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Azab et al. Mini-invasive Surg 2020;4:88 I http://dx.doi.org/10.20517/2574-1225.2020.75 Page 5 of 11
[32]
damage, and was also associated with better cosmetic results . Many decades after these pioneering works,
Axel Perneczky popularized the supraorbital keyhole approach through an eyebrow incision and solidly
demonstrated the importance of endoscopic assistance in this approach through many published large
series of vascular and tumor cases [18,19,29,33] .

Compared to classic transcranial approaches, the supraorbital keyhole approach minimizes brain
retraction, tissue dissection and length of the skin incision. Temporalis muscle dissection is also very
limited so that temporalis muscle atrophy and the consequent mandibular pain and chewing problems
are almost nonexistent. Needless to say, the approach yields much better cosmetic results than classic
approaches [18,33,34] . Much more than what is expected from a mini-craniotomy, the supraorbital eyebrow
exposure offers a larger field of view with increasing the distance from the craniotomy as is the case with
any keyhole approach [29,34,35] . Especially with endoscopic assistance or control, the approach truly provides
an excellent view of anterior skull base meningiomas. Additionally, optic nerve decompression is possible
[36]
under endoscopic view when angled scopes are used . Recently, the pure endoscopic (endoscope-
controlled) supraorbital keyhole approach has been used in place of the endoscope-assisted method with
promising results [36,37] .

ROLE OF THE RIGID ENDOSCOPE IN THE SUPRAORBITAL KEYHOLE EYEBROW APPROACH
The idea of endoscopic assistance in cranial surgery emerged out of the need to operate via a small opening
and yet obtain an appropriate visualization and control of the structures within the field, that is, to perform
a minimally invasive yet maximally effective surgery. A closer look at the early beginnings of adopting
this surgical philosophy sheds the light on how rigid endoscopes were capable of fulfilling this goal by
surmounting the hurdle of suboptimal visualization when a small exposure is used.

In 1974, Werner Prott - an otosurgeon from the University of Würzburg- used a rigid endoscope to
explore and operate within the cerebellopontine angle. At that time, he preferred a transpyramidal
retrolabyrinthine approach via Trautmann’s triangle. A bone flap of 1 cm diameter was made after
performing a mastoidectomy and the endoscope was inserted through this narrow space confined between
the sigmoid sinus, the superior petrosal sinus, the posterior semicircular canal and the endolymphatic sac
[38]
without damaging any functional structure of the inner ear or of the cerebellum . The same approach
was used by Falk Oppel in 1981 for sectioning the sensory root of the trigeminal nerve, glossopharyngeal
nerve, and cranial part of the vagus to treat an intractable facial pain in one patient with recurrent deeply
seated carcinoma of the upper jaw. The patient’s general condition did not permit surgical exploration of
[39]
the posterior fossa and a “minor” procedure was therefore necessary .
Indeed, one of the greatest advantages of a rigid endoscope is that it brings light inside the surgical field, a
feature that results in a very highly illuminated area of interest. Moreover, the close proximity of the light
source to the structures being viewed eliminates shadows within the field, adding to the extreme clarity of
the viewed images. Such superiority of the endoscopic view is also brought about by the high color fidelity
and image definition capabilities of today’s state-of-the-art rigid endoscopes. Notably, one of the very
important optical properties of the rigid endoscope is the greater depth of focus, which simply means that
the viewed objects remain in focus within a greater range of distances from the viewing lens. This means
lesser need to adjust the focus of the endoscope during the procedure, a feature that results in a seamless
operative workflow. The use of angled scopes also enables “looking around the corners”, and thereby adds
further to the efficacy and safety of the procedure as it brings concealed tumor remnants into view and
obviates the need for retraction of neurovascular structures.

On the contrary, the microscope in keyhole surgery requires frequent changing of the viewing angle
[40]
to allow illumination and visualization of the area of interest deep in the surgical field , an inevitable