Finite Element Modeling of the First Ray of the Foot: A Tool for the Design of Interventions

Subject-specific material properties of the heel pad: An inverse finite element analysis

Individuals with diabetes are at a higher risk of developing foot ulcers. To better understand internal soft tissue loading and potential treatment options, subject-specific finite element (FE) foot models have been used. However, existing models typically lack subject-specific soft tissue material properties and only utilize subject-specific anatomy. Therefore, this study determined subject-specific hindfoot soft tissue material properties from one non-diabetic and one diabetic subject using inverse FE analysis. Each subject underwent cyclic MRI experiments to simulate physiological gait and to obtain compressive force and three-dimensional soft tissue imaging data at 16 phases along the loading-unloading cycles. The FE models consisted of rigid bones and nearly-incompressible first-order Ogden hyperelastic skin, fat, and muscle (resulting in six independent material parameters). Then, calcaneus and loading platen kinematics were computed from imaging data and prescribed to the FE model. Two analyses were performed for each subject. First, the skin, fat, and muscle layers were lumped into a single generic soft tissue material and optimized to the platen force. Second, the skin, fat, and muscle material properties were individually determined by simultaneously optimizing for platen force, muscle vertical displacement, and skin mediolateral bulging. Our results indicated that compared to the individual without diabetes, the individual with diabetes had stiffer generic soft tissue behavior at high strain and that the only substantially stiffer multi-material layer was fat tissue. Thus, we suggest that this protocol serves as a guideline for exploring differences in non-diabetic and diabetic soft tissue material properties in a larger population.

Fit of fire boots: exploring internal morphology using computed tomography

Fit of fire boots is a crucial factor in the safety and performance of firefighters on the hostile fireground. Firefighters have reported that ill-fitting fire boots restrict their lower body movement and sometimes cause very dangerous situations by falling off behind the wearer. By using computed tomography, this study demonstrates the potential to quantify and visualize the fit of fire boots, which previously relied on subjective feedback from the wearers. The high-resolution three-dimensional (3D) models of two fire boot products allowed a detailed observation and measurement of the internal space of the boots. Also, the boot's internal dimension was compared to the foot measurement of local firefighters, showing the significant differences between the two boots. Lastly, simulation wrapping the 3D scanned foot with the boot revealed large void spaces around the toe box and ankle, as well as the narrower ball width of the boot than the foot.

Disease-Specific Finite element Analysis of the Foot and Ankle

Finite-element analysis is a computational modeling technique that can be used to quantify parameters that are difficult or impossible to measure externally in a geometrically complex structure such as the foot and ankle. It has been used to improve our understanding of pathomechanics and to evaluate proposed treatments for several disorders, including progressive collapsing foot deformity, ankle arthritis, syndesmotic injury, ankle fracture, plantar fasciitis, diabetic foot ulceration, hallux valgus, and lesser toe deformities. Parameters calculated from finite-element models have been widely used to make predictions about their biomechanical correlates.

Finite Element Modelling of Planus and Rectus Foot Types for the Study of First Metatarsophalangeal and First Metatarsocuneiform Joint Contact Mechanics

Evaluating the contact mechanics of human joints is an important element in understanding the pathomechanics of orthopaedic diseases. Although physical testing is essential in the evaluation process, reliable computational models can augment these experiments by non-invasive predictions of biomechanical or surgical variables. The objective of this study was to perform verification of a framework for developing a medial forefoot finite element. Verification was conducted by comparing computational predictions to experimental measurements of first metatarsophalangeal and first metatarsocuneiform joint contact mechanics. A custom-built force-controlled cadaveric test-rig was used to derive measurements of contact pressure, force, and area. A quasi-static finite element was developed and driven under the same boundary and loading conditions. Calibration of cartilage moduli and mesh sensitivity analyses were performed. Mean errors in contact pressures, forces, and areas were 24%, 4%, and 40% at the first metatarsophalangeal joint and 23%, 12%, and 19% at the first Metatarsocuneiform joint, respectively. Verification of a medial forefoot finite element model development framework was presented and found to be within 30% for contact pressure and contact force of both joints. This study presents a method to verify and simulate realistic physiological loading to investigate orthopaedic diseases of the medial forefoot.

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.

