Retreatment: Fractured Implants Due To Biomechanical Overload

The strength of osseointegration, the biologic and biomechanical union of bone to alloplastic materials such as titanium, is well know and thoroughly documented in the dental literature. From early clinical trials to the present modes of implant treatment, a high success rate has been published for implant and prosthesis stability in the anterior mandible.

Long term successful function of this prosthodontic reconstruction has been reported for both the two-stage protocol as well as immediately loaded implants such as the TEETH IN A DAY protocol.

The long-term function of an implant-supported prosthesis

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The mechanical factors to consider in such rehabilitations include the number, length, diameter, and alloy of the implants, as well as their position in the jaw relative to the anticipated occlusal scheme. Leverage factors that are generated by cantilevers can place enormous stress on the implant as well as the bone implant interface. Leverage forces on implants also increase in proportion to the vertical height of the prosthesis. In patients with minimal bone loss, multiple long implants will easily support a prosthesis that has a relatively normal coronal height. However, with severely advanced alveolar bone atrophy, the prosthetic height, from the top of the implant to the incisal or
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occlusal table, has been measured in excess of 30 mm. This volume of prosthetic material is generally required in order to fulfill functional performance and restore vertical dimension to the lower third of the face.

The engineering design for the implant prosthesis in cases with severe alveolar atrophy must consider the potential for parafunctional loading, the off axis loading forces that are applied to the implants and bone by a variety of cantilevers, and the intense muscular force generated on "prosthetic levers" during normal masticatory function. The catastrophic outcome of inadequately engineered implant supported prostheses can have
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enormous negative effects on both the patient and the treating doctor. Functional overloads on implants can be generated by both fixed detachable as well as overdenture prostheses. Such overloads may lead to prostheses fracture, abutment loosening or fracture, implant fracture, or loss of osseointegration.

Warning signs of impending catastrophe can be observed clinically in the form of prosthetic screw loosening, abutment loosening, and occasionally excessively rapid attrition of the incisal and occlusal tables.