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Journal of the Anatomical Society of India

A Review of Otosclerosis With Special Reference to Measurements of Bony Part of External Auditory Canal for Corrective Surgery

Author(s): S.Nandi, S.Palit, A. Ray, P. Mukherjee, A. Tapadar, N. Dutta

Vol. 56, No. 1 (2007-01 - 2007-06)

(1)NRSMC, Kolkata ,IPGME&R, Kolkata, (2)MMC, Paschim Midnapur, W.B.


The present study was conducted among 20 cases of otosclerosis in the Institute Of Post Graduate Medical Education and Research(IPGMER), Kolkata and the Vivekananda Institute of Medical Sciences(VIMS), Ramkrishna Mission Seva Pratisthan, Kolkata An elaborate history of all patients was taken and clinical examination was done to confirm the diagnosis of otosclerosis Majority of patients in this study belonged to the age group 31-40 yrs, male:female ratio was found to be 3:7Most of them had undergone stapedectomy under operating microscope where posterosuperior bony overhang of external auditory canal was measured before and after curettage80% of patients in the study had bilateral disease of which 11 were female70% of patients had curved external auditory canal near the annulus and only 30% had straight type Moderate to severe conductive deafness was noted in pure tone audiometry ; impedance audiometry shows normal A type curve in most cases After elevation of tympanomeatal flap only lenticular process of incudostapedial joint was visible in most cases The aims and objectives of this study is to evaluate the degree of overhang of posterosuperior bony meatal wall which needs to be measured for proper exposure of oval window and long process of incus for stapes surgery Except for rare instances, bony overhang of external auditory canal was much more in the straight variety than in curved type Curvature of posterosuperior bony meatal wall near annulus was also noted to see if any preoperative clinical assessment can be made regarding bony overhang.

Key Words: Otosclerosis, Bony Overhang Of Postero Superior Canal Wall


Otosclerosis is a hereditary localised disease of derived from the otic capsule, characterised by alternating phases of bone resorption and formation. Here mature lamellar bone (around the oval and round windows) is removed by osteoclasts and replaced by new bone of greater thickness. This leads to ankylosis of footplate of stapes and conductive deafness,tinnitus,vertigo. If parts of labyrinthine capsule are involved, sensorineural deafness and vestibular anomalies would also result. The disease is commonly found in Caucasian races and is a frequent cause of hearing loss in Europe, the Balkans, the Middle East and in the Indian subcontinent 85% of lesions of otosclerosis are situated in the oval window area,in between anterior part of stapedial footplate,processus cochleariformis and the bulge of promontory. 70-80% of cases showed both temporal bones affected and there is striking similarity of location and extent of lesion in both ears.

The surgery of choice is stapedotomy / stapedectomy,where part of footplate of stapes is removed and replaced with prosthesis. Adequate exposure of incudostapedial complex is mandatory, failing which, there can be a lot of complications, even_permanent loss of hearing (Paparella et al,1986) Smith and Roberson(1994) observed that amount of bony external auditory canal overhang varies markedly in the posterosuperior quadrant. The removal of bony overhang should be sufficient enough to provide wide exposure of the long process of incus ,the stapes and the pyramid as far posteriorly as the base(Smyth,1997) Glascock and Shambaugh(1990) observed that 2-4mm of posterosuperior bony meatal wall must be removed with the angled middle ear curette to gain adequate exposure of the oval window and the stapes. The aim of the study is to evaluate the amount or degree of overhang of posterosuperior bony meatal wall which is required to be removed for proper exposure of oval window in stapes surgery. Curvature of the posterosuperior bony meatal wall near annulus was also noted to see if any pre operative clinical assessment can be made regarding bony overhang.

Materials and methods:

