In his Ph.D. thesis, Stenfelt ⁸ discussed the different bone conduction theories of von Békésy, Barany, Tonndorf, and others. Most of the studies cited refer to patient groups with a normal ear, which is seldom the case for Baha patients. One exception is after acoustic tumor surgery, in which the eighth cranial nerve has been sacrificed, but the ear canal, middle ear, and cochlea are normal. Research has proven hearing through bone conduction to be highly complex. First, for frequencies less than 1.5 kHz, the relative movement of the ossicular chain dominates the bone conduction response; this is the inertia of the middle ear ossicles. Even with damage to the middle ear and ossicles, however, there is a bone conduction response in the low frequencies because the skull moves like a solid body. Second, for frequencies greater than 1.5 kHz, the response is attributed to the compression of the labyrinth. A third means of bone conduction is due to the relative movement of the mandible. Air in the external ear is set in motion and transmits sound through the intact drum and ossicles to the cochlea.

Several other factors probably also play a role, such as the oval and round window release, inner ear fluid inertia, and cochlea aqueduct effect. These factors are often interrelated, and it is difficult to isolate one from another. Almost regardless of the location of bone stimulation, the waves of the basilar membrane travel from the base of the cochlea, where the membrane is stiffer, toward the helicotrema, as is the case with air conducted sound. This means that the cochlea has difficulties in differentiating between bone conducted and air conducted sound. Cancellation experiments have verified this difficulty. Stenfelt ⁸ also showed that bone conducted sound from 0.1 to 10 kHz for levels up to 77 dB HL is linear; this is important because the distortion level of the bone conducted sound would be low.

OSSEOINTEGRATION

The term osseointegration was coined by Brånemark and refers to direct contact between bone and an implant that can withstand a functional load. Numerous papers on osseointegration were published in the late 1970s.⁹^–¹² Osseointegration is a fundamental prerequisite for direct bone conduction; without it, the Baha would not function. Among the factors identified by Brånemark as key to establishing osseointegration was choice of implant material. Commercially pure titanium has been the material of choice since the start. Commercially pure titanium has a purity of 99.75%. The level of ferromagnetic contamination is 0.02% to 0.05% in the bulk metal, but no traces are found in the oxide layer, which means that the implant would not jeopardize magnetic resonance imaging (MRI).^13,¹⁴ Because of the difference in density between bone and titanium artifacts, less than 1 mm would appear around the implants.

The screw-shaped design provides initial stability. Micromovements during the healing phase, if too large, could result in soft tissue encapsulation and prevent osseointegration.

When a commercially pure titanium implant is machined, the surface is covered with an oxide layer within milliseconds. The implant can be regarded as a ceramic. The oxide layer has unique biocompatible properties; it is also dense and adheres to the bulk metal. Proteins from the host would not denature when in contact with the oxide. Titanium oxide has a high electrochemical value, which means that the surface attracts foreign material that could contaminate the implant.¹¹ This is why the implant should be handled with the utmost care, avoiding contact even with sterile particles from gloves and draping during surgery.

The surgical procedure, which is described in detail subsequently, is focused on reducing surgical trauma as much as possible. Excessive heat interferes with healing, and an osteocyte tolerates only 42° C for 1 minute before apoptosis. If too many osteocytes are damaged, there is a risk of fibrous tissue healing.

In the treatment of edentulousness with implants, early loading often caused loss of osseointegration. This was the reason why the two-stage procedure was initially recommended for the Baha.¹⁰ The load produced by the Baha is very low, however, compared with the forces produced during chewing. Based on a study comparing a one-stage with a two-stage procedure in adults, the one-stage procedure is now recommended because no difference in osseointegration could be found.¹⁵ In irradiated bone, a two-stage procedure with a 6-month interval is suggested, often in combination with hyperbaric oxygen treatment.¹⁶

PATIENT SELECTION

Two groups of patients are potential candidates for the Baha: (1) patients with conductive or mixed hearing loss, and (2) patients with single-sided deafness.

