Current Orthopaedics (1998) 12, 51-57
© 1998 Harcourt Brace & Co. Ltd
Biomechanics
Materials in total joint replacement
S. Santavirta, Y. T. Konttinen, R. Lappalainen, A. Anttila, S. B. Goodman, M. Lind,
L. Smith, M. Takagi, E. Gdmez-Barrena, L. Nordsletten, J.-W. Xu
METALS Stainless steel is now rarely used in new designs of
replacement of the major load-bearing joint. These
Total joint replacement (T JR) is one of the most materials are iron-based alloys containing appreciable
efficacious and cost effective procedures in surgery. amounts of chromium, nickel and molybdenum.
However, issues concerning the optimal materials for Stainless steel is usually annealed, cold worked or
TJR have been a major concern. The materials chosen cold forged to improve the strength of the alloy.
must be biocompatible (both the material itself and Although this material is relatively biocompatible,
any breakdown products), strong, resistant to fatigue, strong and cheap, it does undergo slow corrosion in
wear and corrosion, easily accessible and reasonably the body. The grain size and the inclusion of metal
priced. For many years, the materials chosen by impurities must be carefully controlled, because these
Sir John Charnley in the early 1960s (namely stain- characteristics adversely influence the strength and
less steel, polyethylene and polymethylethacrylate) resistance to corrosion. One other potential problem
remained the 'gold standard' for comparison. 1 More is the relatively high modulus of elasticity of stainless
recently, there has been a trend for modern joint pros- steel, which is approximately 200 Gpa. This high
theses to employ one of the newer metallic alloys of value is much greater (10 times) than for bone. Thus,
cobalt chrome or titanium. In general, these newer stainless steel prostheses have generally been used
alloys are stronger and more corrosion and wear resis- with a lower modulus polymer cement such as poly-
tant than stainless steel)~ This theoretical benefit has methylmethacrylate both for fixation of the implant
yet to be converted into improved clinical outcome. in bone and to mitigate the stress shielding effects of
the high modulus material.
Cobalt-chromium alloys are stronger and more
corrosion resistant than stainless steel. For joint
S. Santavirta MD PhD. Professor, Department of Orthopaedics
and Traumatology, Helsinki University Central Hospital, Helsinki, implants, these alloys are usually composed of
Finland, Y. T. KonttinenMD. PhD, Senior Investigator of the 30-60% cobalt, 20-30% chromium, 7-10% molybde-
Finnish Academy, Department of Anatomy, Institute of num, and various amounts of nickel (as high as 37%,
Biomedicine,University of Helsinki and Division of Rheumatic
Diseases, Department of Medicine, Helsinki University Central in some cases). With the application of heat and high
Hospital, Finland, R. Lappalainen PhD, Senior Researcher, pressure, the strength of cobalt chrome alloys can
Department of Physics, Helsinki University,Helsinki, Finland, be increased almost twofold compared to casted
A. Anttila PhD, Professor, Department of Physics, Helsinki
University, Finland, S. B. Goodman MD, Associate Professor, materials. Cobalt-chromium alloys have an elastic
Division of Orthopaedic Surgery, Stanford University School of modulus that is slightly higher than for stainless steel
Medicine, CA, USA, M. Lind MD, PhD, University of Aarhus, (approximately 240-250 Gpa for cobalt chromium
Denmark, L. Smith PhD, Stanford University,Stanford, CA, USA,
M. Takagi PhD, MD, Department of Orthopaedic Surgery, alloys compared to 200 Gpa for stainless steel).
Yamagata University School of Medicine, Yamagata, Japan, Polymethylmethacrylate is commonly used for fixa-
E. G6mez-BarrenaMD PhD, Traumatologia Y Cir. Ortop., tion of joint implants made of cobalt-chrome alloys.
Hospital 'Ntra Sra de Gracia', Zaragoza, Spain, L. Nordsletten
MD, PhD, Orthopaedic Surgeon, Orthopaedic Department, Stress shielding of the periprosthetic bone is also a
Ullevhl University Hospital, Oslo, Norway and Institute for potential problem, especially when the material is
Surgical Research, Rikshospitalet, Oslo. Norway. &-W. Xu MD, used as a cementless femoral component. Although
PhD, Post-Doctoral Fellow, Department of Orthopaedics and
Traumatology, Helsinki University Central Hospital, and Research prosthetic breakages are rare now that modern
Unit ORTON. Invalid Foundation, Finland. processing techniques are commonly employed, the
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