These are fractures of the palmar/plantar eminences of the middle phalanx. Because they are uniaxial fractures, there is not significant instability in the palmar/plantar direction.
The horses present as acute onset, and severe lameness, not accompanied by instability. Localizing signs are minimal, but maybe include mild soft-tissue swelling and pain on direct palpation on the pastern region.
All breeds can be affected, but horses engaged in Western performance activities may be overrepresented.
Often times, localizing the source of lameness requires regional anesthesia of the digit.
Degenerative disease of the proximal interphalangeal joint as well as fractures of the middle phalanx are seen in most breeds of horses. However horses engaged in western performance activities seem predisposed to injuries involving the proximal interphalangeal joint, especially fractures of the middle phalanx, likely due to the bending and torsional forces generated during abrupt stops and changes in direction.
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Complete radiographic evaluation of the pastern region reveals an uniaxial avulsion fracture of the palmar/plantar eminence, most evident on the dorsopalmar and oblique views.