Ossiculoplasty

Anatomy and Physiology

The ossicles (malleus, incus, and stapes) are three bones in the middle ear that assist in the mechanical transduction of sound energy.  The sound energy is collected by the tympanic membrane, transmitted across the middle ear along with the three middle ear bones and into the fluid-filled cochlea.  Collectively, the middle ear bones are known as the ossicular chain.  Embryologically, the malleus and incus develop from Meckel’s cartilage of the first pharyngeal arch, the stapes from Reichert’s cartilage of the second pharyngeal arch, and the footplate and annular ligament from the otic capsule.[3][4] 

The malleus is divided into the head, neck, manubrium, anterior process, and lateral process. The incus is divided into the body, short crus, long crus, and lenticular process. The stapes is divided into the head (capitulum), neck, anterior crus, posterior crus, and footplate.[1] The ossicles are held into position and suspended within the middle ear by a variety of ligaments, muscles, and structural attachments. Laterally, the malleus is attached to the tympanic membrane at the lateral process, manubrium, and umbo. 

Medially, the stapes footplate is secured to the otic capsule by an annular ligament.  The intermediate-positioned incus is joined by the other two ossicles by the incudomalleolar joint and the incudostapedial joint, which are synovial diarthrodial joints.  When sound energy is transmitted across the ossicular chain, it results in vibratory motion of the stapes within the oval window to the cochlea. The classical description of the stapes movement is similar to a piston. However, other vibration patterns and characteristics have been described for the stapes footplate. The differential pressure and impedances at the oval and round windows are required for the movement of cochlear fluid, which forms the basis of neural stimulation for hearing.[5]

The muscles of the middle ear include the tensor tympani and stapedius.  In addition to the suspensory and stabilization function, it provides to the ossicular chain, the muscles protect the inner ear from potentially injurious loud sound levels. The tensor tympani attaches to the manubrium of the malleus close to the neck and retracts it medially to stiffen the tympanic membrane.  In this manner, it can reduce vibratory transmission across the tympanic membrane.  The stapedius attaches to the posterior aspect of the stapes head adjacent to the neck and dampens vibrations into the oval window when the acoustic reflex is triggered by loud sounds.[3] If the aforementioned muscles are detached from their respective ossicles or lose function from disease or surgery, there is no way to restore the protective effects conferred by the muscles during ossiculoplasty.

If sound energy that travels in an air medium enters the external auditory canal directly to the stapes footplate in the absence of a tympanic membrane, middle ear, and ossicles, an impedance mismatch will occur with only 0.1% of acoustic energy reaching the inner ear.[2]  In order to overcome this mismatch, the ossicular chain contributes to an impedance matching system and is responsible for transmitting over 90% of acoustic energy into the inner ear. The mechanism of how the middle ear overcomes the impedance change is primarily acquired through the area effect of the tympanic membrane and the lever ratio of the ossicular chain.

The area effect is the difference between the surface area of the tympanic membrane relative to the surface area of the stapes footplate where sound must travel.  Sound is concentrated by a factor of 17 to 20-fold based on the ratio of surface areas of the recipient tympanic membrane to target stapes footplate (between 17 and 20 to 1).[2] The ossicular lever effect is the ratio of the malleus manubrium length to the incus long process length that provides a mechanical advantage.  Based on the relative lengths, a ratio of 1.3 to 1 has been reported.[1] When taking both area and lever effects into consideration, the overall middle ear mechanical advantage conferred is 22 to 1 (the product of the area ratio and lever ratio).  Overall, approximately 20 to 30 dB gain in sound pressure is produced by the impedance matching function of the ossicular chain.[2]

When ossiculoplasty is performed, continuity of the ossicular chain may be surgically restored, but it is not possible to replicate the lever effect.