Clinical Outcomes After First Metatarsophalangeal Joint Arthrodesis by Flat Cut Joint Preparation With Individual Adjustment for Sagittal Alignment

Sagittal misalignment is a major cause of patient dissatisfaction and re-operation after first metatarsophalangeal (MTP) joint arthrodesis. The stereotypical application of the fixed angle would be undesirable, especially in cases of flat or cavus foot. We retrospectively reviewed 31 cases (27 patients) in which first MTP joint arthrodesis was performed using the flat cut joint preparation technique with reference to the plantar clearance beneath the pulp of the toe while simulating weightbearing by pushing a board against the sole. The most common underlying cause of surgery was rheumatoid arthritis (22 cases [71%]). Clinical outcomes were evaluated by the Japanese of Surgery of the Foot (JSSF) hallux scale and the self-administered foot evaluation questionnaire (SAFE-Q). Twenty-three cases were also examined by pedobarography to evaluate postoperative walking plantar pressure. At the most recent follow-up of a mean 19.6 months, the toe-to-floor distance of the hallux in static standing posture was a mean of 2.5 mm (range, 0-10 mm). All but 1 foot (97%) achieved bone union. There were no complications or revisions due to misalignment of the fused MTP joint. JSSF hallux scales improved significantly from 47 preoperatively to 82 postoperatively. All subscale scores except general health and well-being in the SAFE-Q improved significantly at final follow-up versus preoperative period. Plantar pressure under the hallux was correlated with the toe-to-floor distance but not radiographic parameter. In conclusion, first MTP joint arthrodesis achieved good clinical outcomes when using toe-to-floor distance and Kirschner wire template for flat cut joint preparation.

Influence of first proximal phalanx geometry on hallux valgus deformity: a finite element analysis

Hallux abducto valgus (HAV), one of the most common forefoot deformities, occurs primarily in elderly women. HAV is a complex disease without a clearly identifiable cause for its higher prevalence in women compared with men. Several studies have reported various skeletal parameters related to HAV. This study examined the geometry of the proximal phalanx of the hallux (PPH) as a potential etiologic factor in this deformity. A total of 43 cadaver feet (22 males and 21 females) were examined by means of cadaveric dissection. From these data, ten representative PPHs for both genders were selected, corresponding to five percentiles for males (0, 25, 50, 75, and 100%) and five for females. These ten different PPHs were modeled and inserted in ten foot models. Stress distribution patterns within these ten PPH models were qualitatively compared using finite element analysis. In the ten cases analyzed, tensile stresses were larger on the lateral side, whereas compressive stresses were larger on the medial side. The bones of males were larger than female bones for each of the parameters examined; however, the mean difference between lateral and medial sides of the PPH (mean ± SD) was larger in women. Also the shallower the concavity at the base of the PPH, the larger the compressive stresses predicted. Internal forces on the PPH, due to differences in length between its medial and lateral sides, may force the PPH into a less-stressful position. The geometry of the PPH is a significant factor in HAV development influencing the other reported skeletal parameters and, thus, should be considered during preoperative evaluation. Clinical assessment should evaluate the first ray as a whole and not as isolated factors.

Constitutive formulation and numerical analysis of the biomechanical behaviour of forefoot plantar soft tissue

The aim of this work is to provide a numerical approach for the investigation of the mechanical behaviour of the forefoot soft tissues. The development of reliable numerical models of biological structures requires the definition of constitutive formulations that actually interpret the mechanical response of the constituent biological tissues and their structural arrangement. A specific visco-hyperelastic constitutive model is provided to account for the typical features of soft plantar tissue mechanics, as geometric and material non-linearity, almost-incompressible behaviour and time-dependent phenomena. Constitutive parameters are evaluated by the analysis of experimental data from compression and stress relaxation tests on tissue samples. A three-dimensional finite element model of the forefoot region is developed starting from the analysis of biomedical images, leading to the evaluation of overall structural response. The reliability of model and analyses is assessed by the comparison of experimental and numerical results pertaining to indentation tests. The numerical model developed allows to evaluate the mechanical response of plantar soft tissue in terms of stress and strain distribution.

What has finite element analysis taught us about diabetic foot disease and its management? A systematic review

Background

Over the past two decades finite element (FE) analysis has become a popular tool for researchers seeking to simulate the biomechanics of the healthy and diabetic foot. The primary aims of these simulations have been to improve our understanding of the foot’s complicated mechanical loading in health and disease and to inform interventions designed to prevent plantar ulceration, a major complication of diabetes. This article provides a systematic review and summary of the findings from FE analysis-based computational simulations of the diabetic foot.

Methods

A systematic literature search was carried out and 31 relevant articles were identified covering three primary themes: methodological aspects relevant to modelling the diabetic foot; investigations of the pathomechanics of the diabetic foot; and simulation-based design of interventions to reduce ulceration risk.