Conductive or Mixed Hearing Loss

Patients who have mixed hearing loss with a conductive element greater than 30 dB may not be helped by conventional air conduction aids, but could benefit from the Baha.

Chronic Ear Disease

The Baha was originally designed for patients with bilateral or unilateral conductive or mixed hearing loss who needed amplification, but who could not be helped using reconstructive surgery, and who could not use an air conduction hearing aid. Suitable candidates included patients with ears that drain despite trials to make the ear dry, and patients whose ears start to drain when the external ear canal is occluded with a hearing aid mold. Patients with dry radical cavities who experience acoustic feedback when an air conduction aid is tried may also benefit from the Baha.

Congenital Malformations

Patients with bilateral atresia often have normal or near-normal cochlea function and are ideal patients for the Baha. Because reconstructive atresia surgery is complex and high risk, a lasting hearing improvement is not always possible to achieve. In patients with a Jahrsdoerfer rating of 7 or worse, we often suggest a bone-anchored hearing implant instead of reconstructive trials.¹⁷ One advantage of the Baha for these patients is that the treatment does not interfere with atresia surgery if this becomes necessary later on. Although regulations regarding the minimum age for implantation vary from country to country, Baha has been successfully used in children 2 years old. Children can benefit from hearing with the Baha until they are old enough for an evaluation of the final anatomic situation to be made. The final decision can be made whether or not atresia surgery should be recommended.

Unilateral external ear canal atresia has been said to have little or no importance for child development. In a review published in 2004, Cho Lieu ¹⁸ found, however, that speech development and the learning capacity of these children are at risk, even when hearing is normal in the contralateral ear.

Conductive Loss in Only Hearing Ear

A conductive loss in a patient with a deaf contralateral ear could be a problem. If the conductive component is large, an air conduction hearing aid may have difficulty overcoming this gap. A Baha bypassing the middle ear could be an alternative. As in all middle ear surgery, there is always a risk of damage to the cochlea; the author uses Baha in these cases. A patient with a deaf ear on one side, from stapes surgery, and a maximum conduction loss on the other side is an excellent candidate for a Baha.

Chronic External Otitis Preventing Use of an Air Conduction Hearing Aid

Some patients in need of amplification experience problems with air conduction ear canal molds. Several different materials have often been tried, but for some patients the mold causes irritation, infection, and inflammation. As a result, these patients can use their hearing aid only for very limited periods or not at all. For them, the Baha is a good alternative. This group represents about 10% of the Baha patients at our Implant Unit.

Down Syndrome

Conductive hearing impairment is common in Down syndrome. Narrow external ear canals and middle ear malformations are prevalent in patients with Down syndrome. Serous otitis media is also frequent in these children and can be hard to treat sometimes. It is clinically proven that the Baha improves the quality of life for these patients.¹⁹

Single-Sided Deafness

Cochlear deafness in one ear is not unusual. One cause is acoustic tumor surgery. Patients with single-sided deafness often experience verbal communication problems even when the opposite ear functions normally. Problems with speech understanding are most commonly experienced in noisy surroundings. One reason is the head shadow effect, but several other factors are involved. Processing is highly complex and is not yet fully understood. One finding is that patient satisfaction is often greater than indicated by test results.²⁰^–²⁵

PATIENT COUNSELING

Patients considering a Baha should be thoroughly counseled in all aspects. The patient must have realistic expectations and be aware that this sound processor also has its limitations. The surgical procedure should be described, and possible intraoperative and postoperative complications should be discussed. A dummy implant may be helpful for showing to the patient the small size of the implant when discussing the implant procedure. Many patients experience some postsurgical numbness around the implant site. This numbness is more common in patients in whom a large amount of soft tissue has been removed. The patient should also be able to come for regular follow-up visits. One of the main causes of adverse skin reactions is inadequate hygiene. The necessity of a high level of personal hygiene must be stressed preoperatively.

The sound processor requires proper maintenance. Detailed instruction of its function and physical care should be given.

