If you’re not already a member of the Thoroughbred Health Network, please subscribe FREE TODAY to receive the latest e-news and research reviews.
Research review outcomes will be displayed as 3 progressive layers of information:
1. Key messages – organised into ‘what we have good evidence for’, ‘what we have some evidence for’ and ‘what we don’t have evidence for but would benefit from’.
2. Summarised findings – expand the boxes to view the research that supports that key message.
3. Hyper-links – click the numbers to view the scientific papers themselves*. Our aim is to improve our general understanding of the strength of evidence available on each health issue, and consequently identify knowledge gaps. *Some papers may remain copyright protected according to the journal in which they are published and therefore, may not be accessible in full.
Preventing tendon injuries is better than trying to treat tendon as the structural nature of the tendon matrix means injured tendon will not repair to its original structure or function and therefore, re-injury is likely.
Collagen molecules are arranged hierarchically into micro-fibrils, sub-fibrils, and fibrils, which are grouped into fascicles and aligned longitudinally, separated by the more loosely organised endo-tendon (Figure 1) (31).
Figure 1: Illustration of the hierarchical structure of tendon which contributes to the mechanical function of the tendon. (Source: Liu, et al, 2008).
When the surface of a tendon is viewed at an angle, the presence of a wave-form or crimp within the fascicles can be detected (Figure 2).
Figure 2: Image showing the crimp-like alignment of tendon fibres
This crimp pattern plays a beneficial role in the elasticity of tendon during the early stages of strain loading (Figure 3). Therefore, any elimination in the crimp formation of tendon is thought to partly contribute to increased stiffness of tendon and lower structural elasticity (31).
Figure 3: The crimp alignment of tendon fibres play an important role in responding to the strains upon tendon during exercise. Copyright, EVJ Ltd (2015). Reproduced with the permission of the editor.
Lower structural elasticity within the tendon leads to increased risk of injury
Micro-damage within the tendon structure results in irreversible changes to the tendon matrix such as, altered crimping, irregular arrangement of the collagen fibrils and reductions in the density of collagen (203).
In the racehorse, the Superficial Digital Flexor Tendon (SDFT), which contributes to locomotion (movement) by storing energy, has a low safety margin and is frequently injured. Tendons which merely position the limb, including the opposing Common Digital Extensor Tendon (CDET), are rarely injured (Figure 4).
Following injury, the healing process will always result in tissue that is structurally different to original tendon tissue which has reduced elasticity; often referred to as “scar tissue”.
Figure 4: The Superficial Digital Flexor Tendon is the most frequently injured tendon in racehorses
(Source: Novobrace, 2015).
Tendon injuries limit long term performance due to their re-occurring nature.
Although sudden trauma can occur for example, due to a blunt strike with another limb, the degeneration of tendon tends to be progressive and therefore, will predominantly take place during training.
This presents the opportunity to prevent tendon rupture if recognised early.
Reduced crimp alignment of tendon fibrils occurs during the degeneration of tendon and therefore, is a precursor to injury.
The crimp pattern in tendon structure helps to impart elasticity to the tendon during the early stages of strain loading depicted as the ‘toe region’ in Figure 5 (31).
Figure 5: The mechanical deformation of tendon.
Changes within tendon crimp may be associated with exercise, age and their regional location within the tendon itself. Such changes are indicative of micro-trauma and would be detrimental to tendon strength.
Whilst spontaneous sudden injuries do occur in tendon disease, there is evidence that such injuries are often preceded by degeneration of the tendon matrix prior to onset of clinical signs (40).
Racehorses that commence training younger have a decreased risk of developing a tendon injury. Extensive research indicates that appropriate exercise regimes prior to 2 years of age, which are not excessive, may help to improve the quality of tendon, thereby reducing the incidence of injury during ageing or subsequent athletic career.
Severity of tendon injury (tendonitis) increases with age (52).
Horses aged 4 and 5 years were more likely to incur injury to the SDFT than 2-year-olds (59).
The risk of SDF tendonitis in the forelimb is higher in 3 year olds, compared to 2 year olds (64).
It has been suggested that the reduced rate of tendon injuries in 2-year-old horses is because they have not accumulated sufficient exercise related micro-damage to result in tendon injury (57).
In a Hong Kong study, the age of horses that retired due to tendon injury was the highest within 3 year olds and risk appeared to decrease with age. However, this was a retrospective study reporting the reason for retirement rather than the prevalence of injury, and therefore it is plausible that the apparent reduction in the number of older horses retiring of tendon injury, is due to their retirement being recorded as poor performance or age (57).
