Dale Lee Robinson

Bone-anchored prostheses (BAPs) address many of the shortcomings of socket prostheses, driving the adoption of the technology for lower limb amputees. However, higher femoral fracture rates have been observed in these patients compared to socket users. It is believed that this increased fracture rate is due to the direct transmission of loads to bone through the bone-anchored implant. While low-impact activities, such as walking, have been extensively studied using finite element (FE) models, other more demanding low-impact activities such as ambulating on stairs and inclines remain poorly understood. Most previously reported subject-specific FE models of femurs fitted with osseointegrated implants were not validated against experimental data. The current study aimed to validate an FE modelling methodology using implanted cadaveric specimens and to calculate the fracture risk of implanted femurs during various activities of daily living. Strain responses of cadaveric femurs fitted with press-fit osseointegrated implants under load were recorded and compared to strain predicted by specimen-specific FE models created from CT data. Five different published relationships between Hounsfield units and Young’s Moduli were investigated to determine which relationship gave the most accurate model predictions. The validated FE models of the implanted femurs were later used to simulate in-vivo force measurements during five routine activities of daily living. Lastly, the fracture risk of the implanted femurs during these activities was quantified using factor of safety (FOS), where FOS = 1 denoted fracture was predicted at the given load. The validated FE model predicted the experimental strain responses with a coefficient of determination (R2) of 0.91 and a root mean square error (RMSE) of 81.27 με. The FOS for the activities of daily living of individual specimens ranged from 1.8 to 5.0, with walking up an incline having the lowest average FOS (1.9) across all specimens. Compared across different environments, the average FOS for level walking (4.3) and ambulating on stairs (4.0) were similar but almost double that of ambulating on an incline (2.2). Male BAP FE models predicted the experimental bone strains and suggested that peak loads during ambulation on level ground, stairs, and inclines are unlikely to cause bone fracture.

Bone-anchored percutaneous implants, commonly referred to as osseointegrated implants, are gaining popularity as an alternative to sockets for attaching a prosthetic limb to an amputated femur. While these implants have several advantages, femoral fractures are relatively common in these patients, occurring in 6.3% of femurs within 4 years. This study aimed to develop a novel method to quantify femoral strength under all possible external loads acting on the distal implant. Finite element analysis was used to explore combinations of forces and moments that, when applied on the bone-anchored implant, were just sufficient to fracture the femur. The 6-dimensional envelope generated in the current study provided a quantitative description of the strength of the femur-implant construct. All the femoral fractures were predicted in the trochanteric region, corresponding to where most in vivo fractures have been reported. The 6-dimensional envelopes developed in the current study may be useful in the design of fail-safe devices that could reduce femoral fractures in patients with bone-anchored percutaneous femoral implants. The framework presented may also be used for determining patient suitability for bone-anchored percutaneous implants, and for evaluating the functional performance of implant designs.

Personalised fracture plates manufactured using 3D printing offer an improved treatment option for unstable pelvic ring fractures that may not be adequately secured using off-the-shelf components. To design fracture plates that secure the bone fragments in their pre-fracture positions, the fractures must be reduced virtually using medical imaging-based reconstructions, a time-consuming process involving segmentation and repositioning of fragments until surface congruency is achieved. This study compared statistical shape models (SSMs) and contralateral mirroring as automated methods to reconstruct the hemipelvis using varying amounts of bone surface geometry. The training set for the geometries was obtained from pelvis CT scans of 33 females. The root-mean-squared error (RMSE) was quantified across the entire surface of the hemipelvis and within specific regions, and deviations of pelvic landmarks were computed from their positions in the intact hemipelvis. The reconstruction of the entire hemipelvis surfaced based on contralateral mirroring had an RMSE of 1.21 ± 0.29 mm, whereas for SSMs based on the entire hemipelvis surface, the RMSE was 1.11 ± 0.29 mm, a difference that was not significant (p = 0.32). Moreover, all hemipelvis reconstructions based on the full or partial bone geometries had RMSEs and landmark deviations from contralateral mirroring that were significantly lower (p < 0.05) or statistically equivalent to the SSMs. These results indicate that contralateral mirroring tends to be more accurate than SSMs for reconstructing unilateral pelvic fractures. SSMs may still be a viable method for hemipelvis fracture reconstruction in situations where contralateral geometries are not available, such as bilateral pelvic factures, or for highly asymmetric pelvic anatomies.

Summary Due to limitations of the predominant clinical method for diagnosing osteoporosis, an engineering model based on a dedicated CT scanner for bone density and structure was applied in fracture patients and controls. Improved diagnostic performance was observed, which supports its potential use in future research and clinical practice. Introduction Dual-energy X-ray absorptiometry (DXA), the predominant clinical method for diagnosing osteoporosis, has limitations in identifying individuals with increased fracture risk. Peripheral quantitative computed tomography (pQCT) provides additional information and can be used to generate finite element (FE) models from which bone strength properties can be estimated. We investigated the ability of pQCT-FE properties to distinguish peripheral low-trauma fracture patients from healthy controls, by comparison with DXA and standard pQCT. Methods One hundred and eight fracture patients (77 females aged 67.7 ± 7.9 years, 31 males aged 69.7 ± 8.9 years) were recruited from a hospital fracture liaison service. One hundred and twenty healthy community controls (85 females aged 69.8 ± 8.5 years, 35 males aged 68.9 ± 7.2 years) were recruited. Results Significant differences between groups were observed in pQCT-FE properties, especially at the 4% tibia site. Fracture odds increased most per standard deviation decrease in pQCT-FE at this location [shear stiffness estimate, k_shear, in females, OR = 10.34, 95% CI (1.91, 43.98); bending stiffness estimate, k_bend, in males, OR = 8.32, 95% CI (4.15, 33.84)]. Area under the receiver operating characteristics curve (AUROC) was observed to be highest with pQCT-FE properties at 4% the tibia site. In females, this was 0.83 for the pQCT-FE variable k_shear, compared with 0.72 for DXA total hip bone density (TH aBMD) and 0.76 for pQCT tibia trabecular density (Trb vBMD); in males, this was 0.81 for the pQCT-FE variable k_bend at the 4% tibia site, compared with 0.62 for TH aBMD and 0.71 for Trb vBMD. There were significant differences in AUROC between DXA and pQCT-FE variables in both females (p = 0.02) and males (p = 0.03), while no difference was observed in AUROC between primary pQCT and pQCT-FE variables. Conclusions pQCT-FE modeling can provide enhanced diagnostic performance compared with DXA and, given its moderate cost, may be useful in clinical settings.