Age is not a contraindication for the Baha per se. The oldest patient who has undergone surgery at the Sahlgrenska unit was more than 90 years old, and the youngest was 18 months old. Because surgery becomes easier and less risky with age, it is suggested that the Baha Softband be used until the child is 3 years old (Fig. 33-1) (see subsequent section “Baha in Children ”.)

FIGURE 33-1 A 3-year-old girl with a Soft-Band wearing the transducer in her own way.

When clinical and audiometric criteria have been evaluated, and the final decision to proceed with implantation has been made, the patient should be informed that the Baha procedure is completely reversible. If the patient is dissatisfied with the Baha, it is easy to remove the abutment and, if so desired, the implant, returning the patient to his or her original state. Very few procedures in otology offer this possibility.

Psychiatric disease can be considered a relative contraindication. When in doubt about a patient’s condition, psychiatric consultation could be very helpful for avoiding pitfalls.

AUDIOMETRIC CRITERIA

Generally, the better the cochlea function, the better the result. The air-bone gap is not important. As of September 2007, there was only one manufacturer of direct bone conduction cochlea stimulators, Cochlear Bone Anchored Solutions (a division within Cochlear Ltd, Göteborg, Sweden). Two ear-level devices and one body-worn device are available. The major difference between the ear-level devices is that they target different patient groups. The Baha Divino (Fig. 33-2) is for patients whose average bone conduction thresholds are at 0.5 to 3 kHz should be better than 30 to 35 (45) dB.²⁰ The Divino also has a directional microphone setting. The other ear-level device, the Baha Intenso (Fig. 33-3), is recommended when the cochlea reserve is worse. Patients with a bone conduction threshold of 40 to 45 (55) dB can benefit from this hearing instrument.

FIGURE 33-2 A patient with the Baha Divino.

FIGURE 33-3 A patient with the Baha Intenso.

The body-worn device, the Baha Cordelle II (Fig. 33-4), has the transducer in a separate housing that is worn at ear level, connected to the Baha coupling. The microphone, electronics, and batteries are in the body-worn unit. Patients with cochlea hearing loss of 50 to 55 (60) dB could benefit from this device. The levels of hearing impairment discussed here are only guidelines. Some patients have not been satisfied even when well within the audiometric criteria, whereas others are satisfied even though their hearing capacity is lower than technically required.

FIGURE 33-4 Body-worn aid Baha Cordelle II.

In the preoperative evaluation, a testband with a Baha can be very helpful. The Baha is attached to a steel spring that the patient wears over his or her head (Fig. 33-5). With this test device, the patient still has skin between the transducer and the skull bone. If the patient is satisfied with this testband, the chances of implant surgery being successful are very high. The test rod can also be used as an alternative means to give the patient an idea of the outcome (Fig. 33-6). The Baha is attached to the coupling of a plastic rod and pressed onto the skin over the mastoid. To reduce the damping effect of the skin, the patient can hold the rod firmly between his or her teeth, which would simulate direct bone conduction.

FIGURE 33-5 Test band with Baha.

FIGURE 33-6 Test rod with Baha.

CONTRAINDICATIONS

There are no absolute contraindications to providing a patient with a Baha. Psychiatric disease or psychological problems rendering the patient unable to follow instructions and participate in follow-up could be discussed, however. The same could be said for patients with extremely poor general hygiene. Patients with diabetes, psoriasis, scleroderma, or other skin problems could experience a slightly increased frequency of adverse skin reaction, which should be mentioned during patient counseling. Bone diseases, such as osteogenesis imperfecta and Paget’s disease, might constitute a higher risk of implant losses.

SURGICAL TREATMENT

The surgical technique is presented in the accompanying DVD. For the surgical technique in children, see the section on Baha in children.

Preparation of the Patient

The surgical procedure is simple, generally performed under local anesthesia and as a day procedure. Premedication with morphine and scopolamine is often, but not always, given. No antibiotics or steroids are used, unless the patient has been irradiated or has artificial implants elsewhere, such as a hip prosthesis.

