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. 2021 Aug;62(8):834–838.

Cryotherapy provides transient analgesia in an induced lameness model in horses

Vivian Quam 1,, Jonathan Yardley 1, Mikkel Quam 1, Cahuê Paz 1, James Belknap 1
PMCID: PMC8281941  PMID: 34341594

Abstract

The analgesic effect of cryotherapy in an induced lameness model was evaluated. Lameness was induced with solar pressure from a custom-made shoe in a 10-horse, cross-over study. The degree of lameness was recorded with a commercial non-invasive inertial sensor. The distal limbs were maintained in an ice and water slurry (cryotherapy) or at ambient temperature (control) for 1 hour. Lameness was assessed serially over the following hour. Lameness at each time point was compared to the baseline induced lameness, within and between groups. Lameness had improved significantly in all horses 5 minutes after treatment but remained improved 10 minutes after treatment for the cryotherapy group only. Fifteen minutes after treatment, lameness in the cryotherapy group was improved relative to the control. Cryotherapy produced moderate, transient analgesia. Additional research is required to determine if altering the method, duration, or temperature of cryotherapy, as well as the targeted pathology and anatomy, alters the analgesic effect.

Introduction

Equine musculoskeletal lameness produces substantial economic losses for owners and is a welfare concern for the equine athlete (1). Severe musculoskeletal injuries halt the athletic career of some horses, whereas others compete with low-grade pain and reduced performance caused by conditions such as injured ligaments or tendons or degenerative joint disease (2,3). Lameness also has safety implications as it increases the chance of horses stumbling (2,4). Options for managing low-grade pain are limited in events sanctioned by organizations including the Fédération Equestre Internationale (FEI) and The United States Jockey Club (5,6), based on pharmacokinetic properties of non-steroidal anti-inflammatory drugs (NSAIDs) and their influence on soundness and performance (7,8). Ligament and tendon injuries are most common in the palmar metacarpus and digit, anatomical regions amenable to cryotherapy (3,9). Cryotherapy is used as an analgesic in humans and often as an alternative therapy for elite athletes that face drug screening (1013). Owners and trainers seek permissible therapeutics, including cryotherapy, to improve performance in the equine athlete. Understanding effects of cryotherapy on lameness and performance is essential for determining guidelines for its use in competition.

Studies on cryotherapy in horses are largely limited to the treatment of laminitis, for which anti-inflammatory effects are reported (14). Cryotherapy prevents tissue injury when digital hypothermia is initiated early in the disease process, decreasing inflammatory mediators and the progression of digital lamellar tissue injury (15). However, effects of cryotherapy on digital pain were not reported in those studies. Cryotherapy was effective at cooling the deep structures in the distal limb and was safe up to 48 h (9,16). The commercial marketplace offers several equine cryotherapy products, including those marketed for sport horse management and those used in laminitis cases (17). The goal of this study was to assess analgesic effects of cryotherapy in a framework that could be applied in the future to the sport horse in competition or the hospitalized patient. To avoid confounding from an anti-inflammatory effect, a method of lameness induction with minimal inflammatory stimulus was selected. We hypothesized that horses are less lame 1 h after cryotherapy compared to horses in the control group, demonstrating an analgesic effect of cryotherapy.

Materials and methods

Subjects

All procedures were approved by the university’s Institutional Animal Care and Use committee. Ten mature horses from the university research herd were selected for inclusion, based on normal physical examination and minimal observed lameness at the time of initial examination, defined as lameness grade < 3/5 on the American Association of Equine Practitioners’ (AAEP) scale (18). The horses were aged 10 to 22 y and weighed 468 to 615 kg. Breeds represented were Standardbred (n = 3), Thoroughbred (n = 3), Warmblood (n = 2), Quarter Horse (n = 1), and Arabian (n = 1). Horses were housed in stalls for the duration of the study and offered free choice grass hay and water. Ten subjects were selected for the study, based on a review of the existing literature, in which similar studies evaluated analgesia in a lameness model employing the same commercial non-invasive inertial sensor (CNIS) (Lameness Locator; Equinosis, Columbia, Missouri, USA) (19,20). A certified journeyman farrier applied steel keg shoes to both forefeet with modifications designed to create solar pressure, a proven model for experimental lameness induction (21). In all shoes, the medial and lateral shoe branches had pre-drilled, threaded holes for lameness induction via screw application (Figure 1). Shoes were set 1 wk before the first treatment and left on for the duration of the study. Due to loosening of the shoe, 2 horses required resetting of 1 shoe each during the study. Each horse was assigned to a treatment group, cryotherapy or control, during the first experiment period by a random number generator. All horses had a 2-week washout period before the second experiment period, when horses crossed over to the other treatment group. Each horse, therefore, acted as both control and cryotherapy subjects.

