n bicortical fixation in mandibular molar areas is thus far limited.
Load bearing capacity
The net effect of all the considerations discussed above governs what dynamic load the fixtures are able to bear. It is the quality of all perifixtural bone and the total interface surface of all fixtures that determine the loadbearing capacity. Consequently, four 15 - mm - long fixtures may be capable of carrying the same load as six 10mm long ones, provided all are strategically well placed.
The long - term fixture survival rate is slightly smaller for the maxillae than for the mandible. These results are likely to reflect differences in the load - bearing capacity between the jaws. Such differences could theoretically require a greater fixture/bone interface in the maxillae for adequate load distribution, that is, more or possibly longer fixtures than in the mandible.
Trauma to the host tissues
l. Handling of the covering soft tissues. The leading surgical principle of the osseointegration method has always been to minimize trauma to the host tissues. Incisions should be placed to reduce interference with vascularization and in areas where there is little frictional (denture) load. Consequently, individual modifications for earlier reinsertion of a temporary denture or easier access by placing the incision palatally or on the top of the crest, respectively, entail greater risks than the originally recommended placement of the incision in the buccal vestibule. Flaps should be raised by strict subperiosteal, almost superficially intracortical, dissection to preserve maximam osteogenicity of the periosteum (for review see Adell). Their watertight closure is of greater importance when implants have been installed than if the flaps rest entirely on vascularized bone. An easily overlooked area is the mandibular anterior region. If a vertical mattress suture technique (Fig 7-2) or a double-layer closure is not used, even an experienced clinician may have to face later dehiscences due to the considerable pull from the mentalis muscle.
2. Handling of the bone tissue. All aspects of the process of drilling in bone have been carefully scrutinized to avoid frictional heat, and strict recommendations have been published. Clinical studies in dense mandibular bone have confirmed that if these recommendations are followed, the frictional heat at drilling for fixtures will stay below the threshold level of 47°C for l minute. Frictional heat above this level will prevent osseointegration from taking place.
All investigations on this item have been performed with pretapping of the fixture sites. If
self -tapping fixtures are inserted indiscriminately in dense mandibular cortical bone - a purpose
for which they are not intended - frictional heat could be generated. It should also be kept in mind
that extremely dense, poorly vascularized bone, frequently seen in severely resorbed mandibles especially in the symphysis region, requires both a minimum of flap exposure to prevent devascularization and an extra precutting with a 3.15 mm drill to make the subsequent pretapping possible and/or prevent excess heat at this stage.
Fig 7-2 Correct method watertight closure in the mandibular anterior region by means of vertical mattress sutures.
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