Development and Validation of a Computational Model to Study the Effect of Foot Constraint on Ankle Injury due to External Rotation

Toe-out landing reduces anterior talofibular ligament strain while maintains calcaneofibular ligament strain in people with chronic ankle instability

BACKGROUND

The anterior talofibular ligament (ATFL) and the calcaneofibular ligament (CFL) are vulnerable to be torn or ruptured during lateral ankle sprain (LAS), especially in people with chronic ankle instability (CAI). This study aims to determine whether landing with a larger toe-out angle would influence ATFL and CFL strains in people with CAI, aiming to contribute to the development of effective landing strategies to reduce LAS risk.

METHODS

Thirty participants with CAI (22 males and 8 females, age: 21.6 ± 1.5 years, height: 175.3 ± 7.1 cm, body mass: 70.8 ± 7.1 kg, mean ± SD) were recruited. Each participant landed on a specialized trap-door device with their unaffected limbs on a support platform and their affected limbs on a movable platform, which could be flipped 24° inward and 15° forward to mimic LAS conditions. Two landing conditions were tested-i.e., natural landing (NL, with natural toe-out angle at landing) and toe-out landing (TL, with toe-out angle increased to over 150% of that under the NL conditions). Kinematic data were captured using a 12-camera motion analysis system, and ATFL and CFL strains were calculated using a 3D rigid-body foot model. Paired sample t tests and Pearson's correlations were used to analyze data.

RESULTS

Compared to NL conditions, ATFL strain decreased (p < 0.001, d = 2.42) while CFL strain remained unchanged (p = 0.229, d = 0.09) under TL conditions. The toe-out angle was negatively and strongly correlated with ATFL strain (r = -0.743, p < 0.001) but not with CFL strain (r = 0.153, p = 0.251). Compared to NL conditions, participants exhibit a lower ankle inversion angle (p < 0.001, d = 0.494), a higher plantarflexion angle (p < 0.001, d = 1.101), and no significant difference in external rotation angle (p = 0.571, d = 0.133) under TL conditions.

CONCLUSION

Toe-out landing may reduce ATFL strain while maintaining CFL strain in people with CAI, thereby reducing the risk of LAS.

Contribution of the plantar fascia and long plantar ligaments to the stability of the longitudinal arch of the foot

The contribution of the Plantar Fascia (PF) and Long Plantar Ligament (LPL), two ligaments extending from the hindfoot to the forefoot, to arch stability has been studied in the past using in vivo, in vitro, and in silico methodologies. In silico studies were based on one single model obtained from one single subject and did not account for the known inter-subject morphological and biomechanical variations. In the present study, we developed computational dynamic models of nine different legs obtained from nine different individuals to evaluate the role of the LPL and PF in arch support, accounting for biological differences between subjects. These models were validated by comparing the simulation results against experimental results from the corresponding cadaver legs. After validation, we simulated body weight conditions for each model by applying a vertical load to the tibia, starting from zero and increasing linearly to 720 N. Kinematic and dynamic parameters, including the variation of the medial arch angle and of the navicular height, as well as the passive forces developed by the LPL and PF, were used to evaluate the contribution of these ligaments to arch support under body weight. The results indicate that a total collapse of the medial longitudinal arch occurred only when both the LPL and PF were absent, but a stable arch was maintained when either one of these two ligament structures were present. The results varied significantly among the specific models, highlighting the importance of using multiple models to account for inter-subject morphological differences.

A numerical investigation of stress, strain, and bone density changes due to bone remodelling in the talus bone following total ankle arthroplasty

Total ankle arthroplasty is the gold standard surgical treatment for severe ankle arthritis and fracture. However, revision surgeries due to the in vivo failure of the ankle implant are a serious concern. Extreme bone density loss due to bone remodelling is one of the main reasons for in situ implant loosening, with aseptic loosening of the talar component being one of the primary reasons for total ankle arthroplasty revisions. This study is aimed at determining the performance and potential causes of failure of the talar component. Herein, we investigated the stress, strain, and bone density changes that take place in the talus bone during the first 6 months of bone remodelling due to the total ankle arthroplasty procedure. Computed tomography scans were used to generate the 3D geometry used in the finite element (FE) model of the Intact and implanted ankle. The Scandinavian Total Ankle Replacement (STAR™) CAD files were generated, and virtual placement within bone models was done following surgical guidelines. The dorsiflexion physiological loading condition was investigated. The cortical region of the talus bone was found to demonstrate the highest values of stress (5.02 MPa). Next, the adaptive bone remodelling theory was used to predict bone density changes over the initial 6-month post-surgery. A significant change in bone density was observed in the talus bone due to bone remodelling. The observed quantitative changes in talus bone density over 6-month period underscore potential implications for implant stability and fracture susceptibility. These findings emphasise the importance of considering such biomechanical factors in ankle implant design and clinical management.

Clinically useful finite element models of the natural ankle - A review

BACKGROUND

Biomechanical simulation of the foot and ankle complex is a growing research area but compared to simulation of joints such as hip and knee, it has been under investigated and lacks consistency in research methodology. The methodology is variable, data is heterogenous and there are no clear output criteria. Therefore, it is very difficult to correlate clinically and draw meaningful inferences.

METHODS

The focus of this review is finite element simulation of the native ankle joint and we will explore: the different research questions asked, the model designs used, ways the model rigour has been ensured, the different output parameters of interest and the clinical impact and relevance of these studies.

