This research refers to a prosthesis made of CoCr with porous area "Madrepore macro", recovered during a revision surgical intervention due to femoral stem's fracture. Such a fracture appears to prosthesis distally well fixed, but proximally mobile, leading to fracture through stem's middle or proximal third. In the case of this prosthesis, object of the present study, the goal is to find the cause leading to the implant's failure. The macroscopic observations showed that the stem's fracture occurred on a transversal plan, proximally close to the distal area of the porous zone, due to bending efforts, not preceded by a plastic strain. Based upon microscopic observations, this research shows that this process manifested itself as a fragile fracture with highlight on a pronounced trans-crystallization.

In the fracture area and in its adjacency there were detected conspicuous oxidation aspects (corrosion). Therewith, during microscopic observation performed on both pieces (the femoral head being assembled through a shrinking-on process on the flange femoral stem) it was detected that they have the same type of microstructure, made of a solid solution δ, and a relatively coarse compounds network, wich represents the structure as it was casted. In the stem's marginal area as well as middle area, numerous casting defects were identified, like microporosities and microfissures. These defects communicated with the stem's exterior, and after the implant, they allowed body's organic components diffusion, which leaded to tribocorrosion. The present research highlights also the existence of a major casting defect around the tiredness efforts' action area, a defect that facilitated the stem's fracture.

Keyword:uncementless hip prosthesis, stem stability, porous implant surfaces, breakage

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