Osseointegrated Bone-Conducting Hearing Protheses

Anatomy and Physiology

The auditory system is anatomically divided into outer, middle, and inner ear compartments, each with its unique role in the process of audition.

The outer ear is composed of the auricle or pinna, the external auditory canal, and the tympanic membrane. The main function of the outer ear structures is to collect and localize sound and provide a passageway into the middle ear cavity. The tympanic membrane serves to divides the outer and middle ear and convert sound waves into vibration.

The middle ear is a bony cavity that is localized inside the temporal bone. It is a cavity filled with air that contains a chain of three bones known as the ossicles with their respective muscles and ligaments. The names of the ossicles are malleus, incus, and stapes. The malleus is attached to the TM laterally and the incus in its medial portion; the incus is attached to the stapes, which itself is attached to a membrane known as the oval window in the cochlea. The role of the middle ear is to amplify the vibration created by the tympanic membrane and deliver them to the inner ear. The inner ear contains the organ for hearing and balance.

The inner ear is housed by an outer bony and inner membranous labyrinth at the petrous portion of the temporal bone. The outer bony labyrinth is filled with perilymph fluid, and the inner membranous labyrinth is filled by endolymph fluid. The inner ear organs involved in sound transduction are the cochlea and the vestibulocochlear nerve. The cochlea is a snail-like organ that has three anatomical divisions: vestibular duct, cochlear, and tympanic duct. At the vestibular duct, the vibration transmitted at the oval window enters and travel along the cochlea toward the tympanic duct. The pressure exerted at the oval window causes a change in perilymphatic fluid pressure, which travels across the outer ducts and triggers a vibration of the basilar membrane at the cochlear duct. This vibration at the basilar membrane causes the displacement of hair cells situated at the organ of Corti. The hair cells are task with creating neural impulses from mechanical impulses. Once mechanical impulses are created at the given frequency, the vestibulocochlear nerve serves as the final pathway towards the central nervous system.[5]

Indications

Patients who are candidates for osseointegrated bone conduction prosthesis must meet audiometric and clinical criteria.

Audiometric criteria[6]

1. Pure tone audiometry demonstrating conductive or mix conductive hearing loss with bone conduction less than 45 dB hearing thresholds and a speech discrimination score equal to or greater than 60%.
2. Single-sided deafness with a normal hearing threshold at the unaffected ear.

Medical indications[6]

1. Chronic ear disease (history of chronic otitis externa or otitis media suppurativa that are exacerbated by conventional hearing aids).
2. Congenital or acquired malformations. This included patients with a defect such as external auditory atresia, canal exostosis, or canal osteoma and patients who have undergone otological surgery such as radical mastoidectomy or meatoplasty who have uncontrolled acoustic feedback with conventional hearing aids.
3. Patients unable to withstand non-ideal features of hearing aids such as discomfort due to occlusion effect.
4. Conductive hearing loss at the only functional ear.

Contraindications

Absolute Contraindications[7]

1. Children less than 5 years age
2. Reduce hearing and bone conduction threshold greater than 55 dB
3. Patients with a history of developmental delay, psychiatric disease, or autism spectrum who are unable to maintain adequate hygiene. 

Relative Contraindications[7]

1. Patients with conditions that are predisposed to an abnormal bone formation such as Paget's disease, osteoporosis, or previous radiation
2. History of keloid formation
3. Patient with pathology close to ear requiring MRI surveillance

Equipment

Equipment required for surgical implantation of osseointegrated bone conduction prosthesis includes but is not limited to:

1. Marker
2. 27 gauge needle
3. Hemostat
4. Ruler
5. A 10 ml syringe with 1% Lidocaine with epinephrine 1:100,000
6. Blade 15 with the blade holder
7. Bipolar cautery
8. Self Retractors
9. Surgical instruments: Dummy device, raspatorium, dissector, drill, drill indicator, torque wrenches, screwdriver, and abutment inserter
10. Company-specific drill console
11. Drilling disposable: guide drill and countersink drill (3-4 mm widening).
12. Implants: Abutment and Fixture (multiple length and width provided by the individual manufacturer).
13. Punch Biopsy (measurement 5 mm)
14. 4-0 Vicryl suture
15. 5-0 fast absorbing plain gut suture
16. Thin nonadherent dressing 
17. Bacitracin antibiotic ointment
18. Healing cap

Personnel

Otolaryngologists and audiologists are the personnel directly involved with the management of surgical implantation of osseointegrated bone conduction prosthesis. Otolaryngologists are physicians who are medically trained to treat disorders related to the ear, nose, and throat. Their role in the implantation of osseointegrated bone prosthesis consists of performing adequate history and physical examination to determine if the patient meets both clinical and audiometric criteria for successful prosthesis implant.

Finally, otolaryngologists determine if implantation needs to be performed in a single or two-stage procedure. A two-stage procedure is recommended in children less than 9 years of age or adults with an abnormal temporal bone. The audiologist role includes performing an adequate complete audiometry analysis before surgery. The audiologist also plays a key role after surgical management. Their role involves the management of a sound processor. They evaluate the patient and are task with the fitting of the sound processor, making sure that surgery was adequate and that the sound processor does not come in contact with the skin to avoid acoustic feedback. Additionally, the audiologist task with helping the patient familiarize with the instruction regarding the processor device, how to change the battery, and different control buttons.

