 5.19 (a–d) Techniques for facial nerve preservation. If the galea is separated from the temporalis muscle down to the zygoma without using a facial nerve preservation technique, the nerve is injured (a top left). There are three surgical techniques for facial nerve preservation: (b, top right) interfascial flap (for orbitocranial or orbitozygomatic approaches) and (c, bottom left) subfascial flap (for orbitocranial or orbitozygomatic approaches), and (d, bottom right) combined myocutaneous flap (for pterional craniotomy).


Combined Musculocutaneous Flap

In the combined musculocutaneous flap, the temporalis muscle is not separated from the galea. They are lifted together as a single flap (▶Fig. 5.20 and ▶Fig. 5.21).

Fig. 5.20 (a, b) Combined musculocutaneous flap. After incision, the skin–galea flap is slightly elevated and the scalp clips are placed for hemostasis (a). The temporalis fascia and muscle are cut down to the bone slightly anterior to the skin incision to facilitate suturing of the temporalis fascia when closing the layers (b).
Fig. 5.21 (a, b) Combined musculocutaneous flap. The muscle is then sharply detached subperiostally from the bone in an inferior-to-superior direction that preserves the muscle fibers and vascularization (a). All layers are also subperiostally detached from the STL and frontally until the frontotemporale (see Chapter 2, Landmarks) and the zygomatic process can be palpated with the finger. After elevating the myocutaneous flap, the entire flap is turned anteriorly to expose the frontal and temporal bone at the site of the craniotomy (b). The advantage of the myocutaneous flap is its ease of use and excellent protection of the facial nerve. It is the standard technique for the pterional approach. The disadvantages are (1) the bulk of muscle that may narrow the working space and restrict the view along the sphenoid ridge and the lower margin of exposure for some approaches (i.e., for large anterior communicating [Acom] artery aneurysms) and (2) the fixation of the muscle at the zygoma. Thus, it is not suitable for an orbitozygomatic approach and provides only limited exposure of the lateral orbital rim during orbitocraniotomy or orbital extensions of other craniotomies.


Subfascial Dissection

In the subfascial dissection, the tissue is cut down including the deep layer of the temporalis fascia, but not the temporalis muscle. All layers above the muscle fibers are elevated with the flap (▶Fig. 5.22 and ▶Fig. 5.23).

Fig. 5.22 (a, b) Subfascial dissection. The scalp (skin–galea) is cut. Scalp clips are placed. The superficial fascia of the temporalis muscle is exposed. The scalp flap is elevated until the interfascial yellow fat pad and/or the frontal branch of the STA are visible. The superficial layer of the temporalis muscle, the interfascial fat, and the deep layer of the temporalis muscle are cut (a). Care has to be taken not to damage the surface of the temporalis muscle. The deep temporalis fascia is dissected from the temporalis muscle. All layers above the muscle fibers are elevated (b).
Fig. 5.23 (a, b) Subfascial dissection. The temporalis muscle and the orbital rim are exposed (a). Fish hooks are used to retract the other layers of the flap. Care should be taken to apply only enough tension to hold the flap in place, because excessive traction may also injure the branches of the facial nerve. Now the temporalis muscle is cut at the posterior border parallel to the posterior skin incision line and detached from the bone in an inferior-to-superior direction (b). It is also detached from the STL and the orbital rim and reflected inferiorly. A suture with loose tension holds the muscle in place. This maneuver moves the temporalis muscle away from the anterior cranial base as is required for orbital rim or orbitozygomatic approaches.


Interfascial Dissection

In 1984, Gazi Yaşargil described the interfascial dissection technique and how its use for pterional craniotomy allowed sufficient basal exposure and retraction of the temporalis muscle when needed. Use of the interfascial dissection technique has markedly reduced, but not eliminated, the incidence of frontal muscle palsy. This is because surgeons sometimes mixed up the fat pads or did not correctly follow the concept and description. Interfascial dissection is similar to subfascial dissection, but the deep layer of the temporalis fascia covering the temporalis muscle is left intact (▶Fig. 5.24 and ▶Fig. 5.25).

Fig. 5.24 (a, b) Interfascial dissection. After incision, the skin–galea flap is elevated until the yellowish fat pad appears (a). The superficial layer of the temporalis fascia and the interfascial fat pad is incised. The deep temporalis fascia covering the muscle becomes visible. Now, the fat and the layers above are mobilized anteriorly using a swab. A typical error is to dissect too superficially within the suprafascial fat pad where the facial nerve runs. Therefore, the subfascial technique is preferred because dissection may be easier as there is a clear border between the deep layer of the temporalis fascia and the temporalis muscle. Interfascial or subfascial dissection is only required when the lateral orbit or the zygoma has to be exposed, or where the temporalis muscle is likely to obstruct the surgical field.
Fig. 5.25 (a, b) Interfascial dissection. The temporalis muscle and the orbital rim are exposed. Fish hooks are used to retract the combined flap. Care should be taken to apply only as much tension as is required to hold the flap in place, because excessive traction may also injure the branches of the facial nerve (a). Now the temporalis muscle is cut at the posterior border parallel to the posterior skin incision line and detached from the bone in an inferior-to-superior direction (b). It is then also detached behind the orbital rim and reflected inferiorly. A suture with loose tension holds the muscle in place. In this example, a muscle cuff is left at the STL to enable better refixation by suturing the muscle to this cuff during closure.


