6 Fundamentals of Cranial Neurosurgery

6.1  Principles of Mayfeld Head

Holder Positioning (Fig. 6.1)

•Variable holding pressure is used to fx the head holder and is defned by four tension rings on the outer aspect of each single pin, which should correspond to 20 Lbs/in2 for each ring.

•Suggested holding pressure: adults 60 Lbs/in2, children 30/40 Lbs/in2.

•Pediatric holding pins have a smaller pinpoint compared to adults’ one and they should be used for children aging up to 5 years.

•Mayfeld head holder should not be used for children younger than 3 years.

•Maximum holding pressure allowed by the system: 80 Lbs/in2.

•Mayfeld head holder should not be used in case of skull fracture after head trauma.

6.1.1 Pins Positioning

•Pins should be placed away from

○The course of the skin incision.

○Pneumatized sinuses (e.g., frontal sinus, mastoid).

○Pterion and cranial sutures (considered as points of least resistance).

○Dural venous sinuses and temporal artery because of risk of vascular damage.

•The line connecting the single pin and the center

of the double pin clamp should bisect the intersection

between the main sagittal and coronal diameters of the skull (Fig. 6.2).

Fig. 6.2 Principles of Ma ld head holder positioning.

Blue lines show the sagittal and coronal planes. Red lines with dots represent the Ma ld holder with its pins.

Fig. 6.1 Principles of Ma ld head holder positioning.

Abbreviations: CS = coronal suture;

FS = frontal sinus; LS = lambdoid suture; M = mastoid; PT = pterion; TS = transverse

sinus; TSq = temporal squama; SS = sigmoid sinus; SSS = superior sagittal sinus. Colors: Blue area = dural sinuses; Purple area = areas of least resistance.

6.2  Types of Skin Incisions (Fig. 6.3)

•Linear incisions. Incision should run parallel to the direction of the principal subcutaneous arteries (i.e., temporal and occipital arteries) to preserve the regional vascular supply.

Fig. 6.3 Skin incisions.

Lines: Blue dotted lines (linear incisions); Green dotted line (C-shaped incision); Red dotted line (question mark incision); Yellow dotted line (horseshoe incision).

•Horseshoe incision. AKA U-shaped incision. The concavity of the incision trajectory must be directed downward to preserve the regional vascular supply. It reduces the mechanical tension on the skin as compared to linear incision.

•Question mark incision. It is usually performed for surgical approaches to the fronto-temporal region and skull base. It starts <1 cm in front of the tragus at the level of the zygoma, runs posteriorly around the superior margin of the ear and turns anteriorly after reaching the posterior aspect of the pinna. Incision ends on the midline just behind the hairline.

•C-shaped incision. It is usually performed for lateral approaches to the posterior cranial fossa and cerebellopontine angle. The incision starts 1 cm above the ear, at the lower temporal region, runs around the pinna toward the mastoid tip and turns downand forward until it reaches the anterior margin of the sternocleidomastoid muscle. The concavity of incision trajectory is tailored according to the approach, which has to be performed.

6.3  Extracranial Soft Tissues

Dissection (Fig. 6.4)

6.3.1  Extracranial Soft Tissues

Encountered During Superfcial

Dissection:

•Skin

•Subcutaneous fat tissue

•Galea capitis (aka galea aponeurotica)

•Loose connective tissue

•Pericranium

Fig. 6.4 Cranial soft tissues dissection. Abbreviations: FB = frontal bone; GC = galea capitis; PC = pericranium; TA = temporal artery; TF = temporal fascia; TM = temporal muscle; TN = temporal nerve.

6.3.2  Specifc Anatomical

Considerations

•The superfcial temporal artery runs anteriorly to the tragus, in the subcutaneous tissue, lying on the superfcial temporal fascia and bifurcates into its frontal and parietal branches at the temporal region, just 2 cm above the zygomatic arch.

○The corresponding vein and the fronto-temporal branch of the facial nerve run anteriorly to the artery.

○To preserve these structures, the skin incision is generally performed 0.5 to 1 cm anteriorly to the tragus.

•In the temporal region, the galea capitis divides into

○The deep temporal fascia is located beneath the temporal muscle (TM) on the bone surface. It carries the blood supply to the temporal muscle.

○The superfcial temporal fascia, which covers the entire TM, goes from the zygomatic arch to the superior temporal line. The anterior third of the fascia is composed of two layers separated by the interfascial fat pad (a sickle-shaped layer of fat); here the frontal branch of the facial nerve and the deep temporal vessels run. The two layers can be recognized at the temporal attachment on the orbital rim.

6.4  Fundamental Techniques of

Temporal Muscle Dissection

(Figs. 6.5–6.7)

•Three diferent methods of temporal muscle dissection are described: interfascial, submuscular, and sub-fascial. We remind the reader to the dedicated chapter to see the nuances of the diferent techniques and their advantages

(see Chapter 8).

6.4.1  Main Principles

○Allows to preserve the frontal branch of the facial nerve and to optimize temporal bone and zygomatic arch exposure.

