MRI for Diagnosing Sesamoidean Ligament Desmitis

The high degree of detail seen with MRI has made it possible for veterinarians to find equine injuries they've never seen before. One example of this--MRI evaluation of desmitis in the oblique and straight distal sesamoidean ligaments--was discussed at the 2007 American Association of Equine Practitioners Convention, held Dec. 1-5 in Orlando, Fla. Sarah Sampson, DVM, doctoral graduate student in equine surgery and MRI at Washington State University (WSU), discussed the anatomy of these ligaments and a study of findings and treatment success in 27 horses.

"The distal sesamoidean ligaments include four ligaments (straight, paired oblique, paired cruciate, and paired short distal sesamoidean ligaments) that comprise the distal suspensory apparatus," she explained. "That apparatus resists extension of the fetlock and holds the suspensory bones to the first and second phalanges. The distal sesamoidean ligaments are susceptible to overload injury from hyperextension.

"Initially, diagnosis of desmitis in these ligaments relied on observations of swelling over the palmaroproximal (upper rear) aspect of the digit or on ruling out other causes of lameness in the area with radiographs," she noted. Ultrasound has been helpful in some cases, but she said this area is hard to evaluate with ultrasound.

However, MRI was found to be a very good way to identify these injuries, much more so than ultrasound or swelling. After describing the anatomy and imaging of these ligaments in great detail, Sampson described a retrospective study of affected horses. Of the 27 horses, most (15, 55.6%) were jumpers or dressage horses, and the age range of the whole group was 2 to 13 years. All were lame--six for less than four months, 19 for longer, and two for an unknown length of time. Seventeen horses had hind limb lameness (left, right, or bilateral), while the rest had forelimb lameness.

MRI confirmed injuries of the oblique (ODSL) or straight distal sesamoidean ligament (SDSL) in all study horses, but only one horse had palpable swelling in the distal sesamoidean ligament region. Only two horses' injuries were detected with ultrasound by referring practitioners (four horses were reevaluated with ultrasound at WSU).

Most of the horses (24/27, 89%) had injuries of the ODSL, and these were more frequently in the hind limb (16/24, 67%). The total number of medial (inside) and lateral (outside) branch injuries was about the same overall, but forelimbs tended to have medial branch injuries (87% of forelimb injuries) and hind limbs tended to have lateral branch injuries (71%).

Sampson noted that blocking the digital flexor tendon sheath eliminated most lameness due to distal sesamoidean ligament injury after 30 minutes. Thus, this injury should be considered when this block alleviates lameness.

Returning to Work

The primary treatment for all cases was six months of rest and a rehabilitation program. Twenty-two horses had initial hyaluronic acid and methylprednisolone acetate injections into the digital flexor tendon sheath to combat inflammation, swelling, and fibrous tissue formation. Two horses also underwent ligament splitting.

All six horses with short-term lameness (less than four months' duration) were sound at follow-up (one to three years after diagnosis). Eight of 12 horses (67%) with lameness for more than four months at the time of diagnosis were sound. Ten horses had mild lesions, and four of the five available for follow-up (80%) were sound. Thirteen of 14 horses with moderate lesions were available for follow-up, and of those 10 (77%) were sound. Two of three horses with severe lesions were sound (67%). Finally, 16 of 21 horses (76%) that were available for follow-up were competing at the same or a higher level of performance than before the injury. The remaining five were lame and had been retired.

"The ability of most of these horses to return to athletic performance indicates that the prognosis is not as poor as previously reported," Sampson noted. "High-field strength MRI is capable of early diagnosis of distal sesamoidean ligament injury, and appropriate treatment soon after injury may provide a better prognosis for returning to performance.

"Injury to the ODSL or SDSL can occur without palpable abnormalities in the region," she concluded. "These injuries do not need to be severe to cause performance-limiting lameness in athletic horses, and they should be considered in the differential diagnosis when examining horses with lameness that blocks out in the area of the pastern or metacarpophalangeal/metatarsophalangeal region. High-field strength MRI is a valuable way to evaluate ODSL and SDSL desmitis."

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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