Gait Analysis for the Quantification of Lameness

Chapter 22 Gait Analysis for the Quantification of Lameness



Kevin G. Keegan


Evaluation of lameness in horses is a skill first learned by training and then sharpened with experience over time. The standard of practice is an initial subjective evaluation of the horse while moving to detect and then localize the source of pain causing lameness to the affected limb or limbs. It is a difficult endeavor in horses with mild lameness, when only subtle changes in movement from normal are present. There is substantial disagreement, even between experienced clinicians, in the identification and localization of the lame limb(s) in horses with mild lameness.1-3 There is also disagreement between experts in the amount of improvement in lameness after nerve blocks,4 and equine veterinarians, because they are human, have been shown to be biased. The amount of disagreement between veterinarians on the results of subjective evaluation of lameness in horses is what would be expected for any difficult diagnostic test.


A contributing source of disagreement is the limited sensitivity of the human eye, which has an estimated time resolution of about 10 to 15 samples per second.5,6 Events in stride of a horse trotting at 4 meters per second, which is about 1.5 strides per second, occur at a frequency of twice the stride rate, or about 3 times per second.7 To prevent significant errors in detection of signal amplitude, the sampling frequency should be, at a minimum, 5 times the frequency of the event being detected.8 Therefore the natural capability of the human eye is below or just at the minimum required to detect important asymmetry in motion events used to judge lameness in horses. All the objective methods of lameness evaluation discussed in this chapter sample data at frequencies higher than that of the naked eye, and thus are theoretically capable of higher accuracy. An objective, precise, and accurate method of lameness detection and quantification in horses is certainly justifiable.


The purpose of this chapter is to introduce and summarize the gait analysis methodology currently being studied or used. Emphasis is given to those that could be used by private practitioners. To be effective, any such system has to be easy to use, and data collection and analysis must be quick. It must be affordable and capable of providing the veterinarian with adequate return on investment. It may be most useful to veterinarians as an aid in detecting and evaluating horses with subtle lameness and lameness in several limbs. Natural stride-to-stride variability must be overcome, so that small differences in subtle lameness can be detected. For example, to detect subtle differences after diagnostic analgesia or after treatment, any such system must be capable of collecting data from multiple contiguous strides. Lameness over ground may be different from lameness on a treadmill; therefore any system must be capable of collecting data when the horse is moving over ground. Also, ideally, any system should be capable of detecting and evaluating lameness in all limbs simultaneously.


There are two general approaches to using gait analysis to detect and measure lameness in horses: kinetics and kinematics. A kinetic technique measures ground reaction forces. A kinematic technique measures motion of the body. There are advantages and disadvantages for each general approach.



Kinetics


Kinetics can rightly be considered a more direct method (compared with kinematics) for detecting and measuring lameness in horses. If a horse has pain during the weight-bearing portion of the stride because of lameness, it bears less weight on that limb, resulting in lower peak vertical ground reaction forces (GRFs) on that limb. Certain lameness conditions may decrease horizontal or transverse GRFs, but the effect on the vertical GRFs is usually most prominent.


Kinetic methods to measure GRFs include a stationary force plate, pressure-measuring pads, instrumented horse shoes, and force-measuring treadmills. A stationary force plate is the most commonly used and cited method, but each method is briefly discussed. A stationary force plate, because of its widespread use in lameness research centers around the world, is discussed in more detail.


Pressure-measuring pads are force or pressure distribution measurement systems consisting of force sensor elements installed between matting surfaces. A horse is led over the pressure-sensitive pad to collect data from single or, if the pad is long enough, a few consecutive strides. Pressure-sensitive pads can be cut and customized to fit the bottom of a horse’s foot and then placed between the bottom of the foot and a shoe in an attempt to collect data from multiple contiguous strides.9,10 This technology is ideal for mapping the force or pressure profile of a surface, but peak force or pressure can also be quantified. With further development and marketing this technology may become useful for routine evaluation of shoeing techniques designed to alter force distribution within the foot. Currently there are only a few equine practices in the United States using this technology clinically to evaluate lame horses.