Indications

The goal of ossiculoplasty is to reestablish ossicular chain continuity and improve the air conduction thresholds in patients suffering from conductive hearing loss. Chronic otitis media and its sequelae such as cholesteatoma and ossicular erosion are responsible for over 80% of ossicular chain disruption cases.[6] 

The remaining cases are due to blunt and penetrating trauma causing disarticulation, congenital ossicular fixation, acquired ossicular fixation from tympanosclerosis, congenital malformations, congenital absence of ossicles, otosclerosis, and middle ear neoplasms.[1][7] 

The incus is involved in the majority of these cases, particularly the long process, followed by the stapes.[8][9] Approximately half of the cases have more than one ossicles involved.[10][11] In up to 55% of conductive hearing loss cases, ossicular discontinuity or fixation was found to be responsible.[12] 

The restoration of ossicular chain continuity is vital as prolonged conductive hearing loss can lead to poor language development, cognition, and learning.[13][14]

Contraindications

Ongoing infection of the middle ear (otitis media) is a contraindication to ossicular chain reconstruction. However, relative contraindications include factors that would result in suboptimal hearing improvement, such as a reduced middle ear space, repeated surgical failures using similar prostheses, stapes fixation/malformation, and an only hearing ear.[7]  The option of hearing assistive devices, such as conventional hearing aids and bone-conduction devices, must be offered to the patient as an alternative to hearing habilitation.

A stable, well-aerated middle ear environment is critical for optimal sound conduction. A contracted middle ear space can restrict the movement of the tympanic membrane, ossicular chain, and round window, resulting in a 35 to 55 dB conductive hearing loss.[15] Middle ear pathology resulting from poor ventilation is significantly more detrimental to ossiculoplasty success than surgical technique and prosthetic factors.[16][17] 

Negative middle ear pressure, poor mastoid pneumatization, effusion, persistent Eustachian tube dysfunction, granulation tissue, middle ear fibrosis, and adhesive disease, and tympanic membrane perforations can all affect the efficiency of sound transmission and must be addressed prior or in conjunction with ossiculoplasty.[2][18]

Reduced stapes footplate mobility, absence of stapes superstructure, or an improperly aligned stapes superstructure can result in the suboptimal ossicular coupling, increased risk of prosthesis displacement, and surgical failure.[19][20]

Equipment

The following equipment may be needed:

• Operating microscope
• Ossicle holder
• Microdrill for sculpting
• Otologic instrumentation tray
• Ossicular prosthesis

There are three types of synthetic ossicular prostheses: partial ossicular replacement prosthesis (PORP), total ossicular replacement prosthesis (TORP), and incus interpositional or incus replacement prosthesis. PORP links an intact stapes suprastructure to the malleus or tympanic membrane. TORP links the stapes footplate to the malleus or tympanic membrane. An incus interpositional prosthesis is used when the long process is compromised.  It connects the stapes superstructure to the remnant incus long process. An incus replacement prosthesis is used when the incus is missing, and the prosthesis is placed between the manubrium and stapes capitulum. Ossiculoplasty improves hearing with a success rate of 75% for PORP and 68% for TORP at 12 to 18 months.[20] The success at five years for PORP and TORP is 66% and 33%, respectively.[16]

The use of autologous versus artificial synthetic materials for prostheses has been debated since the 1950s. Autografts were first used in ossicular reconstruction in 1957 by Hall.[21] The advantage of using autografts from ossicles, cortical bone, and cartilage is the low risk of extrusion.[7] The sculpted incus body is the most commonly used autograft. It offers convenient availability in the same surgical field. The disadvantages of autografts include insufficient ossicular body mass if significant erosion has occurred secondary to disease, prolonged intraoperative time for sculpting, future resorption by osteitis, fixation, and risk of residual cholesteatoma.[7][8]

The first use of artificial ossicular prostheses dates back to 1952 with the use of vinyl-acryl plastic. Over the last few decades, artificial prostheses have been manufactured from plastics (polyethylene, polytetrafluorethylene, Teflon, plastipore, proplast, silicone), metals (stainless steel, titanium, platinum, gold, tantalum), and biomaterials (aluminum oxide ceramic, bioglass, Ceravital, hydroxyapatite, carbon).[2][7][22] 

Artificial prostheses must be inert in the middle cavity to prevent foreign body reaction and chronic inflammation as they can result in an osteolytic process that leads to ossicular remnant necrosis.[23] Disadvantages of allograft materials such as polyethylene, plastipore, Teflon, and Ceravital include inflammatory middle ear reactions, biodegradation, prosthesis extrusion, displacement, and migration into the inner ear.[8][22][24][25] 

