Cover image courtesy of Scone Equine Hospital

Over the past few decades, the thoroughbred industry has observed widespread improvements in the prevention and diagnosis of skeletal injuries thanks to significant advancements in veterinary imaging technology.

Not only are radiographs a vital tool in the routine management of equine athletes, they also have considerable influence on the global thoroughbred sales market. The ongoing debate regarding the controversy surrounding presales radiographs gained another level of complexity last month following an extensive study in regards to the significance of stifle lesions in horses.

The paper titled ‘Subchondral lucencies of the medial femoral condyle in yearling and 2-year-old Thoroughbred sales horses: Prevalence, progression and associations with racing performance’ is the most extensive piece of published research of its kind to date. Co-authors Frances J Peat, Christopher E Kawcak, C Wayne McIlwraith, David P Keenan, Jeffrey T Berk and Daniel S Mork conducted extensive research with the aim of uncovering the significance of subchondral lucencies (SCLs) in the distal aspect of the medial femoral condyle (MFC) (stifle) of young thoroughbred horses.

Subchondral means area beneath the cartilage. While lucency is a decrease in bone density compared to the surrounding bone. The bone supports the cartilage and a decrease in the bone density beneath the cartilage can lead to weaknesses and the development of bone cysts causing lameness in some horses. The significance of such lucencies in the subchondral bone of the stifle is a widely discussed topic but limited scientific evidence regarding the progression and impact on racing careers of thoroughbreds has not been known.

The study involved a sales-based sample of over 2500 yearlings offered at the 2016 Keeneland September Yearling Sale and over 430 from various North American 2-year-old breeze-up sales. The fact that over 70 per cent of consignors authorised researchers to access repository radiographs of both yearlings and 2-year-olds reflects the appetite within the industry to gain evidence regarding the significance of MFC SCLs in sale horses. While the study predominantly focused on the outcomes of stifle lucencies in regards to prospects for racing, it also uncovered some food-for-thought in regards to their significance on the pinhooking market.

The marketplace’s general hesitation around such lesions has many consignors reluctant to offer prospects with blemishes that may be deemed ‘unacceptable’ in the sales ring.

Four veterinarians analysed the sales radiographs of the sample size and applied a rating to sample images using a scale of 0 to 3. Given the reluctance of consignors to offer youngstock with radiographic blemishes, it was unsurprising that over 90 per cent of all yearlings analysed were assessed as having grade 0 MFC SCLs.

Table: Distribution of MFC SCL grades at 2-year-old sales relative to the respective MFC SCL grade at yearling sale

While only 45 of the yearlings analysed were offered as 2-year-olds, the numbers revealed that the overwhelming majority of yearlings with grade 1 MFC SCLs remained unchanged or resolved (80 per cent) by the time they were offered as 2-year-olds.

Sixty per cent of the yearlings noted as having grade 2 MFC SCLs as yearlings remained unchanged by their respective 2-year-old sale (six to nine month period), while 20 per cent improved to a grade 1 and a further 20 per cent deteriorated to a grade 3. Of the yearlings with grade 3 lesions, two (66 per cent) improved to a grade 2 while one remained unchanged (33 per cent).

While it is a relatively small sample size, there is certainly enough evidence to suggest that MFC SCLs do not generally deteriorate from yearling age through to their 2-year-old year.

Graph: Proportion of yearlings starting in at least one race by their 4-year-old season grouped by MFC SCL grade at yearling sale time

In regard to the prospects for racing, 85 per cent of all sampled yearlings started in at least one race while 15 per cent had not made their debut by the end of their 4-year-old season. While the likelihood of yearlings with grade 3 MFC SCLs making a race start was slightly less (78 per cent) than that of yearlings with a MFC SCL grade 0-2 (84–91 per cent) starting in a race, there was no statistical difference in the probability of starting in a race for any category of MFC SCL.

Of all yearlings represented, those with grade 2 MFC SCLs were the most likely of any category to start in a race (91 per cent). Furthermore, all five horses with grade 3 MFC SCLs at 2-year-old sales started at least once and had an average total of 15 career starts.

While it is important to note that in order to draw significant conclusions, research would need to be conducted across a larger sample size given that this particular study focused on just over 10 per cent of the entire North American crop for that year.

Dr David Garth of Sydney Equine wasn’t particularly surprised with the outcomes of the study pointing out that “the findings of the study confirm that we are often over cautious around such issues”.

From an end-user perspective Garth emphasised that “while the study points to a negligible reduction in likelihood of racing for yearlings with grade 3 MFC SCLs, it is essential that veterinarians communicate the potential risk of such horses with prospective clients”.

At the end of the day, the thoroughbred industry is shaped around one purpose and that is to produce horses fit for our end product which is racing. While buyers are both entitled and encouraged to conduct their due diligence in assessing prospective purchases, determining what is significant in the context of the overall objective of purchasing thoroughbreds to race is a very grey area.

This research revealed that the presence of a grade 3 MFC SCL increased the probability of a horse not making it to the races from a one in seven chance to a one in five chance of not racing.

Most people make decisions based on pre-existing habits and the likelihood of a buyer purchasing a horse with blemishes will boil down to their risk-tolerance. If an end user takes a punt on purchasing a yearling with a grade 3 MFC SCL and such yearling turns out to be a Group 1 winner, their tolerance for such blemishes will generally be far higher than the average buyer.

While more objective data is required to provide consignors, veterinarians and buyers with greater evidence-based research regarding the significance of certain radiological findings, the study has inspired plenty of discussion among industry professionals. We encourage readers to actively reach out with their feedback, thoughts and opinions in this thought-provoking debate.