| Background:The calcaneus is an odd-shaped bone. The superior surface consists of three articular facets (the anterior, middle, and posterior) that articulate with the talus. The posterior facet is the major weight-bearing surface and the largest facet. The middle facet is anterior and medial and is located on the sustentaculum; it is often contiguous with the anterior facet. The sustentaculum sits under the talar neck and is medial to the calcaneal body. It is attached to the talus by the interosseous talocalcaneal ligament and by the deltoid ligament medially. The flexor hallucis longus tendon runs below the sustentaculum. Laterally, the peroneal tendons run obliquely along the lateral wall of the calcaneus and sit in two shallow grooves, with a bony prominence between them known as the peroneal tubercle. The entire calcaneal surface behind the posterior facet is known as the posterior tuberosity. On its plantar surface, it has two processes, the lateral and medial. The medial process is the origin of the abductor hallucis muscle and the major weight-bearing structure in the hindfoot. Finally, the Achilles tendon inserts on the posterior surface of the tuberosity.Calcaneal fractures are the most common tarsal bone fractures, accounted for60%of the tarsal bone fractures, accounting for2%of the body fracture, about75%refers to intra-articular fractures,20%to45%with calcaneocuboid joint injury. due to the complexity of the calcaneus anatomy and surrounding anatomical structures, poor quality of local soft tissue coverage, it is difficult to treat with much complications and poor outcome. During the past20years, with the development of antibiotics, imaging technology, internal fixation and minimally invasive techniques, as well as the research of biomechanics of calcaneal fractures, its clinical efficacy and continuously improve. Although a lot of method with differentiated indications and advantages and disadvantages has been applied to treat calcaneal fractures, it is advised that the solution should be made according to the patient’s degree of injury, fracture type, systemic and local circumstances and sections of the technical conditions.Calcaneal fractures often related to calcaneus shorten in the length, width increases, reduced height, uneven subtalar joint the calcaneal axis lateral angled, the the calcaneal tuberosity joint angle (Bohler angle) decreases, disappeared or contra-angle Gissane angle narrowing or increased talar tilt angle to shrink and disappear. This will cause the arch collapse, affect the overall shape of the foot and mechanical stability, can form traumatic flatfoot. Loss of normal relations between the articular surface of the calcaneus three articular surface uneven and the shift is bound to cause a change of the force and movement of the subtalar joint, causing secondary injury subtalar joints in and around the joints, such as trauma arthritis. Calcaneocuboid articular surface fractures will seriously affect the normal movement of the hock function and subtalar joint eversion activities caused calcaneocuboid joint traumatic arthritis, enough to affect normal walking. Serious subtalar joint surface damage and cartilage defects can occur spontaneously arthrodesis, the loss of the active function. After pronation, calcaneal widening shortening, reduced height and lateral wall of the outer expansion is not only caused by wearing shoes difficult, and the peroneus longus and brevis tendon entrapment. In addition, the decline lateral malleolus direct contact or collision the calcaneus outside sidewall calcaneofibular impingement syndrome, long-term form pseudarthrosis. Achilles tendon point enough, leading to weakness of the gastrocnemius and soleus. Hind foot line of force to change the calf muscles can pull direction, torque and muscle changes, the final occurrence of foot and ankle pain and dysfunction. In addition, the abnormal movement of the ankle joint force line and bilateral limb length inequality will affect the normal function of the entire lower extremity and spine, premature low back pain and neck pain. Therefore, the purpose of calcaneal fractures after the resumption of normal biomechanics of the foot and functions to avoid undesirable consequences.With the development of digital simulation modeling techniques and finite element method, the digital virtual gradually applied to the clinical diagnosis, treatment, and experimental studies. Compared with the previous biomechanical experiments, the finite element method is a useful numerical tool geometry and digital nonlinear complex structure of internal stress and strain analysis. Can complete the load cannot be achieved by other research methods and constraints, the specimens can also be corrected to simulate any pathological state. The finite element model can provide the normal physiological experiments cannot be that to get the objective entities experiments difficult to get results. By changing the load loading mode, change the material properties of individual stress analysis.Objective:1. With the help digital technology, to establish the calcaneus and surrounding structures digital virtual simulation model to explore the of digital simulation Medicine modeling method and significance2. Establish normal gait the calcaneus and around the structure of three-dimensional finite element model to analyze the the calcaneus biomechanics normal gait changes.3. To analysis of the operative indication of conservative treatment as well as the intra-articular calcaneal fracture with the change of Bolher angle. Single-leg stance state, calcaneus Bohler angle decreased by5°,10°,15°,20°and25°of the subtalar joint, to test talus calcaneocuboid joint plantar stress distribution changes.4. To analysis of the operative indication of conservative treatment as well as the intra-articular calcaneal fracture with the change of Bolher angle. Single-leg stance state, calcaneus height decreased by5mm,10mm,15mm and20mm, talus calcaneocuboid joint plantar stress distribution changes.5. Calcaneal locking plates and anatomical plate in the treatment of Sanders Ⅳ type calcaneal fractures subtalar joint, talus, calcaneus fracture fragments, with the the calcaneocuboid joint stress distribution and displacement