5.2.2 Superficial Temporal Artery Preservation during Frontolateral Approaches

Andreas Raabe and Peter Vajkoczy

See ▶Fig. 5.26, ▶Fig. 5.27, ▶Fig. 5.28, ▶Fig. 5.29, ▶Fig. 5.30, ▶Fig. 5.31, ▶Fig. 5.32, ▶Fig. 5.33, ▶Fig. 5.34, and ▶Fig. 5.35.

Fig. 5.26 (a,b) Preserving the superficial temporal artery: anatomy of the skin flap. When the lower (temporal preauricular) part of the frontotemporal skin incision is reached (e.g., pterional, frontolateral, or anterior temporal approaches), the surgical technique should be adapted to preserve the superficial temporal artery (STA). The skin flap above the fascia of the temporal muscle consists of two basic layers: the skin with the subcutaneous tissue (superficial layer) and the galea layer containing connective tissue where the STA is embedded above the galeal aponeurotic fascia.
Fig. 5.27 (a, b) Subcutaneous dissection of the two skin flap layers. Using fine scissors, a horizontal dissection is done between the two layers. This creates a space allowing for separate incision of the superficial skin layer leaving the galeal layer with the STA intact.
Fig. 5.28 (a, b) Exposing the deep galeal tissue layer. The deep layer is visualized. The connective tissue containing the STA and the galeal aponeurotic fascia is visible.
Fig. 5.29 (a, b) Selective supragaleal skin incision for preserving the superficial temporary artery. Now that the two layers are separated, the superficial subcutaneous tissue layer is cut. The STA is embedded in the deep layer.
Fig. 5.30 (a, b) Exposing the deep galeal tissue layer with the STA. The deep layer containing connective tissue and the STA is visible.
Fig. 5.31 (a, b) Freeing the superficial temporal artery. The scissors are used to dissect horizontally as described in ▶Fig. 5.27 but immediately below the remaining connective tissue plane and above the STA. The artery is separated from the surrounding connective tissue.
Fig. 5.32 (a, b) Exposure of the superficial temporal artery. The remaining connective tissue plane is cut above the STA.
Fig. 5.33 (a,b) Exposure of the superficial temporal artery bifurcation and the smaller branch. The STA bifurcation can sometimes be found at the level of the incision. To finish the incision, one branch has to be coagulated and divided. Usually the smaller branch or, if the branches have the same diameter, the shorter branch is sacrificed. In this illustration, the frontal branch is smaller.
Fig. 5.34 (a, b) Exposure and coagulation of the smaller branch. The smaller branch—in this case, the frontal branch—is coagulated and cut a few millimeters away from the STA. Staying away with the bipolar coagulation from the bifurcation prevents it from shrinking during coagulation and allows for safe recoagulation of the stump of the frontal branch if bleeding occurs after cutting it.
Fig. 5.35 (a, b) Finishing galea incision. The galea can now be cut to finish the incision. Although the situations requiring an emergency bypass is exceptionally rare, preserving the STA is required for maintaining optimal skin blood flow, a neat and gentle surgical approach, and a true microsurgical mindset.


Checklist




  • Identify the deep galea layer of the skin flap.



  • Dissect the space above the galea layer.



  • Cut the superficial layer.



  • Use the same technique to free the STA from connective tissue in the deep layer.



  • Identify the smaller branch when one branch has to be sacrificed to continue skin incision.



  • Keep 3 to 5 mm away from the main STA trunk when coagulating the smaller branch.



5.2.3 Supraorbital

Nikolai Hopf and Robert Reisch

See ▶Fig. 5.36, ▶Fig. 5.37, ▶Fig. 5.38, ▶Fig. 5.39, ▶Fig. 5.40, ▶Fig. 5.41, ▶Fig. 5.42, ▶Fig. 5.43, ▶Fig. 5.44, ▶Fig. 5.45, ▶Fig. 5.46, ▶Fig. 5.47, ▶Fig. 5.48, ▶Fig. 5.49, and ▶Fig. 5.50.

