Explicit Finite Element Models Accurately Predict Subject-Specific and Velocity-Dependent Kinetics of Sideways Fall Impact

Abstract

The majority of hip fractures in the elderly are the result of a fall from standing or from a lower height. Current injury models focus mostly on femur strength while neglecting subject-specific loading. This article presents an injury modeling strategy for hip fractures related to sideways falls that takes subject-specific impact loading into account. Finite element models (FEMs) of the human body were used to predict the experienced load and the femoral strength in a single model. We validated these models for their predicted peak force, effective pelvic stiffness, and fracture status against matching ex vivo sideways fall impacts (n = 11) with a trochanter velocity of 3.1 m/s. Furthermore, they were compared to sideways impacts of volunteers with lower impact velocities that were previously conducted by other groups. Good agreement was found between the ex vivo experiments and the FEMs with respect to peak force (root mean square error [RMSE] = 10.7%, R 2 = 0.85) and effective pelvic stiffness (R 2 = 0.92, RMSE = 12.9%). The FEMs were predictive of the fracture status for 10 out of 11 specimens. Compared to the volunteer experiments from low height, the FEMs overestimated the peak force by 25% for low BMI subjects and 8% for high BMI subjects. The effective pelvic stiffness values that were derived from the FEMs were comparable to those derived from impacts with volunteers. The force attenuation from the impact surface to the femur ranged between 27% and 54% and was highly dependent on soft tissue thickness (R 2 = 0.86). The energy balance in the FEMS showed that at the time of peak force 79% to 93% of the total energy is either kinetic or was transformed to soft tissue deformation. The presented FEMs allow for direct discrimination between fracture and nonfracture outcome for sideways falls and bridge the gap between impact testing with volunteers and impact conditions representative of real life falls. Specimens: n = 11 (6 female, 5 male). BMI = body mass index (subject weight divided by the subject height squared); T ST,GT = thickness of soft tissue over the greater trochanter; aBMD total hip = areal bone mineral density of the proximal femur.

Elucidating failure mechanisms in human femurs during a fall to the side using bilateral digital image correlation

“…De Bakker et al (2009) reported 7 neck fractures and 5 intertrochanteric fractures. Interestingly, Fleps et al (2019) also reported a similar distribution of fracture types (11 bones, 3 non-fractures, 3 intertrochanteric, 1 basicervical, 1 subcapital, 3 pelvic) using a more complex and biofidelic setup that included pelvis compound and soft tissue mimicking in a drop test. aBMD at the femoral neck was a good predictor of peak force (R 2 =0.67, RMSE=739 N, figure 3), consistently with previous literature findings (Rezaei and Dragomir-Daescu, 2015).…”

The Influence of Fall Direction and Hip Protector on Fracture Risk: FE Model Predictions Driven by Experimental Data

“…The models also showed good agreement with low height impact experiments performed with young volunteers, which supports their validity across impact velocities. 17 The fall posture of the subjects was not changed with respect to the impact angles. Although we consider the current posture as realistic for all simulated impact angles, it is likely that fall postures will vary between fall direction and impact angles in real life, with certain trends being common for certain fall and impact directions.…”

“…Recently, we validated subject-specific finite element (FE) models which allow for the simulation of falls to the side. 17 These models accurately represented ex vivo fall simulations with respect to impact force (RMSE = 10.7%), stiffness (RMSE 12.9%), and fracture outcome (10 out of 11). Furthermore, these FE models allow for the analysis of internal forces 15 and falls with different impact conditions.…”

Body Anthropometry and Bone Strength Conjointly Determine the Risk of Hip Fracture in a Sideways Fall

“…2 Other studies 4,32 focused specifically on the fall dynamics, showing that the hip load at touch down is a function of body anthropometry (height and weight), stiffness of the hip, the nearby soft-tissues, the pelvis and the floor. Yet, to the best of the authors' knowledge, only a few studies have replicated hip fractures in vitro by accounting for variations of body fall dynamics and compliance of the soft tissues surrounding the hip, [16][17][18][19] showing that the compliance of the soft tissues surrounding the hip is an important co-factor for the risk of fracture of the hip. 19 Here, we hypothesize that variations of body anthropometry, conjointly with the bone strength, determine the risk of hip fracture.…”