Risk Factors for the Occurrence of Desmopathy of the Collateral Ligaments of the Distal Interphalangeal Joint in Equines – A Cross-Sectional Study

Abstract Desmopathy of the collateral ligaments of the distal interphalangeal joint (CL-DIPJ) has been increasingly relevant in equine sports medicine. However, the risk factors for the development of such desmopathy are not well established. In the literature, what has been described about it, is based on the clinical conditions observed in animals diagnosed with this disorder. Considering this scenario, the aim of this study was to verify which possible extrinsic or intrinsic risk factors may influence the occurrence of disorders in the collateral ligaments of the distal interphalangeal joint in polo horses. Twenty-five horses from the same equestrian team, all competing in polo, were evaluated via ultrasonography. The outcome was defined as changes in echogenicity and/or boundaries of the CL-DIPJ of the thoracic limbs of the horses evaluated. The intrinsic factors considered in this study were age, the presence of angular deviations and gender. As extrinsic factors, the parameters taken into consideration were the handicap in which the animals played and the type of training to which these horses were previously submitted in their professional athletic career. The prevalence ratio was used to verify the association between risk factor and outcome. The result of this evaluation was considered significant when the confidence interval value did not involve the value 1. Positive and significant association was observed only for age. Playing in different handicaps did not influence the presence of ligament disorders. The other factors evaluated revealed a positive but not significant association. Thus, it was concluded that animals aged nine years or older present a higher risk of CL-DIPJ disorders.


INTRODUCTION
Collateral ligaments of the distal interphalangeal joint (CL-DIPJ) of horses are one of the most affected structures in the hoof region, being the second most disturbed [1]. These lesions occur due to biomechanical stress caused by distal phalanx collateromotion movements at the moment of hoof support on the ground. These movements, when exacerbated, may lead to CL-DIPJ desmitis [2].
The probable risk factors for the occurrence of this condition are based on the common clinical fi ndings in the affected animals. Dyson et al. [3] reinforce the need for studies that aid to establish them. Denoix et al. [4] verifi ed in all the reported cases of CL-DIPJ desmitis the presence of chronic medial-lateral hoof imbalance, which was also pointed out by Zubrod et al. [5]. The results of the study of Zubrod et al. [5], also suggest that angular deformities could predispose their occurrence, since this feature was observed concomitantly with the manifestation of desmitis. Previously in a series of 18 cases described by Dyson et al. [3], it was noted that the affected animals had an average age of nine years (5-14 years) and practiced different sports. The practice of equestrian sports that consist of performing circular movements in conjunction with sudden stops and exercises on uneven ground lead to irregular support of the hoof, is also considered a cause of desmitis in these structures [2,5].
Epidemiological studies have been conducted in order to seek and clarify what infl uences the occurrence of skeletal muscle injuries in equine athletes. Although much has already been done, none of these targeted CL-DIPJ. Risk factors for changes in the hoof region were assessed by Parkes et al. [6], who verifi ed that the lesions of the podotrochlear apparatus of the studied population were infl uenced by the increase of age and gender. Many authors have also explored for which intrinsic and/or extrinsic characteristics predispose to fl exor tendons and suspensory apparatus (SA) lesions. Thus, it was found that, for these structures, increased age [7][8][9][10] and animal gender [9] predispose to the occurrence of lesions. In addition, studies regarding the distal limb conformation show the relationship between the presence of angular deviations and the occurrence of fl exor tendon changes [11], as well as their infl uence on their athletic career [12,13].
Considering the relevance of the CL-DIPJ and the lack of information about which risk factors infl uence the ligament injury process, the aim of this study was to verify which possible extrinsic or intrinsic risk factors may infl uence the occurrence of collateral ligament disorders in the distal interphalangeal joint of polo horses.

Ethics Commission
The experiment was carried out under Project License from the Ethics Commission of the Federal University of Pampa, Uruguaiana, Brazil (process number 027/2017).

Study population
Twenty-fi ve polo horses were used, with average age of 8 ± 3 years, height of 1.53 ± 0.4 m and weight of 470 ± 28.67 kg, being 7 geldings and 18 females. All were in polo season and came from the same property. Thus, food and sanitary management, hoof care, training program and fl oor type were equal in the population studied.