FINDINGS

The 72 published studies explored in this review demonstrate wide variability in approach. Many studies demonstrated a preference for simplicity when representing different tissues, with the majority using linear isotropic material properties to represent the bone, cartilage and ligaments; this allows the models to be complex in another way such as to include more bones or complex loading. Most studies were validated against experimental or in vivo data, but a large proportion (40%) of studies were not validated at all, which is an area of concern.

INTERPRETATION

Finite element simulation of the ankle shows promise as a clinical tool for improving outcomes. Standardisation of model creation and standardisation of reporting would increase trust, and enable independent validation, through which successful clinical application of the research could be realised.

Personalised statistical modelling of soft tissue structures in the ankle

BACKGROUND AND OBJECTIVE

Revealing the complexity behind subject-specific ankle joint mechanics requires simultaneous analysis of three-dimensional bony and soft-tissue structures. 3D musculoskeletal models have become pivotal in orthopedic treatment planning and biomechanical research. Since manual segmentation of these models is time-consuming and subject to manual errors, (semi-) automatic methods could improve the accuracy and enlarge the sample size of personalised 'in silico' biomechanical experiments and computer-assisted treatment planning. Therefore, our aim was to automatically predict ligament paths, cartilage topography and thickness in the ankle joint based on statistical shape modelling.

METHODS

A personalised cartilage and ligamentous prediction algorithm was established using geometric morphometrics, based on an 'in-house' generated lower limb skeletal model (N = 542), tibiotalar cartilage (N = 60) and ankle ligament segmentations (N = 10). For cartilage, a population-averaged thickness map was determined by use of partial least-squares regression. Ligaments were wrapped around bony contours based on iterative shortest path calculation. Accuracy of ligament path and cartilage thickness prediction was quantified using leave-one-out experiments. The novel personalised thickness prediction was compared with a constant cartilage thickness of 1.50 mm by use of a paired sample T-test.

RESULTS

Mean distance error of cartilage and ligament prediction was 0.12 mm (SD 0.04 mm) and 0.54 mm (SD 0.05 mm), respectively. No significant differences were found between the personalised thickness cartilage and segmented cartilage of the tibia (p = 0.73, CI [-1.60 .10^-17, 1.13 .10^-17]) and talus (p = 0.95, CI[ -1.35 .10^-17, 1.28 .10^-17]). For the constant thickness cartilage, a statistically significant difference was found in 89% and 92% of the tibial (p < 0.001, CI [0.51, 0.58]) and talar (p < 0.001, CI [0.33, 0.40]) cartilage area.

CONCLUSIONS

In this study, we described a personalised prediction algorithm of cartilage and ligaments in the ankle joint. We were able to predict cartilage and main ankle ligaments with submillimeter accuracy. The proposed method has a high potential for generating large (virtual) sample sizes in biomechanical research and mitigates technological advances in computer-assisted orthopaedic surgery.

Arbitrary Prestrain Values for Ligaments Cause Numerical Issues in a Multibody Model of an Ankle Joint

Experimental studies report that ligaments of the ankle joint are prestrained. The prestrain is an important aspect of modern biomechanical analysis, which can be included in the models by: applying symmetrical, arbitrary prestrains to the ligaments, assuming a strain-free location for the joint or by using experimental prestrain data. The aim of the study was to comparatively analyze these approaches. In total, 4 prestraining methods were considered. In order to do so, a symmetrical model of the ankle with six nonlinear cables and two sphere–sphere contact pairs was assumed. The model was solved in statics under moment loads up to 5 Nm. The obtained results showed that the arbitrary prestrains caused an unbalanced load for the model at rest, and in turn modified its rest location in an unpredictable way. Due to the imbalance, it was impossible to enforce the assumed prestrains and thus cartilage prestrain was required to stabilize the model. The prestraining had a significant effect on the angular displacements and the load state of the model. The findings suggest that the prestrain values are patient specific and arbitrary prestrains will not be valid for most models.

A Review of Finite Element Models of Ligaments in the Foot and Considerations for Practical Application

PURPOSE

Finite element (FE) modeling has been used as a research tool for investigating underlying ligaments biomechanics and orthopedic applications. However, FE models of the ligament in the foot have been developed with various configurations, mainly due to their complex 3D geometry, material properties, and boundary conditions. Therefore, the purpose of this review was to summarize the current state of finite element modeling approaches that have been used in the ?eld of ligament biomechanics, to discuss their applicability to foot ligament modeling in a practical setting, and also to acknowledge current limitations and challenges.

METHODS

A comprehensive literature search was performed. Each article was analyzed in terms of the methods used for: (a) ligament geometry, (b) material property, (c) boundary and loading condition related to its application, and (d) model verification and validation.

RESULTS

Of the reviewed studies, 80% of the studies used simplified representations of ligament geometry, the non-linear mechanical behavior of ligaments was taken into account in only 19.2% of the studies, 33% of included studies did not include any kind of validation of the FE model.

CONCLUSION

Further refinement in the functional modeling of ligaments, the micro-structure level characteristics, nonlinearity, and time-dependent response, may be warranted to ensure the predictive ability of the models.

