Juvenile Bowed Tendons and Racing Prognosis

“Juvenile bowed tendons, or ‘baby bows,’ are not uncommon in yearlings and weanlings,” said Johanna Reimer, VMD, Dipl. ACVIM, Dipl. ACVC (cardiology), of the Rood and Riddle Equine Hospital in Lexington, Ky., at the 2002 American Association of Equine Practitioners convention. In her presentation “Enlarged Superficial Digital Flexor Tendons in Immature Thoroughbred Horses: Prognosis for Racing,” she discussed her seven-year field study of Thoroughbred yearlings with visually enlarged superficial digital flexor tendons. Her goal was to define the prognosis for racing for immature, untrained Thoroughbreds with this problem.

These horses were presented for examination with enlargement of one or both forelimb superficial digital flexor tendons, generally with a homogenous (uniform) appearance on ultrasound examination. Reimer calculated the cross-sectional area of each tendon at its widest part, and selected horses with a cross-sectional area of 1.7 cm2 or greater in either or both forelimbs for the study. (She noted that a cross-sectional area of 0.8-0.9 cm2 is generally normal for a superficial digital flexor tendon in these young Thoroughbreds.) The distribution of enlarged superficial digital flexor tendons between right, left, and both forelimbs was fairly even.

For a control group to compare with the study group, she took the race records of the selected horses’ maternal siblings from The Jockey Club Information System. The likelihood of racing, age at first start, and earnings were evaluated for both groups.

There was no significant difference in likelihood of racing at all or racing at two, three, or four years of age compared to the control group. There was also no difference in average earnings per start between the two groups.

Implications and Management Recommendations

Reimer concluded that the prognosis for racing for immature horses with thickened superficial digital flexor tendons appears to be favorable based on this research. However, she cautioned that this field study had limitations in that she had no knowledge of whether the siblings used as controls were unaffected by enlarged superficial digital flexor tendons. Also, she suggested that the use of the subjects (pinhooking vs. racing, for example) might have been dictated by the enlarged tendon(s), as well as training regimens. In other words, the subjects could have been managed differently because of the enlarged superficial digital flexor tendon(s), making direct comparison between groups possibly erroneous.

The cause of these enlarged superficial digital flexor tendons is unknown. “It is possible it is a normal developmental phenomenon, or a result of pathology and faulty conformation,” Reimer said. Although all of the subjects were sale prospects which had been carefully scrutinized for conformation and pedigree (thus ruling out severe conformational defects), she added, “Histopathologic evaluation of affected superficial digital flexor tendons may shed light upon the underlying mechanism.”

The onset of this problem appears to be insidious (developing slowly and thus avoiding detection until it is fairly well developed). Reimer suggested that increased use of walkers to exercise and “muscle up” these youngsters intended for sale might increase the incidence of this problem, citing an increased number of these cases in the past year in her practice.

Her suggestions for avoiding and managing enlarged superficial digital flexor tendons in young horses are as follows:

  • Avoid heavy preparation of affected horses for 2-year-old in training sales. This can aggravate the problem.
  • Don’t anticipate heavy 2-year-old racing for affected horses.
  • Monitor affected horses’ superficial digital flexor tendons carefully during training and modify the training regimen if the problem appears to be getting worse.

About the Author

Christy M. West

Christy West has a BS in Equine Science from the University of Kentucky, and an MS in Agricultural Journalism from the University of Wisconsin-Madison.

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