A canalplasty is usually performed to widen a narrowed external auditory canal either due to congenital / acquired causes. The reasons for performing this procedure are as follows:
1. To improve access to middle ear and mastoid cavities during mastoid surgeries
2. To remove bony / soft tissue growths / scar tissue occluding the external canal
3. To treat aural atresia
The adult external auditory canal is about 2.5 cms long and is composed of lateral cartilaginous (1/3) and medial bony (2/3) portions. The medial bony portion of the external canal consists of the tympanic bone which is a ringed lateral projection of temporal bone. There is a notch in the superior portion of the tympanic bone known as the notch of Rivinus which is located at the junction of tympanosquamous and tympanomastoid suture lines.
Sensory innervation of external auditory canal:
1. Auriculotemporal nerve (from the mandibular branch of the trigeminal nerve) provides sensory innervation to anterior, posterior walls and the roof of external canal.
2. The posterior wall and floor of the canal is supplied by the auricular branch of vagus (Arnold nerve)
3. The tympnaic plexus also supplies some areas
1. Posterior auricular artery
2. Deep auricular branch of the maxillary artery
3. Superficial temporal artery
Important anatomic relations that should be borne in mind during surgery:
Anterior to the bony portion of external auditory canal lie the temporomandibular joint and the parotid gland. During canalplasty care should be taken not to injure these structures. Posterior and inferior to the bony external canal lies the mastoid portion of the temporal bone and it contains the facial nerve. Facial nerve courses usually lateral to the annulus in the posteroinferior quadrant of the tympanic membrane.
Function of external canal:
1. It serves as efficient conduit for transmission of sound from the environment to the ear drum
2. Protects the middle ear and inner ear from environmental insults
1. Hearing loss due to the presence of osteoma
2. To improve self cleansing mechanism of external canal in the presence of exostosis
3. To improve visualisation of ear drum while performing tympanoplasty
1. Presence of acute infections in the external auditory canal
If otitis externa is present then the patient should be treated for the same by administration of topical antibiotic ear drops. A combination of antibiotic and steroid ear drops would actually help.
This surgery is ideally performed under general anesthesia. In congenital external canal atresia facial nerve monitoring is used and hence long acting paralytics should not be used. Xylocaine 1% mixed with 1 in 100,000 adrenaline is used to infiltrate the external canal. Infiltration is usually given in the cartilaginous, hair bearing portion of the external canal. This is done to reduce bleeding during the procedure.
The patient is ideally positioned supine on the Operating table with the head turned away from the surgeon. The table is turned 180 degrees away from the anesthesiology team to allow proper positioning of the microscope.
The following approaches are possible:
2. Post aural
Typically a postaural approach combined with endaural incision is used to remove exostosis and medial canal fibrosis. Endaural / endomeatal incision may be preferred for osteoma as they often have a stalk that facilitates easy removal.
Endaural incision is made in the external canal as far medial as possible. A laterally based vascular strip is developed in the external auditory canal skin. After completion of this step the post aural incision is given. It is usually given 7 mm behind the post aural sulcus. The incision is continued through the auricularis posterior muscle down to temporalis fascia. Periosteum over the mastoid is incised and elevated anteriorly to the external canal. The endaural incision is found from the post aural approach, and the two incisions are joined. The external auditory canal skin is carefully elevated off the bony external canal and then retracted forward with the auricle.
In external canal exosotis, the skin over the exosotosis is elevated with a round knife and elevated toward the ear drum. The exosotosis is drilled down using a cutting / diamond burrs in a lateral to medial direction. Curettes can also be used to dissect bony edges. Canalplasty for acquired external canal stenosis needs drilling of the anterior bony canal. When drill is used care should be taken to avoid contact with the ossicular chain as it could cause conductive hearing loss.
While drilling anteriorly care should be takne to avoid penetration into the TM joint. This can be prevented by drilling away bone superior and inferior to the temporomandibular joint first, before carefully removing the buttress of bone overlying the joint. After canaplasty the skin flap is repositioned and the wound is closed in layers. Ideally a stent may be placed to assist adherence of the external canal skin to the external canal.