Retention Mechanisms for Implant Retained Maxillofacial Prostheses

Objectives: Restorations of Maxillofacial defects present many challenges to the prosthodontist. Among them are limited retention, movable tissue bed, skin reaction to adhesives and reduced life span of the prostheses due to daily adhesive applications. Use of sseointegrated implants has increased because they reduce the need for adhesive, allow for esthetic prostheses with thinner margins, and increase patient acceptance and confidence. The purpose of this investigation was to compare the retention and load transfer characteristics of retention mechanisms for implant retained maxillofacial prostheses. Methods: for the following retention mechanisms were studied:Hader bar, magnets, O-Ring attachment, and the new Locator attachment The retention characteristics of these attachments were determined on an Instron test machine. A 3.75 mm diameter 4 mm long craniofacial implant (Nobel Biocare) was embedded in a in 4 cm x 3.5 cm x 0.9 cm block of photoelastic material. The retentive capacity was determined as the force required to dislodge the devices on an Instron test machine. Initial retention and changes with multiple removals simulating long term use were examined. Dislodgement forces were then applied to each of the retention devices with a calibrated load cell in the field of a circular polariscope. Resulting stresses were monitored and recorded photographically. Results: The O-Ring and Locator devices were most retentive (initially 4 and 3.5 lb), the magnet lowest (0.75 lb), and the Hader bar with clips was intermediate (2.75 lb). All attachments, except the magnet, decreased in retention after simulated long term use. Stress generation by the devices was inversely proportional to the retentive capacity: the O-Ring device produced highest stress, magnet the least, with the Locator and Hader with clips intermediate stress. Conclusions: The results suggest that a balance between retention and stress production is necessary in selecting a retention mechanism for the specific requirement of the patient.