Original Research
A Finite Element Model of an Equine Hoof

https://doi.org/10.1016/j.jevs.2014.11.008Get rights and content

Highlights

  • Finite element model of an equine hoof is proposed.

  • It is validated using both experimental and literature data.

  • Biomechanical performance of a laminitic horse hoof is studied using the model.

  • The results confirm softening of the laminar junction in affected horses.

Abstract

One of the most critical equine hoof diseases is laminitis, which can cause lameness. For its assessment and treatment, deep comprehension of the biomechanics of a horse's hoof is crucial. The aim of this research was therefore to create a finite element model of a horse's hoof to understand how laminitis could affect the overall performance of the hoof. The model contains all the relevant tissues, including the distal phalanx, navicular bone, middle phalanx, flexor and extensor tendon, laminar junction, hoof wall, sole, frog, and digital cushion. Material parameters of most of the components are based on available literature data; the least squares fitting on a uniaxial traction test is used in the case of the laminar junction. The model is validated by comparisons with experimental values and data in the literature. The effect of the decreasing stiffness of the laminar junction as a symptom of laminitis on the overall mechanical response of the model is studied. This effect results in the rotation and sinking of the distal phalanx, which is experimentally observed for laminitic horses. That is, the model created allows for the study of the aspects of the behavior of the hoof that are affected by laminitis by varying the material properties of the laminar junction. This approach can be useful for veterinary specialists to assess the severity of the laminitis and the treatment approach that they choose.

Introduction

The laminar junction, playing a key role in the strength and health of the equine hoof, is a highly vascularized dermoepidermal layer between the hoof wall and distal phalanx. The interconnection of dermis and epidermis is mediated via a basement membrane composed of an extracellular matrix. This sheet-like three-dimensional structure is woven in such a way that it creates primary and secondary lamellae interdigitating the epidermal tissue located near the wall horn and dermal tissue located near the distal phalanx. The number and orientation of primary and secondary lamellae change during horse maturation to adapt to the weight and to the needs of mostly dynamic loading of a horse [1]. The orientation and spacing of primary lamellae differ along the hoof wall [2], depending on the magnitude and orientation of mechanical loading. From the mechanical point of view, the distal phalanx is suspended via the laminar junction within the hoof capsule [3]. Forces are transmitted from the weight-bearing skeleton to the relatively stiff hoof wall, causing the suspensory apparatus to bear extensive loads [2], [3], [4]. The lamellar structure of a healthy apparatus ensures highly rigid and strong connection. However, when a horse hoof is affected by laminitis, the laminar junction, which connects the hoof wall and the distal phalanx, may fail, which leads to pathologic changes in the anatomy and, consequently, to the lameness of the horse [5]. A detailed description of laminitis is given, for instance, in the study by Pollitt [6]. The authors of this study investigate relevant subjects such as possible trigger mechanisms, anatomy, and treatment, as well as the laminitis assessment. Three grades of laminitis are introduced based on the lamellar histology of an affected tissue. The study describes the process of the separation of the secondary epidermal and dermal lamellae within the laminar junction because of the disease. The first is the developmental phase, during which the secondary lamellae start to elongate and the basement membrane to separate. During the second stage, the basement membrane is drawn further away and is absent between the bases of the adjacent secondary epidermal lamellae. Total separation of the basement membrane from all the epidermal lamellae is assessed as the third stage. This assessment correlates with the lameness of an affected horse described with the Obel grades. Horses with Obel grade 1 shift weight from one foot to the other. With grade 2, the gait is stilted and shuffling. In grade 3, the horse is reluctant to move, and in grade 4, the horse is immobile and often recumbent.

Laminitis motivates not only studies devoted to its medical aspects but also studies that address the biomechanics of both the normal and the affected hoof. Clearly, the lamellar separation in the laminar junction causes the loss of the mechanical properties of the affected tissue. Kochova et al [7] performed a uniaxial traction test with the healthy tissue of the laminar junction until the tissue rupture. They reported the ultimate stress of 2.09 MPa and the ultimate strain (corresponding elongation) of 59% when the rupture occurred. The ultimate stress and the elongation were determined at the starting point of laminar junction rupture, which is at the point of the highest stress of the stress–elongation curve. The elongation was defined as the percentage length changeε=ll0l0·100,where l0 was the original length of the specimen and l was the actual length of the specimen at some point of the stress–elongation curve. Performing the same test with an affected tissue of a higher laminitic stage, the ultimate stress of 0.57 MPa was determined (unpublished data by Kochova P. and Tonar Z.). This means that the resistance of an affected tissue to breakdown is lower. The effect of laminitis on the biomechanical performance of the whole hoof was studied, for example, by Hobbs et al [8], who observed qualitatively different strain patterns in hoof walls. McGuigan et al [9] focused on the forces that are in the tendons in both normal and laminitic ponies because tenotomy is sometimes used as a treatment method [10].

