Screw Preload Analysis in a Repeated Torque of Variobase Type Abutments

This study aimed to quantify preload efficiency (PE%) and tightening/releasing energy (Et/Er) during ten repeated torque cycles in four Variobase type implant-abutment assemblies.

Twelve Straumann implants were assembled with Variobase type abutments representing four implant-abutment interface configurations: Regular-Neck Crown (RNC), Regular-Neck Bridge (RNB), Regular CrossFit Crown (RCC), and Regular CrossFit Bridge (RCB). Each assembly underwent ten consecutive tightening-loosening cycles at 35 Ncm using an ElectroPuls E10000 linear-torsion testing machine. Friction fit was assessed after the 5th and 10th cycles.

All groups demonstrated a progressive decrease in PE% with repeated tightening, except the RN interface, which showed transient torque exceeding applied tightening torque during the 2nd-3rd cycles. Bone-level (RC) assemblies exhibited greater variability and lower predictability compared with tissue-level (RN) configurations. Et/Er values remained relatively stable across cycles, although RC groups showed more pronounced energy loss and friction-fit-dependent deviations. Friction fit occurred exclusively in RC groups and significantly influenced Et/Er behavior.

New assemblies generated higher PE% than those subjected to repeated cycles, with the exception of RN connections, which maintained superior preload stability and predictability. RC interfaces demonstrated lower and more variable PE% and energy profiles. Repeated tightening did not substantially reduce overall energy containment; however, friction fit altered torque-angle signatures and influenced mechanical performance.

Understanding preload decay and friction-fit effects in different implant platforms provides clinicians with guidance for selecting abutment types and recognizing platform-specific variations may help minimize screw loosening and improve long-term prosthetic stability.