Acute severe lameness associated with proliferative bone on the dorsal middle phalanx (19 cases)

Summary Background Injury resulting in proliferative new bone formation on the dorsal aspect of the middle phalanx (P2) has been recognised by equine veterinarians and is generally referred to as capsulitis. However, no retrospective case series has been published on the condition. Objectives To describe the clinical features, treatment, and outcome of this condition. Study design Case series ( n = 19). Methods Clinical cases in which the horse presented for lameness and was diagnosed with acute formation of proliferative bone on the dorsal P2 diaphysis were included in the study (cases collected from 2005 to 2022). Signalment, history, presenting clinical signs, diagnostics, treatments administered, and outcome were evaluated. Results Horses presented with unilateral lameness and were all classified as either a moderate lameness at a trot ( n = 6) or a moderate to severe lameness at a walk ( n = 13). A forelimb was involved in 79% of cases and a hindlimb in 21%. The presentation of lameness was recorded as acute in 15/19 horses and not reported in the remainder. Bone proliferation on the dorsal aspect of P2 was diagnosed on radiographic evaluation in all horses (as part of the inclusion criteria). In horses with sequential radiographs made from the time of acute presentation, bone production developed rapidly over a period of a few weeks to 2 months. A variety of therapeutic options were implemented, and 13/19 (68%) of the cases were subject to humane euthanasia, with only one horse returning to its previous athletic function. Milder cases were more likely to have a better outcome. Main limitations Small sample size and incomplete medical records. Conclusions Acute new bone proliferation on the dorsal aspect of P2 is associated with moderate to severe lameness and carries a poor prognosis for return to athletic function and survival.