Biomechanics of Medial Ankle and Peritalar Instability

The deltoid and spring ligaments are the primary restraints against pronation and valgus deformity of the foot, and in preserving the medial arch. The posterior tibial tendon has a secondary role in plantar arch maintenance, and its biomechanical stress increases considerably when other tissues fail. A thorough understanding of the anatomy and biomechanics of the deltoid-spring ligament is crucial for successful reconstruction of the tibiocalcanealnavicular ligament, hence, to restore ankle and medial peritalar stability. Although effective in correcting the deformity, tibionavicular tenodesis might be critical, as it blocks physiologic pronation of the hindfoot, which may result in dysfunction and pain.

Bone Position and Ligament Deformations of the Foot From CT Images to Quantify the Influence of Footwear in ex vivo Feet

The mechanical behavior of the foot is often studied through the movement of the segments composing it and not through the movement of each individual bone, preventing an accurate and unambiguous study of soft tissue strains and foot posture. In order to describe the internal behavior of the foot under static load, we present here an original methodology that automatically tracks bone positions and ligament deformations through a series of CT acquisitions for a foot under load. This methodology was evaluated in a limited clinical study based on three cadaveric feet in different static load cases, first performed with bare feet and then with a sports shoe to get first insights on how the shoe influences the foot's behavior in different configurations. A model-based tracking technique using hierarchical distance minimization was implemented to track the position of 28 foot bones for each subject, while a mesh-morphing technique mapped the ligaments from a generic model to the patient-specific model in order to obtain their deformations. Comparison of these measurements between the ex vivo loaded bare foot and the shod foot showed evidence that wearing a shoe affects the deformation of specific ligaments, has a significant impact on the relative movement of the bones and alters the posture of the foot skeleton (plantar-dorsal flexion, arch sagging, and forefoot abduction-adduction on the midfoot). The developed method may provide new clinical indicators to guide shoe design and valuable data for detailed foot model validation.

Strain pattern of each ligamentous band of the superficial deltoid ligament: a cadaver study

BACKGROUND

There are few reports on the detailed biomechanics of the deltoid ligament, and no studies have measured the biomechanics of each ligamentous band because of the difficulty in inserting sensors into the narrow ligaments. This study aimed to measure the strain pattern of the deltoid ligament bands directly using a Miniaturization Ligament Performance Probe (MLPP) system.

METHODS

The MLPP was sutured into the ligamentous bands of the deltoid ligament in 6 fresh-frozen lower extremity cadaveric specimens. The strain was measured using a round metal disk (clock) fixed on the plantar aspect of the foot. The ankle was manually moved from 15° dorsiflexion to 30° plantar flexion, and a 1.2-N-m force was applied to the ankle and subtalar joint complex. Then the clock was rotated every 30° to measure the strain of each ligamentous band at each endpoint.

RESULTS

The tibionavicular ligament (TNL) began to tense at 10° plantar flexion, and the tension becomes stronger as the angle increased; the TNL worked most effectively in plantar flex-abduction. The tibiospring ligament (TSL) began to tense gradually at 15° plantar flexion, and the tension became stronger as the angle increased. The TSL worked most effectively in abduction. The tibiocalcaneal ligament (TCL) began to tense gradually at 0° dorsiflexion, and the tension became stronger as the angle increased. The TCL worked most effectively in pronation (dorsiflexion-abduction). The superficial posterior tibiotalar ligament (SPTTL) began to tense gradually at 0° dorsiflexion, and the tension became stronger as the angle increased, with the SPTTL working most effectively in dorsiflexion.

CONCLUSION

Our results show the biomechanical function of the superficial deltoid ligament and may contribute to determining which ligament is damaged during assessment in the clinical setting.

Analyzing Uncertainty of an Ankle Joint Model with Genetic Algorithm

Recent studies in biomechanical modeling suggest a paradigm shift, in which the parameters of biomechanical models would no longer treated as fixed values but as random variables with, often unknown, distributions. In turn, novel and efficient numerical methods will be required to handle such complicated modeling problems. The main aim of this study was to introduce and verify genetic algorithm for analyzing uncertainty in biomechanical modeling. The idea of the method was to encode two adversarial models within one decision variable vector. These structures would then be concurrently optimized with the objective being the maximization of the difference between their outputs. The approach, albeit expensive numerically, offered a general formulation of the uncertainty analysis, which did not constrain the search space. The second aim of the study was to apply the proposed procedure to analyze the uncertainty of an ankle joint model with 43 parameters and flexible links. The bounds on geometrical and material parameters of the model were set to 0.50 mm and 5.00% respectively. The results obtained from the analysis were unexpected. The two obtained adversarial structures were almost visually indistinguishable and differed up to 38.52% in their angular displacements.

Estimating the stabilizing function of ankle and subtalar ligaments via a morphology-specific three-dimensional dynamic model

