Comment on "Three-dimensional finite element modeling of guided ultrasound wave propagation in intact and healing long bones," [J. Acoust. Soc. Am. 121(6), 3907-3921 (2007)].

Protopappas et al. performed finite element (FE) studies on the propagation of guided ultrasound waves in intact and healing long bones, and found that the dispersion of guided modes was significantly influenced by the irregularity and anisotropy of the bone. A time-frequency (t-f) method was applied to the obtained signals and several wave modes were identified. However, this technique was unable to quantify their observations and provide monitoring capabilities. One possible reason of this shortcoming may come from the inherent disadvantage of the t-f method. The objective of this comment is to demonstrate that it is necessary to combine other techniques with FE simulations for the extraction of significant quantitative ultrasonic features. Individual guided modes in an isotropic pipe have been theoretically examined using the normal mode expansion (NME) method, and many modes that are missed by the t-f analysis have been identified. It is concluded that in order to extract quantitative ultrasonic features, FE simulations should be supplemented by other techniques such as the NME.