The author prefers magnifying lenses to the otomicroscope because this makes it easier to find the exact angle of direction for the different steps of the procedure. An indicator for the Baha is used to make a mark for the implant site (Fig. 33-7A). It is important that the sound processor not touch the pinna because this could cause acoustic feedback. The implant should be placed as close as possible to the external ear canal opening, however. This is often about 50 to 55 mm from the center of the external ear canal opening. A suitable position in the cranial-caudal direction is at the level of the superior auricular fold, or just below the linea temporalis. A mark for the implant site is made with a pen, and the area is shaved. Before cleaning and draping the patient, a small scratch with a needle allows the surgeon to identify the implant site even if the ink mark has disappeared. Cleaning and draping is done in the same manner as for standard mastoid surgery. If there is secretion from the ear, the pinna could be folded anteriorly and kept in place with some adhesive draping to seal off the ear canal.

FIGURE 33-7 A-F, Steps in the surgical procedure. See text for details. G-L, Steps in the surgical procedure. See text for details.

Surgical Instruments

The instruments used consist of both general surgical instruments and instruments specially designed for this type of implant surgery. These include a drill unit with high-speed and low-speed functions and a feature to reverse the direction of rotation. The torque for the low speed can be adjusted according to the quality of the bone. The special instruments, implants, and sound processors are produced by Cochlear Bone Anchored Solutions (Göteborg, Sweden).

SURGICAL TECHNIQUE

The surgical procedure is illustrated in Figure 33-7B through L.

Preparing Thin Hairless Flap

The implant site is clearly marked including the skin for the Baha dermatome flap. The author prefers to have the flap pedicle anteriorly. This is a matter of routine, however, and of no importance. The flap should be called a pedicle graft because there is no clear arterial blood supply to the flap. The area indicating the amount of soft tissue to be removed is also marked. The size of this area depends on the amount of subcutaneous tissue. Less soft tissue has to be removed in a lean patient than in an obese patient. The aim is for the edges around the flap to slope gently down to the implant site.

A local anesthetic of 10 mL of lidocaine with epinephrine is injected, making sure that some of the anesthetic is administered in and under the periosteum, and that it also reaches the periphery of the intended surgical field. The disposable sterile blade for the Baha dermatome is taken out of its pack. There are two grooves on one side of the base of the blade and only one on the other side. The side with the single groove should face upward when placed in the dermatome. The driver pin is placed in the handpiece, and the dermatome is firmly secured to the handpiece. The thickness and width of the dermatome are set, and no adjustment is needed; the width is 25 mm, and the thickness is 0.6 mm. This thickness produces a flap where the hair follicles are left for removal in conjunction with soft tissue reduction (see Fig. 33-7B).

The drill is set to 2000 rpm. A little liquid paraffin is applied to the flap area to make the dermatome slide gently. A small cut with a knife along the starting point also facilitates creating the flap. A slight pressure is applied as the dermatome is pushed forward. It is important to maintain the correct angle of the dermatome—neither too shallow nor too steep. When the intended base of the flap is reached, the engine is stopped, and the dermatome is withdrawn. The hair remaining on the flap must be removed by scraping with a blade.

The next step is to expose the implant site. A cut is made along the three sides of the subcutaneous flap leaving the base still attached (see Fig. 33-7C). The cut should be down to, but not through, the periosteum. Two small self-retaining retractors keep the flap out of the implant area. The periosteum is trimmed especially close to the actual implant site. The outer layer of the periosteum should be removed, leaving only the innermost layer in place. A hole about 6 mm in diameter is made in the thinned periosteum for the implant.

Guide Hole Drilling

The sterile packed guide drill is attached to the handpiece (see Fig. 33-7D). The white plastic spacer is initially left in place. This spacer prevents drilling deeper than 3 mm. The drill speed is 2000 rpm. During drilling, generous cooling is used to reduce heat trauma to the bone. The drill should be moved up and down in short sequences about 3 seconds long. In addition, it is important to widen the hole to be able to inspect and check the bottom for any underlying soft tissues, such as the dura mater or the sigmoid sinus. The width should be almost 3.75 mm, which is the diameter of the implant. This widening also allows the irrigation to reach the area where the cutting is taking place. The drill flutes should be kept free from bone because this would otherwise induce heat. If there is still bone at the base of the implant site when the spacer is all the way down, the spacer is removed to allow an additional 1 mm of drilling so that the 4 mm implant can be used.

