BACKGROUND: Total ankle replacements (TARs) have a risk of early mechanical failure. Clinical studies have shown that implants with more robust tibial fixation-i.e., stemmed designs-have a lower early mechanical failure risk. We aimed to compare the micromotion between 2 tibial components with different fixation features, using a cadaveric robotic gait simulator. We hypothesized that more robust fixation accommodates loads more effectively, resulting in less micromotion at the bone-implant interface for stemmed tibial implants when compared to low-profile tibial implants.
METHODS: Specimens from 10 cadaveric matched pairs (from 6 men and 4 women, age: 48 ± 11 years) were implanted with either a low-profile or stemmed TAR. Post-TAR, gait was simulated using a validated 6-degrees-of-freedom robot. A digital image correlation camera system captured the relative motion between the tibial bone and the implant via surface markers. Implant micromotion was calculated by performing rigid-body transformations on segmented geometries of the tibia and implant derived from computed tomography imaging. Peak and mean micromotion were defined as the maximum and mean nodal displacement, respectively, on the implant surface across all time points in stance. Wilcoxon signed-rank tests compared micromotion of stemmed and low-profile implants between matched pairs.
RESULTS: On average, peak and mean micromotion values were greater for low-profile implants by 150 µm (95% confidence interval [CI]: 98 to 203 µm) and 30 µm (95% CI: 9 to 52 µm), respectively, compared with the matched stemmed implants (p < 0.001 for both). Peak micromotion of low-profile implants typically occurred at the posterior aspect of the implant. In contrast, stemmed implants demonstrated a more evenly distributed pattern with peak micromotion located at the tip of the stem.
CONCLUSIONS: This study identified significantly greater time-zero micromotion of low-profile tibial implants compared with stemmed implants. The micromotion distribution suggests that the stemmed implants were further compressed into bone, whereas the low-profile implants demonstrated a rocking motion. Our data support the notion that early mechanical failures of low-profile tibial implants may be due to excessive early micromotion.
LEVEL OF EVIDENCE: Therapeutic Level IV. See Instructions for Authors for a complete description of levels of evidence.
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