6 Fundamentals of Cranial Neurosurgery • Variable holding pressure is used to fix the head holder and is defined 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. • Mayfield head holder should not be used for children younger than 3 years. • Maximum holding pressure allowed by the system: 80 Lbs/in2. • Mayfield head holder should not be used in case of skull fracture after head trauma. • 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. 6.1 Principles of Mayfield head holder positioning. Fig. 6.2 Principles of Mayfield head holder positioning. Blue lines show the sagittal and coronal planes. Red lines with dots represent the Mayfield holder with its pins. • 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. • 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 down- and 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. • Skin • Subcutaneous fat tissue • Galea capitis (aka galea aponeurotica) • Loose connective tissue • Pericranium Fig. 6.4 Cranial soft tissues dissection. • The superficial temporal artery runs anteriorly to the tragus, in the subcutaneous tissue, lying on the superficial 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 superficial 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. • Three different 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 different techniques and their advantages (see Chapter 8). ◦ 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 superficial 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 superficial and the deep fascia, starting from the pterion. • 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 reflected anteriorly with the skin flap.
6.1 Principles of Mayfield Head Holder Positioning (Fig. 6.1)
6.1.1 Pins 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)
Lines: Blue dotted lines (linear incisions); Green dotted line (C-shaped incision); Red dotted line (question mark incision); Yellow dotted line (horseshoe incision).
6.3 Extracranial Soft Tissues Dissection (Fig. 6.4)
6.3.1 Extracranial Soft Tissues Encountered During Superficial 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 Specific Anatomical Considerations
6.4 Fundamental Techniques of Temporal Muscle Dissection (Figs. 6.5–6.7)
6.4.1 Main Principles
6.5 Technique of Supraorbital Nerve Preservation (Figs. 6.8, 6.9)
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