Knowledge of the stabilizing role of the ankle and subtalar ligaments is important for improving clinical techniques such as ligament repair and reconstruction. However, this knowledge is incomplete. The goal of this study was to expand this knowledge by investigating the stabilizing function of the ligaments using multiple morphologically subject-specific computational models. Nine models were created from the lower extremities of nine donors. Each model consisted of the articulating bones, articular cartilage, and ligaments. Simulations were conducted in ADAMS™ - a dynamic simulation program. During simulation, tibia and fibula were fixed while cyclic moments in all three anatomical planes were applied to the calcaneus one-at-a-time. The resulting displacements between the bones and the forces in each ligament were computed. Simulations were conducted with all ligaments intact and after simulated ligament serial sectioning. Each model was validated by comparing the simulation results to experimental data obtained from the specimen used to construct the model. From the results the stabilizing role of each ligament was established and the effect of ligament sectioning on Range of Motion and Overall Laxity was identified. On the lateral side, ATFL provided stabilization in supination, CFL restrained inversion, external rotation and dorsiflexion and PTFL limited dorsiflexion and external rotation. On the medial side, PTTL restrained dorsiflexion and internal rotation, ATTL limited plantarflexion and external rotation, and TCL limited dorsiflexion, eversion and external rotation. At the subtalar joint, ITCL limited plantarflexion and its posterior-lateral bundle restrained subtalar inversion. CL restrained plantarflexion/dorsiflexion, and internal and external rotation. The large inter-model variability observed in the results indicate the importance of using multiple subject-specific models rather than relying on one "representative" model.

Influence of population variability in ligament material properties on the mechanical behavior of ankle: a computational investigation

Biomechanical behavior of ankle ligaments varies among individuals, with the underlying mechanism at multiple scales remaining unquantified. The present probabilistic study investigated how population variability in ligament material properties would influence the joint mechanics. A previously developed finite element ankle model with parametric ligament properties was used. Taking the typical external rotation as example loading scenario, joint stability of the investigated population was consistently shared by specific ligaments within a narrow tolerance range, i.e. 62.8 ± 8.2 Nm under 36.1 ± 5.7° foot rotation. In parallel, the inherent material variability significantly alters the consequent injury patterns. Three most vulnerable ligaments and the consequent rupture sequences were identified, with the structural weak spot and the following progressive stability loss dominated by the relative stiffness among ligaments. This study demonstrated the feasibility of biofidelic models in investigating individual difference at the material level, and emphasized the importance of probabilistic description of individual difference when identifying the injury mechanism of a broad spectrum.

Experimental and finite element investigation of total ankle replacement: A review of literature and recommendations

This paper briefly reviews the different methodology, technology, challenges, and outcomes of various studies related to TAR prosthesis based on numerical and experimental techniques. Very less in-vitro experimental studies on TAR have been found than finite element (FE) studies. Due to the invasive nature of the experimental approach, inadequacy and less clinical information, computational modelling has been widely used by the researchers. This paper critically examines the part related to FE modelling and experimental analysis. Some recommendation related to modelling of bones, cartilages, ligaments, muscles, and implant-bone interface condition were discussed for better understanding the results and better clinical significance.

Effects of implant orientation and implant material on tibia bone strain, implant–bone micromotion, contact pressure, and wear depth due to total ankle replacement

The aim of this study is to investigate the effects of implant orientation and implant material on tibia bone strain, implant–bone micromotion, maximum contact pressure, and wear depth at the articulating surface due to total ankle replacement. Three-dimensional finite element models of intact and implanted ankle were developed from computed tomography scan data. Four implanted models were developed having varus and valgus orientations of 5° and 10°, respectively. In order to determine the effect of implant material combination on tibia bone strain, micromotion, contact pressure, and wear depth, three other finite element models were developed having a different material combination of the implant. Dorsiflexion, neutral, and plantarflexion positions were considered as applied loading condition, along with muscle force and ligaments. Implant orientation alters the strain distribution in tibia bone. Strain shielding was found to be less in the case of the optimally positioned implant. Apart from the strain, implant orientation also affects implant–bone micromotion, contact pressure, and wear depth. Implant materials have less influence on tibia bone strain and micromotion. However, wear depth was reduced when ceramic and carbon fibre–reinforced polyetheretherketone material combination was used. Proper orientation of the implant is important to reduce the strain shielding. The present result suggested that ceramic can be used as an alternative to metal and carbon fibre–reinforced polyetheretherketone as an alternative to ultra-high molecular weight polyethylene to reduce wear, which would be beneficial for long-term success and fixation of the implant.

Comparative Evaluation Between Anatomic and Nonanatomic Lateral Ligament Reconstruction Techniques in the Ankle Joint: A Computational Study

Biomechanical studies have indicated that the conventional nonanatomic reconstruction techniques for lateral ankle sprain (LAS) tend to restrict subtalar joint motion compared to intact ankle joints. Excessive restriction in subtalar motion may lead to chronic pain, functional difficulties, and development of osteoarthritis (OA). Therefore, various anatomic surgical techniques to reconstruct both the anterior talofibular and calcaneofibular ligaments (CaFL) have been introduced. In this study, ankle joint stability was evaluated using multibody computational ankle joint model to assess two new anatomic reconstruction and three popular nonanatomic reconstruction techniques. An LAS injury, three popular nonanatomic reconstruction models (Watson-Jones, Evans, and Chrisman–Snook) and two common types of anatomic reconstruction models were developed based on the intact ankle model. The stability of ankle in both talocrural and subtalar joint were evaluated under anterior drawer test (150 N anterior force), inversion test (3 N·m inversion moment), internal rotational test (3 N·m internal rotation moment), and the combined loading test (9 N·m inversion and internal moment as well as 1800 N compressive force). Our overall results show that the two anatomic reconstruction techniques were superior to the nonanatomic reconstruction techniques in stabilizing both talocrural and subtalar joints. Restricted subtalar joint motion, which is mainly observed in Watson-Jones and Chrisman–Snook techniques, was not shown in the anatomical reconstructions. Evans technique was beneficial for subtalar joint as it does not restrict subtalar motion, though Evans technique was insufficient for restoring talocrural joint inversion. The anatomical reconstruction techniques best recovered ankle stability.

