Abstract

Statement of problem: A hemi-engaging abutment design has been suggested to improve the stability of the implant-to-abutment interface compared with that of a fully nonengaging design to restore implant-supported fixed partial denture. However, controversy persists regarding the benefit of using a hemi-engaging abutment design and prompts the need for specific mechanical testing on the effect of these designs on screw preload under simulated clinical conditions.

Purpose: The purpose of this in vitro study was to determine whether significant differences in preload values of the screw before and after cyclic loading exist between hemi-engaging and nonengaging abutment fixed partial denture designs.

Material and methods: Twenty-four conical connection implants measuring 4.3×10 mm (Nobel Biocare Replace Conical Connection; Nobel Biocare) were mounted in acrylic resin blocks. Specimens were divided into 2 groups. An experimental group included 12 three-unit fixed partial dentures with a hemi-engaging design; a control group included 12 three-unit fixed partial dentures with the conventional design of 2 nonengaging abutments. A digital screw torque meter was used to measure screw torque values per the manufacturer's recommendation of 35 Ncm. Reverse torque value was measured before cyclic loading and referred to as initial preload. After cyclic loading, reverse torque value was measured and referred to as final preload. The effect of cyclic loading was evaluated by averaging the reverse torque value across the 2 screws in each specimen and then calculating the changes between the initial preload and final preload. The difference between initial and final preload was referred to as reverse torque difference. An additional reverse torque difference, referred to as reverse torque difference-nonengaging, was calculated for the nonengaging screws in each experimental specimen and for 1 randomly selected screw of the 2 in each control specimen. Preload efficiency before and after cyclic loading was also calculated. All groups went through cyclic loading using a universal testing machine. The specimens went through axial loading first, and then the reverse torque value was measured. Twenty-four new abutment screws were then used, and the specimens then went through lateral loading at 30 degrees. Load was applied to the units (1.0×106 cycles) for each loading axis. The statistical significance of differences between the axial and lateral reverse torque difference and between the 2 groups of reverse torque difference and reverse torque difference-nonengaging were assessed using Mann-Whitney U tests (α=.05).

Results: A comparison of reverse torque difference between loading types revealed no significant difference (P=.773). Therefore, data for the 2 loading types were combined before comparing the reverse torque difference and reverse torque difference-nonengaging values between the 2 groups based on abutment design (12 hemi-engaging designs in the experimental group and 12 fully nonengaging designs in the control group). The experimental group mean reverse torque difference was -0.65 ±1.95 Ncm (range -4.0 to 2.4 Ncm), and the control group mean reverse torque difference was -2.5 ±5.44 Ncm (range -15.3 to 5.3 Ncm). No significant difference was found (P=.340). Furthermore, no significant difference was found between the reverse torque difference for the nonengaging screw in each of the 12 implants with a hemi-engaging design versus 1 randomly selected nonengaging screw in each of the 12 implants with a fully nonengaging design (P=.355).

Conclusions: No significant difference was found in screw preload between a hemi-engaging and a full nonengaging 3-unit fixed partial denture supported by conical connection implant configurations before and after cyclic loading.