Analysis of changes in implant screws subject to occlusal loading: a preliminary analysis

Loosening of the abutment screw can present a problem for the stability of the implant-supported crown or prosthesis. The purpose of this study was to understand the parameters of screw loosening, using an in vitro model, including loss of torque, screw head rotation, changes in screw dimension, and distortion of the implant-abutment joint. Implants (4 x 10 mm) were potted in autopolymerizing blocks. Abutments were placed with screws tightened with a 35-Ncm torque and standardized crowns fabricated. There were 3 implant systems used: Nobel Biocare USA, Inc. (Yorba Linda, CA), 3i Implant Innovations, Inc. (Palm Beach Gardens, FL), and Bio-Lok International, Inc. (Deerfield Beach, FL). Seven samples were tested for each system. Samples were loaded with 300-N loads for 50,000 cycles at 1 Hz. Torque turn audits were performed at 10,000, 25,000, and 50,000 cycles. At the conclusion of the loading, counterclockwise rotation of the abutment screw was measured. The screws were retrieved and measurements made compared with the controls. Finally, 1 sample from each group was embedded in resin, sectioned longitudinally, and examined under the standard error of the mean. The Nobel Biocare system showed a 9.4-Ncm loss of torque from the loading protocol. This result was accompanied by a counterclockwise rotation of 7 degrees and a 200-ìm elongation of the screw. Finally, there was compression and distortion of the longitudinally sectioned joint architecture observed with the standard error of the mean. From the 3i and Bio-Lok International groups, no loss of torque, counterclockwise rotation, or lengthening of the screws was observed. Intimate adaptation of the joint without distortion was seen in the longitudinal sections. Screw loosening appears to follow specific parameters that include counterclockwise rotation, lengthening of the screw, and distortion of the screw joint. This process is likely associated with both the physical properties of the screw as well as its configuration.