Mechanical and functional assessment of the wrist affected by rheumatoid arthritis: A finite element analysis

Finite Element Modeling of the Human Wrist: A Review

Background  Understanding wrist biomechanics is important to appreciate and treat the wrist joint. Numerical methods, specifically, finite element method (FEM), have been used to overcome experimental methods' limitations. Due to the complexity of the wrist and difficulty in modeling, there is heterogeneity and lack of consistent methodology in the published studies, challenging our ability to incorporate information gleaned from the various studies. Questions/Purposes  This study summarizes the use of FEM to study the wrist in the last decade. Methods  We included studies published from 2012 to 2022 from databases: EBSCO, Research4Life, ScienceDirect, and Scopus. Twenty-two studies were included. Results  FEM used to study wrist in general, pathology, and treatment include diverse topics and are difficult to compare directly. Most studies evaluate normal wrist mechanics, all modeling the bones, with fewer studies including cartilage and ligamentous structures in the model. The dynamic effect of the tendons on wrist mechanics is rarely accounted for. Conclusion  Due to the complexity of wrist mechanics, the current literature remains incomplete. Considering published strategies and modeling techniques may aid in the development of more comprehensive and improved wrist model fidelity.

Scaphoid numerical simulation of the critical loading until fracture

The numerical study of the scaphoid fracture, although it is relatively unexplored, can be of great clinical interest since it is highly common and can result in temporary or persistent disability. In this manuscript, seven combinations of boundary conditions and contacts between adjacent bones, together with four different loads, simulating real hand movements, are assessed. Three different fracture criteria for bones are employed to study the failure of the scaphoid with the aforementioned combination of interaction conditions. The results offer an interesting view of the accuracy of the possible interaction between adjacent bones. For future calculation, it would be possible to choose a combination of the balance between precision and computational cost savings. This study provides a comprehensive assessment into the modeling of the scaphoid bone and its interactions with adjacent bones. The findings reveal that various choices of interactions can yield similar results, allowing for flexibility in selecting interaction models based on desired accuracy or computational efficiency. Ultimately, this study establishes a foundational understanding for future research on modeling scaphoid motion.

The effect of rheumatoid arthritis on upper extremity functions: A kinematic perspective

AIM

To examine the global upper extremity kinematics in 3D while performing "jar opening motion" in Rheumatoid Arthritis (RA) and to compare these with healthy individuals.

METHOD

Twenty-four women (12 healthy, 12 RA) were included. Evaluations were made with a JAMAR dynamometer, Health Assessment Questionnaire, and 3D kinematic analysis of global upper extremity during "jar opening motion." The time taken during "jar opening motion" was analyzed in 2 parts (Part 1, Part 2), with total time: part 1 + part 2. In addition, shoulder-to-table distance; elbow flexion angle; wrist extension angle; the area scanned and angular rotation by arm, forearm and hand were used in the analysis.

RESULTS

Between groups, there was a statistical difference in: bilateral hand grip strength; part 1, part 2, total time; shoulder-to-table distance; elbow flexion angle; the area scanned by hand; angular rotation of arm and hand in favor of the healthy group (P < .05). In stepwise multiple regression analysis, the most predictive variable for disability was elbow flexion, explaining 53.9% of disability.

CONCLUSION

Compared to healthy individuals, individuals with RA have slower motion, more elbow flexion, less hand grip strength, circular pattern in hand, rotation in arm and hand. Increased disability may result in greater load on elbow flexion.

Assessment of Rheumatoid Hand Function as a Characteristic Feature of Rheumatoid Arthritis in Patients Treated with Methotrexate or Methotrexate with Biological Agents with and without Deformation of Hands

BACKGROUND

The hand is an excellent work tool that provides the functional ability to mechanical work. The hand is affected in rheumatoid arthritis (RA) patients, it is a significant problem in the functional sphere as a result of deformities, the grasping function limitation and muscle strength.

OBJECTIVES

The aim of the study was the assessment of grip strength, endurance and manipulation abilities of rheumatoid hands with or without deformities treated with methotrexate (MTX) or MTX plus biologics (MTX+BIO).

