EQUUS - - Conformation Insights -

What used to be called nav­ic­u­lar “dis­ease” is now known to be an ar­ray of dif­fer­ent prob­lems which cause pain in the pos­te­rior part of the hoof cap­sule. “Nav­ic­u­lar” gets its name be­cause patho­log­i­cal changes to the shape of the nav­ic­u­lar bone—ac­tu­ally a sesamoid—are usu­ally prom­i­nent. Prickly look­ing ex­os­to­sis of the front and rear as­pects of the nav­ic­u­lar sesamoid is char­ac­ter­is­tic. Ex­os­to­sis im­pinges the ten­don of the deep dig­i­tal flexor mus­cle where it passes over the nav­ic­u­lar bone to its in­ser­tion upon the semilu­nar crest on the un­der­side of the cof­fin bone. The ten­don rub­bing on bony prick­les cre­ates in­tense “is­chemic” pain—pain that an ex­pe­ri­enced vet­eri­nar­ian once de­fined for me as “pain out of all pro­por­tion to the size of the le­sion.”

Rolf is the only horse among the four an­a­lyzed here whose skele­ton shows ev­i­dence of nav­ic­u­lar syn­drome; Ethan Allen’s nav­ic­u­lar bones are pre­sented for com­par­i­son as he is en­tirely free of “nav­ic­u­lar.”

Prickly ex­os­to­sis as­so­ci­ated with “nav­ic­u­lar dis­ease” (yel­low ar­rows) is ev­i­dent on joint sur­face of the sesamoid (A) and the volar sur­face over which the ten­don of the deep dig­i­tal flexor mus­cle passes (B).

Close-ups of Ethan Allen's nav­ic­u­lar show no ev­i­dence of pathol­ogy. The rough ap­pear­ance of some of the edges is due to post­mortem rot­ting, not dis­ease.

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