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Plastic Horseshoe Pads Do Not Attenuate Hoof Wall Vibrations in Trotting Horses.
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1. Introduction
When the limb contacts the ground during a horse’s stride, the limb decelerates, and high-frequency vibrations are created [1]. These vibrations have been considered to possibly be involved in the etiology of osteoarthritis in man [2,3], laboratory animals [4,5], and performance horses [6,7] as well as in other conditions in horses, such as navicular disease [8].
Concern about the effect of impact-related vibrations has led efforts to dampen them through the use of horseshoes [9] and especially, horseshoe pads. Among their hypothesized benefits, pads are theorized to protect the horse’s foot by reducing vibrations [10]. For this study, a commercially available plastic pad with an attached inflatable air bladder [a] was tested under a variety of conditions to see if impact vibrations were affected.
2. Materials and Methods
Animals
Eight adult horses of mixed breeding and various ages were used for this study. Each horse was trimmed by a farrier on entering the study and was kept in an inside stall for 24 h before the initiation of the study.
Instruments
Hoof vibration during impact was measured by attaching a custom-made, 150-g (1 g = 9.8 m/s2) accelerometer [b] transducer with a 8-kHz sampling frequency to the dorsal hoof wall. The accelerometer signal was sampled, conditioned (filtered and amplified), and transmitted using a microcontroller [c] and a Bluetooth Class radiotransceiver [d] in an integrated-circuit design. The sensor, microcontroller, and Bluetooth transceiver were powered by an on-board 3.7-V lithium polymer battery [e]
The whole transducer was packaged in methylmethcrylate for protection and to facilitate firm application to the dorsal hoof wall. The transducer was 1 X 1.5 X 0.25 in and weighed 30 g. The data station consisted of a tablet personal computer with a Bluetooth-enabled USB adapter.
Pre-study calibration and hoof-mounting fidelity were performed using a scanning laser vibrometer; these tests were conducted by attaching the transducer to a cadaver foot bolted to a metal frame and striking the frame with a sledge hammer. The transducer was attached to the hoof by potting in methylmethacrylate formed to the shape and surface contour of the dorsal hoof wall, attaching the methylmethacrylate form to the hoof wall with acrylic, and then wrapping cloth tape over it.
Procedure
The transducer was attached to the hoof in the horse in the same way as used in pre-study calibration. Hoof vibration in the right forelimb was measured in each horse while trotting at a slow speed along a 96-ft-long asphalt runway. Horses were led by a handler running alongside and restraining the horse with a lead shank. Measurements were obtained under four conditions.
- Control (CON). The horse was shod on both forelimb feet with standard steel horseshoes.
- Pad-uninflated (NON). Both forelimb feet were shod with regular steel shoes and a commercially available plastic horseshoe pad with an attached air bladder; the air bladder was not inflated.
- Pad-right inflated (RIG). Both forelimb feet were shod with the same shoes and pads, and the air bladder was inflated on the right forelimb only.
- Pad-both inflated (BOT). Both forelimb feet were shod with the same shoes and pads, but the air bladder was inflated on both forelimbs.
One-half of the horses were evaluated with shoes but without pads, and one-half of the horses were evaluated with shoes and pads. After pads were applied to the horse, the order of treatments (NON, RIG, and BOT) were randomized. All trimming, shoeing, and pad applications were performed by an experienced farrier.
Data Analysis
Three trials were conducted for each horse under each condition for a total of 96 trials (8 X 4 X 3 = 96 trials). Nine measures of hoof vibration were calculated from the raw-hoof acceleration data for each of the three trials per horse and condition; these measurements were then added and averaged.
- Stride rate (SR; the number of strides analyzed per time period selected for analysis). The stride rate is a measure of the speed consistency between trials.
- The number of selected strides (SN) analyzed. This is a measure of the consistency in the number of strides selected for each trial.
- The frequency at which 50% of the total impact vibration signal (measured in Hz) was below (50P).
- The 50% frequency area in Hz X s (i.e., the area under an amplitude versus frequency plot to the left of the 50% frequency).
- The frequency at which 90% of the total impact vibration signal (measured in Hz) was below (90P).
- The 90% frequency area in Hz X s (i.e., the area under an amplitude versus frequency plot to the left of the 90% frequency).
- Impact duration (ID; measured in s).
- The peak positive acceleration (g) after initiation of impact (PA).
- The peak negative acceleration (-g after initiation of impact (NA).
All calculations were performed using custom-written Matlab script that required the user to upload raw-data acceleration into a graphical user interface. The user then manually selects calculations using the characteristic shape of the raw-acceleration signal (Fig. 1) and the beginning and end of impact for each stride. Automatic peak-detection algorithms were employed to calculate SR, number of strides and peak negative and positive accelerations. Fast Fourier Transform was used to convolute the signal into the frequency domain before calculation of the 50% and 90% frequencies. Time of impact was calculated from user-selected beginning and end points for impact of each stride. The first two strides at the beginning and end of each trial were discarded. Any strides in which the foot experienced >150 g, which caused the sensor to max out, were discarded.
Figure 1. Vibrational analysis of three strides.
General linear model analysis of variance was performed to evaluate difference in treatments. Before data collection, the significance level was set at α >0.05. For non-significant effects, the power of the test was calculated for finding a 10% difference between treatments (Δ = 0.1).
3. Results
No significant effects between treatments were found for any measure (Table 1). For all measures, the power of the test for calculating a 10% difference in treatments was >90%.
