Failure strength of 2.0 locking versus 2.0 conventional Synthes mandibular plates: A laboratory model

This laboratory study compared the performance of locking versus nonlocking Synthes (Synthes, Paoli, PA) 2-mm mandibular fixation plates. The purpose of the study was to determine if there are physical or mechanical properties to support the use of one rigid fixation device over the other.

A bovine rib model was selected for this study because they closely approximate characteristics of the human mandible. Two experimental groups composed of 10 locking and 10 nonlocking 2-mm plates were secured to randomly selected osteotomized bovine ribs using 10-mm monocortical screws. The specimens were loaded using a 4-point bending system to the point of failure using a MTS model 309.00 servo-hydraulic testing system (MTS Systems Corp, Eden Prairie, MN) with a custom fabricated fixator. The data was obtained with TestStar 2 version 4.0 and TestWare Sx 4.0 software (MTS Systems Corp), and analyzed to determine if there was any difference in performance between the 2 mandibular plate designs.

Overall, there were no statistically significant differences between the locking and conventional 2-mm mandibular plate. Of the 20 samples tested, 1 (nonlocking) had a unique early screw failure (pullout) inconsistent with any other samples and was not analyzed. Of the remaining 19 samples, 17 produced a yield failure, bend, or stretch rather than a plate fracture. Both of the plate fractures occurred in the nonlocking plates. Although the data suggested that this mode of failure was more common in nonlocking plates, the difference between both groups in the mode of failure did not achieve statistical significance (chi2 = 2.0, P = .16). Thus, yield was the predominant mode of failure for both types of devices. The force at which each device failed was also similar in both groups. The force of failure for the nonlocking plates was 559.9 N (SD = 247.9), whereas the locking plate failure strength was 637.8 N (SD = 276.3; t (17) = 0.6, P = .53). The results indicated that there was no suggestion that one plate might be stronger than the other.

In this laboratory model, no significant differences were found between the 2 types of mandibular plates. Although studies have shown the locking system to be more rigid, this study suggested that the type and degree of failure are more likely related to bone quality and surgical technique when using the 2-mm mandibular plate. Additional prospective studies are needed to correlate these results in patient models.