The Deformation and Stress Distribution of Human Foot after Plantar Ligaments Release

A cadaveric study and finite element analysis

LIANG Jun¹; YANG Yunfeng²; YU Guangrong²; NIU Wenxin³; WANG Yubin¹

Orthopaedic department of Shanghai East hospital, Tongji University, Shanghai, China

Orthopaedic department of Tongji hospital, Tongji University, Shanghai, China

School of Biological Science and Medical Engineering, BeiHang University, Beijing, China

Background: Most[1] of the foot deformities relate with the arch collapse or instability, especially the longitudinal one. Though the function of the plantar fascia to the arch height has been investigated by some authors, the stress distribution is still unclear. The purpose of this study was to explore the role of the plantar ligaments in the foot arch biomechanics. Methods: A geometrical detailed three-dimensional (3-D) finite element model (FEM) of the human foot and ankle, was constructed by 3-D reconstruction of CT images, which was comprised of most joints of the foot and consisted of bone segments, major ligaments and plantar soft tissue. Plantar fascia and other ligaments releases were simulated to evaluate the corresponding biomechanical effects on load distribution of the bony and ligamentous structures. These intrinsic ligaments of the foot arch were sectioned to simulate different pathologic situation of the plantar ligaments injury and to explore the bone segments displacement and stress distribution. 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. Results: Plantar fascia release may decrease arch height but did not necessarily cause total collapse of the foot arch. The longitudinal foot arch was ruined when all the four major plantar ligaments were sectioned simultaneously. Plantar fascia release was compromised by increased strains of the plantar ligaments and intensified stress in the midfoot and metatarsal bones. Load redistribution among the centralized metatarsal bones and focal stress relief at the calcaneal insertion were predicted. Conclusions: The FE model suggested that plantar fascia release may provide relief of focal stress and therefore could relieve associated heel pain. However, these operative procedures may pose a risk to arch stability and clinically may produce dorsolateral midfoot pain. The initial strategy for treating plantar fasciitis should be nonoperative.

Keywords－biomechanics; finite element analysis; foot arch; ligament; stress

                                                                                                                                                                 I.          Introduction

The plantar fascia, or plantar aponeurosis, is the investing fascial layer of the plantar aspect of the foot. It is part of the retinacular system, which consists of a network of connective and adipose tissues whose main functions are to support and protect underlying vital structures of the body. The anatomy of the plantar fascia has been well described by Sarrafian^[1] and others ^[1-4]. Partial or total plantar fascia release may relieve the metatarsal and calcaneal stresses and the painful heel syndromes of plantar fasciitis. However, reduction of plantar fascia stiffness may have a significant impact on arch stability, resulting in a more deformable longitudinal arch.

Cadaveric studies have been done to investigate the biomechanical consequence of plantar ligaments release. Huang ^[6] reported that the average vertical displacements between the talar neck and supporting platform were 7.3 and 8.4 mm, respectively in 12 cadaveric feet with intact plantar fascia and fasciotomy under a load of 690 N. Kitaoka et al.^{[ 7, 8]} noted the high tensile loads required for failure of the plantar fascia in a biomechanical cadaver study. He also found the majority of failures, or ruptures, during testing occurred at the plantar fascia origin from the os calcis, the most common site of clinical plantar fascial rupture and symptoms of plantar fasciitis. Daly et.al ^[9] found evidence of flattening of the arch in 16 feet in 13 patients who underwent plantar fasciotomy for intractable plantar fasciitis. Because of the intrinsic structural and material variability of cadaver specimens and experimental limitations, systematic evaluations of the stress distribution and bony segments movement of the ankle-foot complex after plantar fascia release are difficult to achieve. Some researchers have turned to the computational approach to acquire these biomechanical parameters, but all these mathematic model were not verified with cadaveric experiment in the same pathologic situation.^[12,13]

The objective of this study was to establish a detail FEM of a normal human adult foot and analyze the foot arch deformation and stress distribution after plantar fascia release based on FEM prediction and cadaveric experiment verification.

