Determination of calcaneal ultrasound properties ex situ: Reproducibility,effects of storage, formalin fixation, maceration, and changes in anatomicmeasurement site

The objective of this study was to determine the reproducibility of ultraso nic bone properties with a system for measuring calcanei ex situ; the influ ence of changes of the measurement site; and the effects of fixation, stora ge, and maceration. We examined 14 fixed calcanei and 12 fresh bones. Ultra sonic measurements were performed ex situ after degassing, using an Achille s+ system and a special positioning device. The instrument precision was 0. 16% for speed of sound (SOS), 1.4% for broadband ultrasonic attenuation (BU A), and 1.8% for the stiffness index (ST). The short-term precision was 0.5 4%, 1.9%, and 2.8%, respectively. A defined shift of the measurement site ( 5 mm distal of the middle) led to unpredictable changes in ultrasound (US) properties (r = 0.65 for SOS, 0.82 for BUA, and 0.75 for ST). Embalment wit h 4% formalin/96% alcohol caused a systematic decrease in SOS, an increase in BUA, and a decrease in SI (mean = -12.7 units; P < 0.001), the effect in creasing with time. However, values at 6 months of fixation and later were highly correlated with those in fresh specimens (r = 0.95 for the ST). Two weeks storage in degassed and normal solution had only modest effects on ul trasound properties. Maceration did not lead to a systematic increase or de crease of ultrasound variables, but introduced unpredictable changes (r = 0 .64-0.94). We conclude that in comparative biomechanical studies it is feas ible to measure calcaneal specimens embalmed in formalin/alcohol ex situ, i f the primary interest is not in the absolute values but in the correlation with mechanical failure loads at other skeletal sites.