| 1. |
- Grassi, Lorenzo, et al.
(author)
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Elucidating failure mechanisms in human femurs during a fall to the side using bilateral digital image correlation
- 2020
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In: Journal of Biomechanics. - : Elsevier BV. - 0021-9290. ; 106
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Journal article (peer-reviewed)abstract
- An improved understanding of the mechanical properties of human femurs is a milestone towards a more accurate assessment of fracture risk. Digital image correlation (DIC) has recently been adopted to provide full-field strain measurements during mechanical testing of femurs. However, it has typically been used to measure strains on the anterior side of the femur, whereas in both single-leg-stance and sideways fall loading conditions, the highest deformations result on the medial and lateral sides of the femoral neck. The goal of this study was to measure full-field deformations simultaneously on the medial and lateral side of the femoral neck in a configuration resembling a fall to the side. Twelve female cadaver femurs were prepared for DIC measurements and tested in sideways fall at 5 mm/s displacement rate. Two pairs of cameras recorded the medial and lateral side of the femoral neck, and deformations were calculated using DIC. The samples exhibited a two-stage failure: first, a compressive collapse on the superolateral side of the femoral neck in conjunction with peak force, followed by complete femoral neck fracture at the force drop following the post-elastic phase. DIC measurements corroborated this observation by reporting no tensile strains above yield limit for the medial side of the neck up to peak force. DIC measurements registered onto the bone micro-architecture showed strain localizations in proximity of cortical pores due to, for instance, blood vessels. This could explain previously reported discrepancies between simulations and experiments in regions rich with large pores, like the superolateral femoral neck.
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| 2. |
- Jones, Benedict C, et al.
(author)
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To which world regions does the valence-dominance model of social perception apply?
- 2021
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In: Nature Human Behaviour. - : Springer Science and Business Media LLC. - 2397-3374. ; 5:1, s. 159-169
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Journal article (peer-reviewed)abstract
- Over the past 10 years, Oosterhof and Todorov's valence-dominance model has emerged as the most prominent account of how people evaluate faces on social dimensions. In this model, two dimensions (valence and dominance) underpin social judgements of faces. Because this model has primarily been developed and tested in Western regions, it is unclear whether these findings apply to other regions. We addressed this question by replicating Oosterhof and Todorov's methodology across 11 world regions, 41 countries and 11,570 participants. When we used Oosterhof and Todorov's original analysis strategy, the valence-dominance model generalized across regions. When we used an alternative methodology to allow for correlated dimensions, we observed much less generalization. Collectively, these results suggest that, while the valence-dominance model generalizes very well across regions when dimensions are forced to be orthogonal, regional differences are revealed when we use different extraction methods and correlate and rotate the dimension reduction solution. PROTOCOL REGISTRATION: The stage 1 protocol for this Registered Report was accepted in principle on 5 November 2018. The protocol, as accepted by the journal, can be found at https://doi.org/10.6084/m9.figshare.7611443.v1 .
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| 3. |
- Kok, Joeri, et al.
(author)
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Augmenting a dynamic hip screw with a calcium sulfate/hydroxyapatite biomaterial
- 2021
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In: Medical Engineering and Physics. - : Elsevier BV. - 1350-4533. ; 92, s. 102-109
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Journal article (peer-reviewed)abstract
- Internal fixation failure in hip fractures can lead to reoperation. Calcium sulfate/hydroxyapatite (CaS/HA) is a biomaterial that can be used for augmenting fracture fixation. We aimed to determine whether an injection of 2 ml CaS/HA increases the fixation of a dynamic hip screw inserted in synthetic and human trabecular bone. The study consists of two parts: 1) synthetic bone blocks (n = 74), with three subgroups: empty (cannulated screw, no injection), cannulated, and fenestrated; and 2) osteoporotic human femoral heads (n = 29), with the same subgroups. The heads were imaged using µCT. Bone volume fraction, insertion angle, and head diameter were measured. Pullout tests were performed and peak force, stiffness, and work were measured. The fenestrated group showed increases in pullout strength compared to no injection in the synthetic blocks. The cannulated group showed a higher pullout strength in low-density blocks. In the femoral heads, the variation was larger and there were no significant differences between groups. The bone volume fraction correlated with the peak force and work, and the insertion angle correlated with the stiffness. CaS/HA can improve the fixation of a dynamic hip screw. For clinical use, spreading of the material around the threads of the screw must be ensured.
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| 4. |
- Kok, Joeri, et al.
