A femoral component of a knee prosthesis has spaced condyle surfaces defining a notch therebetween. The notch defines an elongated cam housing having an anterior cam and a posterior cam at opposite ends of the housing. The tibial component of the knee prosthesis includes a platform and a bearing supported on the platform, the bearing defining bearing surfaces configured to articulate with the condyle surfaces. The tibial component includes a spine projecting superiorly from the bearing that defines an anterior face and a posterior face. The posterior face and the posterior cam define complementary curved surfaces configured for cooperative engagement when the femoral component and the tibial component are at a predetermined flexion angle. The cam housing is configured to form a gap between the posterior cam and the spine when the knee is normally extended. In another feature, the spine includes a stiffening pin extending therethrough.