LABORATORY INVESTIGATION J Neurosurg 131:1561–1570, 2019 Key anatomical landmarks for middle fossa surgery: a surgical anatomy study Maria Peris-Celda, MD, PhD,1 Avital Perry, MD,1 Lucas P. Carlstrom, MD, PhD,1 Christopher S. Graffeo, MD,1 Colin L. W. Driscoll, MD,1,2 and Michael J. Link, MD1,2 Departments of 1Neurologic Surgery and 2Otolaryngology–Head and Neck Surgery, Mayo Clinic, Rochester, Minnesota OBJECTIVE Middle fossa surgery is challenging, and reliable surgical landmarks are essential to perform accurate and safe surgery. Although many descriptions of the middle fossa components have been published, a clinically practical description of this very complex anatomical region is lacking. Small structure arrangements in this area are often not well visualized or accurately demarcated with neuronavigation systems. The objective is to describe a “roadmap” of key surgi- cal reference points and landmarks during middle fossa surgery to help the surgeon predict where critical structures will be located. METHODS The authors studied 40 dry skulls (80 sides) obtained from the anatomical board at their institution. Mea- surements of anatomical structures in the middle fossa were made with a digital caliper and a protractor, taking as refer- ence the middle point of the external auditory canal (MEAC). The results were statistically analyzed. RESULTS The petrous part of the temporal bone was found at a mean of 16 mm anterior and 24 mm posterior to the MEAC. In 87% and 99% of the sides, the foramen ovale and foramen spinosum, respectively, were encountered deep to the zygomatic root. The posterior aspect of the greater superficial petrosal nerve (GSPN) groove was a mean of 6 mm anterior and 25 mm medial to the MEAC, nearly parallel to the petrous ridge. The main axis of the IAC projected to the root of the zygoma in all cases. The internal auditory canal (IAC) porus was found 5.5 mm lateral and 4.5 mm deep to the lateral aspect of the trigeminal impression along the petrous ridge (mean measurement values). A projection from this point to the middle aspect of the root of the zygoma, being posterior to the GSPN groove, could estimate the orienta- tion of the IAC. CONCLUSIONS In middle fossa approaches, the external acoustic canal is a reliable reference before skin incision, whereas the zygomatic root becomes important after the skin incision. Deep structures can be related to these 2 ana- tomical structures. An easy method to predict the location of the IAC in surgery is described. Careful study of the preop- erative imaging is essential to adapt this knowledge to the individual anatomy of the patient. https://thejns.org/doi/abs/10.3171/2018.5.JNS1841 KEYWORDS arcuate eminence; greater petrosal nerve; internal auditory canal; middle fossa surgery; surgical anatomy HE middle cranial fossa encompasses one of the this region is lacking. The objective of this study is to de- most intriguing and complicated anatomical areas scribe a clear “roadmap” of surgical anatomical structures in skull base surgery. The middle fossa comprises in the middle fossa in order to aid the skull base surgeon Tthe area between the sphenoid ridge anteriorly and the precisely and accurately, particularly in identifying the in- petrous ridge posteriorly. Its concave shape is designed to ternal auditory canal (IAC). cradle the temporal lobe. Numerous neurovascular struc- tures pass through the middle fossa, and the anatomy is further complicated by the structures contained within Methods the temporal bone. Several anatomical studies have been Forty dry skulls (80 sides) from the anatomical board at performed in this region from the classical and surgical our institution were used for measurements. The study was standpoint.9,18,24,26 However, a clearly understandable study previously approved by the biospecimens subcommittee of of practical, anatomical references to guide the surgeon in our institutional review board. One side was excluded from ABBREVIATIONS AE = arcuate eminence; EAC = external auditory canal; FO = foramen ovale; FS = foramen spinosum; GSPN = greater superficial petrosal nerve; IAC = internal auditory canal; MEAC = middle point of the EAC. SUBMITTED January 6, 2018. ACCEPTED May 29, 2018. INCLUDE WHEN CITING Published online November 9, 2018; DOI: 10.3171/2018.5.JNS1841. ©AANS 2019, except where prohibited by US copyright law J Neurosurg Volume 131 • November 2019 1561 Unauthenticated | Downloaded 10/06/21 08:14 AM UTC Peris-Celda et al. measurements from the external auditory canal (EAC) TABLE 1. Measurements opening because of ear canal malformation. Measure- Measurement Mean (range) ments were taken with a digital