Results

Methodological studies illustrated appropriate use of FE analysis for simulation of foot mechanics, incorporating nonlinear tissue mechanics, contact and rigid body movements. FE studies of pathomechanics have provided estimates of internal soft tissue stresses, and suggest that such stresses may often be considerably larger than those measured at the plantar surface and are proportionally greater in the diabetic foot compared to controls. FE analysis allowed evaluation of insole performance and development of new insole designs, footwear and corrective surgery to effectively provide intervention strategies. The technique also presents the opportunity to simulate the effect of changes associated with the diabetic foot on non-mechanical factors such as blood supply to local tissues.

Discussion

While significant advancement in diabetic foot research has been made possible by the use of FE analysis, translational utility of this powerful tool for routine clinical care at the patient level requires adoption of cost-effective (both in terms of labour and computation) and reliable approaches with clear clinical validity for decision making.

The in vivo plantar soft tissue mechanical property under the metatarsal head: implications of tissues׳ joint-angle dependent response in foot finite element modeling

Material properties of the plantar soft tissue have not been well quantified in vivo (i.e., from life subjects) nor for areas other than the heel pad. This study explored an in vivo investigation of the plantar soft tissue material behavior under the metatarsal head (MTH). We used a novel device collecting indentation data at controlled metatarsophalangeal joint angles. Combined with inverse analysis, tissues׳ joint-angle dependent material properties were identified. The results showed that the soft tissue under MTH exhibited joint-angle dependent material responses, and the computed parameters using the Ogden material model were 51.3% and 30.9% larger in the dorsiflexed than in the neutral positions, respectively. Using derived parameters in subject-specific foot finite element models revealed only those models that used tissues׳ joint-dependent responses could reproduce the known plantar pressure pattern under the MTH. It is suggested that, to further improve specificity of the personalized foot finite element models, quantitative mechanical properties of the tissue inclusive of the effects of metatarsophalangeal joint dorsiflexion are needed.

Pain after cheilectomy of the first metatarsophalangeal joint: diagnosis and management

Cheilectomy is commonly performed for osteoarthritis of the first metatarsophalangeal joint and generally has a successful outcome and high rate of patient satisfaction over the short to medium term. Despite the relatively good results achieved in most cases, a proportion of patients have ongoing pain after cheilectomy. This article outlines the potential causes of ongoing pain, including progression of osteoarthritis, neuralgic symptoms, and transfer metatarsalgia. Management strategies for treating the ongoing symptoms are discussed.

Effect of Dorsal Plate Positioning on Dorsiflexion Angle in Arthrodesis of the First Metatarsophalangeal Joint: A Cadaveric Study

BACKGROUND

The relationship between dorsal plate positioning and final dorsiflexion angle after first metatarsophalangeal (MTP) joint fusion has not been well established. The main purpose of this study was to investigate whether changes in dorsal plate positioning along the longitudinal axis affect fusion dorsiflexion angle, as excessive dorsiflexion angles can lead to poor clinical results.

METHODS

Ten cadaver foot specimens were randomly assigned to 2 groups for first MTP joint fusion: 1 group used a straight plate, and the other group used a 10-degree precontoured plate. After routine preparation, the plates were placed in an "ideal" position based on clinical and radiological examination. The plates were then moved proximally 3 mm and 6 mm from the initial site, with repeat imaging completed at each position. The radiological dorsiflexion angle was determined for each position, and the results were assessed.

RESULTS

Placement of both straight and precontoured plates at positions more proximal from the initial position led to significant increases in dorsiflexion angles (P = .04), although the percentage change was larger in the precontoured plate group (P = .01). While placement of the plate 3 mm proximal from the perceived "ideal" position did increase the dorsiflexion angle, the percentage of specimens with dorsiflexion angles in the suggested optimal range changed minimally. Positioning at 6 mm from the starting point, however, led to significantly increased dorsiflexion angles for both plates (P = .004).

CONCLUSION

Positioning the dorsal plate at more proximal locations leads to increasing dorsiflexion angles. Precontoured plates are more likely to lead to excessive dorsiflexion compared with straight plates regardless of plate position.

CLINICAL RELEVANCE

Fusion at excessive dorsiflexion angles can be minimized with appropriate selection and proper positioning of the dorsal fusion plate along the longitudinal axis.

