Abstract: The coupling joint has on the mating contact surfaces between the tooth and the tooth holder the following characteristics: the upper and lower surfaces which define the stabilisation planes comprise a substantially flat central surface and another two flat surfaces, one on each side of the said central surface, so that the interstice between the central flat surface and the corresponding facing surface of the tooth holder is smaller than the respective interstices between the lateral flat surfaces and the corresponding facing surfaces of the receiving cavity of the tooth; the upper and lower rear edges of the lugs of the tooth have a convex transverse structure; the internal vertical rear edge of the rear lugs of the tooth have surfaces of a shape matching the opposed faces of the tooth holder, giving rise to higher bearing surfaces in the case of transverse bearing on the tooth.