The influence of multi-unit abutment angulation on the prosthetic screw-joint stability

The purpose of this in vitro study was to evaluate the effect of the angulation degree of multi-unit abutment (MUA) on the prosthetic screw-joint stability using a reverse torque test of the prosthetic screw and a pull-out test of the bonded coping.

Thirty specimens with a two-implant segmental portion of an implant-supported fixed dental prosthesis design were prepared. Titanium copings (Variobase; Straumann USA) were cemented to the zirconia specimens (ZirCAD; Ivoclar Vivadent) using RelyX Universal Cement and Scotchbond Universal Plus Adhesive (3 M ESPE). The groups were defined as follows: Group S-S: straight implant + 0° MUA and straight implant + 0° MUA, used as the controls; Group S-17: straight implant + 0° MUA and 17° angulated implant + 17° MUA; and Group S-30: straight implant + 0° MUA and 30° angulated implant + 30° MUA. The initial torque value and initial reverse torque values of prosthetic screws were recorded using a digital torque gauge (Model HTGS-15; Imada Inc.). Zirconia specimens and cemented titanium copings were treated with thermal cycling (5000 cycles of 5°C-55°C, dwelling time 20 s). After this limited thermal cycling, all prosthetic screws were replaced with new screws to connect the zirconia plate to MUAs with tightening to 15 Ncm. All specimens were then subjected to masticatory simulation using cyclic loading treatment (300 N, 1.5 Hz in a 37°C water bath for 5000 cycles). At the end of the cyclic loading, final reverse torque values were recorded. A pull-out test of the cemented coping was conducted using a universal testing machine with a crosshead speed of 5 mm/min for all specimens. Failure patterns of the titanium copings and damage to the prosthetic screws were examined under an optical microscope. Mixed-effects linear modeling and estimated marginal means were used to analyze torque value changes. One-way ANOVA was used to analyze retentive force values from the pull-out test at α = 0.05.

The torque value loss percentage of individual abutments ranged from 16.43% to 28.78%. There was a significantly higher torque value loss of Group S-30 when compared to Group S-S before cyclic loading (p = 0.01). After cyclic loading treatment, there is an increased torque value loss in Group S-S compared with before cyclic loading (p = 0.02). No statistically significant differences in pull-out force were observed among groups (p = 0.689). Debonding of titanium coping occurred in Group S-S only, while prosthetic screw deformations occurred 100% in Group S-30.

Prosthetic screw loosening was expected to occur at the same rate in all tested groups after cyclic loading treatment. However, when complications such as debonding of titanium coping or prosthetic screw fracture occurred, the modes of failure differed in each group.