Instrumented horse shoes are custom-built systems designed to either directly, with force transducers, or indirectly, with strain gauges, measure vertical GRFs to the hoof during weight bearing.11-14 Their main advantage is the ability to collect data from multiple contiguous strides in an overground, field-like setting. The main difficulties and disadvantages of instrumented horse shoes include the complexity, size, and weight of the instrumentation, which affect normal hoof and limb movement and gait. Although there are several recent reports of force-measuring horse shoes used successfully in research investigations, currently there are no commercially available systems for a practicing veterinarian to use. Strain gauges glued to the hoof wall, with the resulting strain to the hoof during weight bearing estimating GRF, can act as a surrogate for an instrumented horse shoe. Disadvantages of any hoof-mounted system for measuring GRFs to detect lameness include the need to instrument all four feet simultaneously and, more importantly, the extreme dependence of results on surface characteristics. Because of these disadvantages and the technical and design expertise required for development, it is unlikely that such a device will quickly become adopted for use during routine lameness evaluations.


A force-measuring equine treadmill is a unique piece of equipment consisting of piezoelectric load-sensitive sensors in a treadmill platform (Figure 22-1).15,16 The only working system in regular use is at the University of Zurich in Switzerland. The force-measuring equine treadmill is exceptional for detecting small changes in lameness between treatments because of its ability to collect data from multiple, contiguous strides. It is also exceptional for determining severity and location of compensatory lameness because it is capable of measuring vertical GRFs in all four limbs simultaneously.17 It is a one-of-a-kind, custom-built, installed piece of equipment, and as such is unlikely to be adopted for use as a lameness diagnostic aid in any but the most sophisticated equine private practices or research centers.


image

Fig. 22-1 Treadmill-integrated force-measuring system capable of determining the vertical ground reaction forces and the hoof positions during stance phase of all limbs simultaneously. Fz, vertical ground reaction force in kilo-Newtons. 1, Traction belt; 2, shock absorber; 3, treadmill platform; 4, treadmill frame. R1 through R9, piezoelectric force sensors of the right side of the treadmill.


(Drawing courtesy Dr. Michael Weishaupt, University of Zurich, Switzerland.)


A stationary force plate is the most widely used and available kinetic technique for objective evaluation of lameness in horses.18,19 Under controlled conditions of constant speed the coefficient of variation of vertical GRF between strides in both lame and sound horses is remarkably low.20-22 Thus a lesser number of stride repetitions are required to achieve repeatable results, and small differences between treatments can be detected. A stationary force plate is also sensitive to subtle lameness detection, with some studies suggesting that force plate detection of lameness (decreased vertical GRF) is more sensitive than the human eye in subjective evaluation of lameness.23,24 A stationary force plate, unlike the aforementioned other kinetic techniques, is also capable of measuring horizontal GRFs, which may be helpful in further differentiating type of lameness, that is, acceleratory or deceleratory. There are several force plates worldwide that are used, at least occasionally, for evaluation of lame horses. Despite the high accuracy and precision of a stationary force plate, most experts agree that five or six strikes on the force plate are needed for acceptable results.10,20-22 Because a horse does not always strike the force plate on every attempt, the process is time consuming. Also, the size of commercially available force plates is too small, such that data from only one hoof (occasionally two hooves) are collected at one time. Collection of data from contiguous strides and simultaneous measurement of all four limbs to detect and evaluate compensatory or secondary lameness is not possible. Therefore a stationary force plate has not been readily accepted as a tool for routine, clinical evaluation of lameness.



Kinematics


Using kinematics is an indirect method of detecting and quantifying lameness. Pain of lameness causes the horse to bear less weight on the affected limb. The decreased vertical GRF during weight bearing perturbs, in some way, the normal, expected motion of the torso, head, neck, and limbs, usually by increasing asymmetry of movement between the right and left strides. However, motion of the torso, head, neck, and limbs is perturbed by causes other than lameness as well; for example, conscious movement of the head in a curious or anxious horse. Thus in lame horses motion perturbation is more variable than change in GRF. Therefore kinematic techniques must be able to collect numerous (more than is required for a stationary force plate), contiguous strides to attain the sensitivity required to detect and evaluate mild to moderate lameness. However, because most veterinarians observe changes in motion in a horse’s gait during their subjective evaluation, kinematic techniques generate results that are generally more intuitive and well understood. Important findings in kinematic studies can be easily applied by a practicing veterinarian in a standard lameness evaluation.

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Jun 4, 2016 | Posted by in EQUINE MEDICINE | Comments Off on Gait Analysis for the Quantification of Lameness

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