MATERIAL AND METHODS

The study involved 80 RA women, (40 received MTX+BIO, 40 MTX), treated at the Rheumatology Department of the Central Clinical Hospital of Interior Affairs in Warsaw. VAS-pain, DAS28, SDAI, HAQ, HAQ hands, estimation of hand grip strength, endurance, manipulation ability were analyzed.

RESULTS

In group MTX+BIO values of DAS28 (3.7±1.3 vs 4.3±1.2, p=0.019), HAQ (0.72 ± 0.57 vs 1.08± 0.87, p=0.011) and HAQ-hand (0.85±0.65 vs. 1.19±0.68, p=0.024) were statistically lower than in MTX group. Hand deformations recorded in 35 (43.7%) cases, 16 (40%) in MTX group, 19 (47.5%) in MTX+BIO. Comparison of grip strength, endurance, manipulation ability showed better results in MTX+BIO group with deformities (significance level from 0.013 to 0.046) than in MTX group. Relative differences in hand function in MTX + BIO group ranged from 10.8% (maximal power grip strength) to 127.6% (minimal hand endurance), after disease duration adjustment - from 28.2% (maximal power grip strength) to 148.4% (minimal hand endurance).

CONCLUSION

Measuring grip strength, hand endurance, manipulation abilities are useful in RA patients with hand deformities.

Biomechanical Finite Element Method Model of the Proximal Carpal Row and Experimental Validation

The Finite Element Method (FEM) models are valuable tools to create an idea of the behavior of any structure. The complexity of the joints, materials, attachment areas, and boundary conditions is an open issue in biomechanics that needs to be addressed. Scapholunate instability is the leading cause of wrist pain and disability among patients of all ages. It is needed a better understanding of pathomechanics to develop new effective treatments. Previous models have emulated joints like the ankle or the knee but there are few about the wrist joint. The elaboration of realistic computational models of the carpus can give critical information to biomedical research and surgery to develop new surgical reconstructions. Hence, a 3D model of the proximal carpal row has been created through DICOM images, making a reduced wrist model. The materials, contacts, and ligaments definition were made via open-source software to extract results and carry on a reference comparison. Thus, considering the limitations that a reduced model could carry on (unbalanced forces and torques), the stresses that result in the scapholunate interosseous ligament (SLIL) lead us to a bones relative displacement, which support the kinematics hypothesis in the literature as the distal carpal row moves as a rigid solid with the capitate bone. Also, experimental testing is performed, successfully validating the linear strength values of the scapholunate ligament from the literature.

Finite element analysis of the performance of additively manufactured scaffolds for scapholunate ligament reconstruction

Rupture of the scapholunate interosseous ligament can cause the dissociation of scaphoid and lunate bones, resulting in impaired wrist function. Current treatments (e.g., tendon-based surgical reconstruction, screw-based fixation, fusion, or carpectomy) may restore wrist stability, but do not address regeneration of the ruptured ligament, and may result in wrist functional limitations and osteoarthritis. Recently a novel multiphasic bone-ligament-bone scaffold was proposed, which aims to reconstruct the ruptured ligament, and which can be 3D-printed using medical-grade polycaprolactone. This scaffold is composed of a central ligament-scaffold section and features a bone attachment terminal at either end. Since the ligament-scaffold is the primary load bearing structure during physiological wrist motion, its geometry, mechanical properties, and the surgical placement of the scaffold are critical for performance optimisation. This study presents a patient-specific computational biomechanical evaluation of the effect of scaffold length, and positioning of the bone attachment sites. Through segmentation and image processing of medical image data for natural wrist motion, detailed 3D geometries as well as patient-specific physiological wrist motion could be derived. This data formed the input for detailed finite element analysis, enabling computational of scaffold stress and strain distributions, which are key predictors of scaffold structural integrity. The computational analysis demonstrated that longer scaffolds present reduced peak scaffold stresses and a more homogeneous stress state compared to shorter scaffolds. Furthermore, it was found that scaffolds attached at proximal sites experience lower stresses than those attached at distal sites. However, scaffold length, rather than bone terminal location, most strongly influences peak stress. For each scaffold terminal placement configuration, a basic metric was computed indicative of bone fracture risk. This metric was the minimum distance from the bone surface to the internal scaffold bone terminal. Analysis of this minimum bone thickness data confirmed further optimisation of terminal locations is warranted.