                                                                                                                                                                        II.         methods

The geometry of the Finite Element model was obtained from a 24 year male (age 27, height 175 cm, weight 70 kg) without any foot pathology. A series of coronal CT images of 2-mm intervals in neutral unloaded position were segmented to obtain the boundaries of skeleton and plantar soft tissue. The 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 created and analyzed using a CAE program (ANSYS9.0). The articulations and ligament structures of the foot were created with LINK 10 and Link 12 model respectively, and Shell 93 model was used to construct the plantar soft tissues (Figure1.). The material properties were assumed to be linear elastic. A detail left foot model was built, which consisted of 170426 nodes, including articular cartilage, ligaments, and plantar soft tissue. Static loading of 700N was employed axially through distal tibia to simulate the one foot standing with the heel of the model fixed and the other part of the plantar soft tissue elements restrained in the vertical axis and free in the transverse plane. All nodes on the upper cross-sectional area of the distal tibia were restrained in the transverse plane but free in the coronal axis. Then a rigid plane under the foot plantar soft tissues was established to simulate the ground. The reaction of each bone segment of the foot arch was recorded and analyzed.

Seven fresh adult cadaveric foot (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 (Figure2.). The four major bone segments and stabilizer of the foot arch (plantar fascia, spring ligament, long and short plantar ligament) were identified and marked before experiment. 700N axial loading from the proximal tibia was applied by MTS in the gradient of 100N. Simulation of the ligaments injury was undertaken by ligaments release in different combination and sequence. The displacement of the major bones were collected in gray level images by two digital cameras and recorded in computer. The displacements of the bone segments were calculated by Digital Speckle Correlated Methods and compared with the FEM results for verification. The FEM angular displacement of the major tarsal bones were also analyzed based on the lines passed through the middle points of the articulations, such as the subtalar joint, talonavicular joint, chopart joint, Lisfranc joint, and metatarsophalangeal joints..

                                                                                                                                                                        III.        results

All the bone segments marked moved downward in the sagittal plane under axial load in intact situation.The calcaneus showed plantarflexion and the other tarsal and metatarsal bones appeared dorsiflexion, which lead to the longitudinal and transversal arch of the model flatten. The finite element model and the cadaveric feet showed the same tendancy when 700N load was applied to the distal part of the tibial axialy in intact situation except the calcaneus and the fifth metatarsal. When all the four plantar ligaments were sectioned, all the bone segments appeared to displace in all three global planes, which were dorsiflexion in the saggital plane, abduction in the transverce plane and external rotation in the coronal plane (Figure3). The FEM and the cadaveric experiment showed the same tendancy (Figure4,). The rotation changes between intact, plantar fascia realesed and all four major ligaments released of the finite element model were showed in the Figures (Figure6- Figure8).

The bone segments rotated in the three planes and showed the same tendancy except the calcaneous and the fifth metartasal in the coronal plane. Following the flatten of the longitudinal arch, the foot bones showed dorsiflexion in the sagittal plane, abduction in the transversal plane and external rotation in the coronal plane. The degrees of the rotation changed when the plantar ligaments were released and reach to the top as the four major ligaments were all sectioned (Figure5~ 7). 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 Mises stress was found at the lateral mid-foot region (Figure8). The plantar pressure showed the same tendency (Figure9). The abnormal re-distribution of the fore foot stress may lead to overuse injury of the metatarsal region. Supposing the last condition to be 100%, the contribution of the plantar fascia to the stability of the foot arch was calculated. They were 34.53%, 22.46% and 12.63% in sagittal, transversal and coronal plane respectively.

