Dorello´s canal: a microanatomical study

FELlX UMANSKY, M.D., JOSEF ELIDAN, M.D., AND ALBERTO V ALA REZO, M.D.
Departments 01 Neurosurgery and Otolaryngology, Hadassah University Hospital and the Hebrew
University Medical School, Jerusalem, Israel

The microsurgical anatomy of Dorello's canal has been studied in 20 specimens obtained from 10 cadaver heads fixed in formalin. The bow-shaped canal through which courses the abducens nerve before reaching the cavernous sinos is located inside a venous confluence which occupies the space between the dural leaves of the petroclival afea. The petrosphenoidalligament (Gruber's ligament), which forros the posteromedial wall of the canal, appears as a fibrous trabecula surrounded by venous blood. Canal measurements were performed and its anatomical relationship with the sixth cranial nerve is described. Angulations of variable degrees were observed in the course of the nerve inside and outside the canal. The influence of this relatively tortuous course of the abducens nerve opon its vulnerability in some pathological conditions is discussed.

KEY WORDS . abducens nerve - Dorello's canal - petrous apex - anatomical study




THE anatomical relationships of the abducens nerve with the petrous apex in the petroclival regían are of clinical interest. The relatively fixed position of the nerve within an osteofibrous com- partment known as "Dorello's canal" makes it partic- ularly vulnerable to several pathological conditions. Head injuries with or without fractures of the base of the skull,3,!S,!9,11 space-occupying lesions with shifting of the brain stem,6.7.1S surgical trauma,l! inflammatory disease of the petrous apex {Gradenigo's syndrome),J J
infernal carotid artery (lCA) aneurysms,1,J8 and sphe- noid sinus pathologyS among others may injure the nerve causing paralysis of the external rectus muscle. Recent advances in surgical techniques to approach the skull base 1.9, 13. 17,20 have revived an interest in the study of the microsurgical anatomy of ibis complex regían.

From our series of cadaver dissections we have ob- served variations in the anatomy of Dorello's canal and in the course of the abducens nerve that might influence the vulnerability of the nerve. To our knowledge no microanatomical study of Dorello's canal has been performed previously.

Materials and Methods
Ten cada ver heads (20 specimens) fixed in formalin were dissected for ibis study. The heads were placed in a Sugita head holder, turned 45° from the side of

dissection and extended slightly to simulate the surgical position. A frontotemporal craniotomy was performed and the zygomatic arch together with part of the supe- rior and lateral orbital walls was divided and removed. With the aid of the operating microscope, the dura mater was dissected beginning at the floor of the middle fossa from posterior to anterior and from lateral to medial. The baile forming the floor of the middle cranial fossa was removed, exposing the lateral aspect of the superior orbital fissure, the foramen rotundum, the foramen ovale, and the foramen spinosum. The horizontal partían of the petrous ICA was exposed and further medial retraction of the dura mater revealed Meckel's cave and the point of entry of the ICA joto the inferior wall of the cavernous sinus. The abducens nerve was identified at the point where it crossed the lateral wall of the artery and was followed backward toward Dorello's canal. Further dissection in ibis afea disclosed the petrosphenoidal ligament, the petrous apex, the venous confluence in the upper part of the clivus, and the opening of the inferior petrosal sinus. The configuration, measurements, and contents of Dorello's canal were studied. Special attention was given to the presence of angulations in the course of the sixth cranial nerve within and in clase proximity to the canal. Thereafter, the brain was carefully removed in or- der to fully expose the skull base, and further anatomical data were recorded.


 

FIG. 1. Cada ver specimen (leJí) and artist's drawing (center), posterior view, of a left Dorello's canal after removal of the inner dural layer of the petroclival region. The canal is located inside a venous confluence joined by the posterior part of the cavernous sinus (Ion!? black arrows), the basilar sinus (BS), and the inferior petrosal sinus (open arrow). Note the angulated course of the abducens nerve (VI) and the three points of angulation and possible compression: where the nerve pierces the dura mater (short black arrow), beneath Gruber's ligament (GL), and where it reaches the intracavernous internal carotid artery (ICA). Part of the lateral insertion of Gruber's ligament has been removed to show the petrous apex (PA). PPL = the petrolingual ligament. The while arrow indicates the meningohypophyseal trunk, and the arrowhead shows the dorsal meningeal branch. Scale in millimeters. Right: Diagram showing the general outline of Dorello's canal on the skull base.