Fig. 5.36 (a, b) Positioning. The supraorbital approach is used for frontobasal, subfrontal, parasellar, temporomesial, and midline lesions including the contralateral side and intrasylvian pathologies. The head is in a slightly reclined position with the eyebrow being the highest point, and the head is rotated to the contralateral side to achieve a perpendicular view toward the target. The angle of head rotation varies accordingly between 15° and 45° (30° for most cases).
Fig. 5.37 (a, b) Anatomy overview. Important landmarks are (1) the supraorbital foramen with the supraorbital nerve and artery, (2) the temporal line, (3) the frontal branch of the facial nerve, (4) the level of the frontolateral skull base, and (5) the frontal sinus.
Fig. 5.38 (a, b) Draped position before skin incision. The skin incision should be made within the eyebrow for cosmetic reasons, in particular in patients with thin eyebrows. In patients with no eyebrows, the skin incision may be made in a wrinkle. The length is typically 3 to 4 cm.
Fig. 5.39 (a, b) Skin incision. The skin incision spares the frontal and orbicularis oculi muscle fascia and is made just lateral to the supraorbital foramen extending to the lateral margin of the eyebrow.
Fig. 5.40 (a, b) Subgaleal flap preparation. The skin flap is dissected between the galea and the frontal muscle fascia only superiorly, since the craniotomy will be located entirely superior to the skin incision. The planned incision is outlined.
Fig. 5.41 (a, b) Subgaleal flap preparation. The frontal muscle is cut perpendicular to its fibers with a monopolar device, not more than 2 cm superior to the orbital rim and not extending the temporal line laterally, to avoid injury to the facial nerve. Then, the muscle flap is completed by detaching it from the temporal line, cutting in an inferior direction, and pulling it inferiorly with fish hooks or sutures. The temporal muscle is then detached from the temporal line in a very limited area.
Fig. 5.42 (a, b) Craniotomy planning. Fish hooks or sutures gently pull the detached temporal muscle laterally to keep the anterior superior area of the temporal fossa exposed. The illustration on the right side depicts the planned craniotomy. It must be flush with the floor of the anterior skull base to allow subfrontal access without retraction. The lateral extension is just lateral to the temporal line. The medial extension varies and is typically limited to the level of the supraorbital foramen. Superiorly, it is 1.5–2 cm.
Fig. 5.43 (a, b) Burr hole. The burr hole is made with a cutting or diamond drill. The burr hole is located at the cranial base; therefore, the direction of drilling should be toward the surgeon to avoid entering the orbita. The burr hole should be just big enough to accommodate the craniotome, to achieve a cosmetically acceptable result avoiding sinking of the skin.
Fig. 5.44 (a, b) Craniotomy. The craniotomy is performed from the burr hole medially and parallel to the frontal skull base. The craniotome should be angled slightly inferiorly to avoid further drilling of the tabula externa.
Fig. 5.45 (a, b) Craniotomy. Then, the craniotome is moved back to the burr hole and the craniotomy completed by a C-shaped cut starting from the lateral edge of the burr hole and ending at the medial end of the basal cut. The size of the craniotomy is typically 1.5 × 2.5 cm.
Fig. 5.46 (a, b) Additional bone drilling. After elevation of the bone flap, the inner tabula is drilled with a 4- or 5-mm diamond drill. In so doing, another few millimeters of space is gained to achieve subfrontal access flush with the anterior base to avoid retraction. When completed, the dura should be detached from the base to identify larger bony elevations of the orbital roof, so-called jugae, which may obstruct the view. If present, they should also be flattened with the drill.
Fig. 5.47 (a, b) Dura opening. The dura is opened in a C-shaped manner toward the skull base anteriorly and fixed with tacking sutures. The brain is then covered with wet cottonoids to reduce mechanical damage and avoid exposure to heat from the light source of the microscope or endoscope. The image shows subfrontal access to the anterior cranial base without retraction.
Fig. 5.48 (a, b) Closure: bone flap. After closure of the dura, the bone flap is fixed with sutures or a titanium plate as shown on the left image. For cosmesis, the bone flap should be placed superiorly and medially without any gaps. Bone cement, as shown on the image, or other material should be used to fill larger lateral and inferior gaps. Opening the frontal sinus during craniotomy is of less concern but should be avoided if possible.
Fig. 5.49 (a, b) Closure: soft tissue. The frontal muscle flap is sutured medially and superiorly and to the temporal muscle fascia. Then, the subcutaneous tissue is sutured tightly to achieve a narrow scar.
Fig. 5.50 (a, b) Closure: skin. The skin may be closed with an intracutaneous suture as shown in the left image (a). Alternatively, a monofilament running suture (b), sterile strips, or acrylic glue may be used. Surgeons should check cosmesis during follow-up of the patients to obtain feedback on the results of the technique they are using.


Checklist




  • Palpate the supraorbital foramen and temporal line and estimate the level of the frontal skull base about 1.5 cm superior to the inner edge of the orbital rim.



  • Make the incision within the eyebrow and not above it.



  • Mobilize skin only superiorly.



  • Use a small drill for the burr hole for cosmetic reasons.



  • Direct the burr toward your body when making the burr hole to avoid entering the orbita.



  • Angle the craniotome inferiorly during the basal cut of craniotomy.



  • Drill the tabula interna for better access to the base while leaving the tabula externa intact.



  • Detach the dura mater basally from the orbital roof and check for large jugae that obstruct your view along the anterior skull base.



  • Avoid superior and medial bone gaps when placing the bone flap. Cover the burr hole with temporal muscle.

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May 14, 2020 | Posted by in NEUROSURGERY | Comments Off on 5 Skull Base Craniotomies

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