Preparation

Before surgery, physicians must obtain proper consent forms and proceed to answer any question or concern. The patient is transferred to the OR and placed supine in the bed with the head closest to the highest position of the bed. Time out should be performed. Once local anesthesia with sedative or general anesthesia is administered by anesthesia, turn the OR table 180 degrees away from the anesthesia team for adequate working space. Proceed to turn the head contralateral to where the surgery will be performed for adequate exposure. Preparation of the field is carried out with a beta-dine solution, and the patient is draped, as he would undergo a mastoid procedure.

Technique or Treatment

The procedure is begun by first marking the location of the implant site, which is at the postauricular region just below linea temporalis at 50-55 mm from the EAC opening; a template for the device (dummy) can be used to secure correct placement and avoid sound processor from touching the auricle. Next, mark the vertical incision line, which will roughly extend 4-5 cm long with the proposed implant site in its midpoint. The incision will be 20-30 mm from the posterior auricle border and approximately 10 mm anterior to the implant-marked site. Hair at the postauricular region around the area of incision is shaved. The patient can then undergo prepping in a sterile fashion with adequate drape. Then skin and soft tissue thickness are measured using a 27-gauge needle, which is inserted perpendicularly at the implant site. When the bone is encountered by the needle, it can be marked with a hemostat. Retrieve the needle and measure distance with a ruler to adequately select the abutment length of the implant device. The patient is then injected with 1% lidocaine with epinephrine 1:100,000 at marked sites, and the amount will depend if the procedure is performed under local or general anesthesia.

A vertical incision is performed at the skin with a 15-blade scalpel down to the periosteum; it is important to limit bipolar cauterization in this step. Self-retractors are used to facilitate periosteal exposure. When adequate exposure is achieved, perform cruciate incision at periosteum and raise the edge with raspatorium to expose approximately 6 mm of bone. After obtaining adequate bone exposure for implant, proceed to drill a fixture guide hole. First, attach drill bit to 3mm spacer drill. Then set up the manufacturer drill unit to a high-speed setting of 2,000 rpm. Proceed to place the guide drill in a perpendicular position. Then drill the fixture hole to 3 mm depth; verify that there is bone underneath the drill site and constantly irrigate to avoid osteocyte injury. After 3mm depth is reached, make sure no dura has been exposed; remove the spacer-attachment on the 3-mm guide drill and continue to drill in the same fashion until 4 mm depth is reached. Once adequate implant depth is reached, proceed to use the widening drill, also known as countersink drill, to widen the area of the implant. Before drilling is begun, revise that the drilling unit is at high speed 2,000 rpm.

The countersink drill is performed in the same fashion as above until the countersink is created at a depth of 0.5 mm; avoid making the hole larger than it is as it may risk implant instability. Proceed to place implant and abutment by first setting the drill to low speed 15-30 rpm with adequate torque drill setting 20-40 Ncm (Newton centimeter). Then proceed to attach the hand-piece to the abutment inserter; this will be used to retrieve the abutment-implant device. It is important not to contaminate the implant while retrieving or placing it in the skull. Place implant-abutment and rotate until the adequate depth is reached (drill engine will stop at adequate depth). Remove handpiece and abutment inserter gently. Wound closure is achieved with 4-0 vicryl for the deep layers of skin and is followed by a running 5-0 fast absorbing plain gut. Perform a punch biopsy (5mm) in skin overlying abutment. Once the abutment is exposed, attach a healing cap with a plug. Then place a thin dressing cover in bacitracin ointment below the healing cap. Finally, cover the healing cap with a mastoid dressing.

Complications

Complications of osseointegrated bone conduction prosthesis include bone and soft tissue related complications, with the latter being more common. Soft tissue related complications included: soft tissue reactions ranging from erythema and tenderness to granulation tissue. Other common soft tissue related complications included skin overgrowth and hypertrophic scar and keloid formation. Most complications can be treated on an outpatient basis. To reduce the incidence of soft tissue complications, patients need a multidisciplinary approach with adequate instruction regarding hygiene and an established protocol regarding regular follow-up with their otolaryngologist and audiologist. Failure to do so leads to an increased incidence of preventable soft-tissue complications. Finally, bone-related complications include failure of osseointegration with extrusion of the implant, which requires removal of implant and re-implantation. This complication is common in patients younger than 5 years of age due to their thinner skull or poor bone quality.[8][6]

Clinical Significance

Osseointegrated bone prosthesis offers a simple reversible surgical alternative to a patient interested in rehabilitating their audition without risk of further hearing loss. Patients who are candidates for this type of surgery did not tolerate hearing aids or had a contraindication to conventional hearing aids due to the anatomical deformity of the middle/outer ear or a medical condition such as chronic ear infections.

