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UF Health University of Florida
Veterinary Diagnostic Imaging College of Veterinary Medicine
LA Radiology Overview

Case reveal

Nuclear scintigraphy

  • Proximal right tibial diaphyseal region, severe and focal IRU
  • Right sacroiliac joint, mild and diffuse IRU*
  • Left tarsus at the level of the distal tarsal joints, mild and diffuse IRU*

IRU = Increased radiopharmaceutical uptake

Right Tibia Radiographs:

  • Seen on all projections, at the level of the proximal tibial diaphysis and centered on the medullary cavity, there is a focal, ill-defined, ovoid region of increased mineral opacity (yellow arrows). 

Conclusions:

  • Enostosis-like lesion of the right tibia

*The IRU of the right sacroiliac and left distal tarsal joints may be indicative of osteoarthrosis.

Follow-up:

  • Discharged for conservative management at home with stall rest and NSAIDs.

A little bit more…

Overview and Clinical Presentation

  • Enostosis-like lesions (ELLs) are idiopathic, intramedullary sclerotic bone lesions primarily affecting the long bones of horses. Initially described by Bassage and Ross (1998), these lesions are characterized by focal or multifocal areas of increased bone density within the medullary cavity, often near the nutrient foramen. ELLs have been identified in various breeds, with a predominance in Thoroughbreds, and typically affect horses aged 3–11 years.
  • Clinical signs vary, with lameness being the most common presentation. In some cases, ELLs are incidental findings during imaging for unrelated issues. Lameness associated with ELLs is often mild to moderate (grades 1–3/5), and may be unilateral or bilateral, affecting forelimbs or hindlimbs. Pain localization is challenging due to the intramedullary nature of the lesions, and diagnostic analgesia is often inconclusive, especially for proximal limb lesions.

Diagnostic Imaging

Nuclear Scintigraphy

  • Scintigraphy is the most sensitive modality for detecting ELLs. Lesions appear as discrete focal or multifocal areas of increased radiopharmaceutical uptake (IRU) within the medullary cavity of long bones. Orthogonal views are essential to confirm intramedullary localization and differentiate ELLs from cortical or periosteal pathologies such as stress fractures or exostoses.
    • Commonly affected bones: Tibia, radius, third metatarsal, humerus, and femur.
    • Scintigraphic grading: Lesions are graded based on uptake intensity (mild, moderate, intense) and distribution (focal vs. diffuse).
    • Correlation with lameness: Higher uptake intensity and diffuse patterns are associated with more severe lameness, particularly in humeral and femoral lesions.

Radiography

  • Radiographs reveal focal or multifocal radiopacities within the medullary cavity, often adjacent to the nutrient foramen. These opacities are typically ovoid to irregular and may appear as clusters. Radiographic changes often lag behind scintigraphic findings and may persist after scintigraphic resolution.
    • Radiographic grading: Based on lesion size, density, and number.
      Limitations: Radiography is less sensitive in early stages and may not detect subtle lesions without concurrent scintigraphy.
O’Neill and Bladon (2011)

Epidemiology and Distribution

  • Ahern et al. (2014) reported ELLs in 1.6% of horses undergoing scintigraphy, with Thoroughbreds being overrepresented. Older horses were more likely to be affected. The tibia and radius were the most frequently involved bones, with humeral lesions causing the most severe lameness. Multifocal lesions and bilateral involvement were common.
  • O’Neill and Bladon (2011) found ELLs in 0.01% of horses, with hindlimbs more frequently affected than forelimbs. Repeat imaging revealed new lesions in different bones, indicating potential for recurrence.

Treatment, Prognosis, and Pathophysiological Considerations

Treatment

  • Management of ELLs is typically conservative:
    • Rest: 3–6 weeks of stall rest followed by controlled hand-walking.
    • NSAIDs: Phenylbutazone or similar agents for symptomatic relief.
    • Rehabilitation: Gradual return to work after veterinary reassessment.
      Most horses respond well to this regimen, with resolution of lameness and scintigraphic abnormalities over time. Radiographic changes may persist longer but tend to diminish.

Prognosis

  • The prognosis for return to athletic function is generally favorable:
    • Thoroughbreds: Majority return to racing at previous levels.
    • Other disciplines: Showjumpers and pleasure horses resume full activity.
  • Recurrence: Some horses develop new ELLs in different bones, but these are typically manageable.
    Ahern et al. noted that horses with multiple ELLs at a single examination were less likely to return to racing, suggesting lesion burden may influence prognosis.

Pathophysiology and Differential Diagnosis

  • The exact etiology of ELLs remains unknown. Hypotheses include:
    • Medullary infarction: Possibly due to vascular occlusion or increased intramedullary pressure.
    • Microtrauma: Repetitive stress may lead to trabecular bone remodeling.
    • Inflammatory processes: Similar to canine panosteitis, which shares imaging and clinical features.
  • ELLs differ from stress fractures by their intramedullary location and lack of cortical involvement. They also contrast with bone islands and intramedullary osteosclerosis seen in humans, which are typically asymptomatic and static.

References:

  • Bassage, L. H., and M. W. Ross. “Enostosis‐like lesions in the long bones of 10 horses: scintigraphic and radiographic features.” Equine veterinary journal 30.1 (1998): 35-42.
  • Ahern, Benjamin J., Raymond C. Boston, and Michael W. Ross. “Enostosis-like lesions in equids: 79 cases (1997–2009).” Journal of the American Veterinary Medical Association 245.9 (2014): 1042-1047.
  • O’Neill, H. D., and B. M. Bladon. “Retrospective study of scintigraphic and radiological findings in 21 cases of enostosis‐like lesions in horses.” Veterinary Record 168.12 (2011): 326-326.