Stress Remodeling: Is Pasture Rest the Best Treatment?

Often experience tells us that we can't go wrong with pasture rest, when it comes to healing our horses. But could pasture rest alone be the ideal treatment for stress remodeling in horses' cannon bones? According to the results of a recent study by a team from Rood & Riddle Equine Hospital, in Lexington, Ky., turning a horse diagnosed with stress remodeling--a relatively common cause of lameness--out to pasture could allow him the best chance to return to his previous level of work.

Described by lead researcher Travis M. Tull, DVM, Dipl. ACVS, a "cumulative stress-induced bone injury" throughout the study, stress remodeling is a process in which a bone tries to adapt its structure when exercise demands exceed what it can handle. Such remodeling likely predisposes the horse to injury: "If the bone cannot adapt fast enough to the stress of training and racing, horses can develop microstructural or microscopic pathology, leading to lameness, condylar fracture, and fetlock arthritis," Tull explained.

Stress remodeling is often to blame for lameness and poor performance in Thoroughbred and Standardbred racehorses. Stress remodeling can affect other types of performance horses, however Tull noted that cannon bone involvement is not as common in disciplines besides racing.

Previous studies indicated that stress remodeling in the lower cannon bones yielded a poor prognosis, and only 40%-54% of case horses were able to return to their previous levels of work. These horses were treated with stall rest, and intra-articular (within the joint) and systemic medications (including hyaluronic acid and polysulfated glycosaminoglycan injections).

But Tull and colleague Larry Bramlage, DVM, MS, Dipl. ACVS, wanted to see if a different treatment method could yield better prognoses for horses with lower cannon bone stress remodeling. Specifically, they were interested in whether being turned out in a small pasture or paddock (preferably less than an acre in size, Tull said) with free-choice exercise could be the best treatment option.

The team reviewed the medical records of Thoroughbred racehorses with confirmed lower cannon bone stress remodeling that were treated at Rood & Riddle from January 2000 to January 2009. They narrowed the focus to 55 horses (41 males and 14 females, ages 2 to 6) that had been examined by Bramlage. These horses were diagnosed using a combination of nerve blocks, X rays, and nuclear scintigraphy examinations. Further, the team ensured that the horses had no other sources of lameness. Finally, the horses' recommended treatment was an immediate 60 days of pasture rest.

Fifty-three of 55 horses were immediately turned out to pasture for free-choice exercise followed by a re-examination, per veterinary recommendation. The team noted that three owners or trainers elected to keep three horses in light work programs instead of the free-choice exercise program, however two of the three were turned out after 30 days with no resolution of the issue.

The team obtained follow up information on all the horses in the study. Eighteen of the 55 were examined at Rood & Riddle, and the rest (37) were examined by the referring veterinarians. The examining veterinarians determined that the lameness and stress remodeling in the majority of the horses (N=44) had resolved after the 60-day turnout. The remaining horses were turned out for additional 30 to 90 days before resolution occurred.

Tull and Bramlage then examined the performance records of all 55 horses to determine if the horses were able to return to their previous level of work. They found that:

  • Fifty-two of 55 horses returned to racing after the stress remodeling and treatment;
  • The mean number of days from diagnosis to first race back was 230 (ranging from 98 to 719);
  • Forty-five of 55 raced both before and after injury and treatment;
  • Fourteen of 45 that raced both before and after advanced in racing class (transitioned from handicap races to stakes races, for example);
  • Fourteen of 45 remained at the same racing level; and
  • Seventeen of 45 had a decrease in racing class (transitioned from handicap to allowance, for example).

The team did note that for the horses returning to racing, there was no significant difference in total earnings before and after the injury. Additionally, the horses started significantly more races after injury (Tull noted this was likely due to the fact that the majority of the horses in the study were of a young age, and simply hadn't raced much yet). "The results of treatment surpass any previous reports in the number and quality of return to previous performance," Tull noted. "This would indicate that free-choice exercise in a paddock is a successful method of restoration of athletic soundness.

"Dr. Bramlage and I feel that this treatment is successful because repetitive loading is needed to induce remodeling, and by allowing the horses to control their own rate of exercise their distal cannon bones are encouraged to remodel and heal without resulting in further damage," Tull concluded.

The study, "Racing prognosis after cumulative stress-induced injury of the distal portion of the third metacarpal and third metatarsal bones in Thoroughbred racehorses: 55 cases (2000-2009)," was published in the May 15 Journal of the American Veterinary Medical Association. The abstract is available online.

About the Author

Erica Larson, News Editor

Erica Larson, news editor, holds a degree in journalism with an external specialty in equine science from Michigan State University in East Lansing. A Massachusetts native, she grew up in the saddle and has dabbled in a variety of disciplines including foxhunting, saddle seat, and mounted games. Currently, Erica competes in eventing with her OTTB, Dorado.

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