Countersink Drilling

The next step is to widen the hole to the final diameter of 3.75 mm; this is done with a spiral drill, which also has countersink edges (see Fig. 33-7E). Drill speed is still 2000 rpm. The surgeon should work up and down in two to four steps with generous cooling. The grooves on the side of the drill should be cleaned of bone, which collects here. If the grooves are not kept clean, the drilling temperature may become too high. The countersink edges are designed to provide an even surface for the flange of the implant to improve initial stability. The depth of the countersink should be kept at a minimum, however, because the cortical shell has high density and provides good implant stability; this is especially important if the bone is thin and soft as in children (see section “Baha in Children”).

Subcutaneous Soft Tissue Reduction

The implant site has now been secured, and the soft tissue reduction can start (see Fig. 33-7F). This is one of the most important steps for establishing a lasting and reaction-free skin penetration for the years to come. The soft tissue reduction starts when the implant site has been secured, but before placing the implant. The self-retaining retractors are removed, and two small skin hooks are used. The assistant stretches out the skin flap with the hooks, and by folding the flap back and forth the surgeon can remove the soft tissue under the base of the flap (see Fig. 33-7G). The other three sides are undermined in the same way with the aim of creating tension-free sloping edges down to the periosteum (see Fig. 33-7H).

Placing the Baha Coupling

The drill speed is set on low, which is 15 to 30 rpm, and the torque is adjusted to suit the bone quality. In the sclerotic mastoid of a patient with chronic ear disease, the torque often needs to be 40 to 45 NCM. In soft bone and with a thin cortical shell, a much lower setting should be used, 20 to 25 N/cm², to avoid trauma to the implant site. The Baha abutment inserter is placed in the handpiece. The self-tapping implant with premounted abutment is picked up from its plastic container and should not be touched by anything (see Fig. 33-7I). The implant is kept over the implant site, and the engine is started. With a slight pressure, the self-tapping implant finds its way. After the first few turns, no pressure is needed. When the implant is placed, the engine stops automatically at the set torque. The surgeon should now turn the handpiece about 5 degrees counterclockwise, put his index finger close to the abutment, and remove the abutment inserter from the abutment; this is to reduce any counter lever effect on the implant (see Fig. 33-7J).

Suturing the Flap

The thin flap is positioned over the coupling, and using a biopsy punch, a hole is made in the flap. The coupling is pulled through the hole, and the edges of the flap are sutured down to the periosteum with 5-0 nylon monofilament (see Fig. 33-7K). A healing cap is snapped onto the coupling, and a piece of 1 cm wide cotton gauze, soaked in antibiotic ointment, is very loosely packed under the cap to avoid swelling and hematoma (see Fig. 33-7L). An ordinary mastoid dressing is applied, which is removed after 1 to 2 days.

POSTOPERATIVE MANAGEMENT

In most cases, this surgery is performed as a day procedure. After a few hours in the recovery room, the patient is discharged and can go home. The patient is instructed to remove the mastoid dressing the following day. The patient should clean the area with ordinary soap and water, and leave it without any further dressing. The healing cap with the gauze is left for 7 to 10 days. At this time, the patient comes for the first postoperative visit. The healing cap is snapped off, and the gauze is removed (Fig. 33-8). Often some or all stitches can be removed. If the healing is delayed for some reason, the healing cap can be put back on, and fresh gauze can be loosely placed around the coupling for another few days. Sometimes a healing cap is put on without any gauze just to provide a little protection.

FIGURE 33-8 Implant area 10 days after surgery.