The effects of additional hollow cylinder coated to external fixator screws for treating pilon fracture: A biomechanical perspective

An external fixator is a promising medical device that could provide optimum stability and reduce the rate of complications in treating bone fracture during intervention period. It is noted that the biomechanics behaviour of device can be altered by introducing more features such as material suitability and additional components. Therefore, this study was conducted via finite element method to investigate the effects of additional hollow cylinder coated with external fixator screws in treating Type III pilon fracture. Finite element models which have been validated with experimental data were used to simulate stresses at the pin-bone interface and relative micromovement at interfragmentary fractures during swing (70 N load) and stance phases (350 N load). All bones and external fixators were assigned with isotropic material properties while the cartilages were simulated with hyper-elastic. For the hollow cylinder, polyethylene was assigned due to its properties which are equivalent to the bone. From the results, it is found that stresses at the pin-bone interface for the coated screws were reduced to 54% as compared to the conventional fixator. For the micromovement, there was no difference between both models, whereby the value was 0.03 mm. The results supported previously published literature, in which high stresses are unavoidable at the interface, fortunately, those stresses did not exceed the ultimate strength of bone, which is safe for treating patients. In conclusion, if patients are allowed to bear weight bearing, the external fixator with coated screws is a more favourable option to be fixed into the bone to avoid complications at the interface.

Stress Distributions and Micromovement of Fragment Bone of Pilon Fracture Treated With External Fixator: A Finite Element Analysis

Osteoporosis and osteoarthritis are common pathological problems of the human bone tissue. There are some cases of pilon fractures associated with these 2 pathological conditions. In terms of treatment, for a normal and healthy bone with pilon fracture, the use of the Delta external fixator is a favorable option because it can allow early mobilization for patients and provide stability for the healing process. However, the stability of the external fixator differs when there is low bone stiffness, which has not been previously investigated. Therefore, this study was conducted to determine the stability of the external fixator to treat pilon fracture associated with osteoporosis and osteoarthritis, particularly to differentiate the stress distribution and micromovement of fracture fragment. Three-dimensional finite element models of the ankle and foot bones were reconstructed based on the computed tomography datasets. The bones consisted of 5 metatarsal, 3 cuneiform, and 1 each of cuboid, navicular, calcaneus, talus, fibula, and tibia bones. They were assigned with linear isotropic behavior. The ankle joint consisted of ligament and cartilage, and they were assigned with the use of linear links and the Mooney-Rivlin model, respectively. During simulation of the gait cycle, 70 N and 350 N were applied axially to the tibia bone to represent the swing and stance phases, respectively. The metatarsal and calcaneus bones were fixed to prevent any movement of the rigid body. The study found that the greatest von Mises stress value was observed at the pin-bone interface for the osteoporosis (108 MPa) model, followed by the osteoarthritis (87 MPa) and normal (44 MPa) models, during the stance phase. For micromovement, the osteoporosis model had the largest value at 0.26 mm, followed by the osteoarthritis (0.09 mm) and normal (0.03 mm) models. In conclusion, the greatest magnitudes of stress and micromovement were observed for the osteoporosis bone and extra care should be taken to treat pilon fracture associated with this pathological condition.

Mechanical performance comparison of two surgical constructs for wrist four-corner arthrodesis via dorsal and radial approaches

BACKGROUND

Four-corner arthrodesis, which involves fusing four carpal bones while removing the scaphoid bone, is a standard surgery for the treatment of advanced stages of wrist arthritis. Nowadays, it can be performed using a dorsal approach by fixing a plate to the bones and a new radial approach is in development. To date, there is no consensus on the biomechanically optimal and most reliable surgical construct for four-corner arthrodesis.

METHODS

To evaluate them biomechanically and thus assist the surgeon in choosing the best implant orientation, radial or dorsal, the two different four-corner arthrodesis surgical constructs were virtually simulated on a 3D finite element model representing all major structures of the wrist. Two different realistic load sets were applied to the model, representing common tasks for the elderly.