Optimization of nonlinear hyperelastic coefficients for foot tissues using a magnetic resonance imaging deformation experiment

Accurate prediction of plantar shear stress and internal stress in the soft tissue layers of the foot using finite element models would provide valuable insight into the mechanical etiology of neuropathic foot ulcers. Accurate prediction of the internal stress distribution using finite element models requires that realistic descriptions of the material properties of the soft tissues are incorporated into the model. Our investigation focused on the creation of a novel three-dimensional (3D) finite element model of the forefoot with multiple soft tissue layers (skin, fat pad, and muscle) and the development of an inverse finite element procedure that would allow for the optimization of the nonlinear elastic coefficients used to define the material properties of the skin muscle and fat pad tissue layers of the forefoot based on a Ogden hyperelastic constitutive model. Optimization was achieved by comparing deformations predicted by finite element models to those measured during an experiment in which magnetic resonance imaging (MRI) images were acquired while the plantar surface forefoot was compressed. The optimization procedure was performed for both a model incorporating all three soft tissue layers and one in which all soft tissue layers were modeled as a single layer. The results indicated that the inclusion of multiple tissue layers affected the deformation and stresses predicted by the model. Sensitivity analysis performed on the optimized coefficients indicated that small changes in the coefficient values (±10%) can have rather large impacts on the predicted nominal strain (differences up to 14%) in a given tissue layer.

First metatarsophalangeal joint arthrodesis; what is the best fixation option? A critical review of the literature

First metatarsophalangeal joint arthrodesis can be accomplished using many forms of fixation. Distinguishing the best fixation construct requires evaluation of many variables. A review of the literature provides a starting point for what needs to be assessed and what questions need to be asked. In vivo and in vitro studies attempt to provide answers but frequently reveal shortcomings in the evidence to date. In the end, there is always 1 best fixation technique.

Clinical outcomes after isolated periarticular osteotomies of the first metatarsal for hallux rigidus: a systematic review

Isolated periarticular osteotomy of the first metatarsal has been proposed for treatment of hallux rigidus due to the perceived ability to "decompress" the first metatarsophalangeal joint through axial shortening, as well as plantar displacement of the first metatarsal head to correct purported elevation. Additionally, isolated periarticular osteotomy of the first metatarsal has been proposed for treatment of hallux rigidus because of the perceived safety and efficacy. Furthermore, it has been proposed that undergoing isolated periarticular osteotomy of the first metatarsal does not prevent the ability to perform revision surgery. The author undertook a systematic review of electronic databases and other relevant sources to identify material relating to the clinical outcomes and need for surgical revision after isolated periarticular osteotomy of the first metatarsal for hallux rigidus. Information from peer-reviewed journals, as well as from non-peer-reviewed publications, abstracts and posters, and unpublished works, was also considered. In an effort to procure the highest quality studies available, studies were eligible for inclusion only if they involved consecutively enrolled patients undergoing isolated periarticular osteotomy of the first metatarsal for hallux rigidus, involved a prospective study design, included some form of objective and subjective data analysis, evaluated patients at a mean follow-up ≥12 months' duration, and included details of complications requiring surgical intervention. Four studies involving 93 isolated periarticular osteotomies of the first metatarsal followed up for a weighted mean of 18.6 months were identified that met the inclusion criteria. Peak dorsiflexion range of motion of the first metatarsophalangeal joint for the entire cohort of 93 patients increased 10.4°. The American Orthopaedic Foot and Ankle Society Hallux Metatarsophalangeal-Interphalangeal Scoring Scale for the entire cohort of 93 patients increased 39 points from a weighted mean of 47.2 preoperatively to 86.2 postoperatively. For the two studies that included it, complete satisfaction or satisfaction with reservations was reported in only 55/75 (73.3%) patients, with the remainder being dissatisfied. A total of 21 (22.6%) procedures underwent surgical revision in the form of hardware removal (n = 8), lesser metatarsal surgery for intractable postoperative metatarsalgia (n = 7), no mention of revision procedure (n = 3), Keller resection arthroplasty (n = 2), and treatment of infection with revision of non-union (n = 1). Two studies specified the grade of hallux rigidus that underwent revision surgery after isolated periarticular osteotomy of the first metatarsal as follows: grade I, 16.7% (n = 3/18) and grade II, 30.5% (n = 18/59). Finally, a total of 30.5% (n = 18/59) of patients developed postoperative metatarsalgia or stress fracture. Additional prospective studies involving validated subjective and objective outcome measurement tools with computerized gait analysis and long-term follow-up after isolated periarticular osteotomy of the first metatarsal for the various grades of hallux rigidus, as well as with comparison with isolated cheilectomy and Valenti arthroplasty, would be beneficial. Based on the high incidence of complications until these studies can be completed, routine use of isolated periarticular osteotomy of the first metatarsal for hallux rigidus should be performed with caution or not at all.