Influence of ankle joint plantarflexion and dorsiflexion on lateral ankle sprain: A computational study

Understanding the mechanism of injury involved in lateral ankle sprain is essential to prevent injury, to establish surgical repair and reconstruction, and to plan reliable rehabilitation protocols. Most studies for lateral ankle sprain posit that ankle inversion, internal rotation, and plantarflexion are involved in the mechanism of injury. However, recent studies indicated that ankle dorsiflexion also plays an important role in the lateral ankle sprain mechanism. In this study, the contributions of ankle plantarflexion and dorsiflexion on the ankle joint were evaluated under complex combinations of internal and inversion moments. A multibody ankle joint model including 24 ligaments was developed and validated against two experimental cadaveric studies. The effects of ankle plantarflexion (up to 60°) and dorsiflexion (up to 30°) on the lateral ankle sprain mechanism under ankle inversion moment coupled with internal rotational moment were investigated using the validated model. Lateral ankle sprain injuries can occur during ankle dorsiflexion, in which the calcaneofibular ligament and anterior talofibular ligament tears may occur associated with excessive inversion and internal rotational moment, respectively. Various combinations of inversion and internal moment may lead to anterior talofibular ligament injuries at early ankle plantarflexion, while the inversion moment acts as a primary factor to tear the anterior talofibular ligament in early plantarflexion. It is better to consider inversion and internal rotation as primary factors of the lateral ankle sprain mechanism, while plantarflexion or dorsiflexion can be secondary factor. This information will help to clarify the lateral ankle sprain mechanism of injury.

A numerical study on stress distribution across the ankle joint: Effects of material distribution of bone, muscle force and ligaments

OBJECTIVE

The goal of this study is to develop a realistic three dimensional FE model of intact ankle joint.

METHODS

Three dimensional FE model of the intact ankle joint was developed using computed tomography data sets. The effect of muscle force, ligaments and proper material property distribution of bone on stress distribution across the intact ankle joint was studied separately.

RESULTS

Present study indicates bone material property, ligaments and muscle force have influence on stress distribution across the ankle joint.

CONCLUSION

Proper bone material, ligaments and muscle must be considered in the computational model for pre-clinical analysis of ankle prosthesis.

Determination of the in situ mechanical behavior of ankle ligaments

The mechanical behavior of ankle ligaments at the structural level can be characterized by force-displacement curves in the physiologic phase up to the initiation of failure. However, these properties are difficult to characterize in vitro due to the experimental difficulties in replicating the complex geometry and non-uniformity of the loading state in situ. This study used a finite element parametric modeling approach to determine the in situ mechanical behavior of ankle ligaments at neutral foot position for a mid-sized adult foot from experimental derived bony kinematics. Nine major ankle ligaments were represented as a group of fibers, with the force-elongation behavior of each fiber element characterized by a zero-force region and a region of constant stiffness. The zero-force region, representing the initial tension or slackness of the whole ligament and the progressive fiber uncrimping, was identified against a series of quasi-static experiments of single foot motion using simultaneous optimization. A range of 0.33-3.84mm of the zero-force region was obtained, accounting for a relative length of 6.7±3.9%. The posterior ligaments generally exhibit high-stiffness in the loading region. Following this, the ankle model implemented with in situ ligament behavior was evaluated in response to multiple loading conditions and proved capable of predicting the bony kinematics accurately in comparison to the cadaveric response. Overall, the parametric ligament modeling demonstrated the feasibility of linking the gross structural behavior and the underlying bone and ligament mechanics that generate them. Determination of the in situ mechanical properties of ankle ligaments provides a better understanding of the nonlinear nature of the ankle joint. Applications of this knowledge include functional ankle joint mechanics and injury biomechanics.

The role of shoe design on the prediction of free torque at the shoe-surface interface using pressure insole technology

The goal of the current study was to expand on previous work to validate the use of pressure insole technology in conjunction with linear regression models to predict the free torque at the shoe-surface interface that is generated while wearing different athletic shoes. Three distinctly different shoe designs were utilised. The stiffness of each shoe was determined with a material's testing machine. Six participants wore each shoe that was fitted with an insole pressure measurement device and performed rotation trials on an embedded force plate. A pressure sensor mask was constructed from those sensors having a high linear correlation with free torque values. Linear regression models were developed to predict free torques from these pressure sensor data. The models were able to accurately predict their own free torque well (RMS error 3.72 ± 0.74 Nm), but not that of the other shoes (RMS error 10.43 ± 3.79 Nm). Models performing self-prediction were also able to measure differences in shoe stiffness. The results of the current study showed the need for participant-shoe specific linear regression models to insure high prediction accuracy of free torques from pressure sensor data during isolated internal and external rotations of the body with respect to a planted foot.