Intrinsic Risk Factors
The intrinsic factors evaluated were age, gender, presence of angular deviations and medial lateral hoof imbalance. Age was verifi ed by birth registration and horses were divided between animals under nine years old and horses nine years old or older. Sex was defi ned by external observation of the genitals, being divided into females and geldings.
The presence of angular deviations was defi ned by inspection of the horses in quadrupedal position on a straight fl oor. The evaluator observed the animals in a frontal view, looking for deviations of the thoracic limbs. Whenever there was, it was considered to have valgus or varus angular deviation.
The medial-lateral imbalance of the hoof was verifi ed by measuring the height of the lateral and medial heels. It was considered unbalanced a hoof in which the difference in height of the heels was equal to or greater than 0.5 cm.

Extrinsic Risk Factors
For this study, extrinsic factors as the handicap and training prior to professional athletic career were considered. The handicap in which they played was reported by the veterinarian responsible for the property and was divided between those who played high and low handicap.
To defi ne the training that preceded the professional athletic career, the horses were divided between those born on the property or purchased. Those who were born on the property were tamed at three years of age, underwent an evaluation of their athletic performance between four and six years and, at six years of age, having reached the desired level of exercise, started professional sports activity. Purchased animals were obtained in adulthood, so there was no information on previous training.

Outcome Defi nition
Ultrasound evaluation of CL-DIPJ was performed on the thoracic limbs of all horses according to the previously described technique [14]. Those with alterations in echogenicity and/or boundaries in at least one of the ligaments were considered affected.

Clinical assessment of horses
All horses evaluated underwent a general and specifi c physical examination of the locomotor system. For this, dehydration level and body temperature were considered, heart and respiratory rates were measured, and auscultation of intestinal motility was performed.
For the specifi c examination of the locomotor system, static inspection was fi rst performed in order to investigate possible increases in volume in the region of the ligaments. In sequence, the dynamic evaluation was performed, with the animals being evaluated at walk and trot. In addition, the veterinarian responsible for the animals was also consulted in an attempt to identify possible previous lameness in the digit region.

Data analysis
Data analysis was performed using software (CEMEQ -USP) in which the prevalence ratio was calculated. The number of individuals was used for age, handicap and previous training factors. For the presence of angular deviations and hoof imbalances factors, the number of thoracic limbs was used.
When PR = 1, it was considered that there was no association between the factor and the occurrence of ligament disorder. When greater than 1, it was interpreted as a positive association between the presence of the factor and the occurrence of disorder. Also, if PR<1, it was considered as a protection factor. Thus, the higher the prevalence ratio, the stronger the association was considered. Even if PR> 1 was observed, the association between factor and outcome was only considered statistically relevant when the confi dence interval did not involve value 1.

RESULTS
In the present study, 40% (10/25) of the horses had echogenicity and/or boundaries changes observed by ultrasonography on CL-DIPJ. Of these 10 horses, 50% (5/10) had sonographic changes in the lateral and medial collateral ligaments of the same thoracic limb -20% (1/5) in the right thoracic limb (RTL) and 80% (4/5) in the left thoracic limb (LTL), 30% (3/10) presented changes in only one collateral ligament, both in the LTL-75% (2/3) in the medial collateral ligament and 25% (13) in the lateral collateral ligament. Of the remaining 2 horses, 50% (1/2) presented alterations in the medial collateral ligament of the RTL and the other one presented changes in the lateral collateral ligament of the RTL and in the medial collateral ligament of the LTL.
The frequencies of occurrence of these changes in the exposed and unexposed can be observed in Figure 1 for intrinsic factors and in Figure 2 for extrinsic factors. The factor of medial-lateral imbalance of the hoof was considered a bias factor and disregarded in this study.
The calculation of the prevalence ratio showed that the handicap in which the animals played did not infl uence the occurrence of ligament changes. The other factors were positively associated, as shown in Table 1. Age was the only factor that was statistically relevant, so that animals aged nine years and over are 500% more probable to develop changes in CL-DIPJ when compared to younger ones.   Prevalence ratio; 2 predisposition percentage of exposed horses; 3 Confi dence intervals Regarding the gender, conformation and previous training factors, although having a positive association with the occurrence of injuries, the confi dence interval values do not allow extrapolating these results to the general population. Also, it was noted that females are more predisposed, as well as those animals with angular deviations. Regarding to previous training, animals born on the property presented less possibility of presenting ligament disorders.
None of the horses evaluated showed lameness in the digit region at the time of the study, as well as a history of lameness from that region.

