FIXATION WITH RECONSTRUCTION PLATES UNDER CRITICAL CONDITIONS - THE ROLE OF SCREW CHARACTERISTICS

Mandibular angular ostectomy defects in 12 sheep were bridged with a t itanium plate system (THORP), designed according to the screw-plate-lo cking principle. Two screw designs (hollow and solid) and two rough (p lasma-coated, sandblasted)-surface and one smooth (anodized)-surface s tructure were tested in a critical two-screw anchorage model. Fixation with two screws only per fragment was successful in only one-third of the cases. The fixation results were superior for screws with a rough surface, either sandblasted or plasma-coated. The hollow-screw design did not seem to offer special advantages over the solid screws with i dentical surface structure. In the successful cases, the bone-screw co ntact was maintained continuously over a period of 8 weeks, and anchor age was even enhanced by reinforcement of the cancellous structures. I n the unsuccessful cases, peri-implant bone resorption seemed to be re lated to jeopardized bone perfusion in the vicinity of the implants. I t is presumed that a plating system with locked screws has the advanta ge that only two screws will hold a relatively short condylar process. This study proved the assumption to be questionable. However, in view of the problems related to prosthetic replacement of the temporomandi bular joint, an attempt to salvage the articular portion may still be justified in selected cases, even if the chance of success is as low a s 30%.