Orthopedic implants--a clinical and metallurgical analysis

The tribological difference between biomedical steels and CoCrMo-alloys

In orthopedic surgery, different self-mating metal couples are used for sliding wear applications. Despite the fact that in mechanical engineering, self-mating austenitic alloys often lead to adhesion and seizure in biomedical engineering, the different grades of Co-base alloys show good clinical results, e.g., as hip joints. The reason stems from the fact that they generate a so-called tribomaterial during articulation, which consists of a mixture of nanometer small metallic grains and organic substances from the interfacial medium, which act as a boundary lubricant. Even though stainless steel also generate such a tribomaterial, they were ruled out from the beginning already in the 1950s as "inappropriate". On the basis of materials with a clinical track record, this contribution shows that the cyclic creep characteristics within the shear zone underneath the tribomaterial are another important criterion for a sufficient wear behavior. By means of sliding wear and torsional fatigue tests followed by electron microscopy, it is shown that austenitic materials generate wear particles of either nano- or of microsize. The latter are produced by crack initiation and propagation within the shear fatigue zone which is related to the formation of subsurface dislocation cells and, therefore, by the fact that an Ni-containing CrNiMo solid solution allows for wavy-slip. In contrast to this, an Ni-free CrMnMo solid solution with further additions of C and N only shows planar slip. This leads to the formation of nanosize wear particles and distinctly improves the wear behavior. Still, the latter does not fully achieve that of CoCrMo, which also shows a solely planar-slip behavior. This explains why for metallurgical reasons the Ni-containing 316L-type of steels had to fail in such boundary lubricated sliding wear tribosystems.

Corrosion on an internal spinal fixator system

STUDY DESIGN

Thirteen spinal fixators with 26 stabilization bridges and 52 pedicle screws and a mean length of implantation of 10 months were prospectively examined for corrosion.

OBJECTIVES

To determine the type of corrosion and the correlation between the construction of the spinal fixator and the type of corrosion.

SUMMARY OF BACKGROUND DATA

Evidence of fretting and crevice corrosion is seen in many stainless steel implants in retrieval studies. Such reactions have not been described in the literature on spinal fixator systems.

METHODS

Macroscopic and microscopic alterations in the adjacent tissue were examined, and the corrosive alterations were documented photographically using stereoscopic optical light microscopy. The chemical composition of the implants was determined spectrographically. Microradiography and x-ray fluorescence analysis of the soft tissue were performed.

RESULTS

At surgery, tissue discoloration was found in four cases. Histologic examination showed extensive fibrosis, foreign body reaction and inflammation associated with a small number of metal particles, indicating metallosis in five cases. Corresponding particles were detected by microradiography. Corrosion was found on 13 telescopic rods and on two pedicle screws. The alterations on the telescopic rods could be interpreted as crevice corrosion and the alterations in the pedicle screws as fretting corrosion. The two monobloc fixator bridges did not show signs of corrosion. In these implants, the neighboring tissue was macroscopically inconspicuous, and histologic examination showed minimal fibrosis or presence of metal particles. Spectrographic examination of the spinal fixators showed no structural imperfection.

CONCLUSIONS

The construction constraints of a spinal fixator make it prone to corrosion. New spinal implants should be examined not only in vitro but also in vivo to ascertain whether corrosion and adjacent tissue reaction occur. Corrosion is one reason to explant the internal fixation system after fusion of the spinal fracture.

In vitro corrosion fatigue of 316L cold worked stainless steel

The corrosion resistance of 316L cold worked stainless steel depends upon its thin protective oxide layer; and if this is partially broken down, corrosion resistance depends upon its tendency for repassivation. Since the intended function of stainless-steel implants is to sustain musculoskeletal forces, research toward the stability of the oxide film during dynamic loading in simulated bodylike fluids is warranted. A pilot corrosion fatigue study was, therefore, performed on uniaxial tension fatigue specimens cycled to various maximum stress levels near their yield point while immersed in 37 degrees C isotonic saline solution, and combined with the electrochemical insult of (a) imparting an 800 mV vs. SCE anodic potential for 20 s to stimulate local film breakdown, and then (b) returning to a constant 200 mV vs. SCE anodic potential and maintaining that potential during cyclic loading until the specimens broke in two. During the anodic polarization by continuously monitoring the current it was possible to (a) observe the repassivation and corrosion behavior following stimulation, and (b) detect crack initiation, crack propagation and failure onset. The combined effects of accelerated corrosion and mechanical fatiguing disturbed the repassivation tendency and reduced the crack initiation times and the fatigue lives as compared to air and saline controls. As the maximum cyclic load levels were increased, the fatigue lives were further foreshortened.