DISCUSSION
Biomechanical stress caused by exacerbated phalanx collateromotion movements is reported to cause CL-DIPJ desmitis [2]. However, it is unclear what situations the animals are exposed to and/or individual characteristics predispose them to occur. Zubrod et al. [5], suggested that the practice of sports involving stops and turns, as well as those practiced on uneven terrain, provide this type of biomechanical alteration. Polo is one of the equestrian modalities in which these types of movement are observed during games [15] and, despite this, no description of CL-DIPJ desmopathy was found in animals of this modality, only being reported in horses training, running [3] and jumping [3,16].
Thus, the frequency of CL-DIPJ injuries observed in this study is an important fi nding since no previous reports of its occurrence in polo horses were found. This fi nding becomes even more relevant if it is considered that all LC-AID can be equally affected in animals that practice this sport. So, the presence of changes in ultrasonography images and their distribution in this study, demonstrates that the collateral ligaments of the distal interphalangeal joint and its changes must be included as a differential diagnosis in polo horses. Particularly, in those with lameness in the digit region, one of the regions most affected by the practice of this sport, in which it is not always possible to reach a defi nitive diagnosis [17].
This was the fi rst epidemiological study that sought to defi ne risk factors for CL-DIPJ injuries. The risk factors chosen to be evaluated in the present study were based on conditions previously mentioned, but which had only been observed clinically and, therefore, could not be confi rmed as risk factors [2,5]. Thus, the medial-lateral hoof imbalance [2,5] and angular deviations [5] were included because they were previously reported as a common characteristic in horses affected by the condition. Nevertheless, the medial-lateral hoof imbalances were disregarded as the horses were constantly trimmed and shod, having been shod about 15 days prior to evaluation. Thus, the analysis of this variable made it impossible to defi ne whether such condition is chronic or not, being invalidated for this study.
In addition to characteristics previously observed in cases of CL-DIPJ desmitis, factors that were reported to predispose to other skeletal muscle injuries were also considered. Thus, it was also evaluated age [6][7][8][9][10], sex [6,9] and exercise level [8]. The inclusion of the type of training prior to the athletic career was due to the particular form that the observed property performs at the beginning of the horse's activities. Thus, the authors sought to observe whether these precautions were being appropriate.
The fact that the evaluated horses did not show lameness even with the presence of changes in ultrasonography, makes the fi ndings of this study even more important. As mentioned, studies that suggest possible risk factors in used horses that already had apparent lameness [2,5], so that there is a certain limitation of knowledge of pre-clinical predisposing factors. This study allows veterinarians to have subsidies to include CL-DIPJ as a potential injury site in athlete animals that fall into risk groups. Thus, clinical desmitis of these ligaments can be prevented, along with other degenerative changes common in athletic horses.
One of the main fi ndings of this study was the greater predisposition of animals aged nine years or older. The average age observed in horses in a series of 18 cases of CL-DIPJ desmitis was nine years, corroborating with this fi nding [1]. Increasing age was also found to be a predisposing factor for general skeletal muscle injuries [7,8], for superfi cial digital fl exor tendon (SDFT) tendonitis of the thoracic limbs [9,10] and injuries to the suspensory apparatus (SA) [9] in racehorses. Similarly, considering the podotrochlear apparatus of horses used for dressage, jumping, running, endurance and reining, it was observed that animals aged six to ten years are subject to the occurrence of disorders and the risk increased further between 10-15 years [6].
Nevertheless, and agreeing with Perkins et al. [9], it is necessary to defi ne if age has a direct infl uence on the lesions or if the infl uence of other factors is manifested together with the increase of the same. Given this, it is not possible to say whether the greater occurrence of CL-DIPJ desmopathy is due to changes in the biomechanical and/or biochemical properties that occur in the ligament fi bers in older animals or to the longer time of sport practice and, consequently, greater exposure to repetitive trauma and biomechanical stress. Studies involving ligament biopsy would be necessary to clarify this, which is not feasible in animals that are in the sporting season, as was the case of the population studied.
Regarding sexual predisposition, previous studies have found a greater predisposition of males to the occurrence of lesions in SFDT and SA [9]. Stallions also had a higher number of lesions that had lameness in the hoof region, especially in the deep digital fl exor tendon [6]. Perkins et al. [9], suggest that this fi nding could be related to hormonal issues or the tendency of stallions to have less docile behavior during training [9]. In our study it was observed the opposite, females were 71.4% more likely to develop changes in CL-DIPJ than males, which may be explained by the higher number of females composing the polo team evaluated or may be the fact that the males in this study were castrated and the hormonal characteristics mentioned in previous studies were not manifested in them.
