Fatigue Strength of Cobalt-Base Alloys with High Corrosion Resistance for Artificial Hip Joints

Physical Properties of Electropolished CoCrMo Alloy Coated with Biodegradable Polymeric Coatings Releasing Heparin after Prolonged Exposure to Artificial Urine

In this study, we aimed to determine the effect of long-term exposure to artificial urine on the physical properties of CoCrMo alloy with biodegradable heparin-releasing polymeric coatings. Variants of polymer coatings of poly(L,L-lactide-ɛ-caprolactone) (P(L,L-L/CL)) and poly(D,L-lactide-ɛ-caprolactone) (P(D,L-L/CL)) constituting the base for heparin-releasing (HEP) polyvinyl alcohol (PVA) coatings were analyzed. The coatings were applied by the dip-coating method. Heparin was used to counteract the incrustation process in the artificial urine. The study included tests of wettability, resistance to pitting and crevice corrosion, determination of the mass density of metal ions penetrating into the artificial urine, and the kinetics of heparin release. In addition, microscopic observations of surface roughness and adhesion to the metal substrate were performed. Electrolytically polished CoCrMo samples (as a reference level) and samples with polymer coatings were used for the tests. The tests were conducted on samples in the initial state and after 30, 60, and 90 days of exposure to artificial urine. The analysis of the test results shows that the polymer coatings contribute by improving the resistance of the metal substrate to pitting and crevice corrosion in the initial state and reducing (as compared with the metal substrate) the mass density of metal ion release into the artificial urine. Moreover, the PVA + HEP coating, regardless of the base polymer coatings used, contributes to a reduction in the incrustation process in the first 30 days of exposure to the artificial urine.

Properties of Wrought CoNiCrMo Alloy Protasul-10, a Highly Corrosion and Fatigue Resistant Implant Material for Joint Endoprostheses

Loosening of prosthetic components in the bone is a problem that has not been fully explored as yet. When loosening has occurred, the metallic anchorage stem may become overloaded and in extreme cases it may ultimately lead to stem fracture if the fatigue limit of the metallic material is exceeded.

In order to preclude the possibility of these undesirable fatigue fractures of loosened prosthesis stems requiring re-operations, the Sulzer research laboratories have developed wrought alloys of extremely high strength and corrosion resistance such as Protasul-10.†

The chemical composition, structure, mechanical properties, corrosion resistance and present state of the art of this implant material, which has been in clinical use since 1971, are specified in national and international standards.

Load transfer with the Austin Moore cementless hip prosthesis

More than 1,300 Austin Moore hemiarthroplasties have been reviewed in the literature, with no reports of fracture of the stem. Many patients with these hip implants had good function. The lack of stem fractures in patients with good functions has not been explained and contrasts with stem fractures that have occurred in patients with cemented prostheses of other designs during the same time. We used three-dimensional finite-element analysis and free-body diagrams to explain the lack of fractures for this device by a description of the probable load-transfer mechanisms between the prosthesis and the bone. Results from our finite-element analysis indicate that, with good calcar-collar support, the stresses in the stem are small because the stem portion of the prosthesis and the bone are uncoupled and, consequently, do not share the resultant bending moment of the head and abductor forces. If the stem is coupled to the bone so that the resultant bending moment is shared, high stresses in the stem are predicted; such stresses are inconsistent with the complete absence of fractures of these prostheses. The results of the finite-element analysis further showed that loss of calcar-collar support with proximal fixation through the fenestrations resulted in high stresses in the stem and stress shielding of the proximal medial cortex. The uncoupled prosthesis also may be modeled with a free-body diagram as a three-force member loaded at the head, stem tip, and in the proximal region. With this model, it can be shown that the reaction force of the stem tip, and thus the peak bending stress in the stem, increases as calcar-collar support is decreased. If there is no calcar-collar support, proximal support must be provided by some combination of integration of bone in the fenestrations and wedging due to the lateral-medial taper of the device. Stresses in the stem are largest when there is no wedging, but high stresses develop in the cancellous bone in the fenestrations. When there is wedging, stresses in the stem can be low, but stresses in the supporting cancellous bone can be high; additional proximal support through the fenestrations substantially reduces these bone stresses. If reduced stresses in the cancellous bone are indicative of a stable device, these mechanisms indicate that fractures of the Austin Moore prosthesis have not occurred in normally loaded hips because load was transferred primarily either through the collar or by wedging, with additional support at the fenestrations.

Twenty years of Sulzer experience with artificial hip joint materials

This paper reviews the development of hip prostheses over the past 20 years with particular reference to the materials employed. The material properties required are discussed and the alloys used in different designs of prostheses are described.

A study of corrosion fatigue in cast cobalt-chrome-molybdenum alloys

Fatigue crack growth rates in Ringer's solution have been shown to be significantly higher than in air, and the crack growth threshold level reduced by the presence of a corrosive environment. This demonstrates that cast cobalt alloys are susceptible to corrosion fatigue effects in the human body, at least during crack propagation, and the proposed mechanism for this involves the synergistic effect of crack tip corrosion and the effects of cyclic stress which produce a ‘true corrosion fatigue’ phenomenon.

Clinical experience with Mueller total hip endoprostheses of different design and material

Patients of the Orthopaedic University Hospital in Frankfurt operated between 1970 and 1977 were examined to control the success of implantations of total hip endoprostheses. Ninety-three percent of totally 1497 implanted hip prostheses are Müller standard type, additionally there have been implanted 44 Weber-Huggler prostheses and 54 special long stem prostheses. During the follow-up period of 10 years 7.4% of the primarily implanted prostheses had to be revised. The reasons for revision were loosening of the stem and/or socket in 56% of the cases, 20% infections, 14% technical errors and in 11% of the cases fractures of the stem occured. Fifty-two percent of the implanted Weber-Huggler prostheses with rotational ball heads made of polyester had to be revised in the meantime, 78% of the cases because of loosening of the prosthetic components. It could be shown that the number of infected cases could be diminished after introduction of an ultraclean air operating box. The change of the cross section of the Müller-type-stem from a round medial wedge to a flat one and the introduction of CoNiCrMo wrought alloy Protasul-10 instead of CoCrMo cast alloy Protasul-2 as stem material in 1973 resulted in a much smaller rate of loosened prostheses without fractures of the forged Protasul-10 stems. During the 6 examined years there has been no fracture of a prosthetic stem made of Protasul-10.