Hydroxyapatite is composed of calcium phosphate ceramic and is similar to the mineral of bone and has good biocompatibility.[26] It was first used by Grote in the 1980s.[27] Extrusion rates are 4% to 16% with lower extrusion (<2%) when a cartilage graft is interposed between the implant and the tympanic membrane.[28][29] However, hydroxyapatite is brittle, challenging to sculpt, and difficult to place due to its bulk.[30][31][32][30]

Titanium is lightweight, rigid, biocompatible, and has gained recent popularity.[2][33] It is easier to conform to different shapes and can be more precisely sized compared to hydroxyapatite, facilitating its placement. The extrusion rate is 1% to 2% with similar hearing results as those of hydroxyapatite or autografts.[34][35][36]

Bone cement serves as a good adjunct for ossiculoplasty. It is easy to prepare and apply with fast setting times and good biocompatibility. It can be molded to augment the ossicular remnant, allowing ossicle preservation. Ninety percent of patients who underwent bone cement reconstruction have reported an air-bone gap closure of 20dB or better.[37][38] 

Today, the use of autograft and homograft prostheses has largely been replaced by the more readily available, versatile, and durable artificial synthetic prostheses.[2] A survey by Goldenberg et al. showed 70% of otologists preferring synthetic material to bone (25.15%) or cartilage (4.4%).[17] There is no statistically significant difference in hearing improvement between autograft versus synthetic prosthesis.[18][39]  Surgeons will ultimately opt for the material that has been proven to give the best results in their hands.[40][34]

Personnel

The following personnel may be needed to carry out this technique:

• Otolaryngologist
• Anesthesiologist
• Surgical technician
• Circulating nurse

Preparation

The patient who presents with conductive hearing loss should undergo a complete history and physical with a focused head and neck exam, including an otomicroscopic exam and tuning fork evaluation. Visual inspection of the sound conduction pathways must be performed to determine the cause and contributory factors to conductive hearing loss. This includes ear canal patency, tympanic membrane integrity and position, middle ear aeration, and retrotympanic abnormalities.  Tuning fork tests include Weber and Rinne tests.

In the setting of conductive hearing loss, the Weber test will lateralize to the side of the conductive hearing loss and the Rinne test will suggest improved audibility better via bone conduction than air conduction. Both 512 Hz and 1024 Hz forks are recommended to help estimate the degree of conductive hearing loss. Preoperative pure tone audiogram will assess the nature and degree of hearing loss, as well as to confirm the findings on the tuning fork exams. Impedance audiometry offers information regarding tympanic membrane compliance and acoustic reflex integrity. CT scan of the temporal bones is obtained to provide information on ossicular integrity and continuity and co-existing tympanomastoid disease that will affect surgical planning.[8]

Prior to surgery, informed consent is obtained. Risks include bleeding, infection, ossicular prosthesis displacement (immediate or delayed) or extrusion, hearing loss, dizziness, altered taste, facial nerve injury leading to facial paralysis, stapes subluxation leading to deafness, perilymphatic fistula, and need for additional future surgery.

The alternative options for surgery to improve hearing includes conventional hearing aids or bone conduction device.

Technique or Treatment

Ossiculoplasty may be performed in the setting of IV sedation/ local anesthesia or general anesthesia, depending on whether other otologic procedures are planned.  If surgical eradication of middle ear or mastoid disease (tympanomastoidectomy) is planned, general anesthesia is administered in conjunction with intraoperative facial nerve monitoring.  If the plan is to restore ossicular continuity in an ear without active disease, for example, in a planned staged ear surgery, the tympanomeatal flap may be raised and ossiculoplasty performed under IV sedation/ local anesthesia.

The ear is prepped and draped in a sterile fashion. Access to the middle ear cavity can be obtained either through the transcanal or postauricular approach.  In either approach, a tympanomeatal flap is raised to expose the middle ear and access the ossicular chain. The postauricular approach is better suited for patients who have a narrow external auditory canal or prominent bony prominences where a transcanal approach would be made difficult or if a concurrent mastoidectomy is planned.  

Local anesthetic containing a vasoconstricting agent (for example, 1% lidocaine with 1: 100,000 epinephrine) is injected into the ear canal in both approaches to promote local anesthesia and hemostasis.  This is done under direct visualization of the operating microscope to precisely deliver the injection into the ear canal at the bony-cartilaginous junction in the subperiosteal plane to result in a diffuse blanch.

