Retention force and stress distribution analysis of the cementless double crown–type implant-supported prosthesis

Statement of problem Implant-supported fixed dental prostheses have been classified into screw-retained and cement-retained types, and each retaining type has complications. A novel retentive cementless double crown (CLD) type of the implant-supported fixed prosthesis has been developed. CLD has air pockets in the intaglio surface of the crown and does not require cement or a screw hole. However, studies on the retention force and stress distribution of the system are lacking. Purpose The purpose of this in vitro study was to evaluate the retention force and stress distribution in the CLD system. Material and methods The specimen comprised an implant, a titanium abutment, and a zirconia crown. Retention forces of 10 specimens of the CLD type were measured at no loading and after cyclic loading for 50, 100, 200, 600, 10 000, and 1 000 000 cycles by using a universal testing machine with a custom attachment device. Forty specimens of the stress distribution test were divided into 4 groups based on the retention type (cement-retained or CLD type) and load direction (vertical or oblique). Strain gauges were attached onto the buccal and lingual sides of the implant, and microstrain values were measured. One-way analysis of variance with the post hoc Tukey honestly significant difference test was performed on the retention force data, and the t test was performed to analyze the microstrain value data (α=.05). Results Retention forces after 0, 50, 100, 200, 600, 10 000, and 1 000 000 load cycles were 18.12 ±6.16 N, 20.47 ±5.78 N, 19.79 ±6.61 N, 18.46 ±5.23 N, 19.60 ±6.93 N, 21.75 ±5.03 N, and 40.91 ±9.32 N, respectively, and after 1 000 000 cycles, the retention force was significantly higher than that of other load cycles (P<.05). The mean of maximum microstrain values under the vertical load were similar in the cement-retained type (buccal side, 834.96 ±53.69 μm/m; lingual side, 490.76 ±34.12 μm/m) and the CLD type (buccal side, 814.28 ±71.20 μm/m; lingual side, 479.10 ±30.74) (P>.05), and the mean of maximum microstrain values under the oblique load was also similar in the cement-retained type (buccal side, 1991.04 ±109.89 μm/m; lingual side, -2232.41 ±189.88) and the CLD type (buccal side, 1932.47 ±152.51 μm/m; lingual side, -2097.47 ±130.69 μm/m) (P>.05). Conclusions The CLD type had clinically acceptable retention during 1 000 000 load cycles and had a similar or better stress distribution capability than the cement-retained type.