As regards angular deviations, their infl uence is questionable and may be related to the activity performed. The athletic career of dressage horses was not infl uenced by the presence of angular deviations [13], while such conformation defects may affect and decrease the sporting career of horses, as observed by Ducro et al. [13] on jumping horses. Fugazzola et al. [11] compared radiographic measurements of the distal region of the thoracic limbs of healthy horses and horses with a history of SDFT tendonitis but could not confi rm whether this is a predictor parameter for these disorders. Still, they reinforced that the understanding about its infl uence on the occurrence of locomotor injuries should be better clarifi ed.
However, angular deviations were pointed as an important predisposing factor for SFDT tendonitis in racehorses where those with angular deviations presented a higher occurrence of the disease when compared to well-groomed animals [12]. The results of the present study corroborate this statement because, although not statistically signifi cant, horses with this characteristic were 30.3% more predisposed to changes in CL-DIPJ, and further studies are needed regarding this factor. In addition, it is suggested that horses subjected to the equestrian polo be negatively affected by the presence of this trait.
In the present study, no infl uence of the handicap where the animals played was observed on the occurrence of ultrasonographic changes of the CL-DIPJ. Exercise intensity was not relevant in relation to alterations of SFDT and SA in racehorses [9]. In contrast, even in racehorses, increased exercise intensity (increased distance traveled) promoted a higher risk of skeletal muscle injury [8]. The infl uence of exercise intensity may vary according to the anatomical structure and activity of the horse. Moreover, in sports such as running, it is easy to verify the intensity of the exercise objectively, while in the polo the change of handicap does not always mean greater intensity, corroborating the observations in this work.
Regarding previous training, purchased animals were more predisposed to ligament disorders. Unfortunately, there was no information on the type of training they had undergone, which makes it diffi cult to identify the characteristics of these animals that corroborate the increased occurrence of CL-DIPJ desmopathy. In relation to animals born on the property, they were started at about three years of age and had a lower predisposition to changes in CL-DIPJ. This differs from what was observed by Reardon et al. [10], in which those horses that ran for the fi rst time at three years had a higher risk of developing SFDT tendonitis than those that starting at four years. It may be extrapolated that the fact that animals born on the property were included in a gradual training program may have reduced the predisposition to injury. However, further studies would be necessary to verify if this fact can be attributed to a genetic background, to skeletal muscle adaptation to exercise or to their association.
Regarding the limiting factors of this study, the authors believe that the number of animals should be considered. Nevertheless, it is noteworthy that a complete polo team was evaluated and that all animals were on the same property, received the same attention and trained polo under the same conditions. Thus, it was sought to obtain homogeneity of the studied population, so that the infl uence of the modality, the type of ground and the regularity of the exercise was excluded. These factors were pointed out by Parkes et al. [6] as limiting in their study that verifi ed several risk factors in injuries of the podotrochlear apparatus in horses of different modalities.
Finally, the fact that some of the risk factors in this study were positively associated with the occurrence of alterations of CL-DIPJ without, however, having statistical signifi cance. This can be attributed to the fact that the interaction between intrinsic and extrinsic factors can cause synergistic or antagonistic effects on the injury-forming mechanism [7]. In addition, it is important to emphasize that locomotor disorders are often multifactorial [6]. Unfortunately, it is not possible to verify the interaction between the variables evaluated in a cross-sectional study. Nevertheless, the present study represents an initial effort to clarify the risk factors associated with the occurrence of collateral ligament desmopathies of the distal interphalangeal joint. Thus, it was possible to verify the positive and signifi cant association between increasing age and the presence of disorders. Although not signifi cant, a positive association was also observed between the occurrence of alterations in ultrasonography and the presence of angular deviations, gender and the gradual initiation of exercise in these horses. Thus, further studies with a larger number of animals are suggested to confi rm this association, as well as longitudinal studies that evaluate these factors in a long term.