Three weeks after surgery, the patient is back for the next postoperative check, and, in most cases, a time for fitting the Baha is decided. Most of our patients get their Baha about 6 weeks after surgery (Fig. 33-9). This timing is mainly due to practical and logistical issues and could probably be shortened. During the first 2 weeks, after the healing cap has been removed, the patient should use a mild ointment to keep the implant area clean and soft. After this time, the patient is told to use some mild soap or shampoo every day. The situation 3 months after surgery is shown in Figure 33-10 with the Baha coupling in place. Cleaning is crucial, and a high level of personal hygiene is imperative. Relatives are often asked to participate in the postoperative visits to be instructed on how to help the patient. After this, the patient comes back for follow-up at 6 to 12 month intervals.

FIGURE 33-9 Implant area after 6 weeks.

FIGURE 33-10 Healing is complete 3 months after surgery.

The Baha is fitted by an audiologist who selects and adjusts the device according to the patient’s audiogram and preferences. In this fitting procedure, it is important to take into account the patient’s lifestyle. Six weeks later, the patient returns for what is often a final adjustment.

ALTERNATIVES IN SURGERY

The above-described technique is the one that we use at the Implant Unit in Göteborg. It has been slowly refined over the 30 years that we have been using Baha. Some colleagues use different techniques, however.

Regarding preparing the implant site, the differences are small. Some surgeons do not follow all the steps described previously, but the success rate for establishing osseointegration seems to be very high, at least in adults. Instead of the Baha dermatome, some surgeons use a U-shaped manual flap. One reason cited for this technique is that the size of the implant area could be adjusted to the amount of soft tissue present. The thinning of the flap is the same, however. A straight vertical incision has been suggested, and could probably be used, too, but making the adequate soft tissue reduction would likely be more difficult, especially in an overweight patient.

PITFALLS AND COMPLICATIONS IN SURGERY AND DURING FOLLOW-UP

For a properly trained otologic surgeon, Baha surgery is simple and presents very few risks. Exposure of the dura mater or the wall of the sigmoid sinus is of no significance. Damage to these structures would be handled the same way as in ordinary mastoid surgery. A pedicle periosteal flap would stop cerebrospinal fluid leakage and blood from the low-pressure sigmoid sinus. Entering air cells during drilling is easily handled. If there is bone trabecula to support the implant, it could be used; if not, a new implant site close by is selected. When surgery is performed under local anesthesia, the patient could experience some discomfort because of suctioning.

Flap Necroses

Using the technique described, the frequency of flap necroses during the first 6 postoperative weeks was found to be 3%.²⁶ Only minor necroses (defined as <25% of the flap area) have been noted. Before the introduction of the Baha dermatome, the flap was thinned manually with a knife. With this method, the frequency of minor necroses was 9.2% and 1.3% for medium necroses. A flap necrosis, whether minimal or complete, has a very good prognosis with local treatment with mild ointment. Healing may take a long time, however, up to several months. A medium-sized flap necrosis is shown in Figure 33-11, and the final result after 3 months is shown in Figure 33-12.

FIGURE 33-11 A patient with medium-sized flap necrosis and exposed bone 3 weeks after surgery.

FIGURE 33-12 Same patient as in Figure 31-11, 3 months later.

Adverse Skin Reactions

Adverse skin reactions of any significance are very infrequent during the years after Baha surgery with the manual technique and with the dermatome. In a study with 4 to 8 years of follow-up, a blade was used to thin the flap manually.²⁷ Using the classification system by Holgers and associates,²⁸ only 2.8% of this group, with a grade 2 condition or worse, needed active treatment. With the Baha dermatome, the corresponding figure was 2%.²⁹

FIGURE 33-13 A patient with bilateral radical cavities has been wearing his Baha for more than 10 years.

