Assessment of the internal connection of an angled screw channel system

The angled screw channel (ASC) design has been well accepted for implant prostheses. However, investigation into the behavior of the ASC connection is sparse.

The purpose of this in vitro study was to assess the effect of cyclic loading on the internal connection of an ASC system compared with straight access systems by measuring reverse torque values (RTVs) and using microcomputed tomography (µCT) imaging.

Thirty dental implants (SLActive; Institut Straumann AG) were embedded in autopolymerizing acrylic resin and divided into 3 groups: Group V0, prefabricated abutment with a straight access channel; Group V25, prefabricated abutment with a 25-degree access channel; and Group C, cemented custom abutment with straight access. All specimens were initially secured with 10 Ncm using a digital torque gauge. A subset of specimens from each group (n=4) were randomly selected and assigned for µCT imaging examination and then reimaged after tightening to 35 Ncm. All specimens were subjected to cyclic loading for 5000 cycles with a loading force of 155 ±5 N at 1.2 Hz frequency in a 37 °C water bath using a fatigue machine. Repeat µCT imaging was done after cyclic loading for the representative specimens. The changes to the internal connection were measured on µCT images using a software program, and the RTV measurement of each specimen after cyclic loading treatment was recorded. The ANOVA test was used to analyze the statistical significance of the RTVs among groups (α=.05). The paired t test was used to conduct within-group comparisons of the measurement data.

Mean ±standard deviation of RTVs varied from 29.05 ±2.99 Ncm to 29.75 ±2.61 Ncm, with no statistically significant differences among the 3 groups (P=.820). The distances from the base of the screw head to the apex of the screw-abutment-implant (SAI) system and internal air space volume within the groups V0 and V25 decreased significantly (P<.05) between the 10-Ncm and 35-Ncm torque applications. Off-axis loading resulted in rotational movement of the abutment that might have created a tensional force on the implant screw.

Results for the prefabricated abutment groups (V0, V25) indicated that the RTVs were not affected by the amount of angle correction. A counterclockwise rotational movement of the abutment and screw within the SAI system would be formed during off-axial loading.