Stifle Abnormalities in Young Thoroughbreds

Radiographic abnormalities of the MFC occur in 42% of young Thoroughbred horses and can appear as early as 6 months of age.

Photo: Elizabeth Santschi, DVM, Dipl. ACVS

Thoroughbreds intended for sale or racing are under great radiographic scrutiny. Veterinarians typically X ray their joints as weanlings, yearlings, and anytime they’re entered in a sale.

“A horse sold as weanling, yearling, and 2-year-old in training would likely be radiographed at least four times,” in his first two years of life, said Elizabeth Santschi, DVM, Dipl. ACVS, professor of equine surgery at Kansas State University’s College of Veterinary Medicine, in Manhattan.

This wealth of images has provided researchers with the ability to track radiographic abnormalities (RAs) and their progress in this population of young horses over time. One common finding are abnormalities of the stifle’s medial femoral condyle (MFC).

Santschi recently studied these abnormalities and their prevalence in young Thoroughbreds and presented her findings at the 2016 American Association of Equine Practitioners Convention, held Dec. 3-7, in Orlando, Florida.

The RAs veterinarians typically note in the MFC include subchondral lucencies (SCLs, or areas that appear less “white” than the surrounding structures on X ray, which is indicative of bone destruction), bone cysts, and sclerosis (formation of new or excessive bone, often in response to continued trauma/stress).

“MFC RAs do not always cause lameness, and follow-up of sales horses provides contradictory evidence of an effect on performance,” said Santschi.

Regardless, these findings can have a significant negative effect on horses’ sale prices. “Understanding the progression of MCF RAs may provide strategies to better manage the condition or reduce the occurrence,” she said.

Santschi’s goals for this study were threefold:

  1. Describe MFA RA progression in young horses;
  2. Determine MFC RA prevalence in Thoroughbreds younger than 1 year; and
  3. Determine the prevalence of MFC changes up to 20 months of age.

Hence, her team examined stifle radiographs from three groups of Thoroughbreds.

Group 1 These 11 horses were born in 2013 or 2014, had their first radiographs taken before they were 1 year of age, and had at least three sets of radiographs taken at 60-day intervals. Santschi and her colleagues noted whether the appearance of each horse’s MFC improved, worsened, or remained the same as radiographs taken 120 and 240 days later. Santschi said they found that:

  • On the first set of radiographs, four horses had unilateral (only affecting one limb) RAs and seven had bilateral (both limbs) RAs;
  • At 120 days, four horses showed improvement, five had worsened, and two remained the same; - In nine horses radiographed at 240 days, five improved, three had worsened, and one was the same; and
  • The improved horses all had subsequent healing of lucencies, the worsened ones had enlarged or new lucencies, and the same horse had no change to a shallow lucency.

Group 2 These 638 Thoroughbreds were all younger than 1 year, with an average age of 239 days. Upon evaluating their radiographs, Santschi said 268 (41.8%) had RAs, 106 (39.7%) of which were bilateral. Of the 162 unilaterally affected horses, 108 (67%) were right MFC abnormalities and 54 (33%) were on the left, she said.

“Considering all MFCs, there were significantly more abnormal grades in right MFCs (41.8%) than left (24%),” she added.

Group 3 For this group, the researchers obtained a second set of stifle radiographs from 188 of the Group 2 horses at an average age of 529 days. Of these, 82 horses (43.6%) had a MFC RA, 46 (57.3%) of which were affected bilaterally. Of the 36 unilaterally affected horses, 25 (69%) were right MFC RAs and 11 (31%) were in the left leg. Comparing the two sets of stifle radiographs, 85% of horses with normal MFC at set 1 had a normal grade at set 2, said Santschi.

“However, only 37% of horses with a lucency at set 1 had a lucency at set 2, and 63% healed,” she said. “Horses that healed the lucency between stifle radiograph sets were younger at set 1 than those that retained the lucency.”

Gender did not influence any group’s findings, she added, and the group does not yet know why there was a right-sided predilection for MFC RAs.

In summary, “Radiographic abnormalities of the MFC occur in 42% of young Thoroughbred horses and can appear as early as 6 months of age” said Santschi. “Most RA are minor, but 45% of MFC RAs change appearance—for better or worse—in young Thoroughbreds. MFC lucencies discovered in horses younger than 1 year were more likely to improve than if discovered later,” suggesting that early diagnosis could allow veterinarians and caretakers to implement management strategies, such as restricted exercise, that promote healing.

For people involved in buying and selling young stock, this report indicates that weanlings’ stifles are subject to a dynamic environment resulting in growth, adaption, injury, and healing, said Sanstchi.

“If horses have a normal appearance of their MFC at about 8 months, most (85%) will remain normal,” she said. “If they have evidence of MFC injury, about 50% will improve, especially the younger weanlings, but the remainder will worsen. Breeders of young stock destined for high-level performance should consider monitoring stifles in their weanlings to promote early resolution of injury.”

About the Author

Alexandra Beckstett, The Horse Managing Editor

Alexandra Beckstett, Managing Editor of The Horse and a native of Houston, Texas, is a lifelong horse owner who has shown successfully on the national hunter/jumper circuit and dabbled in hunter breeding. After graduating from Duke University, she joined Blood-Horse Publications as Assistant Editor of its book division, Eclipse Press, before joining The Horse.

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