Case reveal » Veterinary Diagnostic Imaging » College of Veterinary Medicine

Findings:

Within the lateral solar margin of the distal phalanx at the level of the location of the metallic probe, there are two small, well-defined, rounded to mildly angular mineral opaque bodies that are mildly laterally displaced and overlying a small concave defect along the margin of the distal phalanx at this level (circle).

Adjacent to those osseous bodies, a few poorly defined punctate mineral opacities are seen.
At the dorsoproximal aspect of the distal phalanx, a small osteophyte is present.

Lateral penetrating solar injury into the distal phalanx with secondary left distal phalangeal marginal fractures (Type VI), secondary to the reported penetrating nail injury.
Multifocal pinpoint metallic debris adjacent to osseous fragments, likely representing rust/fragments from the nail
Mild left distal interphalangeal joint osteoarthrosis.

In the lateral solar margin of the distal phalanx, at the level of the previously described lucent tract and small fragments, a large non-displaced fragment (asterisk) with a large peripheral zone of osteolysis (blue arrows) is identified.

Medial to it, a much smaller, ill-defined, and oval osseous fragment is noted.
Peripherally within the distal phalanx, there is mild to moderate widening of the vascular channels.

Third phalanx sequestrum formation secondary to the prior penetrating trauma with suspected concurrent septic osteitis.

The presence of chronic lateral solar fractures of the left distal interphalangeal joint (type VI) with secondary sequestrum and draining tract formation was confirmed. No joint involvement was noted.

Aerobic culture of the draining tract: Moderate growth of Escherichia coli, Enterococcus faecium and Providencia rettgeri.
A sequestrectomy was performed following the CT. Post-op radiograph confirming the successful removal of the sequestrum:

Type VI fractures are fractures involving the solar margin of the distal phalanx. They may be secondary to trauma of other underlying pathology, such as laminitis (Nixon et al., Baird et al.).

Sequestrum formation is secondary to disruption of the blood supply to a bone fragment, resulting in necrosis and subsequent bacterial infection of the ischemic bone.
NOTE: Inflammation and infection is referred to as osteitis rather than osteomyelitis because the distal phalanx does not contain a medullary cavity (Nixon et al.).
Radiographically, the sequestrum (e.g. dead bone) is surrounded by a radiolucent zone of granulation tissue, which is in turn surrounded by a radiodense involucrum (Baird et al., Nixon et al.).

The healthy bone near the sequestrum may respond by producing periosteal proliferation; however, this may be less likely observed in horses with sequestrums to the distal phalanx due to the tight association of the lamina with the periosteum on the distal phalanx (Baird et al.).
Sequestrum in horses usually occur secondary to a wound to the distal limb, which disrupts periosteal blood flow (Baird et al.). They may also occur secondary to septic and nonseptic causes of pedal osteitis, such as hematogenous dissemination of bacteria (foals), hoof wall avulsion or laminitis (Nixon et al., Neil KM et al.).
A sequestrectomy with establishment of drainage of purulent material is the preferred treatment of choice with notable immediate improvements in soundness reported (Baird et al.).

Baird AM, Seahorn TL, Morris EL. Equine distal phalangeal sequestration. Veterinary Radiology. 1990 Jul;31(4):210-3.
Cauvin ER, Munroe GA. Septic osteitis of the distal phalanx: findings and surgical treatment in 18 cases. Equine veterinary journal. 1998 Nov;30(6):512-9.
Neil KM, Axon JE, Todhunter PG, Adams PL, Caron JP, Adkins AR. Septic osteitis of the distal phalanx in foals: 22 cases (1995–2002). Journal of the American Veterinary Medical Association. 2007 Jun 1;230(11):1683-90.
Nixon AJ, Ducharme NG, Bertone AL. Fractures of the distal phalanx. Equine fracture repair. 2019 Oct 28:219-41.