The Journal of Bone and Joint Surgery, Vol 77, Issue 6 911-923, Copyright © 1995 by Journal of Bone and Joint Surgery, Inc
Mechanical consequences of bone ingrowth in a hip prosthesis inserted without cement
TM Keaveny and DL Bartel
Department of Mechanical Engineering, University of California, Berkeley 94720-1740, USA.
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.