Evaluation of a subject-specific finite-element model of the equine metacarpophalangeal joint under physiological load

Abstract

The equine metacarpophalangeal (MCP) joint is frequently injured, especially by racehorses in training. Most injuries result from repetitive loading of the subchondral bone and articular cartilage rather than from acute events. The likelihood of injury is multi-factorial but the magnitude of mechanical loading and the number of loading cycles are believed to play an important role. Therefore, an important step in understanding injury is to determine the distribution of load across the articular surface during normal locomotion. A subject-specific finite-element model of the MCP joint was developed (including deformable cartilage, elastic ligaments, muscle forces and rigid representations of bone), evaluated against measurements obtained from cadaver experiments, and then loaded using data from gait experiments. The sensitivity of the model to force inputs, cartilage stiffness, and cartilage geometry was studied. The FE model predicted MCP joint torque and sesamoid bone flexion angles within 5% of experimental measurements. Muscle–tendon forces, joint loads and cartilage stresses all increased as locomotion speed increased from walking to trotting and finally cantering. Perturbations to muscle–tendon forces resulted in small changes in articular cartilage stresses, whereas variations in joint torque, cartilage geometry and stiffness produced much larger effects. Non-subject-specific cartilage geometry changed the magnitude and distribution of pressure and the von Mises stress markedly. The mean and peak cartilage stresses generally increased with an increase in cartilage stiffness. Areas of peak stress correlated qualitatively with sites of common injury, suggesting that further modelling work may elucidate the types of loading that precede joint injury and may assist in the development of techniques for injury mitigation.

Introduction

The metacarpophalangeal (MCP) joint is the site of a large proportion of musculoskeletal injuries in racehorses (Bailey et al., 1999; Parkin et al., 2004). Osteochondral injuries occur predominantly in the palmarodistal aspect of the third metacarpal bone (MC3) and the dorsoproximal articular margin of the proximal phalanx (P1). Subchondral bone damage to the palmar aspect of the MC3 condyle is especially common, resulting in two types of injury: parasagittal fractures of the condyles and palmar osteochondral disease (Barr et al., 2009, Parkin et al., 2006). These injuries are considered fatigue injuries, the result of a high number of stress cycles applied to cartilage and bone, rather than an acute mechanical event (Stepnik et al., 2004, Norrdin and Stover, 2006).

Many factors contribute to fatigue injury, but the magnitude of the stress or strain to which tissues are subjected is critical (Rapillard et al., 2006). The MCP joint experiences the largest loads of the distal limb during locomotion (Merritt et al., 2008, Harrison et al., 2010). MCP joint hyperextension during locomotion results in the storage of elastic strain energy in the long flexor tendons and the suspensory apparatus (Biewener, 1998; Bobbert et al., 2007, Butcher et al., 2009, Harrison et al., 2010, McGuigan and Wilson, 2003, Witte et al., 2004), which is subsequently utilised by the limb to increase the efficiency of locomotion (Butcher et al., 2009, Harrison et al., 2010). However, stretching of the flexor tendons imposes large forces on the MCP joint (Merritt et al., 2008, Harrison et al., 2010). While previous studies have reported on the magnitudes of the resultant forces transmitted by the MCP joint (Merritt et al., 2008, Harrison et al., 2010), the magnitudes and locations of maximum cartilage stresses are unknown.

Finite-element (FE) modelling has been used to determine the distributions of contact force, cartilage pressure, and bone stress across the three-dimensional geometry of human joints (Anderson et al., 2007, Anderson et al., 2008, Anderson et al., 2010, Besier et al., 2008, Heino Brechter and Powers, 2002, Fernandez and Pandy, 2006, Pustoc'h and Cheze, 2009, Fernandez et al., 2011, 1), as this information is difficult to obtain by direct measurement in vivo. Once validated, an FE model can be used to determine the effects of variations in loading, geometry and material properties on joint contact stresses, without the need for additional experimentation. FE models are sensitive to input parameters such as the geometry, kinematics and mechanical properties of bone and cartilage (Besier et al., 2008, Anderson et al., 2010). Subject-specific FE analysis is often considered the gold standard, but routine application of this methodology in a clinical environment is generally impractical due to the technical expertise and time required for model development. The use of simplified features such as a generic anatomical geometry would be favoured, provided there is an acceptable loss of model fidelity.

A validated FE model of the equine forelimb would be a useful tool to investigate dynamic loading during normal locomotion and gain insight into the causes of joint injury. In the present study a subject-specific, three-dimensional FE model of the equine MCP joint was developed and used to determine joint contact loading and cartilage stresses for walking, trotting and cantering. The specific aims were firstly, to evaluate calculations of bone kinematics and tendon and ligament strains derived from the subject-specific FE model against measurements of the same quantities obtained from cadaver experiments performed on the same animal; secondly, to characterise the influence of gait speed on the magnitudes of joint contact forces, mean contact pressures, and locations of maximum cartilage stress; and finally, to determine the sensitivity of the FE model predictions to variations in the model inputs and parameters.