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Findings

There is increased mineral opacity in the navicular trabecular bone and enlargement of several synovial invaginations in the distal border.
Along the periarticular margins of the proximal and distal interphalangeal joints, mild osteophyte formation is present.
The dorsal hoof wall is parallel to the dorsal aspect of the distal phalanx.
In the region of the medial articular margin of the distal phalanx, best seen on the dorsoproximal-palmarodistal oblique projections, a small focal region of decreased mineral opacity is surrounded by moderate sclerosis. There is a focal, concavity in the proximal articular surface of the distal phalanx, seen on the lateral projections.

Osseous cyst like lesion, medial articular margin of the distal phalanx., with mild distal interphalangeal arthrosis.
Mild navicular bone sclerosis and enlarged synovial invaginations.
Mild proximal interphalangeal osteoarthrosis.

Osseous-cyst like lesions- not true bone cyst since they lack an epithelial lining. Most have a fibrous lining and contain fibrous tissue, +/- synovial gelatinous material. May also be filled with fibrocartilage, occasionally partially mineralised and/or mixed with necrotic bone.
True bone cyst reported in horses include aneurysmal and unicameral cysts
The phalanges are the second most common location for osseous cyst-like lesions (most common is the medial femoral condyle)
Most often adjacent to joints, will sometime communicate with joint.

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Multiple etiologies reported:
- genetics, nutrition, rapid growth, hormonal changes and mineral imbalances, osteochondrosis, biomechanics and trauma
- Young horse with bilateral lesions –> likely dvelopmental
- Older athletic horse –> most likely due to trauma
Variable in size (small to large) and shape (dome, conical or spherical in shape).
May be surrounded by a thick sclerotic rim.
Osseous cyst-like lesions of the distal phalanx are reported to be more common in geldings/stallions. Possibly more common in Thoroughbred horses, although some reports also found a higher occurrence in warmblood horses.
Within the distal phalanx, OCLLs have been identified at the following sites:
- extensor process
- along the solar surface
- palmar processes (at the insertion of the collateral ligaments of the distal interphalangeal joint, more commonly medially)
- insertion of the distal
- sesamoidean impar ligament attachment
- within proximal weightbearing surface
Perineural analgesia can be used to localize the site of lameness. Intra-articular analgesia may resolve lameness if the lesion communicates with the joint.
Radiographic views to consider depending on location of lesion:
- weightbearing surface, insertion of the DIP joint collateral ligaments or at the insertion of the DSIL
  - –> dorsopalmar or dorsoproximal-palmarodistal oblique views
- extensor process
  - –> lateromedial views
Surgical debridement preferred due to the higher success rate (70–90%)

Sherlock, C., and T. Mair. “Osseous cyst‐like lesions/subchondral bone cysts of the phalanges.” Equine Veterinary Education 23.4 (2011): 191-204.
Mair, T. S., and C. E. Sherlock. “Osseous cyst‐like lesions in the feet of lame horses: diagnosis by standing low‐field magnetic resonance imaging.” Equine Veterinary Education20.1 (2008): 47-56.
Peter, Vanessa G., et al. “Radiographic identification of osseous cyst-like lesions in the distal phalanx in 22 lame thoroughbred horses managed conservatively and their racing performance.” Frontiers in veterinary science 5 (2018): 286.