Figure 1.

Figure 1

Custom shoe with screw placed for lameness induction.

Baseline assessment and lameness induction

Horses were trotted in the same straight line on the same grass surface for all lameness assessments. A grass surface was selected to prevent excessive solar pressure and pain from the screw. A baseline assessment was performed with the CNIS before lameness induction. For all assessments, horses were trotted a distance to achieve a minimum of 25 strides for interpretation as stipulated by the CNIS manufacturer. Despite having selected horses with lameness grade < 3/5 on the AAEP scale, in some cases a mild lameness was identified by the CNIS (Table 1). When lameness was identified, the screw was applied to the contralateral limb to minimize confounding (19). The screw was placed in the medial or lateral branch of the shoe on the treatment forelimb. Selection of the medial or lateral branch was based on which provided the best contact between the screw and the sole. The screw was tightened or loosened as needed to obtain a consistent grade 3/5 lameness on the AAEP scale, a previously described criterion for adequate experimental lameness induction in context of using the same CNIS (1820). After recording the degree of lameness via CNIS, horses began the treatment hour in cryotherapy or control conditions.

Table 1.

Vector sum (VS) for all horses and separately for cryotherapy and control groups before and after lameness induction (range and median).a

Cryotherapy (median) Control (median) All (median)
Before lameness induction 3.24 to 19.86 (9.35) 4.57 to 23.02 (11.94) 3.24 to 23.02 (9.84)
After lameness induction 23.33 to 75.78 (38.65)b 16.57 to 97.05 (39.59)b 16.57 to 97.05 (39.44)b
a

The VS was based on lameness determination using a non-invasive inertial sensor.

b

Indicates vector sum (VS) was higher (P ≤ 0.05) for all horses and within both groups following lameness induction, as determined by the Wilcoxon signed-rank test. There was no significant difference between cryotherapy and control groups’ VS before or after lameness induction.

Treatment and subsequent lameness assessment

After induction of lameness, those in the cryotherapy group stood for 1 h with the treatment forelimb in a gel orthotic cryotherapy boot (Soft-Ride, Bacliff, Texas, USA). A slurry composed of ice and water was maintained to the level of the proximal metacarpus. Immersion of the complete distal limb in an ice water slurry was selected for this study as it has been reported to achieve a hoof wall surface temperature of < 10°C in less than 1 h (17). The treatment forelimb for those in the control group was maintained in a standard gel orthotic boot (Soft-Ride) at ambient temperature. In both cases, the non-treatment forelimb was kept in a standard gel orthotic boot. Horses were kept in stocks with free choice access to hay and water. The temperature of the ice and water slurry was monitored for those in a gel orthotic cryotherapy boot, and heart rate was monitored in all horses every 10 min. Ice was added to maintain the ice and water slurry temperature below 6°C, a temperature that is achievable with commercial boots with circulating cooled water and has been shown to achieve deep structure temperatures of 10°C in the equine distal limb (9).

The degree of lameness was assessed with the CNIS at predetermined intervals: 5, 10, 15, 20, 30, 45, and 60 min post-treatment. Following the 60-minute post-treatment assessment, the screw was removed. Starting with the 4th horse, lameness was also assessed following removal of the screw (n = 17/20). The additional assessment was added to ensure that the method of lameness induction was not causing excessive solar pressure and that pain would not persist following removal of the screw. Heart rate, although an indicator of pain, was not included during the lameness evaluation, due to the confounding effect of exercise.

Horses were sedated intravenously with acepromazine (VetOne, MWI Animal Health, Boise, Idaho, USA) twice to facilitate the lameness examinations and the time in stocks. The first dose, 0.045 mg/kg body weight (BW), was given before the first baseline jog, and the second dose, 0.025 mg/kg BW was given halfway, 30 min, through the treatment hour in stocks. Use of acepromazine was based on previous research indicating minimal effect on detection of lameness and in 1 case, improvement in consistent lameness detection using the same CNIS device used in the current study (22,23).

