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Equine Case of the Month, June 2022

A 6 year old Thoroughbred mare was presented for routine foot balance radiographs of the right hind pastern region. The mare had sustained a wound to the pastern region 5 years previously. No lameness was observed at the time of presentation.

At the dorsal and lateral aspects of the proximal interphalangeal joint there is marked, bridging periarticular new bone (yellow arrows). There is a well-defined radiolucent line which passes through this, which appears to be a direct continuation of the joint space.

The proximal interphalangeal joint space is markedly reduced, most markedly laterally (red arrows), with almost complete loss of the joint space at the centre of the lateral condyle of the proximal phalanx. There is moderate surrounding sclerosis with several well-defined regions of lucency within the lateral glenoid of the middle phalanx and the lateral condyle of the proximal phalanx. The medial joint space is also narrowed, but to a lesser extent. There are focal regions of lucency medially, but again to a lesser extent than laterally.

There is subtle lateral displacement of the middle phalanx relative to the proximal phalanx. The long axis of the proximal phalanx is deviated laterally towards the fetlock.

Right Hind Foot LateralRight Hind Foot DP


  • Marked proximal interphalangeal joint osteoarthropathy, with marked reduction in joint space laterally and suspicion of partial ankylosis.
  • Marked lateral new bone formation, which may indicate disruption of the lateral collateral ligament of the proximal interphalangeal joint. There is mild lateral displacement of the middle phalanx
  • Unusual angulation of the proximal phalanx – ddx secondary to joint collapse and lateral collateral ligament injury or acquired angular limb deformity.

Despite the severe radiographic changes present in the proximal interphalangeal joint, the mare is not lame. This may be due to the partial ankylosis of the joint. 

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