COST CONSIDERATIONS.
BOI restorations can achieve a favorable ratio of implants to dental elements: with 4 or 5 implants supporting 12 (ceramic) dental elements, the ratio is between 1:3 and 1:2.4. Accordingly, the cost of the materials for fixed dentures for one jaw are quite low ($150 per implant, ie, $600–$750 for each jaw). In the maxilla, it is necessary to use double disks due to inferior bone quality compared with the mandible, increasing the cost by 30%. However, the smaller number of implants is only one of several cost-reducing factors. Others include the short treatment and the need for only one surgical intervention. On average, it only takes a total of 3.5 hours to restore an entire mandible. This includes the surgical intervention, framework tryin, bite registration, and definitive insertion of the superstructure.

TEMPORARY BRIDGE.
The existing complete denture is cut back to the bridge and cemented onto the abutments. If the denture is missing or does not cover the implants, the temporary restoration is constructed using pulldown splints produced in our own laboratory using standard bridges. Although it is sufficient in principle to use three implants for the temporary bridge, inclusion of all four abutments is recommended, particularly if the temporary bridge is to worn for an extended period, ie, for more than 12 days. In the absence of procedural obstacles or significant postoperative swelling, the definitive bridges are integrated as early as on the third or fourth postoperative day. Often a temporary bridge is not needed, further reducing the risk of wound edge contamination.

DEFINITIVE RESTORATION.
The metal framework has to be exceedingly stable. To withstand the bending stress, it should preferably be cast from a nonnoble alloy. This ensures that ceramic material cannot chip off. The physiological movement of tubular bones such as the mandible involves significant traveling of the bone itself. Medial and caudal flexion will bend and twist the mandible by up to 2 mm.8–10 Richter11 therefore postulated that prosthetic structures in the mandible should always be divided into three parts. This prosthodontic approach is supported for crestal implants, which osseointegrate and transmit loads along their vertical axis.
Based on our own clinical experience, however, this is not a compulsory requirement in the case of BOI because the vertical implant segments do not have to osseointegrate for a BOI to be functional. It is only the basal horizontal load transmission surfaces that have to osseointegrate and transmit masticatory loads. The flexible threaded shaft between the basal disk and the prosthetic structure constitutes a perfectly serviceable interface that follows the torsion movements of the mandible.
Another postulate known from crestal implants is to mount the superstructure onto the implants in a tension-free manner. This is difficult to achieve in complete fixed BOIsupported dentures, notably in mandibular dentures that are loaded immediately. For immediate postoperative impression-taking, the mandible is closed as much as possible. The bridge frame is rarely tried in free of tension; in many cases it has to be readjusted. This can be attributed to functional bone dislocation that will cause a limited but persistent deformation of the mandible immediately after surgery. It has been observed that whenever this happens, the anterior segment of the mandibular curve will bend caudally.
When cementing the complete mandibular denture, the implant shafts are first fixated relative to each other. When the relation between bridge and implant is thus defined, the initial elastic movement of the threaded pin relative to the disk should be minimal. While the cement hardens, patients are instructed to slightly close in an occlusal position.
Basal osseointegration is currently the only procedure for treating extreme mandibular atrophy with a fixed prosthetic denture in a single surgical session. In our view, alternative procedures such as osseodistraction and ridge augmentation using bone from the hip, rib, or parietal skull are obsolete. Those techniques use unsuitable implants requiring cumbersome and risky preliminary interventions to obtain the bone used for those implants instead of using implants actually suited for the available bone.
This is not to say that implant and bone loss cannot occur with the basal osseointegration technique. The bone loss can be controlled, however, by frequently recalling patients and ensuring timely and competent intervention. Unlike many treatment failures using crestal implants, the type and extent of those cases was limited enough to allow insertion of a second BOI denture.

REFERENCES.

1. Martin BR, Burr DB. Mechanical Adaptation. Structure, Function and Adaptation of Compact Bonds. New York: Raven Press; 1989:143–185.
2. Epker BN, Frost HM. Correlation of Bone Resorption and Formation with the Physical Behavior of Loaded Bone. J Dent Res. 1965;44:33–41.
3. Linkow LUS. Patent 4,044,467, June 28, 1976.
4. Spiekermann H. Presentation at the 5th International Winter Meeting for maxillofacial surgeons, orthodontists, and implantologists at St. Jakob, Austria; April 25, 2000.
5. Roberts HD. Oral implants. Part I: The Ramus Frame Dent Surv. 1978;54: 20–21.
6. Besch KJ. Konsensus zu BOI. Schweiz. Monatsschr. Zahnmed. 1999; 109:9–12.
7. Brandt HH. Einführung in die Implantologie. Munich: Urban & Schwarzenberg; 1996:114ff.
8. Ashman RB, van Buskirk WC. The elastic properties of the human mandible. Adv Dent Res. 1987;1:64–67.
9. Jung F. Die funktionell-elastische Deformation des Kieferknochens und die Eigenbeweglichkeit der Zähne. Schweiz Monatsschr Zahnmed. 1960; 70:2–30.
10. Grant AA. Some aspects of mandibular movement: Acceleration and horizontal distortion. Ann Acad Med. 1986; 15:305–310.
11. Richter E-J. Prothetik im Unterkiefer. Schweiz Monatsschr Zahnmed 1999;109:117–126.