A framework for parametric modeling of ankle ligaments to determine the in situ response under gross foot motion

Ligament sprains account for a majority of injuries to the foot and ankle complex, but ligament properties have not been understood well due to the difficulties in replicating the complex geometry, in situ stress state, and non-uniformity of the strain. For a full investigation of the injury mechanism, it is essential to build up a foot and ankle model validated at the level of bony kinematics and ligament properties. This study developed a framework to parameterize the ligament response for determining the in situ stress state and heterogeneous force-elongation characteristics using a finite element ankle model. Nine major ankle ligaments and the interosseous membrane were modeled as discrete elements corresponding functionally to the ligamentous microstructure of collagen fibers and having parameterized toe region and stiffness at the fiber level. The range of the design variables in the ligament model was determined from existing experimental data. Sensitivity of the bony kinematics to each variable was investigated by design of experiment. The results highlighted the critical role of the length of the toe region of the ligamentous fibers on the bony kinematics with the cumulative influence of more than 95%, while the fiber stiffness was statistically insignificant with an influence of less than 1% under the given variable range and loading conditions. With the flexibility of variable adjustment and high computational efficiency, the presented ankle model was generic in nature so as to maximize its applicability to capture the individual ligament behaviors in future studies.

A Robot-Driven Computational Model for Estimating Passive Ankle Torque With Subject-Specific Adaptation

BACKGROUND

Robot-assisted ankle assessment could potentially be conducted using sensor-based and model-based methods. Existing ankle rehabilitation robots usually use torquemeters and multiaxis load cells for measuring joint dynamics. These measurements are accurate, but the contribution as a result of muscles and ligaments is not taken into account. Some computational ankle models have been developed to evaluate ligament strain and joint torque. These models do not include muscles and, thus, are not suitable for an overall ankle assessment in robot-assisted therapy.

METHODS

This study proposed a computational ankle model for use in robot-assisted therapy with three rotational degrees of freedom, 12 muscles, and seven ligaments. This model is driven by robotics, uses three independent position variables as inputs, and outputs an overall ankle assessment. Subject-specific adaptations by geometric and strength scaling were also made to allow for a universal model.

RESULTS

This model was evaluated using published results and experimental data from 11 participants. Results show a high accuracy in the evaluation of ligament neutral length and passive joint torque. The subject-specific adaptation performance is high, with each normalized root-mean-square deviation value less than 10%.

CONCLUSION

This model could be used for ankle assessment, especially in evaluating passive ankle torque, for a specific individual. The characteristic that is unique to this model is the use of three independent position variables that can be measured in real time as inputs, which makes it advantageous over other models when combined with robot-assisted therapy.

A real-time computational model for estimating kinematics of ankle ligaments

BACKGROUND

An accurate assessment of ankle ligament kinematics is crucial in understanding the injury mechanisms and can help to improve the treatment of an injured ankle, especially when used in conjunction with robot-assisted therapy. A number of computational models have been developed and validated for assessing the kinematics of ankle ligaments. However, few of them can do real-time assessment to allow for an input into robotic rehabilitation programs.

METHOD

An ankle computational model was proposed and validated to quantify the kinematics of ankle ligaments as the foot moves in real-time. This model consists of three bone segments with three rotational degrees of freedom (DOFs) and 12 ankle ligaments. This model uses inputs for three position variables that can be measured from sensors in many ankle robotic devices that detect postures within the foot-ankle environment and outputs the kinematics of ankle ligaments. Validation of this model in terms of ligament length and strain was conducted by comparing it with published data on cadaver anatomy and magnetic resonance imaging.

RESULTS

The model based on ligament lengths and strains is in concurrence with those from the published studies but is sensitive to ligament attachment positions.

CONCLUSIONS

This ankle computational model has the potential to be used in robot-assisted therapy for real-time assessment of ligament kinematics. The results provide information regarding the quantification of kinematics associated with ankle ligaments related to the disability level and can be used for optimizing the robotic training trajectory.

A Three-Dimensional Ankle Kinetostatic Model to Simulate Loaded and Unloaded Joint Motion

A kinetostatic model able to replicate both the natural unloaded motion of the tibiotalar (or ankle) joint and the joint behavior under external loads is presented. The model is developed as the second step of a sequential procedure, which allows the definition of a kinetostatic model as a generalization of a kinematic model of the joint defined at the first step. Specifically, this kinematic model taken as the starting point of the definition procedure is a parallel spatial mechanism which replicates the ankle unloaded motion. It features two rigid bodies (representing the tibia–fibula and the talus–calcaneus complexes) interconnected by five rigid binary links, that mimic three articular contacts and two nearly isometric fibers (IFs) of the tibiocalcaneal ligament (TiCaL) and calcaneofibular ligament (CaFiL). In the kinetostatic model, the five links are considered as compliant; moreover, further elastic structures are added to represent all the main ankle passive structures of the joint. Thanks to this definition procedure, the kinetostatic model still replicates the ankle unloaded motion with the same accuracy as the kinematic model. In addition, the model can replicate the behavior of the joint when external loads are applied. Finally, the structures that guide these motions are consistent with the anatomical evidence. The parameters of the model are identified for two specimens from both subject-specific and published data. Loads are then applied to the model in order to simulate two common clinical tests. The model-predicted ankle motion shows good agreement with results from the literature.