Transcanal Approach

The tympanomeatal flap is elevated via a transcanal approach by making two 8 mm radial longitudinal incisions lateral to the annulus superiorly at the tympanosquamous suture line (12 o’clock) and inferiorly (6 o’clock). The two incisions are joined by a transverse incision laterally to form a medially-based U-shaped flap. The canal skin and periosteum are then elevated in continuity with the fibrous annulus, which is then raised out of its bony groove. The tympanomeatal flap is then reflected forward to access the posterior mesotympanum and ossicles. 

Postauricular Approach

Ossiculoplasty in the setting of chronic ear surgery usually occurs as the last step after tympanomastoidectomy and removal of obstructive or cholesteatomatous disease.  In this case, the middle ear will be already widely exposed, and ossiculoplasty will be performed along with eardrum grafting (tympanoplasty).  If tympanomastoidectomy is not performed and a posterior auricular approach is chosen, ear canal incisions, similar to that described for the transcanal approach, are created that would allow a tympanomeatal flap to be raised.

A posterior auricular incision is created and carried down to the plane lateral to the temporalis fascia. An incision through the musculoperiosteum is created along the temporal line and another perpendicular musculoperiosteal incision posterior to the cartilaginous ear canal toward the mastoid tip.  The anterior-based musculoperiosteal flap created from these incisions is elevated toward the membranous ear canal and entry is made into the bony ear canal. The posterior canal skin of the bony canal is elevated medially until the tympanomeatal flap incisions are reached.  A Perkins retractor is used to retract the auricle and posterior canal skin forward. The tympanomeatal flap can now be raised and the middle ear entered.  If ear canal patency is compromised by an anterior canal wall bulge or hypertrophic tympanic ring, the canal skin overlying the bony prominence is elevated and reduced with an otologic drill until adequate access to the middle ear is obtained. 

Prosthesis Placement

Once the middle ear cavity is accessed and exposed, the ossicles are inspected for any defects and gently palpated to assess for mobility and continuity.  Identify the most appropriate ossicular reconstruction solution (autologous vs. synthetic/artificial). Stapes footplate mobility is essential for hearing success. If the stapes capitulum is intact, a PORP can be used. If the stapes footplate is intact and mobile, a TORP can be used. The platform of the TORP or PORP can be positioned under the manubrium or undersurface of the native or grafted tympanic membrane. Autologous cartilage is harvested from either the tragus or conchal bowl to be used as a shield or cap over the prosthesis platform to prevent prosthesis extrusion. If the manubrium is intact but medialized or foreshortened, the tensor tympani tendon should be transected and the manubrium lateralized to better accommodate the ossicular reconstruction.

The prosthesis is then trimmed to a length that would span the distance from the mobile stapes superstructure or footplate medially to the tympanic membrane or manubrium laterally.  The prosthesis length must be trimmed to the length that accounts for the thickness of the cartilage shield or cap that will lie on the prosthesis platform.  The final placement of the prosthesis should result in good stability and not prone to dislocation or displacement. Gelatin sponge soaked in ototopical antibiotic drops can be used to pack around the reconstruction to provide additional prosthesis stability. Some metallic prostheses feature a crimpable cage that can be affixed to the capitulum for further stabilization.

The tympanomeatal flap or grafted tympanic membrane is then placed over the cartilage-prosthetic complex. Antibiotic-soaked gelatin sponge is then placed lateral to the tympanic membrane or graft to secure its position.   

Postoperatively, patients are seen 1 week after surgery. Antibiotic drops are started at the first postoperative visit for a 3-week course. Thereafter, the canal is carefully debrided free of all packing material. Audiometry is performed 3 months after surgery to assess hearing results.