caliper and a protractor. Measurements from the middle point of the EAC (MEAC) A* [A] 36.6 (27.1–43.9) opening to osseous references in the skulls were recorded. B† [B] 24 (11.9–38.1) Additionally, the lateral projections in the skull of the pe- C‡ [C] 39.6 (29 – 47.8) trous apex, posterior aspect of the greater superficial pe- D§ [D] 23.5 (18.9–27.1) trosal nerve (GSPN) groove, middle point of the foramen PPP–MEAC 24 (16.9–32.5) ovale (FO), middle point of the foramen spinosum (FS), APP–MEAC 15.8 (7.2–23.5) and middle point of the arcuate eminence (AE) were stud- ied in relationship with the MEAC. The distance between AE–MEAC 6.6 (2.4–11.6) the lateral aspect of the middle fossa (lateral aspect of the GSPN–MEAC [1] 6.3 (1.6–14) temporal bone squama) and these anatomical references FS–MEAC [2] 14.7 (5.8–20.9) was measured (Table 1, Fig. 1). The relationship between FO–MEAC [3] 20.6 (9.6–29) the EAC and the IAC, the angles between the IAC and the TRIG–IAC [4] 5.5 (2.2–10.6) GPSN, and the orientation of the main axis of the IAC in LAT–FO [5] 30.1 (24.4–39.8) relationship to the zygomatic root were studied. A straight 1-mm metallic probe was used to aid with the projections LAT–FS [6] 25.6 (17.8–33.1) and angle measurements involving the IAC and the EAC. LAT–GSPN [7] 25.4 (14.7–33.1) The IAC opening in the posterior fossa in relationship to LAT–AE [8] 20.2 (15–29) the trigeminal impression in the petrous ridge was stud- VERTICAL IAC [9] 4.5 (2.9–6.9) ied. Transillumination techniques were utilized to study LAT–IAC 34.5 (26.6–42.2) the tegmen tympani area. Two formalin-fixed specimens IAC-GSPN MIN [m] 39° (20°–60°) were used to illustrate the anatomy and the technique to IAC-GSPN MAX [M] 78.6° (45°–90°) find the IAC in the middle fossa. Descriptive statistics of the anatomical variables (mean and range) are reported. AZR–FO 2.9 (−6 to 12.4)¶ AZR–FS 8.8 (−1.3 to 16.2)¶ Results AZR–GSPN 17.3 (4.9–24.1) AZR–APP 7.7 (−2.2 to 15.9)¶ The middle fossa, between the anterior and posterior cranial fossae, is a concave structure formed by a medial AE = arcuate eminence; APP = anterior petrosal point; AZR = anterior point part and paired lateral parts. The medial part is centered of the zygomatic root; FO = foramen ovale; FS = foramen spinosum; GSPN = on the sella turcica. The lateral part has an anterior wall posterior aspect of the greater petrosal nerve groove; IAC = internal auditory canal; IAC-GSPN MAX = maximum angle between the IAC and the GSPN; and a floor. The anterior wall is formed by the sphenoid IAC-GSPN MIN = minimum angle between the IAC and the GSPN; LAT = ridge superiorly, which belongs to the lesser sphenoid lateral aspect of the craniotomy; PPP = posterior petrosal point; TRIG = lateral wing, and the anterior aspect of the greater sphenoid wing aspect of the trigeminal impression; VERTICAL IAC = vertical distance or inferiorly. The floor is an almost flat surface tilted ante- depth from the petrous ridge to the IAC. riorly and formed by part of the greater sphenoid wing, Values are in millimeters except for IAC-GSPN MIN and IAC-GSPN MAX, a portion of the temporal bone squama, and the superior which are in degrees, as indicated. The numbers and letters in brackets (e.g., aspect of the petrous part of the temporal bone. The pos- [A]) correspond to the references in green in Fig. 1. * Distance from the MEAC to the anterior aspect of the squamous suture. terior limit of the middle fossa is the petrous ridge, where † Distance from the MEAC to the projection of the posterior aspect of the the petrous temporal bone continues inferiorly at an ap- squamous suture. proximately 90° angle and forms the lateral border of the ‡ Distance from the superior aspect of the EAC to the superior aspect of the posterior fossa (Figs. 1 and 2). squamous suture. The zygomatic root forms part of the temporal bone § Distance from the MEAC to the anterior aspect of the zygomatic root. and is the posterior attachment of the zygoma. It is located ¶ Negative numbers mean that the structure was found anterior to the anterior immediately anterior to the EAC opening and has an ir- point of the zygomatic root. regular triangular shape seen from above, with its wide base attached to the squamous temporal bone. In all skulls studied, we found that if we drew a line between the IAC the petrous ridge, which has 3 important implications: it opening and the EAC opening, both openings could be is the limit between the middle and posterior fossa, the found in the same coronal plane. This means that if we location of the superior petrosal sinus, and the attachment could look into the EAC laterally through the tissue, we of the tentorium.