(author)
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Femoral strength and strains in sideways fall : Validation of finite element models against bilateral strain measurements
- 2021
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In: Journal of Biomechanics. - : Elsevier BV. - 1873-2380 .- 0021-9290. ; 122
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Journal article (peer-reviewed)abstract
- Low impact falls to the side are the main cause of hip fractures in elderly. Finite element (FE) models of the proximal femur may help in the assessment of patients at high risk for a hip fracture. However, extensive validation is essential before these models can be used in a clinical setting. This study aims to use strain measurements from bilateral digital image correlation to validate an FE model against ex vivo experimental data of proximal femora under a sideways fall loading condition. For twelve subjects, full-field strain measurements were available on the medial and lateral side of the femoral neck. In this study, subject-specific FE models were generated based on a consolidated procedure previously validated for stance loading. The material description included strain rate dependency and separate yield and fracture strain limits in tension and compression. FE predicted fracture force and experimentally measured peak forces showed a strong correlation (R2 = 0.92). The FE simulations predicted the fracture initiation within 3 mm distance of the experimental fracture line for 8/12 subjects. The predicted and measured strains correlated well on both the medial side (R2 = 0.87) and the lateral side (R2 = 0.74). The lower correlation on the lateral side is attributed to the irregularity of the cortex and presence of vessel holes in this region. The combined validation against bilateral full-field strain measurements and peak forces has opened the door for a more elaborate qualitative and quantitative validation of FE models of femora under sideways fall loading.
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| 5. |
- Kok, Joeri, et al.
(author)
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Fracture behavior of a composite of bone and calcium sulfate/hydroxyapatite
- 2022
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In: Journal of the Mechanical Behavior of Biomedical Materials. - : Elsevier BV. - 1751-6161. ; 130
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Journal article (peer-reviewed)abstract
- Calcium sulfate/hydroxyapatite (CaS/HA) biomaterials have been investigated for use in several orthopedic applications. However, the mechanical interactions between the composite of CaS/HA and bone at the microscale are still unknown. The aim of this study was to determine if and how augmentation with CaS/HA alters the fracture behavior of bone. Eleven cylinders of trabecular bone were drilled from human femoral heads and cleaned from bone marrow. Among them, five cylinders were injected with CaS/HA to generate composite specimens, while the others were kept intact. One extra specimen of pure CaS/HA was prepared. All specimens were compressed in situ using synchrotron X-ray tomography and imaged at ∼2% strain intervals. Structural properties were calculated from the images in unloaded state and mechanical properties were determined from the load-curves. CaS/HA alone displayed the highest peak force and stiffness and the lowest strain at fracture. All composite specimens had a higher peak force than the pure bone specimens and the composite specimens had higher toughness than the pure CaS/HA specimen. Furthermore, the fracture behavior was analyzed further to characterize the local deformations. The pure bone specimens presented damage in multiple trabeculae and the CaS/HA specimen displayed sharp transition in strains, with low strain in one load step and large cracks in the next. The composite specimens deformed uniformly, with the CaS/HA preventing tissue damage and the bone preventing cracks in the CaS/HA from propagating through the specimen. In conclusion, using tomography with in situ loading, it was possible to show how CaS/HA can help prevent bone tissue damage before global failure.
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| 6. |
- Kok, Joeri, et al.
(author)
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Fracture strength of the proximal femur injected with a calcium sulfate/hydroxyapatite bone substitute
- 2019
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In: Clinical Biomechanics. - : Elsevier BV. - 0268-0033. ; 63, s. 172-178
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Journal article (peer-reviewed)abstract
- Background: Available interventions for preventing fragility hip fractures show limited efficacy. Injection of a biomaterial as bone substitute could increase the fracture strength of the hip. This study aimed to show the feasibility of injecting a calcium sulfate/hydroxyapatite based biomaterial in the femoral neck and to calculate the consequent change in strength using the finite element method. Methods: Five patients were injected with 10 ml calcium sulfate/hydroxyapatite in their femoral neck. Quantitative CT scans were taken before and after injection. Five additional patients with fragility hip fractures were also scanned and the images from the non-fractured contralateral sides were used. Finite element models were created for all proximal femora with and without injection and the models were tested under stance and sideways fall loading until fracture. The change in fracture strength caused by the injection was calculated. Additionally, perturbations in volume, location, and stiffness of the injected material were created to investigate their contribution to the fracture strength increase. Findings: The 10 ml injection succeeded in all patients. Baseline simulations showed theoretical fracture strength increases of 0–9%. Volume increase, change in location and increase in stiffness of the material led to increases in fracture strength of 1–27%, −8-26% and 0–17%, respectively. Altering the location of the injection to a more lateral position and increasing the stiffness of the material led to increases in fracture strength of up to 42%. Interpretation: This study shows that an injection of calcium sulfate/hydroxyapatite is feasible and can theoretically increase the hip's fracture strength.
