Is the Asterion a Reliable Landmark for the Lateral Approach to Posterior Fossa?

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Is the asterion a reliable landmark for the lateral approach to posterior fossa?

. Aysun Uz1, Hasan Caglar Ugur2,Ibrahim Tekdemir1

Departments of 1Anatomy and 2Neurosurgery, Faculty of Medicine, The University of Ankara, SamanpazarO, Ankara, Turkey

Summary An anatomical study was conducted to gain orientation regarding the posterolateral approaches. The asterion is defined as the junction of the lambdoid, parietomastoid, and occipitomastoid sutures. This anatomical point has been widely used as a landmark in lateral approaches to posterior fossa. Although there are many common practices in posterolateral approaches, studies providing accurate anatomical knowledge as to what is the correct point to start a craniotomy are limited in number. Therefore, this study was conducted in an attempt to determine the reliability of the asterion for the posterolateral approaches as surgical landmark. © 2001 Harcourt Publishers Ltd

INTRODUCTION It is highly important for the surgeon to know where the correct localization of the initial burr hole for the lateral approach of the posterior fossa. Correct orientation for any approach begins with consideration of surface anatomical landmarks. If these landmarks are chosen properly, the correct way of gaining access to the lesion can be achieved. Hence, the first step to a successful opera- tion will have been taken. In approaches to the posterior fossa these landmarks are highly required because of their close relation to the transverse sigmoid sinus.1–3 The asterion is defined as the junction of the lambdoid, parietomastoid, and occipitomastoid sutures, and it has been advocated as a primary landmark in per- forming many kinds of posterolateral surgery. Due to its natural variability in location, its reliability can be a debatable issue. Therefore, this study was designed to determine the reliability and usefulness of the asterion as a surgical landmark for lateral approaches of the posterior fossa. Fig. 1 The anatomic position of the asterion placed on the 33-cm grid. MATERIAL AND METHODS DISCUSSION Fifty dried adult human skulls were obtained from the Department of Anatomy for this study. A 2 mm drill bit was externally placed It is an indisputable fact that the success in surgical strategy and over the asterion, and the asterion was drilled through the bone planning mainly relies on the surgeon’s concrete knowledge of the perpendicular to the skull surface. Then, the position of the drill superficial anatomy. Owing to the recent innovations in the micro- hole was determined on the inner surface. In order to record the surgical techniques and technological advances, large craniotomies position of the asterion a 33 cm grid centered over the trans- have been replaced by less invasive key-hole craniotomies. verse sinus and upper curve of the sigmoid sinus was used. The Starting the craniotomy at the right point can provide advantages position was determined to fall within 1 cm2 segments. in the approaches where the key-hole technique in craniotomy is used. Gaining the right orientation even at the initial stages of the surgery, prevention of a large craniotomy, providing shorter RESULTS surgery time, and avoiding serious complications such as sinus Asterion was located over a portion of the transvers-sigmoid sinus injuries are the advantages mentioned above.2–4 Even though lat- complex in 54% (27), above the transverse-sigmoid sinus complex eral approaches to posterior fossa are widely used, a clear concept in 2% (1), and over the posterior fossa dura in 44% (22). The of these approaches has not yet been established and enlightening results are also recorded for both the right and left side and sum- anatomical studies on the same issue remain limited. In these cur- marized in Fig. 1. rent approaches the planning phase are done through the surgeon’s personal experiences. Therefore, each surgeon determines a safe point to start the craniotomy for himself. This study aims at provi- Received 25 February 2000 ding a knowledge of the anatomical orientation for the lateral Accepted 22 March 2000 approaches to posterior fossa which are widely used in neurosurgi- Correspondence to: Professor Dr ibrahim Tekdemir, Ankara Üniversitesi TOp cal practices and contribute to the relevant data pool. FakültesO; Anatomi Anabilim DalO 06100 SamanpazarO, Ankara, Posterior fossa craniotomies can constitute a risk of injury to Türkiye (Turkey). Tel.: ;90 312 3105001; Fax: ;90 312 3106370; the sinus complex due to its location which is either in a groove in E-mail: [email protected] the bone or adherent to the bone, and attached by the emissary

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Asterion as reliable landmark for lateral approach to posterior fossa 147

vein. Positioning the initial burr hole away from the asterion is study show that asterion is not a safe landmark to be used in the expected to reduce the risk of lacerating the sinus complex to a retrosigmoid approaches, and starting the craniotomy medially negligible level by using the method advocated by Rhoton.5 and 2 cm below the asterion can reduce the risk of injury to the According to this method, the hole is positioned 2 cm below the sinus complex to a great extent. asterion, two-thirds behind and one-third in front of the occipitomastoid suture. Similarly, in a study of Day et al., asterion was REFERENCES over transverse-sigmoid sinus in 61% of the skulls.1 Sekhar sup- ports the use of a two-burr hole technique in which the first burr 1. Daj DT, Tschabitscher M. Anatomic position of the asterion. Neurosurg 1998; hole is placed superolaterally, and the second is placed inferome- 42(1): 198–199. dially.6 In his experience, the site of the first burr hole is described 2. Day DJ, Kellogg JX, Tschabitscher M, Fukushima T. Surface and superficial surgical anatomy of the posterolateral cranial base: significance for surgical as one which is usually posteroinferior to the asterion and not planning and approach. Neurosurg 1996; 38(6): 1079–1084. used as a landmark. The second burr hole is made at the most 3. Lang J, Samii A. Retrosigmoidal approach to the posterior cranial fossa. Acta inferomedial corner of the desired bone exposure. Therefore, Neurochir 1991; 111: 147–153. while the exposure of the edge of the sinus complex is achieved, 4. Tedeshi T, Rhoton AL. Lateral approaches to the petroclival region. Surg Neurol 1994; 41: 180–216. no injury is given to the dura. In our study the results were some- 5. Rhoton AL. Surface and superficial surgical anatomy of the posterolateral cranial 2 what similar to the study of Day et al. in that asterion was over base: significance for surgical planning and approach. Neurosurg 1996; 38(6): the transverse-sigmoid sinus complex in 55% of the skulls. This 1079–1084 (comment). indicates that the sinus complex can easily be lacerated in more 6. Sekhar LN. Anatomic position of the asterion. Neurosurg 1998; 42(1): 198–199 than half of the cases if the asterion is selected as a landmark in (comment). the lateral approaches to posterior fossa. Thus, the results of this

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