Rotational stiffness of American football shoes affects ankle biomechanics and injury severity

While previous studies have investigated the effect of shoe-surface interaction on injury risk, few studies have examined the effect of rotational stiffness of the shoe. The hypothesis of the current study was that ankles externally rotated to failure in shoes with low rotational stiffness would allow more talus eversion than those in shoes with a higher rotational stiffness, resulting in less severe injury. Twelve (six pairs) cadaver lower extremities were externally rotated to gross failure while positioned in 20 deg of pre-eversion and 20 deg of predorsiflexion by fixing the distal end of the foot, axially loading the proximal tibia, and internally rotating the tibia. One ankle in each pair was constrained by an American football shoe with a stiff upper, while the other was constrained by an American football shoe with a flexible upper. Experimental bone motions were input into specimen-specific computational models to examine levels of ligament elongation to help understand mechanisms of ankle joint failure. Ankles in flexible shoes allowed 6.7±2.4 deg of talus eversion during rotation, significantly greater than the 1.7±1.0 deg for ankles in stiff shoes (p = 0.01). The significantly greater eversion in flexible shoes was potentially due to a more natural response of the ankle during rotation, possibly affecting the injuries that were produced. All ankles failed by either medial ankle injury or syndesmotic injury, or a combination of both. Complex (more than one ligament or bone) injuries were noted in 4 of 6 ankles in stiff shoes and 1 of 6 ankles in flexible shoes. Ligament elongations from the computational model validated the experimental injury data. The current study suggested flexibility (or rotational stiffness) of the shoe may play an important role in both the severity of ankle injuries for athletes.

Development of an injury risk function for first metatarsophalangeal joint sprains

INTRODUCTION

Sprains of the first metatarsophalangeal (1MTP) joint, also known as turf toe, are debilitating athletic injuries. Because 85% of 1MTP sprains result from excessive hallux dorsiflexion, interventions that limit motion to subinjurious levels would greatly benefit athletes. Hallux dorsiflexion range of motion (hdROM) cannot be overly constrained, however, lest athletic performance be compromised. Therefore, the tolerance of the 1MTP joint to excessive dorsiflexion injury must be quantified before appropriate hdROM limitations may be developed. The purpose of this study was to develop a quantitative injury risk function for 1MTP sprains on the basis of hallux dorsiflexion angle.

METHODS

Twenty cadaveric limbs were tested to both subinjurious and injurious levels of hallux dorsiflexion. Motion capture techniques were used to track six-degree-of-freedom motion of the first proximal phalanx, first metatarsal, and calcaneus. Specimens were examined by physicians posttest to diagnose injury occurrence and ensure clinical relevance of the injuries.

RESULTS

A two-parameter Weibull hazard function analysis reveals that a 50% risk of injury occurs at 78° of dorsiflexion from anatomical zero.

CONCLUSION

Methods presented here drove cadaveric 1MTP joints to various degrees of dorsiflexion, resulting in both noninjurious and injurious trials, which were formed into an injury risk function.

A population of patient-specific adult acquired flatfoot deformity models before and after surgery

Following IRB approval, a cohort of 3-D rigid-body computational models was created from submillimeter MRIs of clinically diagnosed Adult Acquired Flatfoot Deformity patients and employed to investigate postoperative foot/ankle function and surgical effect during single-leg stance. Models were constrained through physiologic joint contact, passive soft-tissue tension, active muscle force, full body weight, and without idealized joints. Models were validated against patient-matched controls using clinically utilized radiographic angle and distance measures and plantar force distributions in the medial forefoot, lateral forefoot, and hindfoot. Each model further predicted changes in strain for the spring ligament, deltoid ligament, and plantar fascia, as well as joint contact loads for three midfoot joints, the talonavicular, navicular-1st cuneiform, and calcaneocuboid. Radiographic agreement ranged across measures, with average absolute deviations of <5° and <4 mm indicating generally good agreement. Postoperative plantar force loading in patients and models was reduced for the medial forefoot and hindfoot concomitant with increases in the lateral forefoot. Model predicted reductions in medial soft-tissue strain and increases in lateral joint contact load were consistent with in vitro observations and elucidate the biomechanical mechanisms of repair. Thus, validated rigid-body models offer promise for the investigation of foot/ankle kinematics and biomechanical behaviors that are difficult to measure in vivo.

Specimen-Specific Computational Models of Ankle Sprains Produced in a Laboratory Setting

The use of computational modeling to predict injury mechanisms and severity has recently been investigated, but few models report failure level ligament strains. The hypothesis of the study was that models built off neutral ankle experimental studies would generate the highest ligament strain at failure in the anterior deltoid ligament, comprised of the anterior tibiotalar ligament (ATiTL) and tibionavicular ligament (TiNL). For models built off everted ankle experimental studies the highest strain at failure would be developed in the anterior tibiofibular ligament (ATiFL). An additional objective of the study was to show that in these computational models ligament strain would be lower when modeling a partial versus complete ligament rupture experiment. To simulate a prior cadaver study in which six pairs of cadaver ankles underwent external rotation until gross failure, six specimen-specific models were built based on computed tomography (CT) scans from each specimen. The models were initially positioned with 20 deg dorsiflexion and either everted 20 deg or maintained at neutral to simulate the cadaver experiments. Then each model underwent dynamic external rotation up to the maximum angle at failure in the experiments, at which point the peak strains in the ligaments were calculated. Neutral ankle models predicted the average of highest strain in the ATiTL (29.1 ± 5.3%), correlating with the medial ankle sprains in the neutral cadaver experiments. Everted ankle models predicted the average of highest strain in the ATiFL (31.2 ± 4.3%) correlating with the high ankle sprains documented in everted experiments. Strains predicted for ligaments that suffered gross injuries were significantly higher than the strains in ligaments suffering only a partial tear. The correlation between strain and ligament damage demonstrates the potential for modeling to provide important information for the study of injury mechanisms and for aiding in treatment procedure.