  1. Place of study:
    • Institute of Post Graduate Medical Education And Research,Kolkata (IPGME&R)
    • Vivekananda Institute of Medical Sciences(VIMS), Ramkrishna Mission Seva Pratisthan, Kolkata
  2. Patient Selection:
    • 20 Indian patients were selected from all patients that visited ENT department, VIMS
  3. Diagnostic criteria for otosclerosis:
    • Deafness(unilateral/bilateral)
    • Vertigo,tinnitus(occasional)
    • Normal tympanic membrane and external auditory canal
    • Rinne’s test negative always, indicating conductive deafness
    • Weber’s test-central or lateralized to deafer ear
  4. Investigations done;
    • Complete hemogram
    • Blood for Sugar /Urea/Creatinine
    • X-ray of paranasal air sinuses
    • Pure tone audiometry- showing conductive deafness with occasional mild mixed component
    • Impedance audiometry-showing A/As curvesAcoustic reflex absent
  5. Preoperative counseling was done in all cases to explain the advantages and disadvantages of surgeryIn bilateral deafness,the deafer ear was selected;the average air-bone gap was 4050 dB in pure tone audiometry
  6. Stapedotomy/Stapedectomy was done under local anesthesiaAfter elevation of tympanomeatal flap,visibility of long process of incus as well as point of crossing of long process and posterior bony meatal wall was noted by means of a piece of gel foamBy using House’s curette or diamond burrs with microdrill,the bony overhang of the posterior meatal wall at level of tympanic sulcus was removedRight angled picks of different dimensions were used to measure the distance between the postero-superior bony canal wall at the level of the base of the pyramid after curetting and the previously noted point of crossing of the meatal wall with the long process of incus before curettingCurettage ensures wide exposure of long process of incus ,stapes and pyramid as far posteriorly as its base to facilitate stapedectomy


Incidence of otosclerosis is maximally found in patients of 31-40 years (8 patients, 40%) at the time of surgery, the next highest number were in the age group of 41-50 years (7 patients, 35% )Regarding the the age of onset of otosclerosis it was recorded that maximum patients (55%) began to complain of deafness between 31-40 years of age, the next highest percentage being in the age group of 11-20 years Occurrence of deafness was never found before 10 years of age But previous study reports stated that onset of deafness was maximum between 20 and 30 years of age which is slightly different from the present study

Regarding the incidence of sex predominance in patients with otosclerosis, the study shows male: female ratio is 3:7, which is marginally higher than previously reported data (2:1) Some authors reported bilateral otosclerosis being more frequent in women which corroborates with the present study (11 out of 16 patients with bilateral otosclerosis were females) Previous studies showed asymmetrical deafness was more common in men, but here it was seen that only 4 out of 14 patients with asymmetrical deafness were males

The study shows 80% of patients with bilateral otosclerosis with hearing loss and unilateral otosclerosis only in 20% patients According to Smyth (1997) in Scott Brown’s Otolaryngology a similar kind of report exists with bilateral hearing loss in 85% patients and unilateral cases were only 15%

Pure tone audiometry shows equal number of patients having moderate (35-45dB air-bone gap) and severe (>45dB air-bone gap) conductive deafness at the time of surgeryOnly 10% of operated ears also have sensorineural component with conductive deafness which in early phases is not a contraindication for surgery

Impedance audiometry shows 60% of patients had type A curve and 25% patients had type As curve in operated ears This indicates that majority of cases of stapedial otosclerosis have normal to slightly reduced compliance of tympanic membrane and normal middle ear pressure; the findings closely tally with those of Hyde (1987)

According to Gray’s anatomy (39th edition, 2005) external acoustic meatus has two structurally different parts, the lateral third is cartilaginous and the medial two- third is osseous It forms an S-shaped curve, directed at first medially, anteriorly and slightly up(pars externa),then posteromedially and up(pars media) and lastly anteromedially and slightly down(pars interna)It is oval in section, its greatest diameter is obliquely inclined posteroinferiorly at the external orifice, but it is nearly horizontal at the medial end The present study shows the incidence of curved pars interna in 70% patients whereas only 25% patients had straight ones

The study shows that in 9 patients (45%) only the lenticular process or incudo-stapedial joint is visible, part of the long process is visible in 5 patients, whole of the long process is visible in 2 patients (10%) and nothing of the long process is visible in 4 patients (20%) The findings closely tally with those of previous authors like Smith and Roberson (1994) who stated that the amount of bony external canal overhang varies markedly and Paparella et al (1986) who opined that usually lenticular process of the incus and a small part of stapes can be seen after elevation of tympano meatal flap

According to the observation of Smyth (1997) the entire extent of the posterosuperior bony overhang was drilled out to expose the whole of the long process of incus; stapes and the pyramid; as far posteriorly as its base In majority of cases (6 patients; 30%) the amount of postero superior bony overhang was found to be 25mm, followed by 225mm (5patients 25%) The maximum overhang found was 3mm (2 patient; 10%); and no overhang was found in 1case (5%) According to the findings of Glascock and Shaumbaugh(1990) the amount of bony overhang was between 2-4mm which corroborates with the present study Also according to O’Connor (1998) the preparation of surgical field continues by curetting away the posterosuperior bony annulus, the amount may vary but bony ponticulus should always be in position.