FINDINGS

Results consistency was assessed by comparing with the literature the force magnitude computed on the carpal bones. The Von Mises stress distribution in the radial and dorsal plates were calculated. Stress concentration was located at the plate-screw interface for both surgical constructs, with a maximum stress value of 413 MPa for the dorsal plate compared to 326 MPa for the radial plate, meaning that the stress levels are more unfavourable in the dorsal approach.

INTERPRETATION

Although some bending stress was found in one load case, the radial plate was mechanically more robust in the other load case. Despite some limitations, this study provides, for the first time, quantified evidence that the newly developed radial surgical construct is mechanically as efficient as the dorsal surgical construct.

Effect of Achilles tendon on kinematic coupling relationship between tarsal bones: a pilot finite element study

BACKGROUND

The procedure of percutaneous Achilles tenotomy (PAT) is an important component of the Ponseti method. However, few studies reported the influence of Achilles tendon on kinematic coupling relationship between tarsal bones. The purpose of present study was to demonstrate the effect of Achilles tendon on the kinematic coupling relationship between tarsal bones, and to illustrate how kinematic coupling relationship between tarsal bones works in term of finite element analysis.

METHODS

A three-dimensional finite element model of foot and ankle was constructed based on the Chinese digital human girl No.1 (CDH-G1) image database using the software of mimics, Geomagic studio, HyperMesh, and Abaqus. The last manipulation of the Ponseti method before the procedure of PAT was simulated. The talus head and the proximal tibia and fibula bone were fixed in all six degrees of freedom, and the outward pressure was added on the first metatarsal head to investigate the kinematic coupling relationship between tarsal bones.

RESULTS

The least relationship of kinematic coupling between tarsal bones was found in calcaneus. Stress concentration was mainly observed at the navicular, talus and the medial malleolus. The difference in displacement of the navicular was only found with the Achilles tendon stiffness of 0 N/mm and others. No difference in the navicular displacement was found in the stiffness of Achilles tendon between 40, 80, 200, 400, and 1000 N/mm. The maximum displacement of navicular was observed at the ankle position of PF-20° (plantar flexion-20°). The difference in displacement of the navicular was greater at the ankle position of PF-20° with the Achilles tendon stiffness of 0 N/mm than that at the ankle position of PF-40° with the Achilles tendon stiffness of 40 N/mm.

CONCLUSIONS

Based on the findings from this study, it was demonstrated that the Achilles tendon existence or not and ankle position had great influence, while increased stiffness of Achilles tendon had no influence on kinematic coupling relationship between tarsal bones. For the cases with severe equinus, earlier implementation of PAT procedure (with the purpose of release the Achilles tendon and reduce the degree of ankle plantar flexion) may be beneficial to the deformity correction.

Computational Simulation of Synovial Fluid Kinematics of the Scapholunate Joint

Background: The interaction between wrist kinematics and synovial fluid pressure has yet to be studied. To our knowledge, this is the first study to determine the effect of scapholunate joint kinematics on synovial fluid pressure change using finite volume method. Methods: The carpal bones of a cadaveric hand were obtained from Computed Tomography (CT) scans. CT images of the carpal bones were segmented and reconstructed into 3D model. The 3D synovial fluid model between the scaphoid and lunate was constructed and then used for computational simulations. The kinematics data of scapholunate joint obtained from radioulnar deviation of the wrist was investigated. Results: It was found that the pressure in synovial fluid varied from -1.68 to 2.64 Pa with maximum pressure located at the scaphoid-fluid interface during the radial deviation. For ulnar deviation, the pressure increased gradually from the scaphoid-fluid interface towards the lunate-fluid interface (-1.37 to 0.37 Pa). Conclusions: This new computational model provides a basis for the study of pathomechanics of ligament injury with the inclusion of synovial fluid.