○The interfascial fat pad marks the separation of the superfcial temporal fascia into its superior and inferior layers.

○The two layers are smoothly separated with a dissector starting from the pterion.

○They are then cut along their junction until their insertion at the superior margin of the zygomatic arch. The fat pad is exposed and the Yasargil vein is recognized.

○The deep temporal fascia together with the muscle are incised starting at the pterion and following the course of the superior temporal line until the zygomatic process of the temporal bone.

○The temporal muscle is subperiostally dissected from the bone, together with the superfcial and the deep fascia, starting from the pterion.

6.5  Technique of Supraorbital

Nerve Preservation (Figs. 6.8, 6.9)

•The supraorbital nerve may exit the skull through either a notch or a true foramen, which may be located at the junction between the medial and the lateral two thirds

of the upper orbital rim. The supraorbital artery runs medial to it.

•If a supraorbital foramen is present, the nerve has to be freed to avoid damage during orbital osteotomies.

•To open the supraorbital foramen a chisel is used, while protecting the nerve and the artery.

•Once freed, the nerve is then gently mobilized and refected anteriorly with the skin fap.

Fig. 6.5 Basic technique of temporal muscle interfascial dissection. Step 1. Abbreviations: CS = coronal suture; FB = frontal bone; FP = fat pad; GC = galea capitis; STL = superior temporal line; TF = temporal fascia; ZP = zygomatic process of the frontal bone.

6.6  Fundamentals of Harvesting  A Pericranial Pedicled Flap  (Figs. 6.10, 6.11)

•The length and width of the pericranial fap that one can harvest mostly depend on the skin incision used for the specifc approach being utilized. Preservation and preparation of this layer is of utmost importance as it represents vascularized tissue that can be used to repair skull base defects. Following the principle that whatever has been opened will have to be closed in the end and that you truly never have enough pericranium, the incision of the pericranium can be extended way beyond the skin incision by elevating the skin

Fig. 6.8 Supraorbital nerve ident tion. Abbreviations: FB = frontal bone; PC = pericranium; SOA =supraorbital artery; SON = supraorbital nerve; TF = temporal muscle fascia.

Fig. 6.9 Dissection of the supraorbital nerve iew.The nerve has been mobilized

from its notch in the frontal bone with the use of a dissector and under no tension. Abbreviations: FB = frontal bone; PC = pericranium; SON = supraorbital nerve; TM = temporal muscle.

and sliding below it as far as it can be done without any skin disruption.

•Pericranium is cut 2 cm posteriorly to the margin of the skin incision and along the superior temporal lines bilaterally. It is dissected from the bone, taking care to preserve its anatomical integrity.

•It is very important to use a wide periosteal elevator and a lot of irrigation to safely elevate this fap, with special attention paid to the areas covering sutures that will be inevitably strongly adherent to the bone.

•It is then refected anteriorly to maintain its blood supply, which arises from frontal vascular pedicles such as the anterior branches of the superfcial temporal, supraorbital and supratrochlear arteries.

Fig. 6.10 Dissection and elevation of the pedicled pericranial

Step 1.

Abbreviations: FB = frontal bone; LTF = left temporal muscle fascia; PC = pericranium; RTF = right temporal muscle fascia.

Fig. 6.11 Dissection and elevation of the pedicled pericranial

Step 2.

Abbreviations: FB = frontal bone; LTF = left temporal muscle fascia; PC = pericranium; RTF = right temporal muscle fascia; SON = supraorbital nerves.

Fig. 6.12 Frontal sinus mucosa removal.

Abbreviations: FD = frontal dura; FS = frontal sinus; LTM = left temporal muscle; M = mucosa; PC = pericranium, SON = supraorbital nerve.

Fig. 6.13 Removal of the posterior wall of the frontal sinus with large rongeurs.

Abbreviations: FD = frontal dura; FS = frontal sinus; LTM = left temporal muscle; PC = pericranium, SON = supraorbital nerve.

6.7  Fundamentals of Skull Base Reconstruction with Pedicled Flap (Figs. 6.12, 6.13)

•Skull base reconstruction with pedicled pericranial fap is indicated in cases of opening of pneumatized sinuses and

skull base reconstruction after skull base surgery, in order to reduce the rate of infection, cerebrospinal fuid leak, pneumocephalus and late mucocele formation.

•If the paranasal sinuses are violated during a craniotomy, the mucosa is exenterated before dural opening and the posterior wall of the frontal sinus is removed (frontal sinus cranialization).

•The ostium of the sinus is plugged with the pedicled pericranial fap, which is held in place with fbrin glue.

•All instruments used during the procedure are kept aside and discarded for the remainder of the operation to lower the risk of infections.

6.8  Burr Holes Placement: Technical

Principles (Figs. 6.14–6.16)

•Burr holes should be placed

○Behind the hairline (aesthetic reason).

Fig. 6.14 Burr holes placement.

Colors and lines: Black dotted line = superior temporal line; Blue area = dural sinuses; Pink area = suggested area for burr holes placement; Red dotted line = hair line.

