Accessing the Stapedius Muscle Via Novel Surgical Retrofacial Approach: A Cadaveric Feasibility Study

Abstract

Hypothesis: Despite the complete embodiment of the stapedius muscle (SM) into the pyramidal eminence, it is possible to safely gain access to the SM belly via a retrofacial approach. This presents a novel approach to directly measure the electrically evoked stapedius reflex threshold (eSRT). Background: Objective fitting of maximum comfortable loudness levels for cochlear implant users can improve the benefit introduced by the device. Sensing SM activity via direct surgical access represents a potential tool for objective eSRT fitting. Methods: Eighteen human temporal bones (TBs) were used. Micro-computed tomography was performed for six TBs. Standard computed tomography for six TBs. Manual 3Dsegmentation of the relevant middle and inner ear anatomy was performed on 12 TBs. Mastoidectomy and posterior tympanotomy allowed the access to middle ear of all 18 the TBs. Once identified the mastoidal segment of the facial nerve (FN), the retrofacial access to the SM was drilled.Results: The total access rate was 72.2%. Only in the first three cases the posterior semi-circular canal was hit. The SM access was identified posterior to the FN at a 4 AE 0.78 mm distance from the stapes' head, almost halfway to the chorda tympani's branching point along the FN direction. The drilling depth to access the SM posterior to the external surface of FN on average was 2 AE 0.30 mm. The exposure took on average of 5 to 8 minutes. Conclusions: The retrofacial approach seems to offer a feasible and reproducible access to the SM belly opening an avenue to electromyographic sensing of eSRT.

Accessing the stapedius muscle via novel surgical retrofacial approach during cochlear implantation surgery: Intraoperative results on feasibility and safety

“…As expected, the location of the drilling access point to the SM was found medial and posterior with respect to the mastoid tract of the FN at halfway between the SM tendon level and the potential branching point of the ChT. This outcome is in line with the results we have obtained from our previous anatomical TBs study [ 18 ]. Moreover, the anatomical landmarks considered intraoperatively such as the mastoid tract of the FN, the stapedial tendon, the height/prominence of the sigmoid sinus confirm our previously described anatomical benchmarks, supporting the formulation of a reproducible surgical approach.…”

“…The high feasibility rate of the retrofacial approach to access the SM in the current data set is comparable and superior to the one achieved on human TBs [ 18 ]. This result validated the crucial importance of practical training on cadaveric samples [ 29 ].…”

“…The surgical access to the belly of the SM was drilled based on our previous results [ 18 , 20 , 21 ]. The access was performed posterior and medial to the mastoid portion of the FN, almost halfway between the stapes head level to the branching point of the chorda tympani (ChT) along the FN direction ( Fig 1 ).…”

“… A and B are the two extreme points of the segment running between the stapes’ head-tendon line (A) and the chorda tympani branching point (B), respectively. Adapted from [ 18 ]. …”

“…This procedure represents an indirect measure of the SM activity [ 15 ] and excludes cases of middle-ear malformations or conditions of compromised biomechanics involving ossicular chain, including the middle ear muscles (e.g., missing stapedius tendon) [ 16 , 17 ]. In our previous study [ 18 ] we described a safe and reproducible surgical approach to directly access the SM tissue in a temporal bone (TB) study. This surgical approach is supported by virtual 3D reconstruction of the temporal bone and allows exposure of part of the SM belly by a posterior, retrofacial access.…”