Patients with permanent single-sided conductive hearing loss surgically treated with osseointegrated bone prothesis describe that the device provides them a greater sound quality and perception in soft and louder environments with increased sound localization use of conventional hearing aids. Additionally, subjective questionnaire response has revealed that the OIBP improves patient quality of life and general well-being with a higher compliance rate, with at least 90 percent of the subjects using the device more than eight hours a day every day of the week. Results for patients with bilateral permanent conductive hearing loss with bilateral OIBP have superior results compared to single OIBP in a patient with bilateral conductive hearing loss. The patient reports both subjective and objective improvement in pure tone audiometry, speech discrimination in noisy and quiet environments, and sound localization, leading to increased compliance with a medical device.[2]

Patients with single-sided deafness have been traditionally treated with the contralateral routing of signal hearing aids (CROS hearing aids) or binaural contralateral routing signal (BICROS) hearing aid. These hearing aids differ from conventional air conductive hearing aids in that a satellite microphone is placed in the affected ear and is transmitted by a wire that runs behind the head into a loudspeaker at the contralateral working ear. CROS is prescribed to a patient with normal pure tone audiometry in the unaffected ear, and BICROS is prescribed to a patient with mild sensorineural hearing loss in the less affected ear. Compliance rates are often poor due to reduced aesthetic, patient acceptance, and reduced general performance in a noisy environment. Recent studies have demonstrated the beneficial effects of osseointegrated bone conduction prosthesis surgical implantation in a patient with deaf ear with multiple controlled trials revealing that the implant provides better speech discrimination in noisy environments compared to traditional CROS hearing aid. However, neither osseointegrated bone conduction prosthesis nor CROS reported any benefits with sound localization.[9][2] 

Osseointegrated bone conduction prosthesis has been surgically implanted successfully in the pediatric population with unilateral or bilateral conductive or mixed hearing loss associated with congenital anomalies or chronic draining ear. However, when one considers an osseointegrated medical implant as a treatment modality for this population, it is important to state that skull thickness is not fully developed until patients are at least 5 years of age. Most patients have skull thickness less than 3mm. Guidelines for implantation in pediatric patients suggest performing surgery after 5 years of age or, in some cases, until 9 years of age. The results reveal that decreased skull thickness may lead to an increased incidence of intraoperative complications (trauma to the dura and/or sigmoid sinus) and postoperative complications (extrusion of the implant) due to failure of Osseointegration.

Compared to the adult population, pediatric patients have a higher rate of soft tissue complications due to decrease hygiene and a higher prevalence of trauma to implant among younger age groups. Results have suggested that children younger than 5 years old may benefit from a trial with conventional bone conductive hearing aids with a soft band while their skull continues to develop. However, audiometric results in the pediatric population were similar to the adult population, with an average conduction threshold improvement from 65 dB to 30 dB postoperatively. Additionally, this surgical treatment allows the patient to normally develop speech during early age while allowing the possibility to undergo reconstructive surgery in the future. Additional studies have been performed comparing early reconstructive surgery as a treatment modality for hearing loss, and OIBP and results have revealed that the average hearing gain for reconstructive surgery was 18 dB, with 93% of patients requiring further sound amplification. While patients undergoing osseointegrated bone conductive prosthesis showed an average gain of 32 dB, results discourage reconstructive surgery for means of hearing loss rehabilitation at an early age.[2][4][10][9][11]

Enhancing Healthcare Team Outcomes

Outcomes of osseointegrated bone conduction prosthesis implant will depend on the otolaryngologist's ability to adequately select patients who benefit from the medical implant and their familiarity with the procedure to maximize the hearing result and minimize complications. Other factors that improve healthcare outcomes include an established follow-up protocol regarding the removal of mastoid dressing at post-op day 1, removal of healing cap post-op day 7, and establish a date for sound processor fitting 3 months after surgery to allow for adequate osseointegration of the implant.

To minimize soft-tissue complications, proper hygiene instruction should be given to the patient in an easily understood manner to maximize results. Outcomes are improved when otolaryngologists and audiologists are part of the same institution and establish a strict protocol to examine implant and adjust according to audiometry results. Failure to do so has been linked to higher dissatisfaction by patients undergoing the procedure.[6]

Nursing, Allied Health, and Interprofessional Team Interventions

The surgical nurse plays a critical role in intraoperative and postoperative care in a patient undergoing osseointegrated bone conduction prosthesis. A surgical nurse must be familiar with the osseointegrated bone conduction prosthesis implant instrument and the drill settings to avoid otolaryngologists taking their eyes off the surgical field, helping minimize intraoperative surgical time. During the operative period, specifically at the drilling portion, constant irrigation is needed to reduce thermal damage on osteocyte and increase the chance of adequate osseointegration of the medical implant. During the postoperative period, nurses should be able to adequately identify signs of complications such as bleeding or CSF leak and notify the physician for prompt decision making.

Nursing, Allied Health, and Interprofessional Team Monitoring

The anesthesiologist role in osseointegrate bone conduction prosthesis implant is important, especially when performing the procedure under local anesthesia. Communication is of extreme importance as pain medication and sedation are important to avoid abrupt movement, which can cause inadequate surgical outcomes.