Mechanical Characterization of Bone Quality in Distal Femur Fractures using Pre-Operative Computed Tomography Scans:​

The main findings from this study was that the median patient age of the 43 cases reviewed was 72 years (IQR = 57 – 81), with 26% males and 74% females. The Young’s modulus in the distal femur was negatively correlated with patient age (R²=0.50, p<0.001). The distribution of patient-specific modulus values was also compared with the compressive modulus ranges for graded polyurethane foams according to ASTM F1839. Bone quality ranged from Grade 25 in younger individuals to Grade 5 in older individuals. These results indicated that no single grade of synthetic polyurethane foam can be used to model all clinically important scenarios for biomechanical testing of distal femur fracture fixation devices. Rather, this data can be used to select an appropriate material for a given clinical scenario. A Grade 25 foam is appropriate for implant longevity, whereas for implant stability, a Grade 5-15 is more appropriate.

An Adaptable Computed Tomography-Derived Three-Dimensional-Printed Alignment Fixture Minimizes Errors in Radius Biomechanical Testing:​

In this study we demonstrated the functionality of a novel, customizable alignment and potting fixture for long bone testing by comparing benchtop torsional test results to specimen-matched finite element models and found a strong correlation (R² = 0.95, p<0.001). Additional computational models were used to estimate the impact of malalignment on mechanical behavior in both torsion and axial compression. Results confirmed that torsion testing is relatively robust to alignment artifacts, with absolute percent errors less than 8% in all malalignment scenarios. In contrast, axial testing was highly sensitive to setup errors, experiencing absolute percent errors up to 50% with offcenter malalignment and up to 170% with angular malalignment. This suggests that whenever appropriate, torsion tests should be used preferentially as a summary mechanical measure. When more challenging modes of loading are required, pre-test clinical-resolution CT scanning can be effectively used to create potting fixtures that allow for precise pre-planned specimen alignment. This may be particularly important for more sensitive biomechanical tests (e.g. axial compressive tests) that may be needed for industrial applications, such as orthopedic implant design.

Boundary Conditions Matter - Impact of Test Setup on Inferred Construct Mechanics in Plated Distal Femur Osteotomies:​

The literature review conducted in this study highlighted the variability in reported outcome parameters. Reported literature values for axial stiffness of laterally plated distal femur osteotomies ranged from 49.3 to 8,736 N/mm. The finite element results showed that construct mechanical performance was highly sensitive to boundary conditions imposed by the mechanical test fixtures. Increasing the degrees of constraint, for example by potting and rigidly clamping one or more ends of the specimen, caused up to a 25x increase in axial stiffness of the construct. The imposed boundary conditions showed a larger effect on the axial stiffness than bridge span. The largest mean difference in axial stiffness between configuration sets was 7,439 ± 2 N/mm (potted/potted vs acetabular cup/cardanic) while the largest mean difference in axial stiffness between bridge span sets was only 1,022 ± 18 N/mm (31mm vs 136mm bridge spans). Transverse motion and gap closure at the fracture line, which is an important driver of interfragmentary strain, was also largely influenced by the constraint test setup. These results suggest that caution should be used when comparing reported results between bench tests that use different fixtures and that standardization of testing methods are needed in this field.

Rethinking the 10% Strain Rule in Fracture Healing:​ A Distal Femur Fracture Case Series:​

In this study, finite element modeling was used to assess the 3D interfragmentary strain in a case series of naturally occurring distal femur fractures treated with lateral plating under load conditions representative of the early post-operative period. The simulations showed that gap strains were within 2-10% only for the lowest load application level, 20% static body weight (BW). Moderate loading of 60% static BW and above caused gap strains that far exceeded 10%, but in all cases, strains in the periosteal region external to the fracture line remained low. Comparing these findings with post-operative radiographs suggests that in vivo secondary healing of distal femur fractures may be robust to early gap strains much greater than 10% because formation of new bone is initiated outside the gap where strains are lower, followed by later consolidation within the gap.

CT-Derived Virtual Mechanical Assessment of Bone Healing in Clinically Diagnosed Tibial Nonunions:​

In this study, a dual-zone material model was developed, implemented, and then compared to the more traditional single zone material model on two tibial model cohorts;​ normal healers and nonunions. A two-way mixed ANOVA revealed that the main effect of the dual-zone material model resulted in a statistically significant difference in axial stiffness (p<0.0005) and in von Mises strain (p<0.0005). The mean difference in axial stiffness and von Mises strain between the two material models was 1.193 ± 0.115 (Nm²/deg) [mean ± standard error], p<0.0005 and 0.009±0.001 (mm/mm), p<0.0005, respectively. No statistically significant difference was found in axial stiffness (p=0.287) and von Mises strain (p=0.558) between the normal healing group and the nonunion group. These results indicate that the single zone material model is overpredicting the mechanical properties of the elements located in the soft callus regions thus artificially stiffening the nonunion models. Additionally, nonunion models that had a scan captured a median of 51 weeks from injury behaved similarly to models of normal healing tibia’s which were captured 12 weeks from injury. This suggests that nonunion intervention may be more effective if undertaken as early as possible, before rigidity has been achieved.

Development and Validation of a Virtual Mechanical Test for Distal Femur Fracture Healing:​

The results from the virtual test of an intact distal femur compared to a “slowly healing” distal femur indicates that the VASA score is capable of capturing the difference in healing responses, compensate for baseline differences between individuals, and can be interpretable by individuals without an engineering background. Acquisition of CT scans of healing fractures with conventional bridge plating and far cortical locking is needed in order to verify that this score can also detect differences between healing results for different fixation groups.