Fig. 6.15 McCarty hole.

Abbreviations: FB = frontal bone; OR = orbital roof; PO = periorbit; STL = superior temporal line; TF = temporal fascia; TM = temporal muscle; TSq = temporal squama.

Fig. 6.16 Dandy hole.

Abbreviations: FB = frontal bone; (OR) = projection of orbital roof; PO = periorbit; STL = superior temporal line; TF = temporal fascia; TM = temporal muscle; TSq = temporal squama.

○Below the superior temporal line (aesthetic reason).

○Away from pneumatized areas (i.e., frontal sinus, mastoid) and dural venous sinuses to avoid opening of paranasal sinuses or vascular damage, respectively.

•Types of pterional holes

○McCarty burr hole is used to expose at the same time the anterior cranial fossa and the orbit. It is located 1 cm behind the fronto-zygomatic suture and along the fron- to-sphenoidal suture.

○Dandy burr hole is generally used for standard pterional craniotomy. It is placed just above the fronto-sphenoidal suture, below the superior temporal line and posteriorly to the fronto-zygomatic suture.

6.9  Craniotomy: Technical

Principles (Figs. 6.17, 6.18)

•General principles

○The pneumatized paranasal cavities, dural venous sinuses and the temporal squama are usually considered areas of concern when performing a craniotomy because of the potential risk of damaging vital structures such as the dural sinuses, of the increase of chances of infections or cerebrospinal (CSF) leaks, or of plunging into the temporal lobe, respectively.

○Before using the craniotome, the dura has to be carefully detached from the inner table of the skull to reduce the risk of dural tearing.

○The craniotome is directed perpendicularly to the skull surface.

○The tip of the instrument is placed between the dura mater and the bone and it is slightly tilted backward 5-10°.

•Principles of craniotomies over dural sinuses

○Craniotomies over dural sinuses should be performed with a clear sequence of burr holes and bone cuts, in order to avoid injuries to the underlying dura.

○Even this needs to follow the principle of doing all the work away from the sinus frst, so that if this would be

Fig. 6.18 Craniotomy over dural sinus. Case example.

CS = coronal suture; SSS = superior sagittal sinus. Colors: Blue area = sinus projection; Orange area = area at risk for draining veins

injured the bone fap could be turned immediately and the bleeding dealt with promptly.

Burr holes

•I: Away from the sinus.

•II: Over the sinus.

Craniotomy cuts

•I: Far from the sinus.

•II: Starting from the sinus and going away.

•III: Over the sinus.

In areas such as the posterior fossa an X-shaped incision is preferred.

•The dura should be opened away from the bone (at least 2-4 mm) to facilitate dural closure.

•The dura is frst incised with a small knife in a “safe point” (away from venous sinuses or underlying vessels, as well as neoplastic masses) and then it is gently cut with scissors, using cottonoid strips to protect the underlying neurovascular structures.

6.10  Dural Tenting Technique (Figs. 6.19, 6.20)

•Dural tenting reduces the risk of progressive detachment of the dura from the bone and the consequent collection of blood in the epidural space. It also prevents the retraction of the dura, which can make the dural closure more challenging.

•Several holes are made on the margins of the craniotomy (about 2 mm in thickness) at a distance of approximately 1 cm from each other, away from venous sinuses and air pneumatized cavities.

•A free blade of smaller diameter to the one used for raising the bone fap is utilized for placing the holes.

•Holes must be placed close to the bone margin and directed obliquely, from the outer cortical table to the spongious bone, protecting the dura with a retractor to avoid damage to the underlying structures.

•Stiches (3/0, non-absorbable) are anchored to the dura, as close as possible to the bony margin, and then passed through the holes.

6.11  Dural Opening (Fig. 6.21)

•The dura is usually opened in a C-shaped fashion, keeping the pedicle toward the cranial base or the dural sinuses.

Fig. 6.20 Stiches for dural tenting.

Abbreviations: DM = dura mater; FB = frontal bone; MMA = middle meningeal artery; PC = pericranium; TM = temporal muscle.

Fig. 6.21 Dural opening.

Abbreviations: FB = frontal bone; MMA = middle meningeal artery; TM = temporal muscle.

•The dura is then refected and gently tented, paying attention not to compress underlying dural sinuses or to stretch draining veins.

References

1.Fossett D, Caputy AJ. Operative neurosurgical anatomy. New York, NY: Thieme Medical Publisher; 2002

2.Sekhar LN, Fessler RG. Atlas of Neurosurgical Techniques. Brain. Volume 1. New York, NY: Thieme Medical Publishers; 2016

3.Snyderman CH, Janecka IP, Sekhar LN, Sen CN, Eibling DE. Anterior cranial base reconstruction: role of galeal and pericranial faps. Laryngoscope 1990;100(6):607–614

4.Yaşargil MG, Reichman MV, Kubik S. Preservation of the frontotemporal branch of the facial nerve using the interfascial temporalis fap for pterional craniotomy. Technical article. J Neurosurg 1987;67(3):463–466