Research has reported exercise increases the cross sectional area (CSA) of tendon during the growth phase:
It appears that while tendon can adapt to the mechanical forces applied to it during growth, there is a lack of evidence suggesting that it can do so after skeletal maturity (approx. 2 years of age). Instead, cumulative fatigue damage causes degeneration, potentially weakening it and increasing the risk of clinical injury (24).
There has been an abundance of research to investigate the impact of exercise on young Thoroughbreds and research has reported:-
Associations between injury and training/ racing surfaces have been found.
Due to fatigue and lack of adequate training for race level:
Currently, evidence is contradictory in this area with research reporting:-
|The risk of tendon injury increases as race distance increases.
The risk of SDF tendinopathy increased in TB racehorses with longer average distance per race (63).
The likelihood of SDF tendinopathy was found to be associated with increased race distance (51).
Increase in race distance allows more chance of injury due to both an increase in exposure time and an increase in the number of fences or hurdles encountered in longer races. Horses are also more likely to be fatigued in longer distance races (5).
It was reported that the likelihood of SDF tendinopathy increased with career length, hypothesised as being a result of increased time at risk of injury and potentially related to increased cumulative tendon pathology over a longer career (53) but it could also be due to the increasing age of the horse.
|Increased cumulative distance/ number of starts has a protective effect against injury.
Racing intensity (number of races run) seems to be a protective predictor of risk (47). The incidence rate of musculoskeletal injury decreased as the number of races increased, suggesting the more exercise horses accumulate, the less likely they are to sustain injuries (47).
The number of races run in the career of each horse was found to be negatively associated with the risk of tendon or suspensory ligament injury suggesting a protective effect when running more races. It was suggested this is due to musculoskeletal tissue remodelling, leading to tendons and ligaments which are more resistant to damage, or may simply be an example of the ‘healthy horse’ effect (50).
Incidence of injury to the Superficial Digital Flexor Tendon (SDFT) or Suspensory Apparatus was not associated with the cumulative distance raced in the last 30 days of a training preparation (59).
Notably the “healthy horse” effect needs to be taken into account when interpreting this
– injury free horses (which may be related to their anatomy or physiology) can run more times than injured ones!
Uphill work increases exercise intensity without increasing force in the SDFT (4).
However, the force patterns were altered (26) therefore, research supports forces may decrease during uphill work but changes are only significant at faster paces.
Those horses with higher official ratings are less likely to incur a tendon injury
Figure 6: The risk of SDFD increases with
increasing fetlock angle (long feet and low heels)
(Source: ANVIL Magazine, 1998)
Figure 7: Horses with Knock-kneed conformation
are at a higher risk of SDFD injury
(Source: Equine Medical Center of Ocal, 2015)
The prevalence of tendon injury was higher in 2 year old entire males (8.1%) than fillies (4.1%) (52).
In a New Zealand study, male horses had a higher risk of injury to the suspensory apparatus and the SDFT than female horses (59).
In a Japanese study, the risk of SDFT increased in male horses compared to females (63).
Another Japanese study concluded that entire males were at greater risk of SDFT and suspensory ligament overstrain in comparison to females and geldings in flat racing (64).
A higher proportion of entire males retired with tendon injuries (18.5%) compared with geldings (13.4%) and females (11.1%) in a Hong Kong Study (57).
Compared with females that had never competed in steeplechase races, males regardless of steeplechase race history and females that had competed in steeplechase races had higher odds of SDFT injury (63).
In contrast a UK study of training yards failed to identify a significant difference in frequency of tendon or ligament injury between male and females. (54).
External protection against over extension
There is some evidence that a contoured palmar splint/ palmar tendon support boot can resist fetlock joint extension and thus, support the deep digital flexor tendon (DDFT) and suspensory ligament (SL) (25).
However, in that paper they advise consideration must be given to balancing this protective benefit with the detrimental accumulation of heat, as increased temperature within the tendon may increase the denaturing of tendon structure and thus, dissipation of heat should be permitted where possible.
Taller horses have an increased risk of tendon injury
A study of training data of racehorses who had retired from racing due to tendon injury, highlighted the importance of training data when evaluating injury risk and concluded that its absence may lead to incorrect assumptions due to key pieces of the jigsaw being missing.
However, currently changes in training intensity and findings of previous clinical examinations could be used to identify horses at risk of tendon injury (55).
Although extensive research has taken place improving our understanding of tendons, their structure and function, further advancement is needed.
Researchers have tried to identify biological markers that could indicate when a horse is at an increased risk of tendon injury. So far they have had mixed success.
Going inconsistencies whilst travelling around the same track would be beneficial to understand i.e. whether travelling from soft, to hard, to soft effects injury risk.
Access to training data and medical history may improve the quality of research undertaken and therefore, further advance our understanding of the degenerati6ve changes that occur prior to injury and in turn, identify horses at risk.