Pain originating in the upper cannon bone area, just below the knee or hock, is common in all types of equine athletes. However, it can be difficult to determine exactly what structure is injured; some injuries can only be seen with high-field MRI. At the 2007 American Association of Equine Practitioners Convention, held Dec. 1-5 in Orlando, Fla., Matthew Brokken, DVM, clinical assistant professor of veterinary clinical sciences at the University of Florida, described a Washington State University study of lameness originating in this area.

"Detection of abnormalities, specifically desmitis (ligament inflammation) of the inferior check ligament (ICL) and proximal suspensory ligament (PSL), has not been possible in some horses (via radiography or X rays, ultrasound, or nuclear scintigraphy)," he explained.

For the retrospective study, 45 horses with lameness localized to the proximal metacarpus/metatarsus (upper cannon bone area) were selected. Most were Warmbloods (20) and Thoroughbreds (12), and the horses ranged from 3 to 17 years of age. All were lame; 22 had been lame for less than four months, 22 for longer, and one had been lame for an unknown time. Forelimb lameness affected 29 horses. Thirty-one horses were grade 3/5 lame, nine were grade 2 lame, and one was grade 4. All were imaged with a high-field (1.0-tesla) MRI system.

"The use of MRI in this area allowed an accurate diagnosis in 44 of 45 horses," Brokken reported. "The transverse proton density sequence was the most helpful in detecting subtle changes in signal and size within the ICL and PSL. Having an accurate diagnosis enabled appropriate treatment in these horses, which helped a high percentage of them to return to their intended use."

Researchers found that 23 horses had desmitis in the upper 4 cm of the PSL--13 had desmitis in hindlimbs and 10 in forelimbs. Sixteen horses had ICL desmitis; 12 had lesions from 1-4 cm below the upper end of the cannon bone and four had them 5-10 cm down. Of the remaining six horses, one had ICL and PSL desmitis in the same limb, one had desmitis in both ligaments but on different limbs, one had an injury to the cannon bone, one had effusion (fluid swelling) of the distal tarsal sheath of the deep digital flexor tendon (DDFT), one had a DDFT injury, and one had no obvious abnormalities.

"It is interesting to note that 59% of horses that had lameness localized to the proximal metacarpal region (forelimb) had ICL damage," noted Brokken. "Previously it was thought that horses that block to the proximal metacarpal region usually have a proximal suspensory injury," but these results prove that this isn't always the case.

Treatments ranged from a six-month rest and rehabilitation program (33 horses) to surgery, injections, and various combinations thereof. Forty-three of the horses were available for followup:

  • Eight of 10 horses (80%) with forelimb PSL desmitis returned to full work.
  • Nine of 13 horses (69%) with hindlimb PSL desmitis returned to full work.
  • Ten of 16 horses (63%) with ICL desmitis returned to full work. Five of eight horses that had ICL desmotomy (complete cutting of this ligament) returned to full work. "The success of ICL desmotomy in these horses further supports the importance of making an accurate diagnosis, because (in) horses with ICL desmitis you have a surgical option that is not appropriate for horses with PSL desmitis," Brokken noted.
  • The two horses with both ICL and PSL desmitis returned to full work, as did the horses with distal tarsal sheath effusion and DDFT injury.
  • Eight horses had ultrasound evaluations, but ultrasound findings were unremarkable or misinterpreted as PSL injury when ICL injury was detected instead by MRI. "Ultrasonographic examination of the PSL is technically challenging, especially in the hind limbs," Brokken commented.

"In conclusion, lameness localized to the proximal metacarpal and metatarsal region has been a diagnostic challenge," Brokken concluded. "Traditional imaging modalities don't always yield an accurate diagnosis. High field strength MRI should be considered in performance horses with lameness localized in the proximal metacarpal and metatarsal areas."

For more information, see "Magnetic Resonance Imaging Features of Proximal Metacarpal and Metatarsal Injuries in the Horse," Veterinary Radiology and Ultrasound, Vol. 48, No. 6, 2007, pp 507-517.

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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