This anchoring rod (4) is elongated and tapered.According to the invention, said anchoring rod (4), from its distal end (4d) towards its proximal end (4p), flares on its palmate side (P) without flaring on its dorsal side (D) and flares on its lateral sides, so as to form a flared proximal portion, this flared proximal portion having a substantially trapezoidal cross-section, with a dorsal face larger than the palmate face.
