Estimation of Mineral Content of the Equine Third Metacarpal by Radiographic Photometry

Abstract

A radiographic photometric technique for estimating bone mineral content (BMC) of the equine third metacarpal (McIII) was investigated. Anterioposterior (AP) and lateromedial (LM) radiographs were made on flesh-covered McIII's of 17 forelegs from equine cadavers. An aluminum (Al) stepwedge was exposed simultaneously in the AP views for use as a reference standard, allowing the conversion of the logarithms of the percentage transmittance (ln % T) of the photometer reading of the bone scans to their radiographic bone Al equivalents (RBAE). The RBAE values for McIII were assessed for their utility in the estimation of BMC. Radiogrammetic measurements of cortex thickness (CT), medullary cavity width (M) and bone diameter (BD) were compared, individually and in various combinations, with BMC for their value as potential components of bone density indices. The correlation between ln % T and thickness of the Al wedge was .99 (P<.01). The RBAE values from transverse photometric scans of McIII AP radiographs correlated to BMC as follows: lateral cortical aspect, .94 (P<.01); midpoint, .92 (P<.01); the medial cortical aspect, .88 (P<.01), and the area under the RBAE curve, .86 (P<.Q1). Correlations between morphological bone characteristics and BMC were: elliptical cross-sectional area index, BDAPBDLM – MAPMLM, .95 (P<.01); circular cross-sectional area index, BDAP2 – MAP2 .88 (P<.01); CTAP + (MAP/4), .85 (P<.01); BDAP, .79 (P<.01); CTAP, .56 (P<.05), and CTAP/BDAP, −.26. Multiple radiographs of a single leg and repetitive photometric scans of single radiographs indicated good reproducibility of the technique. The progressive mineralization of McIII of weanling foals from 120 to 470 days of age was readily followed by radiographic photometry and was found to be primarily curvilinear.