Anatomical Observations
Dorello's canal is an osteofibrous conduit located at the level of the petrous apex through which the abdu- Gens nerve courses to reach the cavity of the cavernous SífiliS. It was a well-defined space in every specimen studied and had a bow-shaped configuration. The canal is located inside a venous confluence which occupies the space between both duralleaves (outer or endosteal and inner or cerebral) of the petroclival afea. The venous channels joining ibis confluence are: the poste- rior part of the cavernous SífiliS, the inferior petrosal SífiliS, and the basilar sinus (Fig. 1). Because of ibis particular anatomical arrangement, the petrosphenoi- dal ligament appears as a fibrous trabecula immersed in venous blood. In some specimens we found the cavity of the venous confluence being crossed by several tra- beculae among which the petrosphenoidal ligament (Gruber's ligament) can always be identified by its largest size, its strength, and its silvery appearance.

Canal Boundaries
The anterolateral wall of Dorello's canal is slightly curved with a posterior concavity. It is formed by the most anterior part of the superior border of the petrous baile, the upper part of the petrosphenoidal suture, ano the lateral part of the upper clivus below the dorsum sellae and posterior clinoid process. The baile in ibis afea is covered by the endosteallayer of the dura mater. The posteromedial wall of the canal corresponds to the petrosphenoidal ligament or Gruber's ligamento This ligament extends from the spina sphenoidalis located on the superior border of the petrous apex to the lateral border of the dorsum sellae and clivus (Fig. 1). The ligament was found to have a butterfly shape, being

wider at its medial (mean :t standard deviation: 4.58 :t 2.2 mili) and lateral (4.38 :t 1.8 mili) insertions and narrow at its midpoint (2.75 :t 1.3 mili). Its mean length was 12.27 :t 2.2 mili. The ligament was dupli- cated in one case (Fig. 2), ossified in another (Fig. 3), and hypoplastic in a third.

Canal Measurements In ibis study the diameter of Dorello's canal varied from 0.5 to 3.0 mm (mean 1.5 :t 0.8 mili) and its length from 4.0 to 13 mm (mean 9.22 :t 2.2 mili).

Canal Contents
The canal, filled with the venous blood of the previ- ously described venous confluence, contained the ab- ducens nerve in 100% of the specimens and the dorsal meningeal branch of the meningohypophyseal trunk in 80% of the cases (Figs. 1 and 3). This vessel contributes to the vascularization of the nerve. In three specimens the dorsal meningeal artery divided into two branches: one coursing beneath and the other above Gruber's ligament. The ostium of the inferior petrosal sinus was found within the venous confluence, either lateral (80%) or medial (20%) to the sixth cranial nerve. It was located below Gruber's ligament and outside Dorello's canal in all cases (Fig. 1).
The most frequent location of the abducens nerve inside Dorello's canal was in its middle third (in 52% of specimens, Fig. 2). In 39% of Gases the nerve was located in the outer third, and in 9% in the inner third of the canal.
In three specimens there was a duplication of the sixth nerve in its course through the canal (Fig. 4). Inside Dorello's canal the abducens nerve was tightly attached to the endosteal dura of the petrous apex and to Gruber's ligament by connective tissue intimately related to the dural sheath surrounding the nerve in ibis reglOn.
 

FIG. 2. Cadaver specimen (lelí) and artist's drawing (right), posterior view, of a right Dorello's canal showing duplication of Gruber's ligament. The main ligament (GL) with its butterfly shape can be seen to the left and a second smaller fibrous band to the right. The abducens nerve (VI) in this specimen is coursing through the middle third of the canal. Scale in millimeters.

Angulations of variable degrees were observed in the course of the nerve inside and outside the canal. The first change in direction occurs where the nerve pierces the dura mater to enter the extradural or "interdural space." This angle is usually obtuse and opens medially and upward (Fig. 1). A second and more significant angulation is present at the level of the petrous apex. Here, the nerve may change its direction rather abruptIy, describing an angle clase to 90° directed down- ward and laterally (Fig. 4). The third and last change in direction is found where the nerve lea ves Dorello's canal to reach the ICA at the level of the inferior wall of the cavernous sinus. This angulation depends on the po si- tion and diameter of the artery and usually less is pro- nounced (Fig. 5).

Anatomical Variations Changes in the canal configuration were due mainly
to anatomical variations of Gruber's ligament. In one

case the ligament was almost completely ossified and the sixth nerve was coursing through a bony canal (Fig. 3). In another specimen, the ligament was hypoplastic, thin, and transparent. In yet another case, the ligament was duplicated (Fig. 2).

