Influence of implant internal thread deformation and repair on screw torque maintenance: An in vitro study

Abutment or screw fracture is a complication of implant-supported prostheses that can damage the internal threads of the implant. Thread tapping can restore the function of the damaged implant but could affect mechanical performance.

The purpose of this in vitro study was to evaluate the influence of the damage to and subsequent repair of the internal threads of dental implants on the maintenance of screw torque after functional loading.

Ten external connection implants (Biomimetic Coral EC; Avinent) with 17-degree angled abutments for cemented restorations were used for the CONTROL group. The recommended torque (35 Ncm) was applied, and initial retrieval torque values (RT PRE) were recorded with a surgical motor (iChiropro; Bien Air). The specimens were subjected to the cyclic loading equivalent of 1 year of in vivo use (300 000 cycles, at 200 N and 2 Hz), and the final retrieval torque values (RT POST) were recorded. The specimens were subjected to internal thread deformation by the pressurized insertion of a drill, and the threads were repaired with a manual thread tapping tool to constitute the repaired implant group (REP). The same variables were recorded as in the CONTROL group. The data were evaluated for normality with the Shapiro-Wilk test; the Student t test was used for the variables that met normality, and the Mann Whitney U-test was used for nonnormal variables (α=.05).

Initial torque values (RT PRE) showed significant differences (P=.002) between the groups, with mean ±standard deviation values of 28.41 ±3.18 Ncm for the CONTROL group and 18.77 ±1.88 Ncm for the REP group. However, no statistically significant differences (P=.499) were observed in the final retrieval torque (RT POST) values between the groups, with mean ±standard deviation values of 18.66 ±3.26 Ncm for the CONTROL group and 20.38 ±5.53 Ncm for the REP group. Statistically significant differences were found in the difference in torque (ΔRT) between the groups (P<.001). No loosening was recorded, but one of the screws in the REP group fractured during cyclic loading.

The deformation and repair of the internal threads of implants negatively impacted the maintenance of the initial torque but did not affect the final torque after functional loading. Within the limitations of this study, thread repair could be an effective method of extending the lifespan of implants with damaged threads, as it is straightforward, conservative, and cost-effective. Further research in this area is required to evaluate the potential increase of stress generated in the screw.

The datasets used and analyzed during the current study are available from the corresponding author on request.