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Abstract We report zero-field low-temperature cycling of saturation remanence (SIRM) produced at 300 or 10 K for crushed natural magnetites in nine size fractions from 0.6 to 135 µm, one set annealed to reduce stress, the other unannealed. Coercivities of isothermal remanence increase tenfold between 300 and 10 K, possibly explaining an apparent transition near 50 K. 300-K SIRM decreases continuously on cooling, losing 60–80% by T V = 120 K (Verwey transition), is constant from 120 to 10 K, then recovers a small memory in warming through T V to 300 K. A dip and recovery of remanence near T V for larger (> 15 µm) annealed grains is probably due to memory of cubic domain structures by monoclinic magnetite below T V, permitting partial recovery of initial remanence. In warming, 10-K SIRM is little affected until lost catastrophically near T V. A small memory is recovered in cooling to 10 K. The contrasting behaviors of 300-K and 10-K SIRMs result from the contrasting anisotropies and domain structures of cubic and monoclinic magnetite. Memories of initial remanences after full temperature cycles are attributed to monoclinic magnetite providing a template for partially regenerating initial cubic domain structures on the second passage through T V. Memory ratios as a function of grain size for our magnetites are too scattered to be granulometrically useful.

Background and purpose — A risk-stratification algorithm for metal-on-metal hip arthroplasty (MoMHA) patients was devised by US experts to help clinicians make management decisions. However, the proposed algorithm did not cover all potential patient or surgical abnormalities. Therefore we adapted the US risk-stratification algorithm in MoMHA patients revised for adverse reactions to metal debris (ARMD) to determine the variability in the revision threshold, and also whether high-risk patients had inferior outcomes following revision. Patients and methods — We analysed 239 MoMHA revisions for ARMD between 2001 and 2016 from 2 centres with pre-revision blood metal ions and imaging. Patients were stratified (low risk, moderate risk, high risk) using pre-revision factors (implant, radiographic, blood metal ions, cross-sectional imaging) by adapting a published algorithm. The risk categories for each factor were assessed against revision year, revision centre, and post-revision outcomes (re-revision surgery, and any poor outcome). Results — Compared with hips revised before 2012, hips revised from 2012 onwards included more high-risk implants (44% vs. 17% pre-2012), high-risk radiographic features (85% vs. 69% pre-2012), and low-risk metal ions (41% vs. 19% pre-2012). 1 centre more frequently revised patients with high-risk implants (48% vs. 14%) and low-risk blood metal ions (45% vs. 15%) compared with the other. All these comparisons were statistically significant (p < 0.05). With the limited sample size available, implant, radiographic, blood metal ion, and cross-sectional imaging risk groups did not statistically significantly affect the rates of re-revision surgery or frequency of poor outcomes post-revision. Interpretation — When applying the adapted risk-stratification algorithm the threshold for ARMD revision changed over time, presumably due to increasing evidence, patient surveillance, and investigation since 2012. Lower blood metal ion thresholds were used from 2012 for ARMD revisions; however, there was evidence that centres attached different importance to metal ions when managing patients. High-risk patients did not have inferior outcomes following ARMD revision.