Mechanical consequences of bone ingrowth in a hip prosthesis inserted without cement.

Long-term biomechanical problems associated with the use of sintered porous coating on prosthetic femoral stems inserted without cement include proximal loss of bone and a risk of fatigue fracture of the prosthesis. We sought to identify groups of patients in whom these problems are accentuated and in whom the use of porous coating may thus jeopardize the success of the arthroplasty. We attempted to develop clinical guidelines for the use of sintered porous coating by investigating the long-term biomechanical effects of bone growth into partially (two-thirds) porous-coated anatomic medullary locking hip prostheses that fit well. More specifically, we used a detailed finite element analysis and a composite beam theory to determine the dependence of proximal loading of the bone and maximum stresses on the stem on the development of clinically observed patterns of bone ingrowth and the dependence of the risk of fatigue fracture of the stem on the diameter of the stem, the diameter of the periosteal bone, and the material from which the prosthesis was made. We found that bone ingrowth per se substantially reduced proximal loading of the bone. With typical bone ingrowth, axial and torsional loads acting on the proximal end of the bone were reduced aa much as twofold compared with when there was no ingrowth; bending loads on the proximal end of the bone were also reduced. The risk of fatigue fracture of the stem was insensitive to the development of bone ingrowth. However, the risk of fatigue fracture of the stem increased with decreased diameters of the stem and the periosteal bone and with increased modulus of the stem. The maximum risk of fracture was found in active patients in whom a cobalt-chromium-alloy stem with a small diameter had been implanted in a bone with a small diameter.