1. Introduction
The incidence of low-energy fractures (e.g., falling from standing height) of the acetabulum has increased substantially (2.4-fold) in developed countries during recent decades [
1,
2,
3]. Low-energy acetabular fractures are common among the geriatric population, and in most cases, they happen due to sideways falling [
4,
5], when the trochanteric soft tissue (TST) hits the ground and the impact force is transmitted through the femoral neck and head to the acetabulum [
6,
7]. Many factors such as bone and soft-tissue quality and body configuration during impact can affect the incidence, severity, and type of low-energy acetabular fractures [
2,
8,
9].
Owing to the higher incidence, the mechanism of proximal femur fractures resulting from low-energy falls has been studied widely [
10,
11,
12,
13,
14]. Through different computational and experimental methods (e.g., mass-spring-damper system, inverse pendulum, pelvis release, and free-fall from standing height), the range of force applied to the femur and its corresponding impact velocities were reported as 1004–9990 N and 580–6070 mm/s, respectively [
15]. Also, Robinovitch et al. [
16] reported that the whole-body kinetic energy range varies from 160 to 387 J in backward and from 6 to 291 J in forward rotation during sideways falls. These force and impact velocity ranges do not necessarily imply the femoral fracture incidence. The typical range of the force that can cause a fracture in a femur during a sideways fall was reported from 1500 to 4000 N [
15], although much smaller (573 N) [
17] and much bigger forces (15,304 N) [
18] have been reported. Even if the mortality rate of geriatric acetabular fractures is higher than that of proximal femur fracture [
19], low-energy acetabular fractures have rarely been investigated [
9,
20]. Shim et al. [
20] studied the acetabular fracture for standing and seating positions and reported the corresponding fracture loads as 3200 and 2300 N. Our previous study [
9] showed that whereas the ground reaction force equal to 2600 N may cause a femoral fracture, it does not lead to acetabular fracture.
Whereas the effect of body configuration at the impact on the acetabular fracture was studied previously [
9], the effect of other variables such as impact velocity (V
Impact), flooring material, and TST stiffness on low-energy acetabular fractures has remained underexplored. V
Impact is positively correlated with the body weight and height and, therefore, it can be speculated that taller and heavier individuals are at higher risk of fracture during a low-energy sideways fall [
15]. Compliant flooring may decrease the incidence and severity of the injuries resulted from low-energy falls [
21]. The role of flooring material in the prevention of bone fractures reflects in its shock-absorbent properties and ability in the attenuation of the force [
22]. The effect of flooring material in low-energy fractures is somewhat unclear. Previous studies on low-energy fractures have indicated that compliant flooring might contribute to fracture prevention within the proximal femur (up to a 76.6% reduction in the peak force in the femoral neck) [
22,
23,
24], whereas it has also been suggested that changing the flooring material does not notably affect the impact force [
25,
26]. Some studies [
27,
28] showed that novel compliant flooring systems can reduce the risk of proximal fracture substantially in comparison with conventional flooring materials. The role of TST thickness and mechanical properties in the attenuation and distribution of the impact load is critical. It was shown that a lower body mass index is correlated with an elevated risk of hip fracture [
29,
30] and Bouxsein et al. [
31] suggested that the persons with a lower body mass index have a thinner TST too and their TST is unable to attenuate the impact force properly. Also, Majumder et al. investigated the effects of TST thickness and hip impact velocity on low-energy proximal femur fractures [
32,
33]. They stated that a decrease in TST thickness and an increase in the V
Impact are strongly correlated with an increase in the risk of proximal femur fracture. Whereas almost all of the studies agree that a higher V
Impact is associated with a higher risk of proximal femur fracture [
15], there is debate on the role of TST thickness. Choi et al. [
34] confirmed via an experimental study that neither stiffness nor dampening properties of the TST are associated with its thickness, as soft-tissue thickness remained almost constant among the young (19–30 years old) and elderly (65–81 years old) groups. In another study, Fleps et al. [
35] concluded that TST thickness alone is not predictive of fracture. Reduction in thickness can result in an overall decrease in the absorption of impact-energy and dissipation capacity of the TST. Therefore, it can be speculated that the failure of the TST in the attenuation of the impact force is mostly attributed to the deterioration of its mechanical properties (not its thickness) due to aging.