Discussion
In 1859, Gruber,12 described the presence of an os- teofibrous canal at the apex of the petrous baile which he called the "foramen petro-sphenoideum." This "fo- ramen" measured 6 to 12 mm in length and 1 to 3 mm in width and contained the abducens nerve and the inferior petrosal sinus. In most of bis specimens, the posterior wall of the foramen was formed by the "liga- mentum petro-sphenoideum or spheno-petrosum pos- terius," but in 1 % to 2% of the examined skulls the ligament was ossified, a feature he found normal in some species of apes (Simia satyrus).
Dorello1o in 1905, stimulated by Gradenigo's work on abducens nerve palsy in cases of inflammatory le- sions of the petrous apex, 11 studied the anatomy of ibis regiDo. He suggested that the most probable mechanism of the nerve palsy in those cases was compression of the sixth nerve by the petrosphenoid ligament of Gruber. The first description in the English literature of Oorello's anatomical findings was by VaiF4 in 1922, who added the results of his own studies performed in eight specimens, induding one fetus of 8 months. He found that inside the canal the abducens nerve was in a lateral position, the meningeal artery was in a medial position, and the inferior petrosal sinus usually overlay the nerve.

Since these early anatomical descriptions, the osteo- fibrous canal, now called "Oorello's canal," has become a reference point and is frequently cited to describe the anatomical bases of sixth cranial nerve lesions in dif- ferent pathological conditions.31425 Several mechanisms have been implicated to explain the vulnerability of the abducens nerve: its long intracranial course, ]6.26 strangulation by transverse branches of the basilar ar- tery,7 pressure against the sharp upper border of the petrous apex,25 and compression by Gruber's lig- amentlOl9 among others. Arias,' described two points of dural fixation in the course of the sixth nerve: one at its entrance and one at its exit from the extradural space. He agreed with the mechanism of injury pro- posed by Takagi, el al.,2' in which the rigid dural tole located at the point of entrance of the nerve into the extradural space acts as a fulcrum, so that the nerve is injured against the ridge ofthe petrous pyramid in cases of asevere midfrontal impacto
In our anatomical dissections we found that the main point of fixation of the abducens nerve is inside Dor- ello's canal. Here, dense adhesions exist between the dural sheath of the nerve, the endosteal dura of the petrous apex, and the petrosphenoidal ligament (Gru- ber's ligament). The angulation of the sixth cranial nerve on its way toward the superior orbital fissure and its fixation inside the relatively narrow Dorello's canal may result in compression and stretching injuries of the nerve in cases of brain-stem shifting due to expand- ing lesions and intracranial hypertension. This might damage the nerve fibers either directly or by interference with their blood supply.
 

FIG. 3. Cadaver specimen (lefi) and artist's drawing (right), posterior view, of a left Dorello's canal showing a partially ossified Gruber's ligament (GL) with the abducens nerve (VI) and the dorsomeningeal artery (arrows) passing beneath the bony ligament. ICA = internal carotid artery. Scale in millimeters.

FIG. 4. Cada ver specimen (le/O and artist's drawing (right), superior view, of a right Dorello's canal showing the duplication of the abducens nerve (VI) and its acute angula- tion at the apex of the petrous bone (PA). The arrow indicates Gruber's ligament; ICA = internal carotid artery; PLL = petrolingualligament. Scale in millimeters.

Some contradictory descriptions can be found in the literature regarding the boundaries of Dorello's canal. The classic works defined the canal as the small space located between the petrous apex and Gruber's liga- ment.IOl2 ather authors have described it as a larger space located between the two duralleaves and extend- ing from the point where the abducens nerve pierces the dura mater to its entrance in the cavernous sinus.4.x We believe that the difficulty in defining the canal in terms of its anatomicallimits lies in the original descrip- tions by Gruber and Dorello, who included the inferior petrosal sinus inside the canal. In our study, we found that the sinus opened in clase proximity to but never within the osteofibrous channel itself. Thus, we agree with the classic description of the canal boundaries but emphasize the fact that it is located inside a large dural space occupied by a venous confluence into which the inferior petrosal sinus opens and in which Gruber's ligament represents a large fibrous trabecula.

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Manuscript received August 28, 1990.
Accepted in final form January 7,1991.
Address reprint requests lo: Felix Umansky, M.O., De- partment of Neurosurgery, Hadassah University Hospital, P.O. Box 12000. Jerusalem 91120, Israel.

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