[Mandibular hybrid dentures with 4 implants. An in-vitro stress analysis]

Four two-component implants (Bonefit) inserted in a mandibular model were fitted with various overdentures and then subjected to tension analysis using wire strain gauges. The implants were positioned between the mandibular foramina of the model; data were obtained for mesial, distal, lingual and buccal compressive and tensile loads of 50 N on duplicates of the prosthodontic base at the level of the first molar, using a measuring wire gauge. The greatest forces were registered for the secondary-splint telescope crown prosthesis; these forces were up to three times as high as the corresponding axial load. In the case of bar attachments, it is not so much the strain on the implant when the prosthesis is loaded that tends to cause problems, but rather the original process of screwing down the overdenture on insertion. During this process, the implants are subjected to forces so high that the use of a torque indicator is deemed to be indispensable as a clinical consequence of this finding. For similar considerations, a conditionally removable bar construction should not be removed during follow-up examinations. Magnetically retained dentures, by contrast, barely exert any tensile forces on the implants.