Analysis

Objective lameness assessment with the CNIS relies on 3 body-mounted sensors: a gyroscopic sensor on the dorsum of the right forelimb pastern, and 2 uni-axial (vertical) 6 g (1 g = 9.8 m/s2) accelerometer sensors on the poll and the dorsum on the pelvis. Data were collected and measured by the CNIS, as described (24). The vector sum (VS) is a general measurement of the asymmetry of head movement defined by the CNIS manufacturer (25). As outputs from the CNIS, the mean difference in maximum head height (in mm) after the stance phases of the right and left forelimb (HDMax) and the mean difference in minimum head height (in mm) during the stance phases of the right and left forelimb (HDMin) were obtained and used to calculate the VS, where:

VS=((HDMax)2+(HDMin)2)( 25).

The VS was compared between treatment groups to ensure degree of lameness did not differ between the treatment groups before or after lameness induction. The change in degree of lameness at each point following treatment (5, 10, 15, 20, 30, 45, and 60 min post-treatment) was calculated based on relative improvement in the induced asymmetry measured by the inertial sensor. For example, at 5 min post-treatment, the lameness improvement score would equal:

1-[(VS at 5min post-treatment)/(VS at baseline screw placement)].

The lameness improvement score was used to compare lameness within and between treatment groups at each time point, where higher scores indicated greater reduction in asymmetry corresponding with lessened lameness. This method accounted for variation in individual lameness (19). Wilcoxon signed-rank tests were performed for all analyses using commercial software (Stata 14; StataCorp, College Station, Texas, USA) as the data were non-parametric per the Shapiro-Wilk test. One-tailed assumptions were based on the hypothesis that the lameness is reduced by the treatment of interest. Statistical significance was set at P ≤ 0.05.

Results

Baseline assessment and lameness induction

The baseline lameness, measured by the VS before application of the screw, did not differ significantly between the cryotherapy and control groups (Table 1). After lameness was induced with screw placement, horses were assessed to confirm appropriate degree of lameness (grade 3/5 AAEP scale) for evaluation (18). For all horses, the median VS was significantly increased relative to the baseline before lameness induction (Table 1). Again, the degree of lameness induced did not differ between treatment groups (Table 1).

The efficacy of this method of lameness induction was further substantiated by the lameness improvement score obtained immediately after removal of the screw. To measure the change in lameness following screw removal from the shoe, the VS from the post-screw removal assessment was compared to the VS at the 60-minute post-treatment assessment. The median [interquartile range (IQR)] VS values were 11.51 (8.21 to 19.03) and 35.28 (21.12 to 54.97), respectively (n = 17). Thus, there was an improvement (P ≤ 0.01) in lameness following screw removal from the shoe, supporting the technique for transient lameness induction.

Treatment and subsequent lameness assessment

Both groups, cryotherapy and control, were maintained without difficulty in stocks for the duration of the treatment hour. The target temperature of 6°C was maintained easily with the application of ice to the gel orthotic cryotherapy boot. No changes in heart rate, an indication of pain, were observed during the treatment hour for either group.

The median lameness improvement score at 5, 10, 15, 20, 30, 45, and 60 min following treatment for each group is presented in Figure 2, with interquartile ranges. Both treatment groups experienced a reduction in lameness following the treatment hour. However, the cryotherapy group had a significant lameness improvement score 5 and 10 min after the treatment period, indicating that the degree of lameness assessed at these time points was significantly less than the degree of lameness initially following screw placement. The ambient group only had a significant lameness improvement score 5 min after the treatment period (Table 2). No significant lameness improvement was noted for either group 15 min post-treatment or in subsequent assessments.

Figure 2.

Figure 2

Cryotherapy (black line) and control (gray line) median lameness improvement score with interquartile ranges (IQR) at each time point post-treatment. Black circles = cryotherapy IQR. White circles = control IQR.

Table 2.

The median lameness improvement score for cryotherapy and control groups.

Time (min) Lameness improvement score

Cryotherapy Control
5 0.67a 0.44a
10 0.46b 0.27
15 0.49c 0.35
20 0.45 0.36
30 0.37 0.07
45 0.19 −0.04
60 0.23 0.17

Time is in minutes post-treatment hour.

a and b indicate improvement in lameness score (P ≤ 0.05 and P ≤ 0.01, respectively) at the given time point relative to the baseline induced lameness as determined by the Wilcoxon signed-rank test within treatment groups.

c indicates greater lameness improvement score (P ≤ 0.05) in the cryotherapy group relative to the control group at the given time point, as determined by the Wilcoxon signed-rank test.

When the lameness improvement score between treatment groups was compared, there was a significantly greater lameness improvement for the cryotherapy group 15 min post-treatment relative to the control group (Table 2), but this significance was not maintained throughout the post-treatment hour.

