Corrosion at the interface. A possible solution to cobalt-chrome heads on titanium alloy stems

The effect of altering head length on corrosion using a material loss method

INTRODUCTION

Corrosion at head neck taper junctions in total hip arthroplasty has increasingly been reported in the literature. Debate persists as to the exact causes and clinical significance of corrosion. Increased offset and head length has been correlated with an increased risk of tribocorrosion due to an adverse mechanical environment. The purpose of this study is to assess the effect of head length on corrosion of a metal-on-polyethylene articulation.

METHODS

Retrievals from a single institution of 28-mm cobalt chromium alloy heads with a 12/14 taper from a single manufacturer were studied. Corrosion of femoral head bores were studied utilising a material loss method. Testing was performed using co-ordinate measuring for maximum linear wear depth.

RESULTS

56 heads were examined with lengths of either -3, 0, +4 or +8 mm and all had been in situ for a minimum of 2 years. There were no significant differences in mean maximum linear wear depth (MLWD) (p = 0.6545). There was no correlation found between MLWD and the time implants were in situ (Spearman coefficient -0.1157) and no significant difference seen between high or standard offset stems (p = 0.1336).

CONCLUSION

In contrast to studies using qualitative methodologies, there was no correlation between head length and material loss when confined to a 28-mm head. Broad application of this outcome should be cautioned against as this study examined 1 taper construct and a metal-on-polyethylene articulation.

Effect of impact assembly on the interface deformation and fretting corrosion of modular hip tapers: An in vitro study

Wear and corrosion at the modular head-neck junction has been recognised to be a potential clinical concern, with multiple reports on adverse local tissue reactions and subsequent early failure of metal-on-metal hip replacements. Furthermore, reports on head-neck taper corrosion are also being described with conventional metal-on-polyethylene bearings. Manufacturing tolerances, surgical technique, non-axial alignment, material combination, high frictional torque and high bending moment have all been implicated in the failure process. There is limited guidance on the force of impaction with which surgeons should assemble modular hip prostheses. This study aims to investigate the effect of impaction force on the deformation and corrosion of modular tapers. Short neck tapers with high surface roughness (average R_z  = 16.58 μm, R_a  = 4.14μm) and long neck tapers with low surface roughness (average R_z  = 3.82 μm, R_a  = 0.81μm), were assembled with CoCrMo alloy heads (smooth finish) under controlled conditions with 2, 4 or 8 kN of impaction force. Material combinations tested included CoCrMo-head/CoCrMo-neck and CoCrMo-head/Ti-6Al-4V-neck. Assessment of surface deformation before and after impaction was made using surface profilometry. Measurement of fretting current during sinusoidal cyclic loading evaluated mechanically assisted corrosion for each assembly load during short-term cyclic loading (1000-cycles) and long-term cyclic loading (5 million-cycles). Deformation on head and neck tapers increased with assembly load. Fretting currents during short term simulation testing showed significantly lower currents (p < 0.05), in 8 kN assemblies when compared to 2 and 4 kN, especially for the short-rough tapers. Long-term simulator testing demonstrated a progressive reduction in fretting corrosion for samples impacted with 4 and 8 kN; however, this reduction was greater for samples impacted at 8 kN even at the start of testing. Based on our results, surgeons could minimise mechanically assisted crevice corrosion by using higher impact loads when assembling the head to the stem in total hip arthroplasty. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:405-416, 2018.

Special modes of corrosion under physiological and simulated physiological conditions

The aim of this article is to review those aspects of corrosion behaviour that are most relevant to the clinical application of implant alloys. The special modes of corrosion encountered by implant alloys are presented. The resistance of the different materials against the most typical corrosion modes (pitting corrosion, crevice corrosion and fretting corrosion) is compared, together with observations of metal ion release from different biomaterials. A short section is dedicated to possible galvanic effects in cases when different types of materials are combined in a biomedical device. The different topics covered are introduced from the viewpoint of materials science, and then placed into the context of medicine and clinical experience.

A methodological study for the analysis of apatite-coated dental implants retrieved from humans

The stability of thermally processed hydroxyapatite coatings for oral and orthopedic bioprostheses has been questioned. Information on the chemical changes, which occur with hydroxyapatite biomaterials post-implantation in humans, is lacking. The purpose of this investigation was to begin to examine post-implantation surface changes of hydroxyapatite-coated implants using scanning electron microscopy (SEM), x-ray microanalysis (EDAX), Fourier transform infrared spectroscopy (FTIR), and x-ray diffraction (XRD). Three retrieved dental implant specimens from humans following clinical failure due to peri-implantitis were examined. Unimplanted cylinders served as controls. Clinically, the retrieved specimens were all enveloped by a fibrous tissue capsule with bone present at the apical extent of the implant. SEM analysis showed that the retrieved surfaces were coated with both calcified and proteinaceous deposits. EDAX scans of the retrieved specimens demonstrated evidence of hydroxyapatite coating loss reflected by increasing titanium and aluminum signals. Other foreign ions such as sodium, chloride, sulfur, silica, and magnesium were detected. XRD of the control specimens showed that the samples were predominantly apatite; however, two peaks were detected in the diffraction pattern, which are not characteristic of hydroxyapatite, indicating that small amounts of one or more other crystalline phases were also present. The retrieved specimens showed slightly larger average crystal size relative to the control sample material, and the non-apatite lines were not present. FTIR evaluation of the retrieved specimens revealed the incorporation of carbonate and organic matrix on or into the hydroxyapatite. Narrowing of and increased detail in the phosphate peaks indicated an increase in average crystal size and/or perfection relative to the controls, as did the XRD results. Based on these results, we conclude that chemical changes may occur within the coating, with the incorporation of carbonate and concomitant reduction in hydroxyapatite coating thickness. Thermodynamic dissolution-reprecipitation of the coating itself and subsequent surface insult by bacterial and local inflammatory components may be involved with these changes.

Radiography of orthopedic trauma and fracture repair

Fracture of bone and bone healing are similar to healing of soft tissues; however, the crystalline structure of bone tissue and its slow but dynamic recovery give some mystique to the healing process. The problem is further complicated by the common misconception that radiology is the best way to assess the changes occurring in bone. It is becoming more apparent that radiology is best only to describe normal or disrupted anatomical conformation. Other established and emerging modalities are competing with radiology for evaluation of the skeletal tissues. It is most important to remember that a clinical examination with evaluation and assimilation of the clinical signs gives a better indication of the physiological and pathological status of bone healing that any of the sophisticated imaging modalities. Imaging should be reserved as an adjunct to the clinical examination.