Stress distribution and bone–implant interface behavior in pick-up implant impression

The purpose of this research was to study the stress distribution at the bone–implant contact in the pick-up implant impression technique. The mechanical properties and the curing dynamics of Pattern Resin LS were previously investigated.

Measurements of the elastic modules of Pattern Resin LS were carried out after complete curing by standard stress–strain tests and a three-point bending technique. A specific laboratory set-up was implemented to monitor the curing dynamics and the linear contraction by using optical imaging methods. A finite element model (FEM) was created and a complete assembly was made with 33 solid bodies representing the main components of the system including the external portion of cortical bone, the internal portion of trabecular bone, six implants, six copings, one bridging structure (modular individual tray), six modules, and six pattern resin layers to fill up the interstitial spaces. Isotropic characteristics for cortical and trabecular bone were incorporated. The stress distribution was measured at the total body of the bone–implant interface, at the external cortical bone surface, at the coping–implant connection area, and at the tray–coping junction. Parametric analysis was also done to study the behavior of two types of internal connection in the impression phase: type A consisting of a 0.4 mm tube-in-tube connection with a triangular head by three cams (Cam-Log Biotechnologies, Germany); type B consisting of a 0.5-mm internal hexagonal connection (Biomet 3i, USA).

The measured values of Young’s modulus and the flexural modulus of the resin were 1.58 ± 0.12 GPa and 2.07 ± 0.24 GPa, respectively. The final volume contraction was found to be strongly dependent on the powder/liquid ratio; specifically, the tests suggested that the highest value of the powder/liquid ratio (2:1) was the most favorable one. In all cases, however, the Pattern Resin behavior was within the linear elastic range, without any permanent deformation or detachment. At individual tray–Pattern Resin interfaces, a stress of about 20.0 MPa was produced for each component; FEM analysis showed that the stress level at the implant–transfer copings connection was higher than that at the bone–implant interface. The trabecular and cortical bone showed the lowest level of stress concentration. B-type transfer showed a higher stress at the transfer coping–implant connection; A-type transfer coping allowed for a higher deformation area, increasing the risk of detachment.

The mechanical properties of Pattern Resin LS seem to be appropriate for use in the pick-up impression technique thanks to the low-volume contraction, the capabilities to dampen misfits, and permanent deformation effects. The shape of the copings affects the values of the stress both in the transfer coping–implant connection zones and at resin–transfer coping interfaces. A thicker clearance of the module–tray connection helps in reducing the mechanical stress. The FEM analysis showed that the removal procedure of the pick-up implant impression tray does not produce any irreversible damage on the cortical and trabecular bone structure. Since the yield threshold was high, the stress distribution at the bone–implant–coping configurations was largely within the safety range for the multiple internal-connection implants, in combination with a modular individual impression tray.