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Sensitivity of the ultrasonic axial transmission technique to bone porosity and cortical thinning is evaluated by means of 2D numerical simulations. The axial transmission configuration is used to measure the apparent Speed of Sound (SoS) of the first signal arriving on receivers set along bone axis. The main effect of porosity on SoS is a linear decrease at a rate of approximately -50 m.s-1 per % of porosity increase. For bone cortical thickness larger than the compressional wavelength (λc) in bone, the first arriving signal (FAS) has been identified as being the lateral wave propagating at the velocity of the compressional wave in bone. We show that lateral wave measurements reflect bone properties over a cortical depth of the order of λc.