The Static Biomechanics Of The Foot Intrinsic Support Of Human Longitudinal Arch

Support of the foot in stance comes from the ligaments and plantar fascia, which stabilizes the longitudinal arch and the intrinsic muscles of the foot. The creation and establishment of the longitudinal arch in the dynamic phase depend on the posterior tibialis and intrinsic and extrinsic muscles of the foot. The gastrocsoleus provides propulsion through sagittal plane motion at the ankle. Weakness in these structures may lead to abnormal biomechanical environment of the foot and help explain an appropulsive gait or instability during stance.Objective:The purpose of this study was to explore the role of the plantar soft tissue in the foot arch biomechanics, especially the plantar fascia, spring ligament complex, short plantar ligament and long plantar ligament through normal adult fresh frozen specimens in different injured condition. Also, a three-dimensional finite element model of a normal left foot was developed which was comprising most joints of the foot and consisted of bone segments, major ligaments and plantar soft tissue. The validity of the three-dimensional finite element model was verified by comparing results with experimentally measured data via the displacement and Von-mise stress of each bone segment. These intrinsic ligaments of the foot arch were sectioned in different sequence in the cadaveric experiment, which simulated the different pathologic situation of the plantar ligaments injury to describe the bone segments displacement and stress distribution, also, to establish a cadaveric flat foot model.Methods:1. Seven fresh adult cadaveric feet (the 1/3 part of the shranks were attached) were tested. The skin and muscles above the ankle joint were detached while kept the ligaments of the ankle intact at the same time. The four major bone segments and stabilizer of the foot arch (plantar fascia PF, spring ligament SL, short plantar ligament SP, long plantar ligament LP) were identified and marked before experiment. Axial loading from the proximal tibia was applied by MTS in the gradient of 100N to 700N, which simulated the foot standing state. At the end of each phase of loading, the gray level images of the specimens were catched by two digital cameras. Then, the simulation of the ligaments injury was undertaken by ligaments release in different combination and sequence. The former procedure was repeated in each pathological situation. All the gray level images were recorded and put into computer. The displacements of the bone segments were calculated by Digital Speckle Correlated Methods (DSCM) and analyzed in Spss13.0 statistics software.2. At the meantime of the former experiments, 10 strain gauges were attached to the bones of the feet, including the calcaneus, navicular, medial cuneiform, the first to fifth metatarsal trunk, the distal part of the tibia and fibular, respectively. At the end of each intact and pathological situation, the surficial strain of these bones was stored by resistance strainmeter.3. An F-Scan insole transducer was calibrated and placed under the plantar of the specimens before experiment. The plantar pressure distribution was recorded and stored simultaneouly in computer when the tests were going. The peak pressure under the plantar in each procedures and total contact area were woked out after the experiment and analyzed in the same statistic software.4. The geometry of the left foot was acquired from a 24 year male without any foot pathology by computed tomography scan. The outer contours of the bone and soft tissue were determined by an automatic contouring program and used to generate the solid models by a CAD program (AutoCADR14.0). The 4-node tetrahedral models were used to mesh the solid model and analyzed using a CAE program (ANSYS9.0). The articulations and ligament structures of the foot were simulated with LINK10 and Link12 model respectively, and Shell93 model was used to construct the plantar soft tissues in the three dimentional finite element model. The material properties were assumed to be linear elastic. A static loading of 700N was employed axially through distal tibia of the model. All nodes on the upper cross-sectional area of the distal tibia were constrained, and a rigid plane under the foot plantar soft tissues was established. The reaction of each bone segment of the foot arch was recorded and analyzed.Result:1. All the bone segments marked moved downward in the sagittal plane under axial load in intact situation, and the movements of them are significant different. The tibia, which represented the foot height, showed the largest displacement when the feet were applied 700N loading by 5.55±0.74mm and the foot showed its stiffness of 128.05N/mm and linear increase was observed according to loading. The damage to the plantar ligaments leaded to increased displacement of the tibia to 11.27±1.77mm and decrease of the foot stiffness. There was no significant difference between intact situation and the state of one ligament released. All the specimens showed visual flat foot deformity when three of the four structures were