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| 7. |
- Kok, Joeri
(author)
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Hip fractures : A biomechanical analysis of fracture strength prediction, prevention, and repair
- 2021
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Doctoral thesis (other academic/artistic)abstract
- Due to the aging population, hip fracture incidence has been increasing over the past decades. Measurements of bone mineral density with dual energy X-ray absorptiometry are the gold standard for hip fracture risk assessment, where patients with a low bone density have a high risk of fracture. However, many people that are not diagnosed to be at risk, still fracture their hip. Calculations of bone strength using subject-specific finite element (FE) models, can improve fracture risk prediction, but further improvement is required.Patients with a high fracture risk are often prescribed pharmaceutical treatment in order to increase bone density systemically. As systemic response to treatment is limited, other options to prevent fractures by improving the bone strength are investigated. One of those options is the injection of biomaterials in the femoral neck. In case of a hip fracture due to a low-energy fall, total hip replacement is generally preferred over joint-preserving methods like fixation using a dynamic hip screw. Screw fixation comes with a risk of screw instability, especially in low-density bone. Bone cements can be used to improve fixation of orthopaedic implants and fracture fixation devices. Calcium sulphate/hydroxyapatite (CaS/HA) is an injectable biomaterial that has been used, for example, to reinforce collapsed vertebrae and to stabilize wrist fractures. The work presented in the thesis aims to improve fracture risk prediction, and fracture prevention and repair methods with use of CaS/HA. This is achieved through a combination of experimental mechanical tests at organ and tissue scale, and development and thorough validation of FE models of the proximal femur.In the first part of this thesis, 12 cadaveric femora were used in an experiment where the bones were loaded until fracture in a configuration developed to replicate a fall to the side. During loading, high-speed cameras were used to image both the medial and lateral side of the femoral neck allowing for full-field strain measurements using digital image correlation. The femora were imaged with clinical CT before and micro-CT before and after mechanical testing. Using the acquired CT images, FE models were developed at two different resolutions to determine their ability to capture the fracture force, fracture location and surface strains. The FE models based on the clinical CT images were able to accurately capture the fracture force and identify regions where the bone would fracture. These models could also capture the strains with high accuracy. However, the strains were not predicted as accurately in regions with high surface irregularity. The models based on the micro-CT images could show with higher accuracy how the strains were distributed around local porosity (e.g., due to vascularization) in the femoral neck and how these influenced the fracture pattern.The thesis continues with an investigation of fracture prevention and repair methods through the use of CaS/HA. The ability of CaS/HA to increase the fracture strength of the proximal femur for fracture prevention and its ability to stabilize a dynamic hip screw used for fracture repair was investigated. The increase in fracture strength was investigated using FE models. These models showed that CaS/HA can increase the fracture strength of the femur approximately 20% when injected close to the cortex in the lateral neck. Pullout tests using a dynamic hip screw were performed on synthetic bone blocks and femoral heads from hip fracture patients. In the synthetic blocks, CaS/HA significantly increased the pullout strength. However, in the human bone the stability of the screw was not improved, because the cement could not easily spread into the threads of the screws. The mechanical behaviour of CaS/HA and bone was further investigated using high-resolution synchrotron X-ray tomography. Cylindrical trabecular bone specimens with and without CaS/HA were imaged with tomography during in-situ loading of the samples. The images revealed that CaS/HA reinforced the bone, and that CaS/HA is a brittle material that will crack before the bone.To conclude, in this thesis FE models are presented showing accurate prediction of fracture strength, which can be used for improved fracture risk assessments. Furthermore, the work provides insight in how CaS/HA behaves mechanically and how it can be used to increase the fracture strength and to stabilize fixation devices in the femur, improving fracture prevention and fracture repair methods.
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| 8. |
- Kok, Joeri, et al.
