Modelling evaluation of the testing condition influence on the maximum stress induced in a hip prosthesis during ISO 7206 fatigue testing

Mechanical Performance of Artificial Hip Stems Manufactured by Hot Forging and Selective Laser Melting Using Biocompatible Ti-15Zr-4Nb Alloy

We investigated the microstructures, tensile properties, fatigue strengths, and durability limits of hot-forged Ti-15Zr-4Nb (Ti-15-4) alloy artificial hip stems. These properties were compared with those of Ti-15Zr-4Nb-4Ta (Ti-15-4-4) and Ti-6Al-4V (Ti-6-4) alloy stems annealed after selective laser melting. The tensile and fatigue properties of test specimens cut from Ti-15-4 stems annealed after hot forging were excellent compared with those of the Alloclassic Zweymüller Stepless (SL) stem, which is used globally. The 0.2% proof stress (σ₀._2%PS), ultimate tensile strength (σ_UTS), total elongation (TE) at breaking, and fatigue strength (σ_FS) after 10⁷ cycles were 919 ± 10, 983 ± 9 MPa, 21 ± 1%, and 855 ± 14 MPa, respectively. The durability limit (P_D) after 5 × 10⁶ cycles of Ti-15-4 stems was excellent compared with that of the SL stem. The σ_UTS values of 90°- and 0°-direction-built Ti-15-4-4 rods were 1032 ± 1 and 1022 ± 2 MPa, and their TE values were 14 ± 1% and 16 ± 1%, respectively. The σ_FS values of annealed 90°-direction-built Ti-15-4-4 and Ti-6-4 rods were 640 ± 11 and 680 ± 37 MPa, respectively, which were close to that of the wrought Ti-15-4 rod (785 ± 17 MPa). These findings indicate that the hot forging and selective laser melting (SLM) techniques can also be applied to the manufacture of artificial hip prostheses. In particular, it was clarified that selective laser melting using Ti-15-4-4 and Ti-6-4 powders is useful for the low-cost manufacturing of custom-made artificial joint prostheses and other prosthetic implants.

EVALUATING THE EFFECTS OF EXPERIMENTAL SETTINGS DURING ISO 7206-4 ENDURANCE AND PERFORMACE TESTS: A FINITE ELEMENT ANALYSIS

ISO 7206-4:2010 is used to determine the endurance properties and performance of stemmed femoral components of hip prosthesis. Experimental set-ups are allowed for tolerances with respect to the desired settings. The effects of the acceptable accuracies are not known nowadays. For this reason, this study aims at evaluating how and how much the precision of the experimental settings can affect the outputs under ISO 7206-4:2010 boundaries. Thus, a finite element model was first defined and verified against experimental test. Then, a sensitivity analysis involving accepted variations for potting level, angle in the frontal plane, angle in the lateral plane and material in use for the cement block, was performed. The results of sensitivity analysis show that both stress and deformation outputs were affected up to 100[Formula: see text]MPa and 0.5[Formula: see text]mm, respectively, for varus configurations. The material properties of the cement block had a main effect on the displacement of the head. The collected information through the performed sensitivity analysis on the verified model against experimental test has a double benefit. It supports the understanding of the potential effects during the experimental set-ups and it is also helpful in case of determining ranges for verification of developed numerical models.

Hip stem fatigue: : The implications of increasing patient mass

General trends of increasing body mass index have been observed in many western countries along with an increasing demand for joint replacement. Standards have been developed for testing the fatigue properties of femoral stems; however, the loads that these apply are based on a historic patient weight and may not be valid in the current patient population. Several fatigue tests were conducted using distally fixed titanium alloy stems positioned according to the ISO standard but with a cyclic load based on a current 75th percentile patient sample. Smaller sized stems (currently not weight restricted) fractured in; 30,000 cycles, while larger sized stems were found to have excellent durability under loads simulating walking and stumbling. The results suggest that while the fatigue properties of medical grade titanium are very good, the ISO pre-clinical durability testing standard does not represent the influence of femoral offset or stem size sufficiently to reflect safe design practice.

Comparative analysis of international standards for the fatigue testing of posterior spinal fixation systems

BACKGROUND CONTEXT

Preclinical evaluation of the long-term reliability of devices for lumbar fixation is a mandatory activity before they are put into market. The experimental setups are described in two different standards edited by the International Organization for Standardization (ISO) and the American Society for Testing Materials (ASTM), but the evaluation of the suitability of such tests to simulate the actual loading with in vivo situations has never been performed.

PURPOSE

To calculate through finite element (FE) simulations the stress in the rods of the fixator when subjected to ASTM and ISO standards. To compare the calculated stresses arising in the same fixator once it has been virtually mounted in a physiological environment and loaded with physiological forces and moments.

STUDY DESIGN

FE simulations and validation experimental tests.

METHODS

FE models of the ISO and ASTM setups were created to conduct simulations of the tests prescribed by standards and calculate stresses in the rods. Validation of the simulations were performed through experimental tests; the same fixator was virtually mounted in an L2-L4 FE model of the lumbar spine and stresses in the rods were calculated when the spine was subjected to physiological forces and moments.

RESULTS

The comparison between FE simulations and experimental tests showed good agreement between results obtained using the two methodologies, thus confirming the suitability of the FE method to evaluate stresses in the device in different loading situations. The usage of a physiological load with ASTM standard is impossible due to the extreme severity of the ASTM configuration; in this circumstance, the presence of an anterior support is suggested. Also, ISO prescriptions, although the choice of the setup correctly simulates the mechanical contribution of the discs, seem to overstress the device as compared with a physiological loading condition. Some daily activities, other than walking, can induce a further state of stress in the device that should be taken into account in setting up new experimental procedures.

CONCLUSIONS

ISO standard loading prescriptions seems to be more severe than the expected physiological ones. The ASTM standard should be completed by including some anterior supporting device and declaring the value of the load to be imposed. Moreover, a further enhancement of standards would be simulating other movements representative of daily activities different from walking.

NATURAL FIBER REINFORCED COMPOSITES FOR FEMORAL COMPONENT OF TOTAL HIP ARTHROPLASTY

This paper describes a theoretical approach to compare two types of fiber reinforced composite materials for femoral component of hip implants. The natural fiber reinforced composite implant is compared with carbon fiber reinforced composite and the results are evaluated against the control solution of a metallic implant made of titanium alloy. With identical geometry and loading condition, the composite implants assumed lower stresses, thus induced more loads to the bone and consequently reduced the risk of stress shielding, whilst the natural fiber reinforced composite showed promising result compared with carbon fibers. However, natural fibers, as well as carbon fibers, lack the power to improve interface debonding due to excessive loads in interface. Nevertheless, natural fiber reinforced composite could be an appropriate alternative given its capability of tailoring and achieving the optimal fiber orientation and robust design.