Specimens were fixed to the base of a servohydraulic testing machine (Model 809, MTS Systems Corporation) with an axial-torsional load transducer (Model No. 662.20-01; Axial capacity of 250 kg, torsional capacity 2.88 kg-m; MTS Systems Corporation). The specimens were set in a vice tilted at 17° in the coronal plane to allow the MTS crosshead to apply an offset axial load simulating supination-adduction loading, which has been described previously (Figure 2).14,15 Load was applied to the inferolateral articular surface of the medial malleolus at 1 mm/s to a crosshead displacement of 6 mm and then cycled back to 0 mm. Load and axial displacement were measured at 60 Hz. The markers on the distal tibia and medial malleolus fracture fragment were tracked using high-resolution video (Fastcam PCI, Photron USA Inc). The motion of the video markers was determined using digitization and motion analysis software (Motus 9, Vicon).
Stiffness was calculated as the slope of the linear portion of the load-displacement curve over a range of 0.5 to 2.0 mm (Figure 3) and reported as mean (standard deviation). The force at 2 mm of fragment displacement was defined as a clinical failure.16,17 Student’s t test was used to determine the difference in construct stiffness and force for 2 mm displacement of the 2 groups. Significance was defined as P < .05. Institutional Review Board approval was not required for this study.
RESULTS
With offset axial testing to simulate supination-adduction force along with video motion analysis, the mean stiffness (± standard deviation) measured 180 ± 48 N/mm for the parallel unicortical screw fixation construct and 360 ± 131 N/mm for the headless compression screw fixation construct (Figure 4A). The headless compression screw fixation construct was over 2 times stiffer than the parallel unicortical construct during initial displacement of the fracture, indicating a statistically significant difference (P < .0001).
The mean force for 2 mm of fracture displacement, defined as clinical failure, reached 342 ± 83 N for the parallel unicortical screw fixation construct and 719 ± 91 N for the headless compression screw fixation construct (Figure 4B). The headless compression screw fixation construct resisted displacement significantly more (P = .0001) than the parallel unicortical screw construct, presenting a 100% increase.
Upon cycling of the servohydraulic testing machine back to 0-mm displacement, the parallel unicortical construct demonstrated no elastic recoil, remaining displaced at 4 mm, whereas the headless compression screw construct rebounded to almost 0-mm displacement, which is well below the clinical definition of fixation failure of 2 mm (Figure 5).
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