The NiTi alloy (Nitinol), with its favorable micro-structured properties and self-passivity (resembling that of pure Ti) is used as an implant material for arterial stents and orthodontic wires. During the long term contact of the alloy with aggressive environment of human body, corrosion by releasing Ni2+ ions can occur. Thus, the usefulness of such material can be dramatically enhanced if its interface structure and surface chemistry are controlled. The octadecylphosphonate interface (ODP) synthesis, which involves a self-assembled covalently (monodentate type) bonded film of octadecylphosphonic acid (ODPA) on the oxide covered NiTi surface, produces stable and corrosion resistant interfaces. This paper introduces integrated approach to the characterization of the NiTi/ODP interfacial architecture as well as the structure of the electrified ODP/solution interface using high-resolution XPS and in situ EIS measurements. The main focus of this work was to determine the influence of the ODPA deposition method (spray and immersion) on the depth-dependent structural characteristics and orientation of ODPA molecules in the surface film by means of angle resolved XPS. Mechanically strong and chemically stable NiTi/phosphonate interfaces have the potential for their successful implementation in stent technologies.
Several cases of alumina ceramic hip replacement failures are reviewed fractographicallly. Three main findings are illustrated. Firstly, there is evidence that surgeons can damage the femoral head bore surface during surgery. Second, three of the failures described are of extended neck designs which are weaker than those of normal or short length in axial laboratory testing. Under physiological loading, such geometry can lead to levering forces, inappropriate localized contact with the metallic stem and stress concentrations. Delayed failure can ensue, with a fracture pattern quite different from that seen in conventional uniaxial testing. Thirdly, while some failures show head bore surfaces which are clean apart from metallic witness marking, others show brown stains and white deposites suggesting poor conformal contact. We suspect either stems becoms damaged during surgery before mounting the head or entrapment of debris, pointing to handling care and cleanliness varying between hospitals.
To improve the long-term performance of hip prostheses, alternative bearings with metal-on-metal and ceramic-on-ceramic couples were introduced. Although currently the results are in the mid-term stage, there have been few comparative studies of these different bearings. From 2000 to 2002, 487 total hip replacements were performed using a Bicon-Plus cup and an SL-Plus stem (Plus Orthopedics AG, Rotkreuz, Switzerland). Three groups were established for study, that differed in the type of bearing: a metal-on-metal group (69 prostheses), a metal-on-polyethylene group (200 prostheses) and a ceramic-on-ceramic group (218 prostheses). Patient demographic data and data on revision operations were evaluated from the hospital computer database. Mean follow-up was 8.5 years (range, 6.9 to 9.9 years). Patient activity was assessed by the UCLA score. The mean patient age was sixty years at the time of the index arthroplasty in the metal-on-metal and ceramic-on-ceramic groups, and seventy-one years in the metal-on-polyethylene group. At the scale of 10, the mean activity level was 6 in the ceramic-on-ceramic group, 5 in the metal-on-metal group, and 4 in the metal-on-polyethylene group. Survival for revision for any reason at 10 years was 0.984, 0.956 and 0.879 for the metal-on-polyethylene, ceramic-on-ceramic, and metal-on-metal groups, respectively. The difference between the metal-on-metal and metal-on-polyethylene groups was statistically significant (p= 0.005). Survival for revision for aseptic loosening at 10 years was 0.995, 0.990 and 0.894 for the metal-on-polyethylene, ceramic-on-ceramic, and metal-on-metal groups. The difference between the metal-on-metal group and the metal-on-polyethylene and ceramic-on-ceramic groups was now statistically significant (p = 0.001 and p = 0.003, respectively). When comparing two groups of patients of similar mean age and mean activity level, ceramic-on-ceramic bearings had a better survival than the metal-on-metal bearings at a mean of 8.5 years; the difference was statistically significant when the revision for aseptic loosening was defined as failure.