Complications

The predominant complications of ossiculoplasty include failure to improve the conductive hearing loss, necrosis of an autologous osseous graft or remnant native ossicles, and prosthesis extrusion/migration. More rare complications include fracture of stapes superstructure, displacement of stapes, disruption of the annular ligament of the oval window leading to perilympahtic fistula, severe or complete sensorineural hearing loss, and vertigo.[7] 

Factors that influence surgical failure include middle ear pathology, type of prosthesis, ossicular chain condition at the time of ossiculoplasty, surgical technique, external auditory canal wall status in cases of canal wall down mastoid surgery, and staging of ossiculoplasty. The most common reason for surgical failure is persistent middle ear disease followed by prosthesis displacement or migration resulting in maximum conductive hearing loss.[2] Middle ear pathology (middle ear drainage, chronic otitis media, cholesteatoma) accounts for up to 56% of all surgical failures with the majority due to atelectasis.[2][16][28][41][42]

Loss of hearing can occur immediately after surgery or be delayed. Poor hearing in the postoperative period may be due to middle ear effusion or ongoing middle ear disease, graft failure, or displaced prosthesis.[43] Delayed hearing loss after an initial period of improvement is most likely due to cholesteatoma recurrence or recidivistic disease, delayed graft failure, prosthesis displacement, or formation of scarring around the prosthesis.[44] 

Traditionally, closure of the air-bone gap on pure tone audiometry below 20 dB has been the marker of surgical success.[43] The Ossiculoplasty Outcome Parameter Staging (OOPS) index was developed to predict hearing outcomes in patients after ossiculoplasty based on preoperative patient characteristic as well as intraoperative findings such as the presence of middle ear drainage,  normal vs. fibrotic middle ear mucosa, presence of malleus, type of surgery (canal wall up vs. canal wall down mastoidectomy), and if it is revision surgery.[45] 

The OOPS index has been useful in helping providers predict surgical success, pinpoint patients at risk for surgical complications, compare different surgical techniques and prostheses, and assess progression in surgical ability.[43] The OOPS has been found to be significant in predicting both short-term and long-term hearing success.[43][45] 

Factors that increase the risk of complications after ossiculoplasty include poor hearing results on the first postoperative audiometry, absence of malleus, middle ear disease, ETD, recurrent otitis media, tobacco smoking, and inability to perform Valsalva.[44][46][47][48][49][50] 

In order to improve ossiculoplasty success rates, a delayed ossicular chain reconstruction has been recommended for patients with a significant disease requiring extensive middle ear and mastoid surgery, canal wall down mastoidectomy, residual cholesteatoma, and excessive middle ear mucosa resection.[2][51]

Postoperatively, patients are followed annually with audiometry to assess the long-term stability of hearing and office otomicroscopic examination to assess whether there is a return of middle ear disease. For patients with poor postoperative audiometric outcomes, the reasons for failure will need to be determined, and decisions will need to be made to see any factors that may be corrected. In some cases, CT imaging may be required to assess prosthesis position and whether there is persistent tympanomastoid disease that might contribute to the poor result. Revision surgery may be necessary.

Clinical Significance

Since the 1950s, ossiculoplasty has been performed in patients with conductive hearing loss associated with ossicular chain disruption, leading to significant improvements in speech, social development, and overall quality of life. However, the rates of air-bone gap closure on audiometry, long-term continuation of postoperative hearing improvement, and overall surgical success have varied.

A myriad of prosthetics and surgical techniques have been described over the last half-century with no overall consensus on the ideal method of treatment. This further highlights the need for additional research to determine the optimal algorithm of hearing habilitation. Healthcare providers that deal with conductive hearing loss and ossicular chain disruption must be well-informed on the etiology of middle ear disease, the anatomy of ear structures, diagnostic practices, chronic ear management, surgical repair techniques, postoperative care, and mitigation of complications.

Enhancing Healthcare Team Outcomes

The management of conductive hearing loss due to ossicular chain disruption requires a multidisciplinary team of otolaryngologists, audiologists, anesthesiologists, and speech therapists. Early diagnosis and treatment of conductive hearing loss are essential for long-term speech fluency and social function.

Patients with chronic ear disease, trauma, and resulting hearing loss should be referred by their primary care physicians to audiologists for hearing evaluation. If ossicular chain disruption is suspected from audiometry and imaging, a referral should be sent to an otolaryngologist. Postoperatively, patients should be followed by an otolaryngologist and an audiologist to monitor surgical outcomes and undergo postoperative audiometry to assess the improvement of hearing and long-term stability of hearing results. For children, speech therapy should continue even after surgical repair to enhance speech outcomes.