comprehensive comparative analysis to arrive at Sanders Ⅳ calcaneal fractures treatment programs and to infer the direction of the internal fixation reasonable improvement.Methods:1.Construction and verification of digital three-dimensional finite element model of foot and ankle:Adopting LightSpeed16-slice spiral CT of the Imaging Center, Nanfang Hospital affiliated to Southern Medical University (scanning parameters:120～140KV,240～300mA, pitch of1.375～1.75, the layer thickness7.5mm, matrix512×512, reconstruction slice thickness0.625mm), scaned the volunteer’s right foot from distal tibia and fibula20cm above the ankle down to the planta, with the right foot remaining neutral position. Then imported the scanned data of DICOM format into Mimics10.01software, by threshold segmentation automatically or manually, to reconstruct the three-dimensional structure of a complete foot and ankle composed of28bones and surrounding soft tissue. Finally, exported the data with point cloud format and reimported into SolidWorks2009, using the guide of grid processing and surface generation to form geometric models and reconfigure them, then import the data into two kinds of finite element analysis softwares:Method1: Imported into Workbench module of finite element analysis software ANSYS12.0to establish a complete finite element model of foot and ankle. To construct0.5mm of articular cartilage on both sides of contact surface according to the joint space, while to use three-dimensional rod elements which were compressed only to simulate other joints cartilage, and to establish125springs to simulate ligaments and crural interosseous membrane, five beam elements to simulate plantar fascia.The material properties were determined with reference to documents. Then refered to Anderson’s method, took the tibia and talus only as a simple model of the ankle joint for test. The normal standing status of ankle joint was simulated by application a vertical load of700N on the upper section of the lower tibia while the talus constrained. To measure contact pressure and contact area of inferior articular surface of the tibia, and to compare the results with the former. Method2:Imported the data into Simulation module of Solidworks to establish a simplified finite element model of the ankle. According to the research needs, to establish a5-bones assembly finite element model containing the tibia, fibula, talus, calcaneus, and navicular, and a9-bones assembly finite element model also including the cuboid bone and the three cuneiform bones in addition to the5bones mentioned above. In the models, to use tension-only springs to simulate ligaments connection. The5-bones assembly contained31springs, while the 9-bones assembly was established42springs to simulate connected structures such as ligaments around the ankle and crural interosseous membrane. To Define the material properties of each tissue, to generate contact pair between each joints automatically or manually, and to set the corresponding boundary conditions. In the5-bone assembly finite element model of ankle, to simulate the state of human body with one foot standing and the states of internal and external rotation of ankle, while in the9-bones finite element model of ankle, to simulate the states of ankle inversion and ankle eversion. Then, regulated the mesh density to generate mesh, and set the simulation examples attribute for solution.2.With the Subtalar joint surface unchanged, wedge osteotomy was applied to the calcaneus to make the Bohler angle gradually decreased in5degrees,10degrees,15degrees,20degrees,25degrees, respectively, then reconstruct six FE models. Assuming the fracture surface completely broken and is in contact state, and the coefficient of friction of0.3. All nodes of the calcaneus, the lower edge of the three-dimensional finite element model of the weight-bearing area of the foot degrees of freedom constrained to zero as boundary conditions, each remote node in the X, Y, Z axis displacement0. Be on the tibial end model700N axial stress calculation of stress distribution and displacement distribution of the model under this stress.3.With the Subtalar joint surface unchanged, wedge osteotomy was applied to the calcaneus to make the calcaneus height gradually decreased in5mm,10mm,15mm and20mm respectively, then reconstruct five FE models. Assuming the fracture surface completely broken and is in contact state, and the coefficient of friction of0.3. All nodes of the calcaneus, the lower edge of the three-dimensional finite element model of the weight-bearing area of the foot degrees of freedom constrained to zero as boundary conditions, each remote node in the X, Y, Z axis displacement0. Be on the tibial end model700N axial stress calculation of stress distribution and displacement distribution of the model under this stress.4.To stimulated Sanders IV fractures of the calcaneus and do osteotomy as well, keep the fragment of the articular surface of the friction relationship. Assuming the fracture surface completely broken and is in contact state, and the coefficient of friction of0.3. Anatomic plate set to frictional contact, locking plate is set to no contact the calcaneus foot weight-bearing area three-dimensional finite element model of the lower edge of the degrees of freedom of all nodes constrained to zero as the boundary conditions. All nodes of the calcaneus, the lower edge of the three-dimensional finite element model of the weight-bearing area of the foot degrees of freedom constrained to zero as boundary conditions, each remote node in the X, Y, Z axis displacement0. Be on the tibial end model700N axial stress calculation of stress distribution and displacement distribution of the model under this stress.Result:1. In the complete finite element model of foot and ankle constructed by method1, the test results of the ankle composed of tibia and talus were taken to contrast with Anderson’s study. It showed that the stress on ankle joint articular surface of the tibia mainly distributed at the central and anterolateral aspects in both of them when ankle joint was in neutral position under the vertical loading, with the maximum stress between2.7MPa and4.0MPa, substantial agreement on distribution area, distribution