released, there was no significant difference between the feet (P>0.05). Length of the medial longitudinal arch prolonged 13.47mm relative to intact when all four ligaments were sectioned. The First metatarsal head changed its position greatly in different procedure, adducted at the first two stages and abducted at the flowing three stages.2. The strain was variance based on different bone segment attached, increased with loading (P<0.05). Tensile force was always found at the medial part of the navicular, the distal part of the tibia and fibular, while the others showed compress all the time. The strain at the sustentaculum tali decreased significant when one plantar structure was released and kept at this level in the flowing phase. As to the intact state, peak strain was found at calcaneus, the second and third metatarsal by -371.35±67.61με, -143.16±24.75μεand -123.54±18.07μεrespectively. Strain on the surface of the bone segments changed greatly with ligament section, though the contur of the foot similar to the intact condition.3. The same increase for the plantar pressure distribution was observed by the F-Scan with gradualness loading. A lateral and anterior shift of the peak pressure was found when the plantar ligaments were released, no matter the sequence or combination of the involved ligaments. When the specimens were intact, the peak pressures were observed at the second, third and first metatarsal head. In the last phase (all ligaments were sectioned) the peak pressure transferred to the region of distal fourth metatarsal and the other metatarsals appeared increased plantar pressure compared to the intact. The total contact areas of all specimens served the same tendency that more area kept up with larger load (P<0.05), especially in fewer loading condition (0-400N), but there was no significant difference (P>0.05) between the intact and damaged situation. 4. A three-dimensional normal foot finite element model with detailed joint characteristics and partial plantar soft tissue was established, including 109572 solid elements, 921 shell elements and 170426 nodes totally. The model was comparable to the spacial reconstruction of CT images. In addition, the finite element analysis softwares (ANSYS9.0) was utilized with the for the quasi-static finite element analysis of human foot arch motion in different situation. The model descended greatly in all phase of plantar fascia released, but moved slightly in other condition. The arch of the model decreased the most when all the ligaments were sectioned and the biggest Von mise stress was found at the lateral mid-foot region. When compared to the experimental data, the model presented more displacement in the sagittal plane, while both had the same tendency generally.Conclusion:1. Digital Speckle Correlated Methods (DSCM), combined with resistance strainmeter and plantar pressure measurement appear to be valid in catching more biomechanical information of the foot longitudinal arch in detail.2. All the four plantar ligaments play an important role in stabilizing the normal foot arch, the stiffness of intact foot shows a linear increasing according to the augmented load. Normal foot arch in the cadaveric foot descend if load is applied, each bone segment appear different displacement at the same load. The medial longitudinal arch subsided greatly. The foot arch, especially the medial longitudinal arch collapses and elongates significantly flowing fore foot abduction and hind foot valgus, when three or more plantar ligaments are released without the function of the tendons and extrinsic stabilizer. It implies that the flat foot deformity in clinic may companied by multiple ligaments injury or dysfunction, which should be emphasized in diagnose and treatment.3. The surficial strain on the bone segments change with any injury to the four plantar ligaments, even the displacements are not significant in some situations. The changed strain may lead to more sever clinical symptom and serve as signal to dysfunction earlier than the displacement of the bone segment. The stress distribution of the foot bones should be cautioned not only the contour of the foot alone in flat foot deformity correction. 4. The plantar pressure distribution is different between vitro and vivo foot. There is a tendancy that the peak preasure shift anterioly and laterally from the second to the third and fourth metatarsal head if the plantar ligaments were injured, while he total pressure area of the specimens do not change significantly. The abnormal distribution of the plantar pressure may be an important factor leading to clinical symptom in patien.5. Though the three-dimensional finite element foot model has some difficulties in simulating the biomechanical behavior of the cadaveric experiment in detail, it shows the same tendancy in the same load condition. The current study proposed a validated three-dimensional foot model which can be modified to simulate other foot conditions in the future. FEM, combined with DSCM and electric resistance strain measurement, provide a valid and digital way to further research of foot and ankle biomechanics.