The biomechanics of low-energy acetabular fracture has remained underexplored due to the uncertainties mentioned above (effect TST thickness vs. its stiffness and the difference between stiff and compliant conventional flooring materials) or to the lower incidence of acetabular fractures in comparison with the proximal fracture. Therefore, we aimed to assess the effect of impact velocity, flooring material, and trochanteric soft-tissue stiffness on the incidence, severity, and type of low-energy acetabular fracture. We performed a parametric study of these variables, changing VImpact, modeling different conventional flooring materials, and changing TST stiffness in sequential steps, using a detailed computed tomography (CT)-based finite element model of a median human pelvis-proximal femur-soft tissue complex with a simple representation of the whole body. The results of this study can be used in designing prevention strategies for low-energy acetabular fractures.
4. Discussion
This parametric simulation study assessed the effect of impact velocity, conventional flooring materials, and trochanteric soft-tissue stiffness on the incidence, severity, and type of acetabular fracture. While the effect of the body configuration on the acetabular fracture was studied previously [
9], to the best of our knowledge, this is the first study evaluating the effect of V
Impact, flooring material, and TST stiffness on acetabular bone failure.
Since this parametric study aimed to investigate the effect of V
Impact, flooring material, and TST quality alterations on the relative incidence and type of acetabular fracture and no direct empirical validation was available, the model was validated against previous experimental studies. Previously, Askarinejad et al. [
75] and Fleps et al. [
13] used cadaveric testing to validate their finite element models of the femur, predicting the crack initiation and propagation. The GRF responses predicted by our models in terms of GRF
max, t
max, and shape of the curves were in good agreement with the previous studies with similar impact velocities [
27,
45,
72]. Also, as discussed in the following paragraphs, the results obtained regarding the type of acetabular fracture and the effect of conventional flooring material were consistent with the clinical findings.
Previous studies [
10,
30,
32,
76] have hypothesized that a greater height of the center of mass in taller persons results in a higher impact velocity, and consequently, an elevated risk of proximal femur fracture. Majumder et al. [
32] showed that by increasing the V
Impact from 1200 to 3170 mm/s, the strain ratio (ε
max/ε
ultimate) increases gradually and that by a further increase to 4790 mm/s, the strain ratio jumps up (more than two times in comparison with 3170 mm/s). The significant effect of V
Impact was also demonstrated by our study for the acetabular fracture. In the current study, a substantial change in the pattern and intensity of compressive bone failure within the acetabulum was observed for V
Impact > 3170 mm/s (
Figure 2a). Also, Majumder et al. [
32] stated that for V
Impact ≤ 1200 mm/s, the strain ratio would be less than one (0.77), which is consistent with our finding indicating no compressive bone failure for V
Impact = 580 mm/s (
Figure 5). Through another study on the lateral compressive failure of the pelvis, Bouquet and Ramet [
77] introduced 8000 N as the threshold impact force for the incidence of obvious pelvic fracture, close to the corresponding GRF
max (8835 N) of the V
Impact = 3170 mm/s in the present study (
Figure 2a). Also, the I
GRFmax for V
Impact > 3170 mm/s was more than two times that of V
Impact ≤ 3170 mm/s (43.63–62.24 vs. 91.38–111.94 N.s) (
Figure 2a). Not only was GRF affected greatly by the impact velocity, but the IRF was increased and t
IRFmax decreased for all joints (including hip) by increasing V
Impact. This can explain why a higher V
Impact can change the incidence and type of acetabular fracture. A higher V
Impact is associated with a bigger impact force exerted in a shorter time (a higher strain rate), which makes the bone prone to fracture during falls [
78]. In a recent study, Cecil et al. [
79] concluded that by increasing the impact energy (velocity), the type of acetabular fracture changes. Also, they stated that the anterior wall fracture is common among elderly people who experience low-energy event trauma. A comparison of the regions with concentrated bone failure (
Figure 5) with the fractured regions depicted in Judet and Letournel’s acetabular fracture classification [
80] (
Appendix A,
Figure A5) showed that the type of acetabular fracture is the anterior wall at low velocities (V
Impact ≤ 3170 mm/s), whereas at higher velocities it changes to both associated columns, representing one of the most prevalent types of acetabular fracture in the senior population [
4,
81]. Overall, it seems that V
Impact ≅ 3170 mm/s can be considered as the threshold value for the incidence of acetabular bone failure. By assuming the body as a lumped falling object and the law of conservation of energy, the corresponding height to this impact velocity would be equal to 51.2 cm. V
Impact = 3170 mm/s was reported by van den Kroonenberg [
82] as the average value for hip impact velocity. This can explain why a sideways fall from a standing height among healthy young adults who have healthy bone remodeling rarely ends up in an acetabular fracture [
83].