Low level laser therapy for reducing pain in rheumatoid arthritis and osteoarthritis: a systematic review

Abstract Introduction: Treatments for rheumatoid arthritis (RA) and osteoarthritis (OA) can reduce, modulate inflammation, and reduce deformities. Low-Level Laser Therapy is a biomodulator and may aid in the clinical picture of these conditions. Objective: To analyze the parameters most frequently used to determine the responses of patients with RA and OA in controlled and uncontrolled clinical trials. Method: This is a systematic review with search of articles in English, Portuguese and Spanish in PUBMED, SCOPUS, LILACS and Web of SCIENCE, of articles published between 2006 and 2018. MeSH terms were used. Inclusion criteria: evaluation of LLLT in the evaluations, evaluation and evaluation of the period, controlled and uncontrolled clinical trials, full publications. The base date of the energy dosimetry and the analysis of mean, median and mode of energy per point and energy per treatment. Results: Three articles on RA and 16 on OA were included in this study. Regarding dosimetry, it was one of the most recent of the pain, being this one with a greater energy dose. In OA, most of the articles presented are of importance, with variability in the dosage applied. Conclusion: There are several reports for patient studies purposes, mainly with doses of 6 J per point and 48 J. In the joints affected with OA and AR, it would be important to publish more scientific articles with better methodological quality and description of dosimetry.

The biomechanical analysis of three-dimensional distal radius fracture model with different fixed splints

BACKGROUND

The distal radius fracture is one of the common clinical fractures. At present, there are no reports regarding application of the finite element method in studying the mechanism of Colles fracture and the biomechanical behavior when using splint fixation.

OBJECTIVE

To explore the mechanism of Colles fracture and the biomechanical behavior when using different fixed splints.

METHODS

Based on the CT scanning images of forearm for a young female volunteer, by using model construction technology combined with RPOE and ANSYS software, a 3-D distal radius fracture forearm finite element model with a real shape and bioactive materials is built. The material tests are performed to obtain the mechanical properties of the paper-based splint, the willow splint and the anatomical splint. The numerical results are compared with the experimental results to verify the correctness of the presented model. Based on the verified model, the stress distribution of different tissues are analyzed. Finally, the clinical tests are performed to observe and verify that the anatomical splint is the best fit for human body.

RESULTS

Using the three kinds of splints, the transferred bone stress focus on the distal radius and ulna, which is helpful to maintain the stability of fracture. Also the stress is accumulated in the distal radius which may be attributed to flexion position. Such stress distribution may be helpful to maintain the ulnar declination. By comparing the simulation results with the experimental observations, the anatomical splint has the best fitting to the limb, which can effectively avoid the local compression.

CONCLUSION

The anatomical splint is the most effective for fixing and curing the fracture. The presented model can provide theoretical basis and technical guide for further investigating mechanism of distal radius fracture and clinical application of anatomical splint.

FUNCTION AND BIOMECHANICS OF UPPER LIMB IN POST-STROKE PATIENTS — A SYSTEMATIC REVIEW

Current clinical services are struggling to provide the most favorable rehabilitation treatment for patients with stroke, which inspired researchers to investigate and explore the use of rehabilitation devices suitable for the patients and rehabilitation therapy. This review paper addresses the importance of biomechanical features in patients who experienced stroke to the upper limb. First and foremost, a review was done on general biomechanical description associated with motor control, shoulder, elbow, wrist and fingers joint. This included the ability of the patients to move their affected arm and the affect on peak joint torque, range of motion, joint forces, grip strength and muscle activities during the activities of daily living. In addition, we also reviewed the material properties and geometrical condition of tissue in stroke patient. The repercussions of post-stroke patient regarding the bone density, stiffness of muscle as well as the thickness of cartilage are described in this review. Based on the findings, the movement of affected stroke hand is associated with the motor control and material properties of tissue. To strengthen the motor control and maintaining tissue properties, early physical training on patients should be conducted in two to four weeks after stroke. In conclusion, this report suggests a new approach for future biomechanical studies in order to enhance the quality of physiotherapy rehabilitation peculiarly for post-stroke patients.

Subject-specific finite element analysis of the carpal tunnel cross-sectional to examine tunnel area changes in response to carpal arch loading

BACKGROUND

Manipulating the carpal arch width (i.e. distance between hamate and trapezium bones) has been suggested as a means to increase carpal tunnel cross-sectional area and alleviate median nerve compression. The purpose of this study was to develop a finite element model of the carpal tunnel and to determine an optimal force direction to maximize area.

