Morphologic changes associated with functional adaptation of the navicular bone of horses

Abstract

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.

Características Planimétricas y Estereológicas de los Nervios Palmar Lateral y Medial en Manos de Equinos con y sin Síndrome Navicular

“…Respecto a las etapas de obtención, diagnóstico de manos y conformación de grupos fueron realizadas según los criterios descritos en la literatura especializada (Stashak;Bentley et al, 2007;Floyd & Mansmann, 2009;Waguespack & Hanson, 2010;Dyson et al;Komosa et al, 2013). La mayoría de los signos macroscópicos observados en manos y los hallazgos radiográficos evidenciaron la existencia de procesos patológicos en el HSD que lo afectan como respuesta a los apremios mecánicos debido a altas exigencias, todos asociados al SN (Komosa et al).…”

Gross and histological evaluation of early lesions of navicular bone and deep digital flexor tendon in horses

“…We suspect that frequent training on heavy, soft ground, which allows the hoof to sink in, leads to problems with navicular bone, as such ground makes DDFT strongly rub against the podotrochlear bursa and flexor surface of navicular bone. This may be accompanied by remodelling abnormalities resulting from uncoupled basic multicellular unit and a greater number of microcracks in bone matrix (1).…”

Estudio Histoquímico de la Distribución de las Fibras de Colágeno en Hueso Sesamoídeo Distal de Equinos con y sin Síndrome Navicular

“…Diversos estudios mencionan que el tejido óseo del HSD en manos de equinos debe soportar una gran cantidad de cargas y presiones (Kinast;Bentley et al, 2007;Floyd & Mansmann, 2009;Waguespack & Hanson, 2010;Dyson et al, 2011;Komosa et al, 2013), sin embargo, descripciones respecto al ambiente mecánico al que se expone el HSD han sido poco estudiadas. En nuestro estudio hemos observado osteonas secundarias en el tejido óseo compacto en equinos con SN y además un aumento y disminución de la birrefringencia del colágeno en el tejido óseo compacto y trabecular, respectivamente, lo que pone de manifiesto la actividad de remodelación ósea inducida probablemente por la excesiva tensión que ejerce el tendón del músculo flexor digital profundo sobre la articulación interfalángica distal y principalmente sobre la cara flexora del HSD.…”