This study showed that, unlike impact velocity, conventional flooring materials did not affect the acetabular fracture remarkably. Only in the case of compliant surfaces such as agglomerated cork and linoleum GRF was slightly lower and I
GRFmax was slightly higher than for the stiffer flooring (
Figure 2b). Also, flooring material affected the IRF
max within the joints, and the bone failure pattern negligibly. These findings are consistent with Lachance et al. [
21], who concluded that while novel compliant flooring materials such as SmartCells and Sorbashock can attenuate the impact force considerably, conventional surfaces cannot notably reduce the exerting force to the hip during a fall. Similarly, Keenan et al. [
84] concluded that conventional flooring can only provide negligible impact on the force attenuation, and novel flooring materials such as rubber with a honeycomb structure can attenuate the impact force up to 25%. Also, Abdul Yamin et al. [
26] stated that there is not an obvious difference in terms of trend, magnitude, and maximum GRF resulting from running on different flooring materials, and Simpson et al. [
25] reported that GRF
max ranges from 11.9 to 12.4 kN for four different flooring materials. This is explained by the fact that the deceleration during impact is influenced by the initial conditions, viscoelastic properties of the ground, and the soft tissue. In terms of impact force attenuation, changing the conventional flooring material may not be as effective as previously thought with regard to the protection of elderly individuals from bone fractures in nursing facilities.
The dissipative role of trochanteric soft tissue can be attributed to its thickness or quality. Bouxsein et al. [
31] and Majumder et al. [
32] concluded that TST thinning increases the risk of proximal femur fracture, but they did not consider TST stiffness changes due to aging, as reported by Lim and Choi [
50] and Choi et al. [
34]. The current study revealed that TST stiffness mostly affects the post-impact phase. Also, it is strongly correlated with GRF
max and IRF
max within the sacroiliac joint, although this is not the case for t
max, I
GRFmax, and IRF
max within the hip or pubic symphysis joints. This shows that soft-tissue stiffening due to aging leads to a loss of the tissue’s inherent ability to dissipate impact force; therefore, the tissue transfers a higher amount of force toward the sacroiliac joint. In combination with osteoporosis, this can explain to some extent why a low-energy sacroiliac fracture is more common among elderly people [
85]. This study showed that changes in TST stiffness do not alter the type of acetabular fracture substantially and do not remarkably affect the bone failure within the acetabulum in comparison with TST thickness as reported by Majumder et al. [
32]. This can be explained by the fact that while a thicker TST can absorb and distribute a bigger portion of the impact force when TST is compressed almost instantly (t
max ≈ 0.015 s), it behaves like a hard tissue [
50], and the contribution of the viscous part of viscoelastic materials is, therefore, negligible [
86]. Although aging affects the bone (as the recipient of impact load) and soft tissue (as the carrier of the impact load) simultaneously, the bone deterioration process through the loss of the bone minerals [
31] and reduction in the trabecular number and connectivity [
87] may play the main role in acetabular fracture.
Overall, it seems that among the studied variables, variation in the impact velocity has a more substantial effect on the occurrence and type of acetabular fracture than the change in TST stiffness or selecting a different conventional flooring material. Also, it appears that the acetabular bone strength is more critical in the prevention of acetabular fracture than the mechanical properties of load-transferring components such as conventional flooring material or TST stiffness.
This study also has some limitations. First, we did not consider muscle activation and recruitment pattern during sideways falls. Second, to simplify the model, only half of the soft tissue (the impact side) was modeled, and due to the abdominal CT images, only the pelvic girdle and the proximal section of the femur were reconstructed directly. Third, owing to the type of study (parametric) and the complexity of the modeling process, only a median model was developed. Fourth, the deformed TST was obtained by rotating the femur from supine to the sideways fall posture, which may not be exactly similar to a real case. Fifth, in this study, only the critical strain criterion was considered for bone failure, whereas using the coupled criterion method (strain and energy) may enhance bone failure prediction. Sixth, the viscous property of the cartilages was not considered for simplification. Finally, the study did not evaluate the effects of gender and osteoporosis, which will be the focus of future studies.