Table 1. Measures of Hoof Vibration | ||||||
Treatment | ||||||
Measure | CON | NON | RIG | BOT | p Value | Power (α = 0.05, Δ = 0.1) |
Stride rate | 1.5/s (0.1) | 1.5/s (0.1) | 1.5/s (0.1) | 1.5/s (0.1) | 0.7860 | 0.99 |
Number of strides | 20 (1) | 19 (2) | 20 (1) | 20 (2) | 0.1112 | 0.99 |
50% frequency | 669 (70) Hz | 688 (62) Hz | 655 (40) Hz | 652 (35) Hz | 0.7581 | 0.99 |
50% frequency area | 9.2 (1.2) mmHz/s2 | 9.4 (1.0) mmHz/s2 | 9.3 (0.7) mmHz/s2 | 9.7 (1.0) mmHz/s2 | 0.8628 | 0.99 |
90% frequency | 2484 (137) Hz | 2502 (105) Hz | 2479 (88) Hz | 2435 (70) Hz | 0.8301 | 0.99 |
90% frequency area | 16.7 (2.2) mmHz/s2 | 16.9 (1.8) mmHz/s2 | 16.8 (1.2) mmHz/s2 | 17.4 (1.8) mmHz/s2 | 0.8628 | 0.99 |
Impact duration | 34 (3) ms | 36 (4) ms | 37 (3) ms | 39 (4) ms | 0.2567 | 0.99 |
peak positive acceleration | 29 (4) m/s2 | 29 (4) m/s2 | 28 (3) m/s2 | 29 (4) m/s2 | 0.7614 | 0.975 |
Peak negative acceleration | -92 (6) m/s2 | -94 (9) m/s2 | -91 (9) m/s2 | -98 (8) m/s2 | 0.7806 | 0.99 |
4. Discussion
This study did not show that the studied pads dampened hoof-impact vibrations at impact during the trot. Thus, the effectiveness of pads in attempting to reduce vibration at impact to treat certain lameness conditions of the limb is questionable. In addition, the findings of this study agree with at least one other previously published investigation [11]. Perhaps this lack of effect is caused by the fact that pads are attached to the horse’s hoof with nails, which pass through the pad and hoof wall. Nails may act as force transmitters to the hoof wall, effectively bypassing the pad.
It may also be that the particular type of pad studied did not reduce impact vibrations. The pad used in this study was a hard-plastic type. The vibration-dampening qualities of hard plastic or of hard plastic with an inflatable air bladder placed over the frog may not be sufficient at the force and acceleration ranges and frequencies experienced in this study.
It is also possible that the effect of pads on hoof vibration (effect size) was too small to be measured by this study. In this study, only eight horses were tested. Our study had negative results with a >90% power to find a 10% difference in any of the variables of hoof vibration that we measured. Effect size is represented by the mean difference in treatments divided by the pooled SD of the groups.
Using recommendations by Cohen [12] for effects sizes (d = 0.2, 0.4, and 0.8 for small, medium, and large effect, respectively), which are used primarily to compare positive results in meta-analyses, we can estimate the comparative power of the negative results in our study [12]. Using these recommendations for effect sizes, this study had an 11.6% chance of finding a small difference (effect size = 0.2), a 60% chance of finding a medium difference (effect size = 0.4), and a 97% chance of finding a large difference (effect size = 0.8). It would require 300 horses to achieve 80% power (a frequently recommended standard) for small effect and 15 horses to achieve 80% power for medium effect. Using Cohen’s [12] recommendations, which should be used with caution, we were capable of finding only a large effect. It is not known how small a reduction in vibrations should be considered clinically relevant.
Our sensor had a maximum capability of measuring accelerations of ± 150 g. While trotting at slow speed on an asphalt surface, some strides in some horses exceeded the sensor’s capacity. This probably increased variability between subjects and decreased our ability to find small differences between treatments. To overcome this problem, a sensor capable of measuring higher g forces should be designed.
This study measured only net vertical acceleration. Although vertical acceleration is the most commonly measured and significant parameter in hoof vibration, it is possible that horseshoe pads have more affect on horizontal or transverse accelerations. To overcome this problem, a three-axis sensor should be designed. To be functional for field use, however, a three-dimensional accelerometer with a higher g range would have to maintain the combination of high sampling rate (8 kHz), wireless transmission, and small size and weight used in this study.
This study measured every stride in horses trotting short distances at a relatively slow speed. Thus, strides may have been captured while the horse was accelerating, traveling at a constant speed, or decelerating. Lower and higher impacts were combined for calculation of trial mean. It is possible that pads significantly dampen vibration only above a specific threshold of force and acceleration and thus, may be effective under conditions different from the ones used in this study.
Based on the results of this study, plastic horseshoe pads, with or without an inflated air bladder, do not seem to alleviate the adverse effects of vibration on the horse’s hooves. Other possible benefits of horseshoe pads, such as protecting the sole or changing the angle of the hoof, still make pads a useful intervention in the management of the horse’s hoof. However, the lack of apparent benefit in reducing vibrations to the horse’s hoof may help explain why, in some cases, horseshoe pads do not seem to help reduce lameness.
This study was funded by Airshod, Napa, California.
Footnotes
- Inflatable air bladder, AIRshodtm, Napa, CA 94559.
- ACH-01, Measurement Specialties, 100 Lucas Way, Hampton, VA 23466.
- PIC18LF452/PQ, 44 Microchip Technology, 2355 West Chandler Blvd., Chandler, AZ 85224-6199.
- Parani-ESD 100, SENA Technologies, 1620 Oakland Rd., Ste D206, San Jose, CA 95131.
- AirShod Pads, 923 Randolph St., Napa, CA 94559.
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