The Use of Model Matching Video Analysis and Computational Simulation to Study the Ankle Sprain Injury Mechanism

Lateral ankle sprains continue to be the most common injury sustained by athletes and create an annual healthcare burden of over $4 billion in the U.S. alone. Foot inversion is suspected in these cases, but the mechanism of injury remains unclear. While kinematics and kinetics data are crucial in understanding the injury mechanisms, ligament behaviour measures – such as ligament strains – are viewed as the potential causal factors of ankle sprains. This review article demonstrates a novel methodology that integrates model matching video analyses with computational simulations in order to investigate injury-producing events for a better understanding of such injury mechanisms. In particular, ankle joint kinematics from actual injury incidents were deduced by model matching video analyses and then input into a generic computational model based on rigid bone surfaces and deformable ligaments of the ankle so as to investigate the ligament strains that accompany these sprain injuries. These techniques may have the potential for guiding ankle sprain prevention strategies and targeted rehabilitation therapies.

Eversion during external rotation of the human cadaver foot produces high ankle sprains

While high ankle sprains are often clinically ascribed to excessive external foot rotation, no experimental study documents isolated anterior tibiofibular ligament (ATiFL) injury under this loading. We hypothesized that external rotation of a highly everted foot would generate ATiFL injury, in contrast to deltoid ligament injury from external rotation of a neutral foot. Twelve (six pairs) male cadaveric lower extremity limbs underwent external foot rotation until gross failure. All limbs were positioned in 20° of dorsiflexion and restrained with elastic athletic tape. Right limbs were in neutral while left limbs were everted 20°. Talus motion relative to the tibia was measured using motion capture. Rotation at failure for everted limbs (46.8 ± 6.1°) was significantly greater than for neutral limbs (37.7 ± 5.4°). Everted limbs showed ATiFL injury only, while neutral limbs mostly demonstrated deltoid ligament failure. This is the first biomechanical study to produce isolated ATiFL injury under external foot rotation. Eversion of the axially loaded foot predisposes the ATiFL to injury, forming a basis for high ankle sprain. The study helps clarify a mechanism of high ankle sprain and may heighten clinical awareness of isolated ATiFL injury in cases of foot eversion prior to external rotation. It may also provide guidance to investigate the effect of prophylactic measures for this injury.

Rotational Stiffness of Football Shoes Influences Talus Motion during External Rotation of the Foot

Shoe-surface interface characteristics have been implicated in the high incidence of ankle injuries suffered by athletes. Yet, the differences in rotational stiffness among shoes may also influence injury risk. It was hypothesized that shoes with different rotational stiffness will generate different patterns of ankle ligament strain. Four football shoe designs were tested and compared in terms of rotational stiffness. Twelve (six pairs) male cadaveric lower extremity limbs were externally rotated 30 deg using two selected football shoe designs, i.e., a flexible shoe and a rigid shoe. Motion capture was performed to track the movement of the talus with a reflective marker array screwed into the bone. A computational ankle model was utilized to input talus motions for the estimation of ankle ligament strains. At 30 deg of rotation, the rigid shoe generated higher ankle joint torque at 46.2 ± 9.3 Nm than the flexible shoe at 35.4 ± 5.7 Nm. While talus rotation was greater in the rigid shoe (15.9 ± 1.6 deg versus 12.1 ± 1.0 deg), the flexible shoe generated more talus eversion (5.6 ± 1.5 deg versus 1.2± 0.8 deg). While these talus motions resulted in the same level of anterior deltoid ligament strain (approxiamtely 5%) between shoes, there was a significant increase of anterior tibiofibular ligament strain (4.5± 0.4% versus 2.3 ± 0.3%) for the flexible versus more rigid shoe design. The flexible shoe may provide less restraint to the subtalar and transverse tarsal joints, resulting in more eversion but less axial rotation of the talus during foot/shoe rotation. The increase of strain in the anterior tibiofibular ligament may have been largely due to the increased level of talus eversion documented for the flexible shoe. There may be a direct correlation of ankle joint torque with axial talus rotation, and an inverse relationship between torque and talus eversion. The study may provide some insight into relationships between shoe design and ankle ligament strain patterns. In future studies, these data may be useful in characterizing shoe design parameters and balancing potential ankle injury risks with player performance.

Capillary-driven automatic packaging

Packaging continues to be one of the most challenging steps in micro-nanofabrication, as many emerging techniques (e.g., soft lithography) are incompatible with the standard high-precision alignment and bonding equipment. In this paper, we present a simple-to-operate, easy-to-adapt packaging strategy, referred to as Capillary-driven Automatic Packaging (CAP), to achieve automatic packaging process, including the desired features of spontaneous alignment and bonding, wide applicability to various materials, potential scalability, and direct incorporation in the layout. Specifically, self-alignment and self-engagement of the CAP process induced by the interfacial capillary interactions between a liquid capillary bridge and the top and bottom substrates have been experimentally characterized and theoretically analyzed with scalable implications. High-precision alignment (of less than 10 µm) and outstanding bonding performance (up to 300 kPa) has been reliably obtained. In addition, a 3D microfluidic network, aligned and bonded by the CAP technique, has been devised to demonstrate the applicability of this facile yet robust packaging technique for emerging microfluidic and bioengineering applications.