ANTERIOR IMPLANTS.
If sufficient vertical space is available, the implants used are usually the ones with two disks. The basal disk has a diameter of 9 or 10 mm, whereas the crestal disk is 7 mm in diameter. If the insertion of double disks fails due to the lack of available bone, a single BOI with a 7- to 9-mm diameter and shafts between 8 and 13.5 mm can be used instead. Disks ,9 mm in diameter should be avoided; larger disks are rarely required.

POSTERIOR IMPLANTS.
The implants used here are usually of a square shape, having a disk of 9 3 12 mm or 10 3 14 mm with shafts of 10 to 13.5 mm in length, depending on the desired vertical dimension and the available horizontal bone. The height of the disk itself is 0.6 mm: this allows the implant to participate in the flexion of the mandible and provides safe ground for the fixed bridge. In general, square implants are an excellent choice because the threaded pin, when inserted from the side, always arrives in a favorable medial position. The absorption of the distal mandible in a centripetal direction can, in part, be compensated during implant placement. The longitudinal shape of the implant results in excellent primary stability. Even wider mandibles no longer require large BOI disks offering rotational symmetry, facilitating minimally invasive implant-bed preparation instead. The best implant site is at the base of the heavily mineralized anterior corner of the ascending ramus that can be easily visualized radiographically.
If the vertical bone available above the mandibular nerve is, 2 mm, infranerve implant insertion (infranerve implant insertion: The disk is introduced below the mandibular nerve; the threaded carrier is located at the side of the nerve.) is indicated. Lengthy implants are virtually useless for this purpose because the nerve will likely be located too far vestibularly. Instead, unilaterally square implants (often 9SG6, 9SG8) or the older round implants are used. In experienced hands, the technique of infranerve implant insertion is no more difficult than supranerve insertion. Using a sharp cutter or a round bur, the bony nerve canal is explored cranially, and the nerve is exposed. The important part is to explore the caudal extension of the nerve canal to prevent caudally displaced nerves from being injured by the cutter. If the nerve in the selected area is lo cated lingually, an infranerve implant cut is prepared from the vestibular aspect. If the nerve is located vestibularly, it is necessary to search for a suitable implant site further distally. In practice, the implant can always be inserted from the vestibular aspect because the mandibular nerve will eventually exit the mandible lingually. Infranerval insertion from the lingual aspect was never necessary, although theoretically this is an option. With BOI, the mandibular nerve rarely needs to be exposed and displaced.
The necessary manipulations in the nerve canal and almost of all the postoperative flap opening in a vestibular direction (tending to require traction and mobilization of the mandibular nerve) result in bland paresthesias, which will rarely persist beyond 3 months; if the paresthesias persists, it will be confined to the area of the chin. Of course, patients need to be informed of this possibility beforehand.
This concept of implant placement is abandoned only if the desired implant site lacks bone substance or if an implant is not an option for other reasons, such as in the presence of excessive periodontal breakdown or previous implant loss. In these cases, it is usually necessary to insert more than four implants for a complete mandibular denture.
Our long-term observation of many cases has shown that implant loss or osteolysis in the anterior mandibular segment is a rare occurrence. The same can be said of crestal implant systems; one factor certainly is that smaller masticatory loads act on a more stable bone. Also, due to the activity of the tongue and the jaw-lowering muscles, this bony environment always tends toward apposition of bone. In those rare cases where osteolysis did occur, these were unilateral in connection with distal BOI, usually due to denture imbalance and overload and frequently combined with infection resulting from poor hygiene. High initial tension acting on the bridge frame with elastic deformation of BOI in the area of the threaded pin can is also implicated.
Occasional unilateral distal implant loss was initially a surprise. After thorough analysis of this phenomenon based on routine follow-up orthopantomographs at 12 months, at which time most patients were either asymptomatic or had only mild complaints, pronounced osteolysis was invariably detected around the disk of the affected distal implant. No other implants were affected. To address this problem, the bridge was shortened and separated from the threaded pin of the affected implant, which would immediately come loose and had to be removed definitively. This indication to remove BOIs was summarized in a “Consensus on BOI.”
Evidently, implants not affected by osteolysis had assumed the whole task of transmitting masticatory loads to the mandible without the bone around the disks being appreciably damaged. Our next step would therefore be to shorten the bridge on the affected side, depending on the bite situation, back to the region of the second premolar, thus leaving the bridge in function.
A new implant is inserted after 6 to 8 weeks depending on clinical or prosthodontic requirements. Forty days are usually allowed for the new implant to heal. Subsequently, the mandibular bridge is either extended distally or is replaced.
Distal implant loss can be attributable to several causes:

1. Overload by unilateral early denture contact. Consequences of this overload manifest themselves mainly within 12 months of inserting the bridge. Later, the basal disks will be ossified to the point where the overload will more likely cause the bar attachment to break. Particularly at risk are patients who seek treatment elsewhere to have the denture rebased or crowns or bridges inserted in the opposite jaw. This is because many dentists lack experience with BOI systems and their correct appearance and will fail to adjust the occlusion accordingly. Patients are therefore informed that in the interest of their own safety, they should only seek dental treatment from dentists who are experienced with BOI.
2. Vertical relation too high. When the mandibular ridge is severely atrophied, dentists tend to be very conservative about the vertical occlusal relation of mandibular complete dentures. This is to prevent the denture from causing interferences as it is moving through the oral cavity and to ensure that the tongue can effectively control it. The use of BOI implants brings back the option to adjust the vertical relation correctly. Although this option is very much in line with the patients’ esthetic needs, the vertical relation will further change as the temporomandibular joint is undergoing postoperative adaptation. The same effect is seen as the biting force recovers, which frequently requires extensive adjustments at follow-up. Patients who cannot cope with the new vertical relation are subjected to gradual subtractive treatment (reducing the occlusal surfaces to increase the vertical dimension) until they are at ease with the bite position. No possibility has yet been found to determine in advance which vertical dimension is tolerated by patients in situ. One of the most important reasons for increasing the vertical dimension is the concomitant reduction in masticatory forces.
3. Poor hygiene. In the earlier days, retention-related infections with the ribbed shafts of the old “disk implants” would sometimes be observed, but such cases are rarely seen with today’s plain shafts.

Our experience does not support the belief frequently encountered in the literature7 that medial and caudal torsion of the (atrophied) mandibular ridge poses an insurmountable obstacle to providing complete dentures. At least for BOI, this belief does not hold true, provided that the threaded vertical portions of the implants are chosen long enough. In our experience, the movement patterns of the atrophied ridge tolerates splinted implants in the anterior mandible and prosthetic support on three or, better still, four sides (circular). Additional implants in the premolar area are relatively often lost (with no damage to the overall structure). Their use was therefore abandoned altogether in favor of safe implant areas.
It is also important to note that the implant is immediately loaded with the prosthetic superstructure, which ensures that the bone regenerates in a functionally sound fashion.