trend, and numerical value. It was preliminarily verified this model valid. The test results of two simplified three-dimensional finite element models of the ankle established by method2showed that they could simulate various load situations of ankle for solution, with moderate number of element, node and suitable amount of computing time. The stress and displacement results in each loading were reasonable and close to the actual situation. Compared with Liacouras and Wayne’s model under the same boundary conditions, it showed that the rotation angle of tibia was3.85°in our model whereas their result was4.28°after application of ankle external rotation load, comparatively close between the two values. In adition, the contact forces of major joints were also consistent with theirs. All confirmed the validity of our models.2. When Bohler angle of the calcaneus decreased by5°,10°,15°,20°,25°, and the stress of the subtalar joint surface peak increase of11.3%,12%,18.5%,19.8%,32.1%; taluspeak stress increased by3%, respectively,12.6%,27.6%,54%,70.6%; calcaneocuboid articular surface stress reduce Bohler angle decreases, the peak stress decreased by2.3%,4.7%,15.4%,20.3%,20.4%; plantar stress concentration at the bottom of the calcaneus, the peak stress values were increased by11.3%.11.6%,18.6%,19.8%,32.1%.3. Calcaneal height to reduce5mm,10mm,15mm,20mm, the subtalar joint surface stress peak increase of10.3%,18.5%,35.4%,54.7%; the talar side peak stress increased by8.9%,9.8%,16.1%,16.7%; calcaneocuboid articular surface stress with the reduction of the height of the calcaneus continue to decrease, the peak stress decreased by2.3%,4.4%,22.4%,67.4%,80.1%;The foot stress mainly concentrated in the heel at peak stress increased10.2%,18.4%,35.4%,54.7%.4. Using locking plate fixation, maximum stress concentration at the plate and screw the junction stress peak to143.49MPa, stress concentrated on the middle of the upper row of screws steel plate in the rear; anatomical plate stress distribution is relatively uniform the screw stress distribution is uniform, the maximum stress in thesteel plate in the rear of the peak stress of102.49MPa; talus and calcaneus each fracture stress peak are locking plate block than anatomic plate. Major displacement of two steel plates fixed model distributed near the articular surface of the screw at which the lock plate displacement is less than the anatomical plate model were0.222mm,0.389mm; locking plate fixation peak displacement of the fracture fragments are less than anatomic steel plate.Conclusions:1. Based on CT scan data, the calcaneus three-dimensional digital simulation models were established using Mimics, Geomagic Studio, and UG software. This approach was feasible, effective, faster and harmless to the human body. The model contained a large amount of information and entities with a similar geometry to the more realistic simulation of the original model.2. The three-dimensional finite element method was a biomechanical study of theories and methods to simulate the geometric model of the structure to give organizations the biological material properties. It can reflect the biomechanical properties of the overall trend, which can be used as a very good supplement for experimental specimen biomechanical study. In this study, according to the actual geometry of the skeleton, which was obtained from3D reconstruction of computed tomography, a three-dimensional (3D) finite element model was developed using Mimics, Geomagic Studio, Hypermesh, Ansys software. The finite element model of had a good geometric similarity. Compared with similar studies reported in the literature, the model had the more refined and uniform grid, the greater the cell density and more accurate results. Furthermore, this model can be disassembled, with great flexibility in the choice of subjects; it can be built on the foot bones of various independent study to further expand the scope of application of the model. In addition, as a whole, compared with the anatomical structure, pathophysiology, clinical research literature, and many other biomechanical researches, it indicated that this model had good physical similarity, more accurate and complete to simulate the anatomy of the calcaneus and its mechanical characteristics. It was beneficial for biomechanical analysis of the calcaneus.3. The change of Bohler angle affect little on the subtalar joint, calcaneocuboid joint, plantar stress changes but more to tibiotalar joint. It is necessary to restore the Bolher angle during operation. If the Bohler angle changes below10°, as no other surgery indication, conservative treatment could be choose; The change of calcaneus height affect more on the subtalar joint, calcaneocuboid joint, plantar stress changes but little to tibiotalar joint. It is necessary to maintain the calcaneus height under15mm, to get better joint function for long term.4. Open reduction and internal fixation for the treatment of comminuted calcaneal fractures, especially for Sanders IV type fracture formation is very important to maintain the articular surface. Locking plate show slightly advantage for the maintenance of the articular surface of the flat, anatomical compression plate can be dispersed fracture stress, the frictional force between the plate and bone can also play a compensatory role. During all bone fragment, set out from the sudden displacement calcaneal plate design needs to set out from the sudden part fixed focus. According to the theory of treatment of tibial plateau fractures distal radial fracture theory, the calcaneal fractures can be designed for two locking screws in the set from the sudden, the calcaneus upper row of screws on the locking screw and compression screw intermittent arranged, parallel to the articular surface. Plantar parts fixed with two locking screws front and rear ends, the rest are ordinary cancellous bone screws. This plate combines the advantage of locking plate and anatomic plate. Whatever within fixed intra-articular bone fragments fast with foot weight and produce different degrees of shift, there is a higher traumatic arthritis risk, it is recommended that an external fixation two arthrodesis treatment. |
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