When new bone forms within long bones, enostosislike lesions (ELLs) can develop–an uncommon but concerning problem. Simply put, ELLs are bony growths within the cavity of the bone. At present, the exact cause of this phenomenon is unknown but veterinarians can diagnose it using nuclear scintigraphy (bone scan): an ELL is visible as one or more focal areas of radiopharmaceutical uptake in the medullary cavity (the innermost part) of one or more long bones. These focal areas are sometimes referred to as "hot spots" on bone scans. At the 2010 American Association of Equine Practitioners Convention, held Dec. 4-8 in Baltimore, Md., Benjamin Ahern, BVSc, a resident in large animal surgery at the University of Pennsylvania, presented a retrospective study of 79 horses affected with ELLs over a period from 1997 to 2009.

Of nearly 5,000 scintigraphic studies performed at the University of Pennsylvania’s George. D. Widener Hospital for Large Animals during the 12-year period, veterinarians identified 85 cases of enostosislike lesions (1.7%) in 79 horses at 157 sites; four of the 79 horses had multiple, separate ELL bouts. Slightly more than half the 79 horses had a single ELL, 28% had two lesions, and 21% had three or more within a single scintigraphic study. A single ELL site didn’t significantly affect a horse’s racing career, but Ahern commented that racing careers were shortened in horses diagnosed with two or more lesions. On discharge from the clinic following diagnosis, veterinarians recommended rest for a 12-week period for racing Thoroughbreds and eight weeks for Warmbloods or nonracing breeds. This recommendation varied depending on severity of lameness.

Interestingly, Ahern noted a marked increase in ELL occurrence in 2008 and 2009, yet ELL incidence in 2010 returned to pre-2008 levels. Ahern said attempts were made to correlate the increased incidence with hyperbaric treatment, various track surfaces, or corticosteroid use, but they weren’t able to make any conclusive associations.

Enostosislike lesions were identified more often in Thoroughbred horses than other breeds, and older horses (of any breed) were more likely affected. ELLs occurred most frequently in the tibia (the bone above the hock) or the radius (the bone above the knee). However, lameness, which occurred in nearly half of affected horses, was most often related to lesions in the humerus (shoulder bone) or femur (the long upper bone of the hind leg, above the tibia). It is noteworthy that, statistically speaking, a horse’s lameness score correlated with the intensity of radiopharmaceutical uptake on scintigraphy. Ahern stressed the importance of differentiating ELL from stress fractures due to differences in management strategies