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Discussion on Research Summary: Exercise and Navicular Disease

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Robert N. Oglesby DVM
Username: dro

Post Number: 19220
Registered: 1-1997
Posted on Saturday, Sep 15, 2007 - 9:25 am:   Edit PostPrint Post

You would have thought that there would have been many studies on navicular bone pathology associatiated with exercise done decades ago but there are few and none have the depth and comparision with controls that this study has. The suggestion that some chronic diseases of the navicular bone and its associated joints come from overloading the bone's capacity to recover from exercise during the early years is not new but never has been better demonstrated than in this summary.

J Anat. 2007 Sep 11;
Morphologic changes associated with functional adaptation of the navicular bone of horses.

Bentley VA, Sample SJ, Livesey MA, Scollay MC, Radtke CL, Frank JD, Kalscheur VL, Muir P.

Comparative Orthopaedic Research Laboratory, School of Veterinary Medicine, University of Wisconsin-Madison, USA.

Failure of functional adaptation to protect the skeleton from damage is common and is often associated with targeted remodeling of bone microdamage. Horses provide a suitable model for studying loading-related skeletal disease because horses are physically active, their exercise is usually regulated, and adaptive failure of various skeletal sites is common. We performed a histologic study of the navicular bone of three groups of horses: (1) young racing Thoroughbreds (n = 10); (2) young unshod ponies (n = 10); and (3) older horses with navicular syndrome (n = 6). Navicular syndrome is a painful condition that is a common cause of lameness and is associated with extensive remodeling of the navicular bone; a sesamoid bone located within the hoof which articulates with the second and third phalanges dorsally. The following variables were quantified: volumetric bone mineral density; cortical thickness (Ct.Th); bone volume fraction, microcrack surface density; density of osteocytes and empty lacunae; and resorption space density. Birefringence of bone collagen was also determined using circularly polarized light microscopy and disruption of the lacunocanalicular network was examined using confocal microscopy. Remodeling of the navicular bone resulted in formation of transverse secondary osteons orientated in a lateral to medial direction; bone collagen was similarly orientated. In horses with navicular syndrome, remodeling often led to the formation of intracortical cysts and development of multiple tidemarks at the articular surface. These changes were associated with high microcrack surface density, low bone volume fraction, low density of osteocytes, and poor osteocyte connectivity. Empty lacunae were increased in Thoroughbreds. Resorption space density was not increased in horses with navicular syndrome. Taken together, these data suggest that the navicular bone may experience habitual bending across the sagittal plane. Consequences of cumulative cyclic loading in horses with navicular syndrome include arthritic degeneration of adjacent joints and adaptive failure of the navicular bone, with accumulation of microdamage and associated low bone mass, poor osteocyte connectivity, and low osteocyte density, but not formation of greater numbers of resorption spaces.
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Brandi Reinert
Username: brandi

Post Number: 111
Registered: 11-2004
Posted on Thursday, Dec 4, 2008 - 1:03 pm:   Edit PostPrint Post

Dr. O, I really appreciate this information, but I need help with it. I can usually make heads 'n tails of these studies, but this one is really over my head. I'm used to seeing the results (n% of horses responded "x" way),so not sure why that part is missing (though you did say it was a summary - is that why?). I see that it says NS horses had formation of intracortical cysts and tidemarks - these are both bad, I assume? "Resorption of space density was not increased" does this mean... I'm struggling to ask the question even after verifying the definition of "resorb". The summary does kind of bring it together, but I'm not sure how they came to "...arthritic degeneration of adjacent joints...", I don't see where that conclusion came from, since I can't interpret that they even looked at adjacent joints. Can you give me a more layman's term summary of what they found?
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