Discussion

After 1 h of cryotherapy, horses in the cryotherapy treatment group had longer duration of lameness improvement relative to control horses, 10 versus 5 min. A difference in lameness improvement between the 2 groups was only significant at 15 min after treatment. Reports from studies on humans suggest there is potential for longer effects (11,12). Due to variation among horses and the low number of subjects, this improvement may not be reflected statistically. It is unlikely that the lack of longer effect was due to an ineffective method of cooling. Using an ice water immersion boot covering the hoof and distal limb, as was used in the current study, was effective at maintaining hoof wall surface temperature below 10°C, the current therapeutic target protecting the submural lamellar tissue in laminitis (17). However, perhaps the duration of cryotherapy used in the current study was too short to obtain maximal effects of hypothermia on the submural soft tissues. Previous studies have demonstrated that wet ice boot application has resulted in hoof wall surface temperature decreasing to equal the water ice temperature (17). However, hoof wall temperature only reaches steady state temperature after 2 h (14). Since horses in this study only received cryotherapy for 1 h, and hoof wall surface temperature was not directly measured, the deeper structures of the foot may not have reached the steady state temperature. A longer duration of hypothermia may be more efficacious in reducing lameness due to solar pressure. In addition, the requisite method and duration of cryotherapy for analgesic purposes will likely vary with the target tissues within the distal limb. In 1 study evaluating the effect of cryotherapy on soft tissues of the distal limb, 1 h of cold therapy was effective in lowering the temperature of the superficial digital flexor tendon to 10°C (9).

There was a brief significant but waning lameness improvement for all horses, cryotherapy and control treatment groups, following the treatment hour, compared to the initial induced lameness. Although this observed improvement was slightly longer in duration in the cryotherapy treatment group, all horses may have had significantly reduced lameness due to the gel orthotic boots that all horses stood in for sole support during the treatment hour. There was no negative control for the gel orthotic boots, so it is impossible to determine if the improvement can be attributed to the boots. Since the commercially designed boots provided for the study included gel orthotics, the control group had to be maintained in gel orthotics as well to examine the effect of ice. If the heel screws were in constant contact with the hard ground, additional pain above the baseline jog may have been elicited in those horses. However, regardless of the reason for the transient improvement in lameness in both treatment groups, improvement in lameness after the 5-minute time point for the cryotherapy group indicated that there is some potential for cryotherapy analgesia in equine athletes and hospitalized patients. Further investigation into the use of cryotherapy for analgesia in distal limb lameness may benefit from longer intervals of treatment for foot lameness models, or possibly the induction of lameness in joint or tendinous structures proximal to the foot, as these structures may reach the target temperature more rapidly. The model of lameness induction used in this study was designed to produce limited inflammation to isolate the effect of cryotherapy on pain from its effect on inflammation. We deemed this method successful given that the lameness noted immediately following screw placement (median: 35.28), did not differ from that observed at 60 min post-treatment (median: 39.44). We also believe the transient nature of lameness induction was demonstrated by resolution of lameness immediately following screw removal. In horses with pain associated with inflammation, the analgesic effect of cryotherapy will likely differ and may be even greater. With a larger study population, different method of lameness induction or longer duration of cryotherapy, the effects of cryotherapy may become more evident.

In conclusion, there was a modest reduction in lameness resulting from 1 h of cryotherapy. Although the therapeutic effect diminished over the post-treatment hour, significant lameness improvement continued at least 10 min after cryotherapy. Given that the reduction of lameness in horses in the cryotherapy group was only significantly greater than the control group at 15 min after treatment in this study, additional research is required to determine the ideal method, duration, and temperature for provision of analgesia. Although actionable clinical recommendations based on the evidence generated by this study are limited, the study laid much of groundwork necessary for designing further research more comprehensively investigating the promising utility of cryotherapy in equine pain management. CVJ

Footnotes

Use of this article is limited to a single copy for personal study. Anyone interested in obtaining reprints should contact the CVMA office (hbroughton@cvma-acmv.org) for additional copies or permission to use this material elsewhere.

Funding was provided to The Ohio State University Equine Research Fund by the Ohio State Racing Commission. Soft-Ride, Inc. provided the 2 ice boot prototypes used in the study. Neither the funders nor Soft-Ride, Inc. played any role in the study design, the collection, analysis, or interpretation of data, or the decision to submit the article for publication(s). None of the authors has any financial or personal relationship that could inappropriately influence or bias the contents of the paper.

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