A Comparison of Torque Stress on Abutment Screw of External Hexagon and Morse Taper Implant.

AIM: This three-dimensional (3D) finite element analysis observed the stress distribution on the prosthetic screws on external hexagon implant and morse taper implant with different tightening loads.

MATERIALS AND METHODS: For this, two different 3D models assembly were obtained from the manufacturer and transferred to finite element analysis software: External hex implant model (EHM), and Morse taper implant model (MTM), both compounds by 3.75 × 7 mm implant, abutment and abutment screw. Bolt pretension force was applied on the shaft next to the threads of the prosthetic abutment. Preload was calculated using the torque on the prosthetic screw as recommended by the manufacturer (EHM30 and MTM20) and 10 Ncm torque above the manufacturer recommendations (EHM40 and MTM30). Maximum von mises equivalent stresses were obtained on the screws.

RESULTS: Preload values results were 243.18N (EHM30), 229.71N (MTM20), 324.24N (EHM40) and 344.57N (MTM30). In EHM30, EHM40 and MTM20 models the maximum stresses were below the yield strength of the abutment screw material. However, the maximum stress in MTM30 model was higher than the reference value.

CONCLUSION: The torque loads above the manufacturer recommendations can cause plastic deformation in the MT abutment screw threads. The screws of morse taper implant can be more sensitive to higher loads than external hexagon implant.

CLINICAL SIGNIFICANCE: The adequate torque can result in screw loosening, while fracture may occur if torque is excessive. Abutment screw suffers many screwing cycles in its lifetime in a way that some tightening forces are above manufacturer recommendations.