Tissue reaction to implant corrosion in 38 internal fixation devices

The corrosion characteristics, metallurgical properties, and clinical performance of 38 retrieved internal fixation devices were correlated with the tissue reaction to these devices. Metallurgical parameters included thin and heavy inclusion content, Rockwell hardness, and grain size. The excised fibrous tissue strip was directly overlying each plate at removal and sectioned between screw-hole sites. The material studied from the 38 plates consisted of 201 screw-hole junctions with associated tissue biopsy sites. Clinical histories were obtained on all 38 patients with hardware removal. The average age at the time of plate insertion was 35.6 years (range, 4 to 75 years). Insertion diagnoses included acute trauma (35 patients), joint dislocations (two patients), and fracture nonunion (one patient). The devices included seven upper extremity bone plates, 19 lower extremity bone plates, and 12 hip screw plates. The devices remained in situ an average of 20.4 months (range, 3 to 60 months). Routine asymptomatic removals were performed on 17 of the patients, while the remaining 21 patients were symptomatic at the time of removal. Included in the reasons for symptomatic removal were pain associated with the implant (eight patients), nonunion (four patients), bursae prominence (three patients), and implant breakage (two patients). Significant correlations were found between average tissue reaction scores and average crevice corrosion scores; a trend of increasing average tissue reaction scores with increasing average screw surface corrosion scores also was observed for the 38 devices, although this relationship was not significant. Average crevice corrosion scores and average screw surface corrosion scores were highly correlated for all removals, and for the asymptomatic and symptomatic removal groups. The metallurgical parameters of thin inclusion content and heavy inclusion content also were significantly correlated for all removals, as well as for symptomatic removals. Similarly, significant correlations were found between the individual tissue reaction scores and crevice corrosion scores from the 201 individual sites, again for all devices and for the asymptomatic and symptomatic removal groups. Tissue reaction scores and time in situ showed significant correlation, with tissue reaction decreasing over time. Crevice corrosion and screw surface corrosion scores were not significantly related to time in situ. The results of this study indicate that there is considerable tissue reaction to the corrosion products of this material.(ABSTRACT TRUNCATED AT 400 WORDS)

In vivo and in vitro studies of the stress-corrosion cracking behavior of surgical implant alloys

Behavior of implant alloys exposed simultaneously to tensile stresses and corrosion environments has been examined. In the in vivo studies, a stainless steel and a titanium alloy exhibited cracklike features when loaded to the yield stress sigma y and implanted for 16 weeks. A cobalt-chromium alloy stressed beyond sigma y exhibited them in plastically deformed areas. A cobalt-chromium-nickel-molybdenum alloy appeared to be immune. In vitro samples loaded to various stress levels were immersed in Ringer's solution at 37 degrees C. Half of them were subjected to applied anodic potentials; the remaining control group was not. The applied potentials were dc potentials of magnitude similar to those generated by bioelectric effects. No attempt was made to duplicate time dependence or wave forms. Cracklike features were observed in the stainless steel and in the titanium alloy loaded to or beyond sigma y and polarized for 38 weeks. None were observed below sigma y. For the controls, no cracklike features were observed at any stress level after 53 1/2 weeks. Polarization measurements and potential versus time measurements were performed to study possible mechanisms for crack propagation. These investigations suggest that the in vivo corrosion environment is more severe than a 37 degrees C Ringer's solution because of the influence of both bioelectric effects and organic constituents. The implications of these studies for the performance of prosthetic devices is discussed.

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.

Performance analysis of total hip prostheses: some particular metallurgical observations

Metallurgical observations of phenomena which influence the clinically successful performance of permanent implants and implant materials are reported. The effect of casting voids in cast cobalt chromium femoral stems of total hip replacements is discussed first. Pitting corrosion occurred in a retrieved stainless-steel implant type AISI 316 containing 2.7% Mo. The fractographic pattern of this device retrieved for fatigue failure shows typical characteristics of corrosion-enhanced fatigue.