METHODS

A planar geometric model of carpal bones at hamate level was reconstructed from MRI with inter-carpal joint spaces filled with a linear elastic surrogate tissue. Experimental data with discrete carpal tunnel pressures (50, 100, 150, and 200mmHg) and corresponding carpal bone movements were used to obtain material property of surrogate tissue by inverse finite element analysis. The resulting model was used to simulate changes of carpal arch widths and areas with directional variations of a unit force applied at the hook of hamate.

FINDINGS

Inverse finite element model predicted the experimental area data within 1.5% error. Simulation of force applications showed that carpal arch width and area were dependent on the direction of force application, and minimal arch width and maximal area occurred at 138° (i.e. volar-radial direction) with respect to the hamate-to-trapezium axis. At this force direction, the width changed to 24.4mm from its initial 25.1mm (3% decrease), and the area changed to 301.6mm² from 290.3mm² (4% increase).

INTERPRETATION

The findings of the current study guide biomechanical manipulation to gain tunnel area increase, potentially helping reduce carpal tunnel pressure and relieve symptoms of compression median neuropathy.

Development of a biomechanical model of the wrist joint for patient-specific model guided surgical therapy planning: Part 1

An enhanced musculoskeletal biomechanical model of the wrist joint is presented in this article. The developed computational model features the two forearm bones radius and ulna, the eight wrist bones, the five metacarpal bones, and a soft tissue apparatus. Validation of the model was based on information taken from the literature as well as own experimental passive in vitro motion analysis of eight cadaver specimens. The computational model is based on the multi-body simulation software AnyBody. A comprehensive ligamentous apparatus was implemented allowing the investigation of ligament function. The model can easily patient specific personalized on the basis of image information. The model enables simulation of individual wrist motion and predicts trends correctly in the case of changing kinematics. Therefore, patient-specific multi-body simulation models are potentially valuable tools for surgeons in pre- and intraoperative planning of implant placement and orientation.

Strain shielding in distal radius after wrist arthroplasty with a current generation implant: An in vitro analysis

A systematic review of peer reviewed articles has shown that the main cause for wrist arthroplasty revision is carpal and radial prosthetic loosening and instability. To improve arthroplasty outcomes, successive generations of implants have been developed over time. The problem with the current generation of implants is the lack of long-term outcomes data. The aim of the present work was to test the hypothesis that the current generation Maestro WRS implant has a stress, strain and stability behaviour which may be associated with a reduced risk of long-term radial component loosening. This study was performed using synthetic radii to experimentally predict the cortex strain behaviour and implant stability considering different load conditions for both intact and implanted conditions. Finite element models were developed to assess the structural behaviour of cancellous-bone and bone-cement, these models were validated against experimentally measured cortex strains. Measured cortex strains showed a significant reduction between intact and implanted states. Cancellous bone adjacent to the radial body component suffers a two to threefold strain reduction, comparing with the intact condition, while along the radial stem, in the axial direction, a strain increase was observed. It is concluded that the use of contemporary Maestro WRS implant changes the biomechanical behaviour of the radius and is associated with a potential risk of bone resorption by stress-shielding in the distal radius region for wrist loads in the range of daily activities.

A biomechanical model of the wrist joint for patient-specific model guided surgical therapy: Part 2

An enhanced musculoskeletal biomechanical model of the wrist joint is presented in this article. The computational model is based on the multi-body simulation software AnyBody. Multi body dynamic musculoskeletal models capable of predicting muscle forces and joint contact pressures simultaneously would be valuable for studying clinical issues related to wrist joint degeneration and restoration. In this study, the simulation model of the wrist joint was used for investigating deeper the biomechanical function of the wrist joint. In representative physiological scenarios, the joint behavior and muscle forces were computed. Furthermore, the load transmission of the proximal wrist joint was investigated. The model was able to calculate the parameters of interest that are not easily obtainable experimentally, such as muscle forces and proximal wrist joint forces. In the case of muscle force investigation, the computational model was able to accurately predict the computational outcome for flexion and extension motion. In the case of force distribution of the proximal wrist joint, the model was able to predict accurately the computational outcome for an axial load of 140 N. The presented model and approach of using a multi-body simulation model are anticipated to have value as a predictive clinical tool including effect of injuries or anatomical variations and initial outcome of surgical procedures for patient-specific planning and custom implant design. Therefore, patient-specific multi-body simulation models are potentially valuable tools for surgeons in pre- and intraoperative planning of implant placement and orientation.

