BIOMECHANICAL FACTORS FOR THE ETIOLOGY OF NAVICULAR DISEASE IN SPORTS HORSES
Keywords: sports horses, navicular bone, navicular disease, joint mechanics
AbstractINTRODUCTION: Navicular disease is a common syndrome in sports horses such as gallopers, jumpers and western horses (especially quarter horses; Stashak, 1987). This syndrome causes forelimb lameness due to pain of the navicular bone (distal sesamoid of the horse digit), navicular bursitis and deep flexor tendon (DFT) affection. When horses develop navicular disease, they can no longer be used for competitive purposes, although they may still be useful for breeding purposes. Yet navicular disease is hereditary, although the mechanisms are still unclear. Nevertheless, distinct morphological variations exist in the navicular bone which are also hereditary (Ueltschi et al., 1995). Our hypothesis on the transmission of navicular disease is that morphological variability causes differences in joint load and bone stress. The aim of this study was to analyze the biomechanical effects of morphological variations of the navicular bone. METHODS: We examined 87 horses radiographically. X-rays were taken of the lateral aspects of the front toes. In the radiographs, the rotation center (center of curvature) of the coffin joint (phalanx II-phalanx III and navicular bone-phalanx II) was determined. Based on this, we constructed the lever arms of the acting forces, taking into account the diameter of the DFT. The lever arms of the proximal DFT-force and of the force of the joint between phalanx III and the navicular bone were calculated relative to the lever arm of the distal DFT-force. Taking into consideration the joint angles, we calculated the tendon and joint forces, and joint surface stresses (quasi-static inverse-dynamic calculation). RESULTS: The decisive factor for the differences in navicular mechanics is the proximal DFT lever arm. The smaller it is - relative to the distal lever arm - the more pressure will be concentrated at the distal end of the navicular bone. The larger it is, the more the pressure maxima will be shifted to the middle, and the more favorable the pressure distribution will be. Unfavorable navicular mechanics will thus occur when the bone is over-stressed in its distal border region where the distal arteries enter the bone. CONCLUSIONS: The varying morphology of the navicular bone is therefore a further explanation for the multi-factorial genesis of navicular disease. This knowledge appears all the more important because selective breeding can prevent the hereditary transmission of unfavorable navicular morphology. The radiological examination of the navicular bone is therefore, besides the classic exterior examination, a sensible screening measure for breeding (Ueltschi et al. 1995). Further, radiological examination is a sensible complement to purchase or aptitude tests. REFERENCES: Stashak, T. S. (1987). Adams’ Lameness in Horses. Philadelphia: Lea & Febiger. Ueltschi, G., Hornig, I., Stornetta, D. (1995). Beobachtungen zur Genetik der Podotrochlose. In P. F. Knezevic (Ed.), Orthopädie bei Huf- und Klauentieren. Stuttgart: Schattauer.
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