This study developed subject-specific, three-dimensional dynamic hindfoot models (1 in vivo, 1 in vitro) using 3D stress MRI data. Each model’s ability to capture mechanical phenomena including those of the healthy hindfoot and the hindfoot with ligament injury was evaluated through subject-specific experimental mechanical analyses (using an arthrometer and a stress MRI technique).
Existing software (3DVIEWNIXTM) was incorporated with software developed in-house (marching cubes program) to obtain the subject’s bone surface geometry, collateral and subtalar ligament insertion data. The bone surface data were then imported into a reverse engineering software package (Geomagic StudioTM) to obtain CAD representations for the bone geometries.
The ligaments’ non-linear structural properties were obtained directly from an existing experimental study or were estimated. Contact forces between bones were modeled using cartilage’s Elastic Modulus and an exponential term to imitate its nonlinear compression characteristics. The ADAMS 2003TM dynamic simulation software generated and solved the dynamic equations of motion under the forcing functions and boundary conditions.
The in vivo experimental kinematic data were smaller than those predicted by the model. This indicates that surrounding soft tissues excluding the ligaments may decrease joint range of motion. The in vitro model captured the experimental kinematic patterns of the ankle joint complex, but did so by under-estimating ankle joint motion and overestimating subtalar joint motion. Better knowledge of the ankle joint and subtalar joint ligament structural properties is necessary.
Similar to experimental data, the in vivo and in vitro models’ ankle joint complex had non-linear load-displacement properties in all directions. They are dependent on the contact of the articulating surfaces and ligament constraints. Sensitivity analyses indicated that kinematic changes caused by altering ligament geometry are smaller than changes caused by lateral ligament removal; therefore the model may be sensitive to predicting the changes that occur during ligament rupture.
The models’ assumptions and limitations include differences between the experimental and modeled boundary conditions, exclusion of the cartilage geometry, estimation of the contact damping coefficient, the contact stiffness and penetration exponent, estimation of the subtalar ligaments’ structural properties, generalized nonlinear properties for the collateral ligaments, and soft-tissue motion during the experiments. Future work must focus on developing a larger group of patient-specific models so that the output data has sufficient statistical power.
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