Finite element analysis of three commonly used external fixation devices for treating Type III pilon fractures

Pilon fractures are commonly caused by high energy trauma and can result in long-term immobilization of patients. The use of an external fixator i.e. the (1) Delta, (2) Mitkovic or (3) Unilateral frame for treating type III pilon fractures is generally recommended by many experts owing to the stability provided by these constructs. This allows this type of fracture to heal quickly whilst permitting early mobilization. However, the stability of one fixator over the other has not been previously demonstrated. This study was conducted to determine the biomechanical stability of these external fixators in type III pilon fractures using finite element modelling. Three-dimensional models of the tibia, fibula, talus, calcaneus, navicular, cuboid, three cuneiforms and five metatarsal bones were reconstructed from previously obtained CT datasets. Bones were assigned with isotropic material properties, while the cartilage was assigned as hyperelastic springs with Mooney-Rivlin properties. Axial loads of 350 N and 70 N were applied at the tibia to simulate the stance and the swing phase of a gait cycle. To prevent rigid body motion, the calcaneus and metatarsals were fixed distally in all degrees of freedom. The results indicate that the model with the Delta frame produced the lowest relative micromovement (0.03 mm) compared to the Mitkovic (0.05 mm) and Unilateral (0.42 mm) fixators during the stance phase. The highest stress concentrations were found at the pin of the Unilateral external fixator (509.2 MPa) compared to the Mitkovic (286.0 MPa) and the Delta (266.7 MPa) frames. In conclusion, the Delta external fixator was found to be the most stable external fixator for treating type III pilon fractures.

Biomechanical evaluation of two commonly used external fixators in the treatment of open subtalar dislocation--a finite element analysis

Subtalar dislocation is a rare injury caused by high-energy trauma. Current treatment strategies include leg casts, internal fixation and external fixation. Among these, external fixators are the most commonly used as this method is believed to provide better stabilization. However, the biomechanical stability provided by these fixators has not been demonstrated. This biomechanical study compares two commonly used external fixators, i.e. Mitkovic and Delta. CT imaging data were used to reconstruct three-dimensional models of the tibia, fibula, talus, calcaneus, navicular, cuboid, three cuneiforms and five metatarsal bones. The 3D models of the bones and cartilages were then converted into four-noded linear tetrahedral elements, whilst the ligaments were modelled with linear spring elements. Bones and cartilage were idealized as homogeneous, isotropic and linear. To simulate loading during walking, axial loading (70 N during the swing and 350 N during the stance phase) was applied at the end of diaphyseal tibia. The results demonstrate that the Mitkovic fixator produced greater displacement (peak 3.0mm and 15.6mm) compared to the Delta fixator (peak 0.8mm and 3.9 mm), in both the swing and stance phase, respectively. This study demonstrates that the Delta external fixator provides superior stability over the Mitkovic fixator. The Delta fixator may be more effective in treating subtalar dislocation.

Biomechanical analysis of the wrist arthroplasty in rheumatoid arthritis: a finite element analysis

The total replacement of wrists affected by rheumatoid arthritis (RA) has had mixed outcomes in terms of failure rates. This study was therefore conducted to analyse the biomechanics of wrist arthroplasty using recently reported implants that have shown encouraging results with the aim of providing some insights for the future development of wrist implants. A model of a healthy wrist was developed using computed tomography images from a healthy volunteer. An RA model was simulated based on all ten general characteristics of the disease. The ReMotion ™ total wrist system was then modelled to simulate total wrist arthroplasty (TWA). Finite element analysis was performed with loads simulating the static hand grip action. The results show that the RA model produced distorted patterns of stress distribution with tenfold higher contact pressure than the healthy model. For the TWA model, contact pressure was found to be approximately fivefold lower than the RA model. Compared to the healthy model, significant improvements were observed for the TWA model with minor variations in the stress distribution. In conclusion, the modelled TWA reduced contact pressure between bones but did not restore the stress distribution to the normal healthy condition.