To provide a better understanding of the complex biomechanics of the whole hoof, including all the relevant aspects, finite element (FE) analysis could be of significant importance. This numerical method represents an application of the theory of continuum mechanics to the specific problem and geometry. The hoof is represented as a three-dimensional object that is composed of elements, each with assigned material properties. By simulating in vivo conditions using an FE model, approximate information about the mechanical response is obtained in terms of stress and strain distributions.

During recent decades, several FE models were proposed to study specific problems that address hoof biomechanics. Hinterhofer et al [11] proposed a model of a hoof capsule that includes the hoof wall, sole, and frog, to predict the effects of farriery. Although the inner structures of the equine hoof are omitted, the geometry of this model has a high level of accuracy. Thomason et al [12] published a different approach including more components, such as the distal phalanx, laminar junction, hoof wall, sole, and solar dermis. The proposed models allow the authors to study the morphology of the laminar junction, as outlined in the study by Thomason et al [2]; however, the geometry of the components is approximated rather roughly, to fill the overall shape of the hoof capsule. Similar model was used in the study by Ramsey et al [13] to assess the influence of the location of the center of pressure on the biomechanics of the hoof capsule. Improvement of the geometry of the hoof wall, distal phalanx, and sole in FE model using magnetic resonance scans was done in the study by Salo et al [14].

The aim of this work was to propose an FE model of the equine hoof that is more detailed compared with already existing models. Motivated by equine laminitis, the primary focus is the mechanical response of the laminar junction and all the structures that could be relevant to this response. Compared with the existing models, the number of components taken into account is higher. In addition, the geometry of the individual components is improved, to obtain a better approximation of the real structure. To ensure a realistic mechanical response, the model is validated using both the data in the literature and the experimental values that were obtained from radiographs of normal hooves.

Section snippets

Model Development

Because of the primary focus of this study, the model was intended to predict motion and deformation of hoof segments under a loading in physiological range. For a given load represented by time-independent forces, corresponding displacements and stresses within the equine hoof were of interest. Hence, quasi-static nonlinear solid mechanics was considered.

The development of the hoof model was based on eight photographs of hoof sagittal sections of 8- to 10-mm thickness (Fig. 1). It was an

Material Parameters of the Laminar Junction Specimens

Results of the uniaxial traction tests, that is, the stress–elongation curves of all three samples of the healthy laminar junction, are plotted in Fig. 5 (asterisks). To fit the experimental data with an Ogden material model, the least squares method was applied. For each individual sample, a particular value of elongation was considered as an upper limit in the fitting process. In sample 1, for instance, the upper limit is approximately 18%. The resulting material parameters are listed in

Discussion

Sagittal sections of 8- to 10-mm thickness were used as a basis for the model. Great detail of inner structures was hence obtained, although the limited number of photographs resulted in a limited resolution in the mediolateral direction. However, relevant components were not omitted because their sizes in the mediolateral direction were higher than the thickness of the sagittal sections. With the missing information interpolated by a veterinary anatomist, the FE model of an equine hoof was

Conclusions

The proposed model is more detailed compared with already existing equine hoof models because it includes all the relevant tissues. The behavior of the model under loading conditions corresponds to the behavior that is observed by authors of other existing models and corresponds to the patterns that are observed in radiographs. Its detail allows analysis that is more accurate. Young's moduli of specific tissues can be altered or even supplied by a stress–strain curve obtained from experiment.

Acknowledgments

Validation of the model was conducted in cooperation with Dr. J. Plachy from the Veterinary Clinic Hermanuv Mestec, Czech Republic, who provided the radiographs of the horses' forefeet. The authors thank Professor H. E. König, Institute for Anatomy, Histology and Embryology, University of Veterinary Medicine, Vienna, Austria, for providing us with physical sections of an equine hoof. This work was supported by the grant project GAČR 106/09/0734 and the European Regional Development Fund,

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