Effect of fence height on hyperextension of the fetlock
Injuries in the equine athlete can mean an increased cost in veterinary fees, loss of use and a reduced welfare of the animal (Murray et al, 2006). Show-jumping horses are particularly at risk of injury due to the large loads transmitted through their distal limbs (Schamhardt et al, 1993). Studies have found that elite show-jumping horses most commonly injure the suspensory ligament (SL), followed by the deep digital flexor tendon (DDFT) (Murray et al, 2006) and demonstrate a higher risk of early retirement due to musculoskeletal injuries than dressage horses (Durco et al, 2009).
Foot balance, track surfaces, fatigue and increased weight have all been linked to an increase likelihood of injuries to the flexor tendons (O’Sullivan, 2007). Several studies have looked at environmental influences, such as fence height, additional weight and varying surfaces, and investigated their effects on landing kinetics and kinematics (Hernlund et al, 2010, Clayton and Barlow, 1989 and Clayton et al, 1997) however these are somewhat limited by small sample sizes and non experimental designs where multiple variables were included which makes drawing firm conclusions difficult.
Understanding of the soft tissue structures and forces transmitted through these structures could aid veterinary professionals in reducing the risk of injury.
Anatomy of the fetlock
The fetlock is supported by the suspensory apparatus, which is composed of the Suspensory ligament, sesamoid bones and the distal sesamoidian ligaments (Bukowiecki et al, 1987), which act to resist extension (Smith et al, 2002). The superficial digital flexor tendon (SDFT), SL and DDFT act to absorb elastic energy and aid efficient transfer of muscular energy for rapid locomotion (O’Sullivan, 2007).
Hyperextension of the fetlock joint during weight bearing is typical in cases of suspensory breakdown (Gibson and Steel, 2002).
Elastic energy storage in tendons and other soft tissues is thought to increase with speed to reduce metabolic demand (Beweiner, 1998). Increased energy transfer and thus strain energy (Back et al 1995a) through the tendon can cause damage either traumatically, in an acute overload injury, or through overstrain injuries where repetitive loads lead to damage at a rate which exceeds the cells ability to repair. This results in accumulative microdamage and eventually presents clinical signs of injury such as lameness (O’Sullivan, 2007; Dahlgren, 2007: Parks, 2003).
Show-jumping horses are at risk of both types of tendon injuries due to their heavy competition schedule, limited recover time and the high athletic demand placed upon them (Dyson, 2002). Protection of these soft tissues are vital in maintaining a sound horse and thus prolonging its career ( Durco, 2009-check this ref).
A good understanding of biomechanics of the fetlock joint and the forces transmitted through its anatomy are key in reducing injury and maintaining a sound horse. By quantifying these forces and energetics it can give us an idea of characteristics, which push soft tissues through their safety margins and result in failure and thus injury and lameness (Thomason and Peterson, 2008). Advice in accordance to these known limitations could be key in minimizing injury through effective training and rehabilitation of a horse.
With Show-jumping growing in popularity there is high financial gain and ethical justification to identify risk factors of injury and devise appropriate exercise and training protocols.
Risk factors of fetlock injuries
With large funding there has been extensive research identifying risk factors of fetlock injury in the racing thoroughbred (Thomason and Peterson, 2008). The effect of track surface, fetlock kinematics and kinetics has been investigated in hope to help prevent injury (Butcher and Ashley-Ross, 2002; Murray et al, 2010). Studies have found younger horses (under two years) may show more acute angles of dorsiflexion because of an immature suspensory apparatus and thus display poorer shock absorption and a greater likelihood of third metacarpal bone bending (Butcher and Ashley-Ross, 2002). Although these results have limited validity except for racing thoroughbreds it has been previously noted that forelimb injuries have been shown to decrease with age, adaptive bone remodeling and specific exercise loading (Reilly 1999; Smith et al, 1999).
The leading forelimb is most susceptible to overloading in the galloping horse (Butcher and Ross, 2001). In jump off’s horses will often be required to gallop however the predominant stress for the show jumper has been found in the landing phase of jumping where the trailing forelimb carries the heaviest loads and demonstrates the largest ground reaction forces and peak flexor joint moments (Schamhardt et al, 1993; Meershoek et al, 2001).
The landing phase exists to absorb the impact of landing and retain horizontal velocity (Powers et al, 1999). Previous research has investigated landing variables and their effect on the distal limb. Kinetic studies have explored joint moments and ground reaction forces (GRF) in the landing phase of jumping finding that GRF increased as fence height increased and that joint moments are consistent among experienced horses (Schamhardt, 1993; Meershoek et al, 2001). Limb displacement from the base of fence increase as the height of a fence increases (Clayton and Barlow, 1989) which may suggest an increase in horizontal velocity and potentially more forces through the distal limb.
The direct measure of tendon forces is difficult due to its invasive nature however the use of inverse dynamic analysis offers some estimation of tendon loading (Meershoek et al, 2001). Although these results need to be interpreted with caution due to joint moment errors, particularly in the coffin joint, they concur with Schamhardt et al (1993) that peak loads can be seen in the trailing forelimb on landing.
Limb contact characteristics have also been studied exploring limb placement. (Clayton et al,1997; Clayton and Barlow 1989), hoof surface interaction (Hernlund et al, 2010) however there is a lack of evidence specifically looking at fetlock extension angles in the landing phase of jumping. It would be logical to assume that if the forces are increased through a joint that its surrounding soft tissues may be put under more strain (Denoix, 1996). Fetlock extension gives an indication of magnitude of GRF (Back et al, 1993) therefore can be used as a useful objective measure to evaluate stresses of soft tissues in the distal limb (Stanschi, 2008). Horses with more upright pasterns appear to have longer careers (Durco et al, 2009), which could be attributed to the smaller moment arm less strain on the soft surrounding tissues (Meershoek et al, 2001).
