In this study, we use the finite element method to analyse the behaviour of cracks emanating from microcavities in the bone cement, binding the cup to the bone, according to their size and position around the cavity, the position of the patient, the cavity's location and the inter-defects distance (cavity-crack, crack-crack). We show that the most unstable stress intensity factor, in mode I, when the crack located in the cement's centre and propagating along this thickness. This instability is all the more important that its size increases, tends towards the cavity, the cracks are located in a vicinity one to other and that the patient is in a squatting position. The predominant fracture mode, in mode I and II, depends on the crack's position priming site around the microcavities. This work allows the better understanding of the interconnection phenomena of the microcavities experimentally observed.