Rare equine hock variant documented in Veterinary Sciences case report
Bottom line
A new case report in Veterinary Sciences describes a rare anatomic variant in the equine hock: a separate os tarsale primum, rather than the more typical fusion of the first and second tarsal bones. The authors, D. Vladova, D. Kostov, and Hristo Hristov, examined 12 equine tarsal joints using gross anatomy and radiography and found six tarsal bones in 11 specimens, with one specimen showing an independent first tarsal bone. The report adds a documented example of a variation that older equine anatomy and radiology references describe as uncommon or sporadic. (ivis.org)
Why it matters: For equine veterinarians, the finding is a reminder that not every unusual structure on a hock radiograph is pathologic. Reviews of equine radiology have long emphasized that normal anatomic variants can be mistaken for fractures, fragments, or other lesions if clinicians don't know what to look for, and standard tarsal imaging already requires careful positioning and interpretation because of the region’s complexity. In practice, this case may be most relevant during lameness workups, prepurchase exams, and teaching, where distinguishing incidental variation from clinically important disease can prevent over-calling abnormalities. (ivis.org)
What to watch: Whether this report prompts more retrospective review of equine hock radiographs, CT studies, or anatomy collections to better define how often a separate os tarsale primum is actually seen in horses. (beva.onlinelibrary.wiley.com)
Key facts
- Article type
- Case report
- Journal
- Veterinary Sciences
- Species
- Horse
- Finding
- Separate os tarsale primum
- Typical anatomy
- The first and second tarsal bones are usually fused
- Specimens examined
- 12 equine tarsal joints
- Observed result
- Six tarsal bones in 11 specimens
- Variant specimen
- One specimen showed an independent first tarsal bone
- Methods
- Gross anatomy and radiography
A case report in Veterinary Sciences puts a spotlight on a small but clinically relevant detail of equine anatomy: a separate os tarsale primum in a horse. In most horses, the first and second tarsal bones are fused, so an independent first tarsal bone is considered unusual. The authors report finding that variant during examination of 12 equine tarsal joints, documenting it with both macroscopic assessment and radiography. (mdpi.com)
That may sound like a narrow anatomy note, but it sits in a broader clinical context. The equine tarsus is one of the more challenging regions to interpret on imaging because of its complex arrangement of bones, joints, and overlapping structures. Standard radiographic evaluation of the hock relies on multiple projections, and equine imaging references stress that both positioning and familiarity with normal variation are essential to avoid false-positive interpretations. (sciencedirect.com)
The background matters here. Older veterinary anatomy texts note that the first and second tarsal bones are usually fused in the horse, while a separate first tarsal bone is described as exceptional. A longstanding AAEP review on normal radiographic variations also argues that incidental skeletal variants are common enough in equine practice that veterinarians need to recognize and dismiss them appropriately, especially in sales imaging and screening exams. This new report appears to extend that principle with a specific, well-documented example involving the distal row of the tarsus. (upload.wikimedia.org)
From the source article summary, the investigators examined 12 tarsal joints and identified six tarsal bones in 11 specimens, with one specimen showing a separate os tarsale primum. The use of both gross and radiographic methods is notable because it helps support that this was a true anatomic variant rather than an imaging artifact or traumatic fragment. That distinction is important in the hock, where radiographic findings can overlap with developmental, degenerative, and traumatic changes, and where supplementary imaging views are often needed to fully characterize bony structures. (sciencedirect.com)
I didn't find a published expert reaction tied specifically to this case report, but the surrounding literature points to why equine imagers would care. Reviews in Equine Veterinary Education and AAEP proceedings emphasize that normal radiographic variants are regularly mistaken for pathology, particularly by clinicians who see these findings infrequently or are working from limited projections. Inference: this case is likely to be most useful as a teaching and reference point for radiologists, equine surgeons, sports medicine clinicians, and practitioners performing prepurchase exams. (beva.onlinelibrary.wiley.com)
Why it matters: For veterinary professionals, the practical takeaway isn't that a separate os tarsale primum is common, but that rare anatomic variation still belongs in the differential when interpreting unusual hock radiographs. Misreading a normal variant as a fracture fragment, osteochondral body, or other lesion could affect case management, client communication, and prepurchase recommendations. In horses with lameness, it also reinforces the need to correlate imaging with the physical exam, diagnostic analgesia, and, when needed, ultrasound or advanced imaging rather than assuming every radiographic irregularity is clinically relevant. (ivis.org)
There's also a research angle. The report is based on a small number of specimens, which is appropriate for a case report but leaves open the question of prevalence. Modern CT-based studies and retrospective imaging reviews could help determine whether separate os tarsale primum is truly exceptionally rare, underreported, or simply overlooked when it causes no clinical signs. (backoffice.biblio.ugent.be)
What to watch: The next step will likely be whether other groups document similar cases in larger imaging or cadaver datasets, and whether future equine radiology references begin listing separate os tarsale primum more explicitly among recognized normal hock variants. (beva.onlinelibrary.wiley.com)