A greater force is required to rupture the suspensory apparatus of a trained horse compared to an untrained horse in vitro (Bukowiecki et al. 1987). It has been shown that at four years horses who have undergone early jump training demonstrate a better technique than their untrained counterparts however they both display equally good technique at five years. Horses with poor technique combined with immature suspensory apparatus (Butcher and Ross, 2001) may be at particular risk of tendon injury in the trailing forelimb when the fetlock reaches maximal extension in landing.
Support boots could help reduce tension in the suspensory apparatus and superficial digital flexor tendon (Kicker et al, 2004). With a sound knowledge of normal fetlock extension angles at different fence heights, training protocols and veterinary advice could be tailored to age, experience and time since injury.
Purpose of study
This study aims to establish if there is a relationship between fetlock extension and the height of a fence. Previous studies have shown that as fence height increases, GRF increases (Meeshoek, 2001b) although have not looked specifically at joint angles and which joints play the most significant role in absorption. One study looked at peak flexor moments finding that SDFT took significantly greater load compared to the DDFT (Meershoek, 2001a). However this looked at only one fence height and gives us limited understanding of the differences of fetlock extension between fences of varying heights and between limbs.
It has been suggested that a reduction of maximal fetlock extension of 1° may, using the inverse dynamic approach of Meershoek and Lanovaz (2001) and Meershoek et al. (2001), for a 500 kg trotting horse result in a force reduction in the suspensory ligament of about 200 N.(Kicker et al, 2004). Further work would need to investigate soft tissue force reduction specifically in the landing phase of jumping however having baseline fetlock extension data for three fence heights would allow us to make rough estimations of relative loading between the fences.
1) As fence height increases, the angle fetlock extension will increase.
2) The trailing forelimb will display larger fetlock hyperextension angles than the leading forelimb at the same fence height.
Materials and methods
A high speed video camera (250 frames/second) will be used, placed perpendicular to the presumed landing site. The horses will jump from right to left across the field of view so that the recordings will be from a left lateral view (Clayton, 1997). Markers will be placed on bony prominences (see below) and angles measured using relevant computer software. Finally, a Global Positioning System will be used to monitor speed.
Ethical approval will be attained from the Royal Veterinary College ethics committee.
Self adhesive white markers, 9mm diameter, with a black spot in the centre will be placed on both forelimbs. The markers will be placed on the skin covering the centres of rotation of the carpus (proximopalmar part of the ulnar carpal bone) and fetlock joint (site of attachment of the collateral ligament,) (Meershoek et al, 2001). Three markers will be fixed to the lateral hoof wall forming a triangle: one close to the heels and the other two parallel to the foot axis (Back et al, 1995a).
Previous studies have found minimal skin displacement in the distal limb (van Weerean, 1988)……
Lateral radiographs will be used to check true representation of bony landmarks ensuring validity of the fetlock angles. This will be conducted on 5 horses at the RVC to ensure researcher skill to place these on the subject horses.
Eight warmbloods competing at affiliated, British Show Jumping Association, level will be used. They will be of similar age, experience and under 165cm in height (16.2hands). Relevant baseline statistical analysis will be completed to establish that there is no significant difference in height and weight.
The same rider, on the same surface and normal training arena will jump them over one fence. The fence will stand at 0.80m, 1.10m, 1.25m at a width of a standard oxer (0.5m). The fence will be positioned in the centre of the arena and will consist of a red and white ground line pole and two red and white verticals standing at the relevant heights. The horses will be regularly trained over these poles prior to the study to reduce any bias caused by an unfamiliar obstacle.
Previous studies have found a change in fetlock kinematics with varying surfaces (Hernlund, 2010). To control this variable the same arena will be used for all horses to minimize any confounding factors skewing results. The arena will be a rubber sand mix which is commonly used in showjumping events. The arena is well maintained and will be raked before every jump.
The horses will be warmed up for ten minutes and allowed two warm up fences as per their normal routine training (Clayton, 1997). The rider will be instructed to jump each fence three times with height increasing from smallest to largest in accordance to normal training. Fetlock extension angles of both forelimbs and hindlimbs will be recorded at maximum extension of stance phase and processed using Statistical Package for the Social Sciences (SSPS).
Three values at each three heights will be recorded with results being discarded if hoof placements fall outside the camera view or the horse fails to clear the fence.
The mean values and standard deviation of fetlock angles of each limb will be calculated for each height of fence.
If data is normally distributed a repeated measure Analysis ofvariance (ANOVA) will be used to compare fetlock angle of each limb between different fence heights. If the data is a non-normal distribution Friedman’s test will be used. Statistical significance will be set at P<0.05.
Results, Discussion and Conclusions will look to form ideas regarding impact of findings and reject or approve the hypothesis.
With high financial gain and concern regarding a horse’s welfare at the elite level it is important to try and reduce the risk of injury. There is substantial evidence investigating kinetics and kinematics of the distal limb in the landing phase of showjumping. With the loads being greatest in the trailing forelimb understanding external influences that may affect these forces will help researchers provide evidence for injury prevention in the future. This study aims to look specifically at hyperextension of the fetlock and the amplitude of change in extension angle between varying fence heights.
|Process||Month||Time expected for completion|
|Research Proposal||February/ March 2011||2 months|
|Gaining ethical approval||April 2011||3-4 weeks|
|Relevant methodological Adaptions||April 2011|
|Data collection||July 2011||1 week|
|Data Input and statistical testing||August 2011||1 month|
|Analysis of results, discussion and conclusions||September-December 2011||3 months|
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