                                                                                                                                                                    IV.        discussion

Computer models have several features that make them more appealing than other types of models ^[10]. An infinite number of computer models can be developed and tested for different conditions and it`s primary characteristic is maintained after exhaustive testing. Also, computer models provide information that cannot be easily obtained using other types of models, such as load distributions within soft tissues, internal stresses, joint reaction forces, and muscle force analysis ^[11]. Once an accurate computer model has been developed and validated appropriately, simulations can be performed quickly and at low cost in diverse situations, such as injury, surgery, dynamic motion simulation and graphical animation of the experimentation and help to enhance the understanding of the underlying mechanisms. The capability of the FE model to predict the internal stress within the bony and soft tissue structures makes it a valuable tool to enrich the knowledge of ankle-foot biomechanics.

Computational analysis of the foot biomechanics has its advantage in providing an overall stress distribution of the foot. It is also more economical than in vitro cadaver experiments. In view of the previously existed computational models, only a detailed representation of the foot geometry and joint characteristics together with realistic loading conditions can depict the internal stress and strain distributions of the foot complex. Many authors have used FEM to quantify the biomechanical role of plantar fascia in load bearing and found the vertical displacement of the foot increased with fasciotomy^{[12, 13]}. In the literature, 3D geometrical detailed FE models have been developed^{[14- 19]}, but were not employed to quantify the biomechanical role of plantar fascia to the tarsal and metatarsal bone simultaneously and did not aim at the one foot standing posture, which is important to support the body weight in walking. Based on the anatomy and computer software, a detail finite element model of a normal adult left foot was established, including the bone segments, articulations, foot intrinsic ligaments and plantar soft tissue. The elements used to establish the foot joints, ligaments and plantar tissue were distinct from the literatures, which could not be used to analyze significant displacement of the foot arch.

Plantar fasciitis is an inflammation of plantar fascia—the strong, fibrous band which originates from the calcaneus and extends distal to the phalanges. It is estimated to affect 2 million people in the United States per year and accounts for about 10% of runner-related injurie^[3]. Plantar fascia release for chronic plantar fasciitis has provided excellent pain relief and rapid return to activities with few reported complications. Cadaveric studies have led to the identification of some potential postoperative problems, most commonly weakness of the medial longitudinal arch and pain in the lateral midfoot.^{[5, 20]}. Sectioning of the plantar fascia led to a pronounced reduction of arch height during load-bearing but did not necessarily result in total collapse of the foot arch even with additional dissection of the long plantar ligament in this study. This is similar to the observations in a cadaver study ^[7]. But later outcome were still unknown. Some patients suffered a flatten foot and occurrence of lateral pain of the mid-foot. The current study simulated the contribution of the plantar fascia in the supportive function to the longitudinal foot arch, which comprised one third of the function of plantar ligaments in sagittal plane and helped to restrict abduction and external rotation of the bone structure. The plantar fasciotomy should be the last option in clinical therapy following the conservative methods.

To simplify the FE analysis, homogeneous and linearly elastic material properties were assigned to the bony and ligamentous structures and the ligaments within the toes and other connective tissues such as the joint capsules were not considered. The current FE model did not account for the surface interactions between bony, ligamentous and muscles structures. The structural simplification of the FE model would result in a reduction of joint stability of the foot arch structures and an increase in predictions of joint and arch deformation. Because of the use of linear truss elements to approximate the nonlinear profile of the plantar fascia structure, assumption of linear material property and the neglection of structural interface between the plantar fascia and surrounding tissue, the predicted plantar fascia strain in this study was likely underestimated

                                                                                                                                                                   V.         Conclusion

All the four plantar ligaments play an important role in stabilizing the normal foot arch, especially the plantar fascia. The medial longitudinal arch subsided greatly. The medial longitudinal  foot arch collapses and elongate significantly flowing fore foot abduction and hind foot valgus following the four plantar ligaments are released without the function of the tendons and extrinsic stabilizer. The current study proposed a validated three-dimensional foot model which can be modified to simulate other foot conditions in the future. This foot model can be useful in observing stress distributions inside the foot, designing footwear, investigating the biomechanical behavior of the foot subjected to different damages and operation design.

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