Morphologic changes associated with functional adaptation of the navicular bone of horses.
Authors: Bentley, Sample, Livesey, Scollay, Radtke, Frank, Kalscheur, Muir
Journal: Journal of anatomy
Summary
# Editorial Summary: Morphologic Changes and Functional Adaptation of the Navicular Bone Navicular syndrome remains a significant cause of lameness in horses, yet the underlying bone remodelling mechanisms that distinguish adaptive success from pathological failure have been incompletely understood. Bentley and colleagues examined histological samples from three cohorts—young racing Thoroughbreds, young unshod ponies, and older horses diagnosed with navicular syndrome—quantifying bone mineral density, cortical thickness, microcrack density, osteocyte populations, and collagen organisation using advanced microscopy techniques. Horses with navicular syndrome demonstrated hallmark maladaptive changes including intracortical cyst formation, multiple tidemarks at the articular surface, high microcrack density, depleted bone volume fraction, and critically compromised osteocyte connectivity and viability, yet notably *without* increased resorption space density. The remodelling patterns—including transverse secondary osteons and collagen alignment oriented laterally to medially—suggest the navicular bone experiences habitual bending loads across the sagittal plane, and when adaptive capacity fails, cumulative microdamage accumulates faster than the bone can repair it. For practitioners, these findings indicate that navicular syndrome involves a specific pattern of skeletal maladaptation characterised by poor bone quality despite maintained mineral density, suggesting that imaging and interventions targeting osteocyte health, microdamage prevention, and load management may warrant greater clinical emphasis than conventional approaches focused primarily on resorption control.
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Practical Takeaways
- •Navicular syndrome involves cumulative cyclic loading damage rather than simple bone resorption; management should focus on reducing repetitive impact and bending loads through farriery and exercise modification
- •Early detection of microcrack accumulation and poor bone quality may be possible through advanced imaging, allowing intervention before extensive cyst formation and cartilage damage occurs
- •The bone's attempt to remodel in response to loading can fail, leading to osteocyte death and poor skeletal integrity; this supports the need for load management in at-risk horses rather than relying on adaptation
Key Findings
- •Navicular bone remodeling in horses with navicular syndrome resulted in transverse secondary osteons oriented lateral to medial, matching collagen fiber orientation
- •Horses with navicular syndrome showed high microcrack surface density, low bone volume fraction, and low osteocyte density compared to young athletic horses
- •Intracortical cyst formation and multiple tidemarks developed at the articular surface in navicular syndrome cases, indicating habitual sagittal plane bending loads
- •Poor osteocyte connectivity and microdamage accumulation occurred without increased resorption space density, suggesting adaptive failure rather than active remodeling