The study shows that the average amount of overhang in patient with curved pars interna is 23mm and it is nearly 225mm with straight external canal; which indicates that the difference in measurement is marginally greater in curved canals; than straight ones But only 1 patient (out of 6) with straight canal and no bony overhang makes the average measurement (for 5 patients) 27mm which is greater than the average measurement in curved ones The clinical experience closely tallies with these findings but no such reference was available from relevant literature.

Fig.1 Exposure of the oval window complex showing a small part of long process of in and inendostapedive joint after elevation of tympano metal flap. ↑ indicates pt. of crossing of long processe of inc and postenior bony meatal wall, marked by all to arm.

Exposure of the oval window complex

Fig.2: Drilling away of posters superior bony meatal wall overhang is being done. ↑ indicates drilling done by diamond burr.

Drilling away of posters superior bony meatal wall

Fig.3 Exposure of the white of the long process of in, the stapes, the horizontal part of facial canal and the pyramid upto its base after removing posutre superior bony overhang.

horizontal part of facial canal

Results and Analysis: Table 1:

Variables studied   No of patients Percentage
AGE ( in years)
11-20 3 15
21-30 2 10
31-40 8 40
41-50 7 35
Male 6 30
Female 14 70
11-20 Years 6 30
21-30 Years 3 15
31-40 Years 11 55
> 40 Years 0 0
IV TYPE OF OTOSCLEROSIS Bilateral Symmetrical 2 10
Asymmetrical 14 70
Unilateral Right Ear 2 10
Left Ear 2 10
V SEVERITY OF CONDUCTIVE DEAFNESS IN OPERATED EAR ( as per tympanometry /impedance audiometry) Mild (20-30 dB AB gap) 0 0
Moderate(30-45dB AB gap) 10 50
Severe (>45 dB AB gap) 10 50
Conductive deafness with mixed component 2 10
As 5 25
Ad 2 10
Cd 1 5
VII VISIBILITY OF LONG PROCESS (LP) of incus after elevation of tympanomeatal flap LP Fully visible 2 10
LP Partly visible 5 25
Only lenticular process / incudostapedial joint visible 9 45
Not visible 4 20
2mm 4 20
225mm 5 25
25mm 6 30
275mm 2 10
3mm 2 10
Straight 5 25
Straight with slight slope 1 5

Table 2:

Direction of external auditory canal Average measurement of bony overhang
Curved (in 14 patients) 230 mm
Straight (in 6 patients)* 225 mm
* In one patient of straight external auditory canal, there was absolutely no overhang. So the average measurement of the overhang of the rest 5 patients of the same category is 27 mm.

Excluding the exceptional case of no overhang, usually we find bony overhang of external auditory meatus in the straight variety much more than in curved ones.


  1. Glascock ME,Shaumbaugh GE: Surgery of the earIn: Operations for otospongiosis (otosclerosis) 4th edition, WB Saunders CoPhiladelphia,1990,pp 393
  2. Hyde ML:Scott Brown’s Otolaryngology In: Objective tests of hearing 5th Edition,Volume 2,Adult Audiology,edited by DStephens, Butterworths,London,1987,pp 272-303
  3. Miglets AW ,Paparella MM,Saunders WH:Atlas of Ear Surgery In:Stapes Surgery in dissection of temporal bone 4th edition,CV Mosby CoMissourie,1986,pp 38
  4. O’Connor AF:Diseases of the ear,edited by HLudman and FWright In:Otosclerosis6th edition,Arnold,London,1998, pp458
  5. Smith M, Roberson J: Otologic surgery,edited by Brackman, Shelton and Arriaga In: Avoidance and management of complications, 1st edition, WB Saunders Co, Philadelphia,1994,pp 362
  6. Smyth GDL: Scott Brown’s Otolaryngology In:Otosclerosis,6th edition,Volume:3, Otology, edited by JBroth, Butterworth-Heinemann ,Oxford,1997,pp 3/14/15
  7. Standring Susan: Gray’s Anatomy-The Anatomical basis of Clinical Practice In:Section 3-Head and Neck-Ear and Auditory and Vestibular Apparatus Chapter38:External and middle ear, edited by Barry KB,Berkovitz 39th edition, Elsevier, Churchill Livingstone Edinburgh, London, New York,Oxford,Philadelphia, St Louis,Sydney,Toronto, 2005,pp 651
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