SUR­PRIS­ING FIND­INGS ABOUT RARE NAV­IC­U­LAR CON­DI­TION

EQUUS - - Eq Medicalfro­nt -

A new study sug­gests that a rare mal­for­ma­tion of the nav­ic­u­lar bone that of­ten re­sults in chronic lame­ness is con­gen­i­tal rather than re­lated to wear or in­jury.

Re­searchers from Utrecht Uni­ver­sity in the Nether­lands and Ghent Uni­ver­sity in Bel­gium re­cently pub­lished a paper doc­u­ment­ing three cases of nav­ic­u­lar bone par­ti­tions---mean­ing the bone is com­posed of two (bi­par­tite) or three (tri­par­tite) sec­tions in­stead of a nor­mal, sin­gle struc­ture. These sec­tions are de­lin­eated by de­fects in the bone, cov­ered by smooth car­ti­lage, and range from shal­low dents to full-depth sep­a­ra­tions con­nected only by fi­brous tis­sue. In each case, the un­usual for­ma­tion was dis­cov­ered through ra­dio­graphs taken to in­ves­ti­gate chronic lame­ness.

These ab­nor­mal­i­ties are of­ten mis­taken for frac­tures or other in­juries, but they would ac­tu­ally be present at birth, says Willem Back, DVM, PhD, DECVS. “At around 100 days of preg­nancy the nav­ic­u­lar bone is still made up of car­ti­lage, while at 3 to 4 months of age, the bone starts to be­come fully os­si­fied,” he says. “We think that de­gree of lame­ness.

“We think these horses de­velop lame­ness when there are more struc­tural, de­gen­er­a­tive changes be­com­ing ap­par­ent in the bone,” says Back. “In cases of a more ad­vanced cyst for­ma­tion, most likely this will re­sult in a chronic lame­ness.”

He adds, how­ever, that if the con­di­tion is dis­cov­ered be­fore the horse be­comes lame, there are ways to try to pre­serve sound­ness. “A con­ser­va­tive ther­apy can be ini­ti­ated us­ing NSAIDs , cor­rec­tive far­ri­ery and a restricted ex­er­cise reg­i­men,” Back says, “but ob­vi­ously still with a poor prog­no­sis. A sur­gi­cal in­ter­ven­tion, such as a neurec­tomy , is not ad­vis­able, as this would even ac­cel­er­ate the cyst for­ma­tion and in­crease the chance of the horse de­vel­op­ing even a patho­log­i­cal nav­ic­u­lar bone frac­ture.” Both the left (A1) and right (B1) front nav­ic­u­lar bones of this horse have an ab­nor­mal tri­par­tite struc­ture, with bone sec­tions con­nected by fi­brous tis­sue. This struc­ture is clearly seen when a cross section of the bone is viewed from above (A2 and B2).

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