Arthrodesis of the first metatarsophalangeal joint: a robotic cadaver study of the dorsiflexion angle

BACKGROUND

Arthrodesis of the first metatarsophalangeal joint is indicated for severe osteoarthritis or as a revision of failed treatment for hallux valgus. The literature suggests that an optimum fused dorsiflexion angle is between 20 degrees and 25 degrees from the axis of the first metatarsal. The purpose of this study was to investigate the relationship between dorsiflexion angle and plantar pressure in the postoperative gait. We assumed that there is a fused dorsiflexion angle at which pressures are minimized under the hallux and the first metatarsal head.

METHODS

Six cadaver foot specimens underwent incremental changes in simulated fused metatarsophalangeal joint dorsiflexion angle followed by dynamic gait simulation. A robotic gait simulator performed at 50% of body weight and one-fifteenth of physiologic velocity. In vitro tibial kinematics and tendon forces were based on normative in vivo gait and electromyographic data and were manually tuned to match the in vitro ground reaction force and tendon force behavior. Regression lines were calculated for peak pressure and pressure-time integral under the hallux and the metatarsal head by dorsiflexion angle.

RESULTS

Peak pressure and pressure-time integral under the hallux were negatively correlated with dorsiflexion angle (p < 0.004), while peak pressure and pressure-time integral under the metatarsal head were positively correlated with dorsiflexion angle (p < 0.004). The intersection of the regression lines that represented the angle at which peak pressure and pressure-time integral were minimized was 24.7 degrees for peak pressure and 21.3 degrees for pressure-time integral.

CONCLUSIONS

Our findings support the hypothesis that an angle-pressure relationship exists following arthrodesis of the first metatarsophalangeal joint and that it is inversely related for the hallux and the metatarsal head. Our results encompass the suggested range of 20 degrees to 25 degrees.

An elaborate data set characterizing the mechanical response of the foot

Mechanical properties of the foot are responsible for its normal function and play a role in various clinical problems. Specifically, we are interested in quantification of foot mechanical properties to assist the development of computational models for movement analysis and detailed simulations of tissue deformation. Current available data are specific to a foot region and the loading scenarios are limited to a single direction. A data set that incorporates regional response, to quantify individual function of foot components, as well as the overall response, to illustrate their combined operation, does not exist. Furthermore, the combined three-dimensional loading scenarios while measuring the complete three-dimensional deformation response are lacking. When combined with an anatomical image data set, development of anatomically realistic and mechanically validated models becomes possible. Therefore, the goal of this study was to record and disseminate the mechanical response of a foot specimen, supported by imaging data. Robotic testing was conducted at the rear foot, forefoot, metatarsal heads, and the foot as a whole. Complex foot deformations were induced by single mode loading, e.g., compression, and combined loading, e.g., compression and shear. Small and large indenters were used for heel and metatarsal head loading, an elevated platform was utilized to isolate the rear foot and forefoot, and a full platform compressed the whole foot. Three-dimensional tool movements and reaction loads were recorded simultaneously. Computed tomography scans of the same specimen were collected for anatomical reconstruction a priori. The three-dimensional mechanical response of the specimen was nonlinear and viscoelastic. A low stiffness region was observed starting with contact between the tool and foot regions, increasing with loading. Loading and unloading responses portrayed hysteresis. Loading range ensured capturing the toe and linear regions of the load deformation curves for the dominant loading direction, with the rates approximating those of walking. A large data set was successfully obtained to characterize the overall and the regional mechanical responses of an intact foot specimen under single and combined loads. Medical imaging complemented the mechanical testing data to establish the potential relationship between the anatomical architecture and mechanical responses and to further develop foot models that are mechanically realistic and anatomically consistent. This combined data set has been documented and disseminated in the public domain to promote future development in foot biomechanics.

First metatarsophalangeal joint arthrodesis and revision arthrodesis

Arthrodesis of the first metatarsophalangeal joint is a powerful procedure that can improve the load-bearing capabilities of the forefoot and assist in medial arch stability. It is the mainstay of treatment for patients with severe arthritic deformity of the great toe joint, because it addresses the importance first ray weight-bearing has on the rest of the foot. In select individuals, fusion can also be effective as a primary procedure in the treatment of hallux valgus. Rather than cause detrimental effects to the function of the foot, this article suggests that first metatarsophalangeal arthrodesis can actually improve faulty mechanics secondary to a dysfunctional joint.