(author)
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The influence of foramina on femoral neck fractures and strains predicted with finite element analysis
- 2022
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In: Journal of the Mechanical Behavior of Biomedical Materials. - : Elsevier BV. - 1751-6161. ; 134
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Journal article (peer-reviewed)abstract
- Hip fractures following a low-impact fall are common in the elderly. Finite element (FE) models of the proximal femur can improve the prediction of fracture risk over current clinical standards. Foramina in the femoral neck may influence its fracture mechanics, albeit the majority of FE modelling approaches do not consider them. This study aimed to show how foramina affect fracture propagation and FE strain predictions in the femoral neck. μCT images were taken of 10 cadaveric proximal femora before and after fracture, following quasi-static mechanical loading representing a sideways fall. The μCT images of the fractured femora were used to determine where the bones fractured in relation to the foramina. FE models were created based on μCT and clinical CT scans of the intact femora. The superolateral side of the femoral neck was modelled with high detail including foramina. Element-specific Young's moduli were assigned and the models were solved quasi-statically. The models predicted high strains inside foramina, agreeing with experimental strain measurements. However, these high strains inside foramina were often not related to the observed fracture location. μCT images also confirmed that the foramina mostly remained intact after fracture. Using a fracture criterion based on local strain averaging improved the accuracy of the predicted fracture location as well as the correlation between the FE predicted fracture forces and the experimentally measured peak forces. To conclude, the presence of foramina can influence the fracture pattern in femoral neck fractures and inclusion of foramina in FE models improves the prediction of local strain concentrations.
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| 9. |
- Raina, Deepak Bushan, et al.
(author)
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A New Augmentation Method for Improved Screw Fixation in Fragile Bone
- 2022
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In: Frontiers in Bioengineering and Biotechnology. - : Frontiers Media SA. - 2296-4185. ; 10
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Journal article (peer-reviewed)abstract
- Pertrochanteric fractures (TF) due to osteoporosis constitute nearly half of all proximal femur fractures. TFs are treated with a surgical approach and fracture fixation is achieved using metallic fixation devices. Poor quality cancellous bone in osteoporotic patients makes anchorage of a fixation device challenging, which can lead to failure of the fracture fixation. Methods to reinforce the bone-implant interface using bone cement (PMMA) and other calcium phosphate cements in TFs have been described earlier but a clear evidence on the advantage of using such biomaterials for augmentation is weak. Furthermore, there is no standardized technique for delivering these biomaterials at the bone-implant interface. In this study, we firstly describe a method to deliver a calcium sulphate/hydroxyapatite (CaS/HA) based biomaterial for the augmentation of a lag-screw commonly used for TF fixation. We then used an osteoporotic Sawbones model to study the consequence of CaS/HA augmentation on the immediate mechanical anchorage of the lag-screw to osteoporotic bone. Finally, as a proof-of-concept, the method of delivering the CaS/HA biomaterial at the bone-implant interface as well as spreading of the CaS/HA material at this interface was tested in patients undergoing treatment for TF as well as in donated femoral heads. The mechanical testing results indicated that the CaS/HA based biomaterial increased the peak extraction force of the lag-screw by 4 times compared with un-augmented lag-screws and the results were at par with PMMA. The X-ray images from the patient series showed that it was possible to inject the CaS/HA material at the bone-implant interface without applying additional pressure and the CaS/HA material spreading was observed at the interface of the lag-screw threads and the bone. Finally, the spreading of the CaS/HA material was also verified on donated femoral heads and micro-CT imaging indicated that the entire length of the lag-screw threads was covered with the CaS/HA biomaterial. In conclusion, we present a novel method for augmenting a lag-screw in TFs, which could potentially reduce the risk of fracture fixation failure and reoperation in fragile osteoporotic patients.
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| 10. |
- Törnquist, Elin, et al.
(author)
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The Hydration State of Bone Tissue Affects Contrast in Neutron Tomographic Images
- 2022
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In: Frontiers in Bioengineering and Biotechnology. - : Frontiers Media SA. - 2296-4185. ; 10
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Journal article (peer-reviewed)abstract
- Neutron tomography has emerged as a promising imaging technique for specific applications in bone research. Neutrons have a strong interaction with hydrogen, which is abundant in biological tissues, and they can penetrate through dense materials such as metallic implants. However, in addition to long imaging times, two factors have led to challenges in running in situ mechanical characterization experiments on bone tissue using neutron tomography: 1) the high water content in specimens reduces the visibility of internal trabecular structures; 2) the mechanical properties of bone are dependent on the hydration state of the tissue, with drying being reported to cause increased stiffness and brittleness. This study investigates the possibility of improving image quality in terms of neutron transmission and contrast between material phases by drying and rehydrating in heavy water. Rat tibiae and trabecular bovine bone plugs were imaged with neutron tomography at different hydration states and mechanical testing of the bone plugs was carried out to assess effects of drying and rehydration on the mechanical properties of bone. From analysis of image histograms, it was found that drying reduced the contrast between bone and soft tissue, but the contrast was restored with rehydration. Contrast-to-noise ratios and line profiles revealed that the contrast between bone tissue and background was reduced with increasing rehydration duration but remained sufficient for identifying internal structures as long as no free liquid was present inside the specimen. The mechanical analysis indicated that the proposed fluid exchange protocol had no adverse effects on the mechanical properties.
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