Implant Stability

The possibility to establish osseointegration for the Baha is high. In an early study with 149 Baha patients, we had a success rate of 97.3% over 1 to 4 years. During the following 4 years, the success rate declined to 94.7%.²⁷ In a study with 144 implants, the success rate was 98.6% over 3 years.³⁰

HANDLING OF COMPLICATIONS

Related to Soft Tissue

A slight redness around the implant, grade 1 according to the Holger classification,²⁸ was noted in 4.3% of observations over a 4 to 8 year period.³¹ The first thing to check in a patient who has skin problems is if the implant is mobile. The internal screw keeping the abutment connected to the implant sometimes may come loose and has to be tightened. Over the same period, 69.7% of the patients did not have one episode of adverse skin reaction, and 87.1% had only one episode. The handling of a grade 1 reaction is often simply to ask the patient to intensify the daily cleaning procedure and apply some mild ointment. Extra outpatient controls are seldom needed. Even in more extensive negative soft tissue reactions, there is seldom any pain. As mentioned earlier, pain is often a sign of loss of implant integration.

In the situation in which there are grade 2 to 4 skin reactions, the surgeon has to be more active. Replacing the healing cap and applying gauze with an antibiotic ointment is often the first step. If irritation continues after 7 to 10 days, a regrafting may be necessary. A thin graft can be taken from the fold behind the ear. Temporary removal of the abutment is often the next step leaving the implant in place. In rare situations, the implant has to be removed. Removal can be done through unscrewing the implant or drilling it out with the guide drill. Culturing and oral antibiotics could be indicated if there are prolonged problems.

BAHA IN CHILDREN

Most authorities agree that hearing is crucial for normal speech, language, and mental development.¹⁸ This also seems to be true for unilateral hearing impairment. If a hearing-impaired child cannot use a conventional air conduction hearing aid and is not a candidate for a cochlear implant, the Baha is a strong alternative.

Surgical Aspects

The placement of the implant is of special concern in children. In children with a malformed ear, it is important to place the implant about 65 to 70 mm from an anticipated external ear canal opening. This position is often much more posterior than expected, but would not jeopardize autogenous external ear reconstruction later on. The dura mater of the middle cranial fossa is often low, and the facial nerve could have a more superficial route. The surgical procedure is basically the same as for adults with the important difference that a two-stage procedure is used. At the first stage, the implant is placed, but the Baha coupling is not attached. The implant is left to integrate for 4 to 6 months, after which the implant is exposed, and the Baha abutment is attached. In children, the dermatome is less suitable, and a U-shaped incision is made manually at the first and at the second stage. The soft tissue reduction is as important in children as in adults, even if the amount of subcutaneous tissue is much less.

The bone in a young child is soft because of a low mineral and high water content compared with adult bone. In drilling, care should be taken not to damage the dura mater or the wall of the sigmoid sinus. It does not matter, however, if these structures are found at the bottom of the implant site. The spiral drill with its countersink edges should be used without much pressure because the sharp edges could easily cut too much soft bone. The implant site often is in bone marrow space, which is of no importance.

The thickness of the temporal bone is crucial for implant integration. If a 3 mm long fixture were to be inserted, a minimum of 2.5 mm bone would be needed. The bone thickness at the implant site has been measured in 30 children.³² The mean thickness, at age of 5 years, was found to be only 2 mm, but with great variations. In patients younger than 4 years, the bone thickness is often reduced, and bone augmentation to be able to install an implant may be needed. A bone augmentation technique of this kind has been described earlier.³³ With expanded polytetrafluoroethylene membranes, appositional bone can be directed to grow under the flange of the fixture. The technique seems biologically sound because it can be used together with standard fixture and abutment placement, and it does not change therapy planning or time needed for osseointegration. If possible, a 4 mm implant should also be used in young children because the failure rate for 3 mm implants is much higher than for 4 mm implants.

If an expanded polytetrafluoroethylene membrane is used, this has to be removed at the second stage. The time between first and second stage of surgery has been extended in the youngest children to at least 6 months. The decision to extend the healing time period was based mainly on clinical parameters, including the thickness of the bone, the softness of the bone, other malformations of the temporal bone, and the primary stability of the implant. When a child is 3 years old, the standard 3 to 4 months between the first and second stage of surgery seems applicable.

In a study of 160 children, the success rate for osseointegration was 94.4%—that is, a failure rate of 5.6%.³⁴ If these figures are broken down, however, it becomes evident that the implant losses for the 3 mm implants were 8%, and the corresponding figure for the 4 mm implant was 1.7%