The quantitative assessment of photodensity of the third carpal bone in the horse

Abstract AIM: To determine whether variation in x-ray-beam angle significantly affected photodensity of the third carpal bone (C3) in the horse using the tangential radiographic view, and indirectly determine whether radioabsorptiometry (RA) could be used to assess differences in bone mineral density (BMD) of C3 between exercised and non-exercised horses. METHODS: The left distal carpal row was isolated post-mortem from 2-year-old Thoroughbred horses that had been either exercised (n=7) for up to 14 weeks using a standard training regimen for racehorses, or not exercised (n=7). The effect of variation in x-ray-beam angle on photodensity was determined on these isolated carpal bones in vitro. Image analysis was used to assess photodensity (compared to a known thickness of aluminium) of four regions of interest (ROI) in C3 and one ROI in the fourth carpal bone (C4) of 14 isolated distal rows of carpal bones of the horse. The isolated carpal bones were placed flat on a x-ray cassette and radiographed at 90° (i.e. with the x-ray beam perpendicular to the cassette). The x-ray-beam angle was varied in the dorsal sagittal plane by 5° increments to a total of 15° from 90° and from a base angle of 60° (the x-ray-beam angle at which the tangential view is taken in clinical cases). RESULTS: Variation in beam angle of <10° from 90° significantly affected photodensity, and photodensity was significantly affected when the angle was varied <5° from 60°. When taken at an x-ray-beam angle of 60°, the abaxial aspect of the radial facet of C3 had a consistently higher photodensity than the rest of C3 and C4. The photodensity of the third and fourth carpal bones was higher in exercised horses than in non-exercised horses. CONCLUSION: As variation in x-ray-beam angle significantly affected photodensity, RA using the tangential view is not considered clinically applicable for assessing BMD of C3 and the accuracy of subjective assessment of BMD of C3 using the tangential view in horses is questionable.